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Z
03-10-2014, 07:26 AM
"THE NEW PACKARD TORSION LEVEL SUSPENSION".
==============================================
(by F. R. McFarland, presented June 15 1955, and in SAE Transactions Vol 64, p284 1956, 560026.)

I made some comments about this paper on the "Roll rates..." thread (linked on previous post, page 25). Below is Figure 1 from the paper, and my interpretation of the system.

In the image the "Main Load Torsion Bars" are longitudinal-(side-pair)-Z-bars. The "Z" shape is quite apparent. IMO, all things considered, this particular layout is possibly the best way to do longitudinal-Z-bar suspension, both on production cars and many types of racecars. So also quite suitable for FSAE.

The entire torsion-bar+lever-arms lies in a horizontal plane at the bottom of the car, so is easy to package and gives a low CG (ie. all the "spring mass" is at the lowest possible position, and big springs, ie. for off-road or luxury cars, can be quite heavy). The front lever-arm cranks outward, allowing easy connection to the wheel, while giving room for the front-wheel to steer. This lever-arm can be in unit with the lower wishbone, or else it can be connected to the suspension by a flexible link (Packard tried both).

The rear lever-arm must now crank inward (to form the "Z"!). So the main body of the torsion-bar angles out towards the rear of the car. This allows the rear lever-arm to connect to any convenient point on the rear suspension. In this case the connection is by a short "pullrod" to the live-axle control-arm (= "Rear Axle Torque Arm"). Similar pullrod-like connection could be used on any independent suspension.

https://lh4.googleusercontent.com/-d7yirHvFrK8/Ux1ImjoEBsI/AAAAAAAAAPs/5kKGPu0_vwE/s800/PackardFig1.jpg

Other comments:
* The "Compensator Bars" at the rear act as simple corner springs (ie. no interconnections), but both can be simultaneously adjusted to reset rear-ride-height (hence "Levelizers"). This is important for a very softly-sprung luxury car, and is described as "an answer to the stylist's prayer"! IMO an adjustable lateral-Z-bar would be much better here.

* The "Rear Stabilizer Link" is a Watts-linkage for lateral control of the live-axle. This was deliberately made quite soft laterally to reduce "harshness". IMO this is poor design for too many reasons to cover here. Well, just one being rear-axle-(over)steer during cornering! Many ways to fix this, but they didn't...

* A "Front Stabilizer" (ie. lateral-U-bar) is also fitted. This, and the "Rear Axle Torque Arms", both act as ARBs, thus stiffening both the Roll AND Twist modes. IMO this shows a lack of deep understanding of the whole system...

* IMO the Packard engineers didn't seem to grasp this whole concept nearly as well as the French, who were doing this sort of thing twenty years earlier. The whole paper seems to be focussed on the side-view, 2-D behaviour of the car in "bounce and pitch". Admittedly, they had better understanding of these motions than is currently the case with the modern concepts of "front and rear ride frequencies" (see extensive ranting elsewhere! :)). But nowhere in the paper is any Twist or Warp-mode behaviour mentioned. Well, except, and only (!), the last Summary point "8. Reduced torsional stresses in frame.").

* Note that to understand Twist-mode motions, you have to think in 3-D, with the four wheelprints starting in a horizontal plane, and their vertical motions taking them out of this plane. By comparison, the "bounce and pitch" motions discussed in the paper are contained entirely in a 2-D, side-view plane. This limited 2-D thinking seems to have prevented the Packard engineers from fully appreciating the advantages of a soft Twist-mode.

* Finally, the paper starts by noting that "automobile developments seem to appear in cycles", such as manual to automatic gearboxes, straight-8 to V-8 engines, and so on. It ends with "The time is ripe for a cycle of development in automotive suspensions ... it would seem that within the next 2 to 5 years, we should see some radical changes in suspension design.". (My emphasis.)

Well, that didn't happen! And a good indicator of why not is in the subsequent discussion to the paper. The other manufacturer's engineers quite clearly did NOT understand the Packard system (not even its dumbed-down, 2-D, side-view version), as they gave some completely false criticisms of it. Having led with this bulldust, they then launched into a marketing spiel about how great their suspensions were!

Ahh, nothing changes... :)

More in a few days...

Z

Z
03-20-2014, 09:53 PM
WHY A SOFT TWIST-MODE???
==========================

It has been suggested (on another thread) that while a soft Twist-mode might be advantageous on bumpy roads, it might offer NO such advantages on smooth, sealed roads, such as those typical in circuit racing.

Those of you who prefer to "follow the numbers", rather than the unquantified opinions of experts, please read on...
~~~o0o~~~

Milliken's RCVD, Chapter 18 "Wheel Loads" starts with,
"The [vertical] loads at each wheel are extremely important in determining a car's maximum steady-state cornering capability." (my emphasis).

The chapter goes on to give examples of how to calculate these vertical wheel loads. Quite reasonably, these calculations are simplified by assumptions such as,
"...steady-state operating conditions - that is, smooth roadway, constant speed cornering, constant longitudinal acceleration, constant grade, etc.
... roll rates, spring rates ... are linear,
... chassis of the car ... is [torsionally] rigid.", and so on.
In fact, there are about 5 pages discussing the importance of a torsionally stiff chassis, because,
"... if the chassis torsional spring is weak, attempts to control the lateral load transfer distribution (and "balance" the car's handling by resisting more of the rolling moment on one track than the other) will be confusing at best and impossible at worst." (my emphasis again).

Equations for calculating the variations to wheel loads from a large number of different factors are then given, including,
* CG position,
* lateral and longitudinal load transfer from horizontal Gs,
* banking,
* crests and dips in the road (albeit in a 2-D vertical-longitudinal plane only),
* aero loads,
* engine torque reaction (for front-engine -> live-rear-axle drivetrain).

It is quite clear, however, from the seven-plus pages devoted to it, that the Millikens believe that Lateral Load Transfer Distribution is the most important factor to be considered when "adjusting handling balance" (which, in this particular area, I agree with). I repeat this for emphasis, if the wheel loads do NOT change as per your intended LLTD (or Claude's "Magic Number"), then the car will not handle the way you expect it to.

At the end of the chapter is "18.11 Summary Example". This works through some of the above calculations for what might be a "sportscar", or perhaps a fairly softly sprung racecar (the corner-spring and ARB rates are a lot less than the tyre rates, so the car is not a very stiffly-sprung aero-car). Right at the very end of the chapter, on page 708 (my older version), is Table 18.1 summarising the changes in wheel loads due to the various factors. For this particular example the "Banking" effect is quite large (ie. oval track racing), the "Aero" effect quite small (ie. no big wings), and, quite clearly, the LLTD is by far the most important effect.

Please go through the RCVD example in more detail, but for now take it that the car is slightly front heavy, but roughly with about 900 lbs weight per wheel. There is a Total LLT of about 800 lbs (from the two inside wheels, to the two outside wheels). This is distributed by the "springs, bars, and RC heights" as +/-430 lbs front, and +/-370 lbs rear, giving LLTD = 54%F, 46%R.
~~~o0o~~~

Now the twist in the story. :) Nowhere in this 40 page chapter is any mention made of any TWIST in the road! All four wheelprints are ALWAYS considered to be lying in a PERFECTLY FLAT plane!

Fortunately, there was a large blank space at the end of the chapter, so I added some more calculations. I imagined that the road is very "smooth", but it is also cambered in the usual manner so that the road surface has a cylindrical shape, which in "end-view" has a radius of about 40 metres. So, if the two edges of the road are 10 metres (30 ft) apart, then the centreline of the road is 0.3 metres (1 ft) higher than the edges (quite typical of real roads).

Driving parallel to the centreline of this road introduces no Twist into the suspension, even if the road curves around a bend. But a car with ~3 metre wheelbase and ~1.5 metre track, driving diagonally across this road at an angle of about 15 degrees to the centreline, has about 7 mm (1/4") of Twist-mode between its four wheelprints (ie. one diagonal pair of wheelprints are up 7 mm, and the other diagonal pair down 7 mm, wrt car-body). Please do the calcs to assure yourselves of this.

Furthermore, if the car is doing 100 mph (~45 m/s) while following this diagonal line from the outside of the road towards the inside "apex", then it will spend almost a full second with its suspension constantly "Twisted" by 7 mm. So the Twist is effectively "steady-state". But when exiting the corner, from inner apex to outside of road, the Twist will be in the opposite direction!

And even furthermore, if the road surface is smoothly cambered "concave up", as is common with banked corners, then the Twist introduced by a diagonal driving line is of the same magnitude as above, but of opposite sign.
~~~o0o~~~

So, the big question:
What does this twist-in-the-road do to your precisely calculated wheel loads?

Based on the (quite soft) corner-spring and ARB rates in the Milliken example, the 1/4" Twist changes the wheel loads by about +/-160 lbs! And depending on which way the Twist is, the LLTD ends up being either 74%F, 26%R (for corner entry of convex-up road), or 34%F, 66%R (corner exit, convex-up road). Put simply, the handling balance changes from massive understeer on corner entry, to massive oversteer on corner exit. Yippeeee!!!

Anyway, there are a whole lot of other effects which should also be considered, some of which lessen the above changes, others which exacerbate them. But the bottom line is that with conventional suspensions, all your precise "handling balance" calculations get tossed out the window as soon as you put the car on a real road. And THE STIFFER THE SPRINGS, especially the Roll and Twist-mode stiffening Lateral-U-Bars (= ARBs), THE WORSE! Please do the calcs.
~~~o0o~~~

Finally, it is worth noting that FSAE's short-wheelbase-small-track cars don't feel the above sort of twist-in-the-road as much as larger cars (because the further the wheelprints are apart, the further the road surface moves out of a flat-plane). But any "twist-in-the-road" will still change the wheel loads of your FSAE car.

How much? Easy to measure! Put your car on its four corner scales, on FLAT ground. Adjust your spring-mounts so that the corner-weights are symmetrical side-to-side. Now slip a 6 mm thick piece of plywood under two diagonally opposite wheels (or a single 12 mm piece under one wheel). This represents a Twist-mode of 3 mm (1/8"), which might represent some parts of some of the "wilder" FSAE tracks. Write down the changes in the four wheel loads.

Now ask yourselves why you bothered doing all those precise "handling balance" calculations in the first place. Because, with conventional suspensions, the road decides what the LLTD is, not you!

Z

MCoach
03-20-2014, 11:23 PM
.......*slow clap*

http://www.youtube.com/watch?v=TAryFIuRxmQ


Z, I will have to take you up on this one and add a page to my calculations.

While the "billiard smooth surfaces" that FSAE runs on may not see this effect very much, many of the testing surfaces that these cars are put on are not. Honestly, Baghdad probably has some roads with a better surface than our test lot.
It interests me how significant the effect may be so logically, I must investigate.

In related news, an article that is somewhat related that I was reading today dealt with ensuring that cars remained stable once a wheel was lifted, either front or rear. The concern was mostly with FWD cars, but it's also applicable to other drivetypes. Whether it's due to road surface, inherent vehicle layout, or whatever it may, the car will eventually lift a wheel in the right conditions -- even if it's an LMP or F1 banging off of kerbs. It's important to learn the basics of what happens at steady-state in the right conditions, but it gets weird when there are exceptions and oddities which very well happen in everyday life.

Thank you for supplying a relevant example of this condition.

murpia
03-21-2014, 01:54 PM
Hi Z,

Presumably if you factor in body slip angle (approx. rear tyre slip angle neglecting toe), the effect is even greater?

Regards, Ian

Tim.Wright
03-22-2014, 07:36 AM
Good read...

I also realised some years ago that for lap simulation, the track shape (im talking about camber, banking and inclination, not bump profiles) is a huge chunk of the overall system. After spending months trying to match a lapsim to track data and failing at one particular corner, I went to the track in question and noted about 7-8deg of banking on this corner (this was a typical european supposedly flat closed circuit, not an oval). I built a 3D track model based on eyeball measurements from a couple of laps I did with a support car and the sim behaved completely differently. Compltely threw out the driver model, but thats another story.

I have to say though... a 300mm rise and fall across 10m is quite an exaggeration. Monza for example is 10m wide (which is extremely narrow for a typical F1 track) for pretty much the entire lap... imagine a 300mm rise in the middle of this. I'd just about class that as offroad.

http://www.gross.it/uploads/pics/Monza.jpg

Tim.Wright
03-22-2014, 08:04 AM
Also regarding the longitudinal Z bar layout, I have to say that image above shows what a bulky system this would be to package. The floor area of any modern car (road or race) is so absolutely full of stuff that there is nowhere to put these torsion bars.

The low CG argument doesn't fly with me either since so many heavy things would need to be RAISED in order to fit these bars, you would probably end up back at square 1 for CGh.

Also, how would you go about getting adequate roll damping?

Buckingham
03-22-2014, 03:39 PM
the handling balance changes from massive understeer on corner entry, to massive oversteer on corner exit. Yippeeee!!!

Funny, thats what just about EVERY NASCAR driver complains about ALL THE TIME. Especially when comparing the "Old Cars" to the "COT" (with it's life savingly strong center section)

dazz
03-23-2014, 10:04 PM
Also regarding the longitudinal Z bar layout, I have to say that image above shows what a bulky system this would be to package. The floor area of any modern car (road or race) is so absolutely full of stuff that there is nowhere to put these torsion bars.

The low CG argument doesn't fly with me either since so many heavy things would need to be RAISED in order to fit these bars, you would probably end up back at square 1 for CGh.

Also, how would you go about getting adequate roll damping?

If only there was a way to create the same result as the Z bars using a different method, I can think of one. Potentially lighter too! (Electronics has nothing to do with it either...)

MCoach
03-23-2014, 10:41 PM
If only there was a way to create the same result as the Z bars using a different method, I can think of one. Potentially lighter too! (Electronics has nothing to do with it either...)


If it involves any long length of hydraulics, keep in mind that fluid has an inertance property that you can use to your advantage, or destroy your day if you over look it.


EDIT: Bad Autocorrect.
This is what I get for commenting from mobile.

mech5496
03-24-2014, 11:18 AM
It can be bars, push-pull rods, wire, hydraulics...many ways to implement it actually!

BillCobb
03-24-2014, 11:50 AM
The only fluid generally recognized to have inheritance potential is .. well you can probably guess (hopefully).

All fluids have inertance, though. It's usually best to exercise good inertance when dealing out your inheritance fluid.

Z
03-25-2014, 09:06 PM
Back in 2005 I posted on this subject on the "Damper Histograms" thread (http://www.fsae.com/forums/showthread.php?3254-Damper-Histograms&p=91827&viewfull=1#post91827). John Bucknell was kind enough back then to post on that thread some images that I sent him. I repost part of John's post here to show that nothing much has changed. (See also the current "Roll Rates in RCVD" thread for more of "nothing much has changed".)

The deep lack of understanding of how these things work is evident in the images below that are taken from various VD/Suspension Design textbooks (and the notes I made when I first read them :)). The lack of understanding of how to calculate modal-spring-rates is seen in the 2CV image. Tim's comments about how "it's just toooo haaaaard to package..." is seen to be utter nonsense from the BLMC images (the interconnecting hydraulic pipe is about the size of brake line). The lack of understanding of Z-bar ERMD is evident in the Olley book (compare with original Packard picture above).

BTW, I had one of the BMC cars, and it rode well over bumps and cornered very flat. The "big-picture" engineering was very well done. However, the company went down the crapper, NOT because of the suspension, but because of pathetic detail design (similar to that seen in a lot of FSAE, especially English and Indian!), totally incompetent Top-Level management, and a lazy, unionised, and militant workforce.

The following is cut-and-pasted from John's 2005 post:
~~~~~o0o~~~~~

http://i16.photobucket.com/albums/b33/john-bucknell/ZBAR2CVsmall.jpg
"Citroen 2CV and BMC, from "New Directions in Suspension Design", Colin Cambell, 1981 (with some comments by Z)

http://i16.photobucket.com/albums/b33/john-bucknell/ZBARPKRDsmall.jpg
Packard, from "Chassis Design...", based on notes by Maurice Olley, pre-1960? (with more comments...)

http://i16.photobucket.com/albums/b33/john-bucknell/ZBARFSAEsmall.jpg
Z-bar sketches by Z"

Z

Tim.Wright
03-26-2014, 06:09 AM
Tim's comments about how "it's just toooo haaaaard to package..." is seen to be utter nonsense from the BLMC images (the interconnecting hydraulic pipe is about the size of brake line). The lack of understanding of Z-bar ERMD is evident in the Olley book (compare with original Packard picture above).

The images don't work for me (I think its because of my work firewall) but if you are introducing hydraulics, then yes the packaging is easy, but you have now significantly increased cost and complexity.

So you either need a simple but bulky mechanical system, or a compact but complex (in relative terms) hydraulic system. Why is it nonsense that manufacturers don't want this added complexity?

Markus
03-26-2014, 09:11 AM
Tim, both are mechanical systems.

The one marked for FSAE is packable albeit likely a bit on the heavy side. Might be a challenge to find a suitable torsion bar - motion ratio combination to achieve high stiffness, low weight and low compliance at the same time. But even if the system is on the heavy side at least everything is down low. Would love to see one of these in action.

The other one might be on the bulky side but as we have seen there's imaginative ways to address packaging problems... :)

Jonny Rochester
03-26-2014, 09:49 AM
QU, Queensland Uni are designing their car for this year with cables interconecting front and rear suspension. Is this related to Z bars in any way?

nowhere fast
03-26-2014, 07:39 PM
QU, Queensland Uni are designing their car for this year with cables interconecting front and rear suspension. Is this related to Z bars in any way?

This sounded intriguing, so after some searching I found this on UQ's website (http://uqracing.com/):

http://uqracing.com/wp-content/uploads/2013/08/1011273_10152281134426197_1374632189_n.jpg

I don't know what their aims are with this system; but I would say this interconnection is related to the discussions on Z-bars, as the cables + front coilovers are working as Z-bars. There are a few other interesting elements on display there too.

nowhere fast
03-26-2014, 08:57 PM
To comment a bit more on the UQ suspension:
Spring-wise this is effectively a 3 Z-bar (2 longitudinal, 1 lateral) setup like those pictured in Z’s sketch above. The connecting cables combined with each front coilover act as the longitudinal Z-bars supporting heave and roll, while the rear coilover acts as the lateral Z-bar supporting heave and pitch.

LLTD could be adjusted by using multiple pick-ups for the cable ends on the bellcranks to change the ratio of rear axle twist to front axle twist when the front springs remain the same length.

There are only 3 dampers pictured, leaving the twist mode is undamped. The design clearly isn’t complete though, so this may change?

Z
03-26-2014, 10:29 PM
Originally posted by Tim (page 26):
The floor area of any modern car (road or race) is so absolutely full of stuff that there is nowhere to put these torsion bars.


Originally posted by Tim (page 27):
Why is it nonsense that manufacturers don't want this added complexity?

(Note contradiction above...)


Originally posted by Z on "Reasoning..." thread:
* Don't argue with a fool. First they drag you down to their level, then they beat you with experience.
Ah, those first "concept" meetings...
~~~o0o~~~

Jonny, Nathan,

Yes.

Z

Jay Lawrence
03-26-2014, 11:58 PM
It's possibly been answered before, but for the benefit of those unburdened with brilliance can someone explain how any of the discussed Z-bar suspensions would allow soft twist or individual wheel mode? Surely if you try to lift any individual wheel you have a Z-bar and a lateral leaf working against it (to use Z's "For FSAE" sketch above).

Also: I understand how the longitudinal Z-bar would work at improving body control for a ride event (as shown under the "Moulton hydolastic suspension" image above), but in a pitch event surely the same bars would create essentially a pro-pitch situation, necessitating heavier ride springs (and exacerbating the twist/single wheel problem)?

Lastly, assuming the cable in the UQ picture is infinitely stiff in tension, any roll event would have to be taken by the coil-overs anyway? This is a bit different to the Packard and Z systems, as they have rotatable torsion bars instead of rigid cables. They don't appear to have any lateral restraint, but as Nathan says it doesn't look finished...

Kindly un-befuddle me...

Z
03-27-2014, 02:06 AM
Kindly un-befuddle me...

Jay,

I think you can best be "fuddled" (or "re-fuddled"?) by looking at the little cars at the very top-right of the "Z-Bar" sketch. (As noted elsewhere, these are shown with 4 x Z-bars for clarity of the "twisting" motion, although only 3 are needed.) Anyway, the central diamond-shaped section represents the rigid chassis, and the four Z-bars form a rectangle connecting the four wheels.

As long as there are flexible pivots between Z-bars and chassis, then the four wheels can move in Twist as shown (top-right). Single-wheel bumps are also possible. Note now that with "rigid" Z-bars and a single-wheel bump of 4 cm height, there will be 1 cm of Twist movement of the "suspension", and also 1 cm of chassis movement in each of Heave, Pitch, and Roll. (Oops, might have just "re-befuddled" you there...)

I made a little model of this sort of stiff-Z-bar suspension back in the late 1990's, to help explain the concept to people. Geoff (BB) recently spent quite some time rolling this model back-and-forth over his finger, to see how it soaked up the "bump". Building a model like this is probably the best way to "fuddle" yourself on how it works. :)
~~~o0o~~~

The "Moulton Hydrolastic suspension" is actually very cheap and simple (contrary to Tim's moaning :)).

The front-upper-wishbone and rear-trailing-arm are connected, at a MR = ~0.3 (or less?), to a pushrod that is rigidly fixed at its top to a shaped "piston" about 10 cm diameter (from memory?). This piston pushes against a rubber diaghragm similar to the "air-bags" commonly used on modern truck suspensions (so the diaphragm material is similar to a tyre sidewall). However, unlike the air-bags, above the Moulton diaphragm is a chamber filled with water + glycol (= antifreeze), hence "Hydro". The front and rear "Hydro" chambers are connected with a simple pipe.

In the "Hydrolastic" version there is a thick rubber disc sealing off the top of the chamber. This deflects upwards, mostly by shear strain, to provide the springing. The later "Hydragas" versions have a heavy-duty rubber "balloon" in the top of the chamber (now capped with a steel dome), with the gas in the ballon providing the springing. Air (or nitrogen) pressure can be adjusted via a conventional tyre filling valve, but is at considerably higher than normal tyre pressures (I think about 10 bars, 150 psi??? Edit: ~10 bars unloaded, going up to ~20 bars with the car's static weight on the system).

Importantly, both "Hydrolastic" and "Hydragas" springs have rising-rates in bump. In addition to this, the "piston" is also shaped to give a tunable rising-rate (from its increasing sectional area against the diaphragm). This gives what I earlier described as the "pendulum" springing. Namely, the rising-rates mean that no other springing AT ALL is needed to control Pitch. Put simply, when the car pitches forward the bump deflection of the front unit causes it to exert a greater force, and the rebound deflection of the rear unit causes it to exert a lesser force. Pitch problem solved!

Lastly, why not just have interconnected air-bags, front and rear (the Hydragas without the "Hydra")? Well, the liquid-filled chamber of each suspension unit is divided horizontally by a steel plate. This has holes in it which are covered by shims and act as the damper valves. Liquid works better at damping than gas.

Interestingly, because this "damper" is so large in diameter, the holes are quite large (large flows) and the shims are again made of rubber. Essentially, they are just largish rubber flaps covering the holes.

Ah, yes Tim, ... "the cost and complexity"...!
~~~o0o~~~

UQ's system has a stiff-cable in the middle, stiff-rockers at each end, a stiff-link at one end, and a squashy-link at the other. Packard's system has stiff-links at each end, stiff-rockers (lever-arms) at each end, and a squishy-torsion-bar in the middle.

They might be different physically, but functionally they are the same (well, similar).

Z

Z
03-27-2014, 02:21 AM
I recently received this PM asking about "Roll Stiffness". I will give my opinions here in case there are other students out there with similar questions. (I leave the OPs name and school anonymous in case they wanted it kept private.)


Hello Z,

I am from ... University in ... USA. I have been struggling with a concept that I believe is very simple, but I cannot find the answer online or in any of my books, I am likely looking in the wrong place because I am ashamed that I do not know. Never the less. Why do we want "some" roll in a race car. As in why do I not want to put my roll center on top of the COG and completely eliminate roll. Or above the COG and make the body "lean into the turn" I know roll bars are used to tune the roll stiffness, what happens when you go to stiff.

My two theories is:

1. Roll allows the dynamic camber gain to take effect
2. If the roll stiffness is too high the suspension is not really working well to isolate bumps.

I don't like either one of these, so I am looking for reason number 3.

Appreciate any help you can give, even if it is just a point in the right direction of where to look. Like I said, I am ashamed I cannot find the answer on my own, but I have spent about 6 hours searching for something I think should be obvious.

(Signed...)

"... why do I not want to put my roll center on top of the COG and completely eliminate roll. Or above the COG and make the body "lean into the turn"..."

Well, if you have independent suspension and a RC at CG height, then you can have quite significant jacking problems, which can be quite bad... You can cure this with stiff Anti-Axle-Heave springs as mentioned on the Swing-Axle sketch and post, quite a few pages ago. But the fact that the "suspension" is effectively rigid in the direction from wheelprint to CG means that you can get the "bouncing on tyres" problems listed in the last section below.

A beam-axle with RC at CG height would behave a bit better, especially if it had a small amount of softish Axle-Bounce and Axle-Roll springing. But, generally, high RCs on beam-axles are NOT good on rough roads, because single-wheel-bumps act through the RC to "kick" the body sideways. The equal and opposite lateral force acting at the wheelprint then makes the tyre lose grip of the road.

But on the smooth FSAE tracks you could get away with this.
~~~o0o~~~

"1. Roll allows dynamic camber gain to take effect."

Camber "gain" (aka camber "recovery", or "compensation"...) is really only a half-baked attempt to keep the wheels at an OK-ish inclination angle during cornering. Most independent suspensions (wishbone, strut, etc.) on most cars have well UNDER 100% "camber recovery". This means they LOSE camber during cornering (ie. the body-roll causes the wheels to adopt bad-ish camber angles). If they had no body-roll, then no camber loss at all (except for tyre squash), so better.

Some suspensions have more than 100% camber recovery, so their wheels actually lean slightly into corners as a result of their body rolling outwards. This makes body-roll advantageous in corners, but these suspensions are in the minority (eg. pre-WW II Mercedes GP De-Dion rear suspension, some modern USA oval-track front suspensions, ...). But there can also be significant disadvantages here in other situations (ie. excessive camber change during heave and pitch motions, and large gyroscopic forces over bumps (though not on the Merc De-Dion ;))).

The "Swing-Axle" and "Swing-Arm" suspensions that I sketched a few pages ago have 100% camber gain, so have NO camber loss during cornering. Importantly, this is regardless of the amount of body-roll, from completely roll-stiff, to lots of body-roll. There is camber change from pitch motions (eg. during accelerating or braking), but there are less of these on an FSAE track than roll in corners, so overall this sort of suspension is suited to FSAE.

Bottom line here, you do not NEED body-roll to have your wheels at a good camber angle. And, for most independent suspensions, body-roll worsens the camber angles.
~~~o0o~~~

"2. If the roll stiffness is too high the suspension is not really working well to isolate bumps."

This is probably the most correct reason. But this is not too important in FSAE because the tracks are so smooth. However, it is certainly one of the main reasons not to have too much roll stiffness in a conventionally suspended road car (or sports car, or "Targa" racecar, etc.).

And, as covered at length above :), excessive roll stiffness of conventional suspensions completely stuffs your handling balance on anything but a near perfectly flat road. Put simply, the more stiffness in your springing (corner and ARB), the greater the variation in your LLTD when on "real" roads (= NOT perfectly flat).
~~~o0o~~~

"I don't like either one of these, so I am looking for reason number 3."

Another good reason for softening a suspension is to suppress "bouncing on the tyres". Cars with effectively rigid suspensions, as have won FSAE in the past (it is a historical fact, Pat), can develop a sideways stick-slip-stick... of their tyres during cornering, which then becomes a bouncing motion of the car on its outer tyres. This bouncing causes a massive reduction in cornering capability. And, importantly, this can happen on a perfectly SMOOTH AND FLAT road.

A small amount of suspension movement is all that is needed to snub out this behaviour. About +/- 5 mm is all an FSAE-sized car needs (+/- 10 mm at most). Having a low spring-rate for this movement means that the damper forces can also be quite low. This type of behaviour is worst when running medium to high tyre pressures. Low tyre pressures, and the internal damping of racing tyres, can sometimes provide enough "suspension" to snub out this bouncing.

Z

Jonny Rochester
03-27-2014, 02:42 AM
The bits of UQR-14 "toad" that worry me, is the chassis looks too light, and is ment to be 23kg. (March 16, facebook). But what would I know, it just looks too light, would flex a bit. The other problem is finding cables that are infinitely stiff in tension. I feel there would be as much spring in the cable as there would be in the actual springs. But I'm sure they have it figured out as Big Bird has been up there.

Pat Clarke
03-27-2014, 03:12 AM
FSAE rules require a minimum 50mm of suspension travel.

Pat Clarke

Jonny Rochester
03-27-2014, 03:39 AM
But do you have to demonstrate the 50mm travel by putting 1 tonne on it to make it move?

Marshall Grice
03-28-2014, 11:50 AM
FSAE rules require a minimum 50mm of suspension travel.

Pat Clarke
the rules require that the suspension be capable of 50mm of suspension travel, they do not require you to use any of it during the event.

Mitchell
03-29-2014, 11:04 AM
The bits of UQR-14 "toad" that worry me, is the chassis looks too light, and is ment to be 23kg. (March 16, facebook). But what would I know, it just looks too light, would flex a bit. The other problem is finding cables that are infinitely stiff in tension. I feel there would be as much spring in the cable as there would be in the actual springs. But I'm sure they have it figured out as Big Bird has been up there.

The UQR design has evolved considerably since those renders. Our design team have been spending many late nights trying to get a design together to meet our deadlines. We have shared bits and pieces along the way, but the final product will be different again. As far as suspension goes it will be mode separated low warp stiffness - just linked differently to what was previously shown. The reason for change is mostly driven by simplicity and availability of springs. The chassis is so light because the vehicle concept allows it to be, this was a major driver of our concept.

As a learning exercise, diving into an unknown concept has taught us more in 3 months than 2 years of iterations. For most of our members the cars competition performance is second to the educational opportunity of developing something from a blank page. That said, we believe it will perform - especially with the weight advantage over a conventional setup. We don't expect to get it right first time (or anytime really), so we are aiming for 4+ months of testing.


FSAE rules require a minimum 50mm of suspension travel.

Pat Clarke
To what datum? What reference plane? What suspension mode? Do all wheels need to travel simultaneously?
More specifically, do you think that any FSAE car has 25mm of wheel compression statically?

Mitch Bessell - UQ Racing

MCoach
03-29-2014, 02:14 PM
Mitch,

Our car has 32mm of wheel compression at static ride height with driver. Just so you know. :)

dynatune
03-29-2014, 07:08 PM
For people like you that want to learn there is a specific tool available. Have a look at it : www.dynatune-xl.com . You can learn about how to design a suspension and see what these parameters can do on full vehicle behavior. You can even create your own tire and learn about what makes the tire work.

Cheers,

Dynatune, www.dynatune-xl.com

Pete Marsh
03-29-2014, 08:39 PM
Mitch,

Our car has 32mm of wheel compression at static ride height with driver. Just so you know. :)

I think he means static compression from the droop stops, at ride height with driver, not available compression travel to the bump stops.

32mm, (or 25mm ) of free droop would produce a ridiculously soft car, and I have never seen one running that soft.

Ironically, the soft twist designs are capable of giving the required 25mm without the low rates if it was ever enforced at scrutineering, but all the conventional cars would be rubbish (or spend a lot of time changing springs to get checked), so that will never happen.

Pete

SomeOldGuy
03-29-2014, 10:30 PM
Some of our cars in recent history ran over 50 mm, close to 60 mm if I remember correctly.

It was done with rising rate rockers so that those cars were not too soft. Of course there is no real reason to have that much droop travel and the rising rate made it a bit harder to tune.

MCoach
03-29-2014, 10:56 PM
Ah, it doesn't compress 32mm from the weight of the driver and chassis, that's only about 20mm. Admittedly, we do run a very soft car with a big drivers. I can think of one one or two teams that run softer.

SomeOldGuy, I was reading a thesis paper recently which analyzed a suspension system and breaking it down to the basic equations again. The goal was optimization of the system as an electrical circuit with varying amounts of resistance, capacitance, and two different types of inductors (someone may be able to figure out why). It turns out that having a 'helper' spring with high spring rates help tire grip when the primary springs are at high spring rates because it allows the system to deform to the road with disturbing the chassis and causing unwanted load transfer. However, the stiff spring rates allow for proper rate of load transfer during maneuvers that occur in high performance situations so the helper springs are only allowed a few mm of travel. If I can dig it up, I can link it here later.

Buckingham
03-30-2014, 01:17 AM
Well, if you have independent suspension and a RC at CG height, then you can have quite significant jacking problems, which can be quite bad...

How significant is 'significant' ??

We built a car with 4" / 7" RC F/R (not at the CG, but plenty above ground) so that we could run soft springs and no ARB because the competition at the time was at a paved baja course known as the Silverdome (04). Virtual swing arm length was equal to 1/2 track width. ARBs seemed really dumb to me... compared to just running more spring - they stiffen 1 wheel bump and roll, but leave pitch soft... at a track that was known to scrape the rivets off the pan under your feet because of bumps and transitions in braking zones.

Was there a lot of Jacking.. yup. Did we figure that out ahead of time.... nope. Was the car super quick in slalomns... yup. Could the car handle rough bumps... like a champ. Was the front outer wheel cambered the wrong way during cornering because of jacking, short swing arm, and soft springs... yup. Did the car win the skidpad event anyway... yup. Pretty sporty in AutoX and Enduro.

Not something I would do for a smooth test track, but if I was going back to Pontiac it would be a good starting point.

Z
03-30-2014, 09:50 PM
For people like you that want to learn there is a specific tool available. Have a look at it : www.dynatune-xl.com.

Dynatune,

Can your software REALLY help Mitchell design the type of suspension he is planning?

That is, can your software model Mitchell's (or UWA's++) type of TWIST-SOFT, interconnected springing???
~~~~~o0o~~~~~

MCoach,


... It turns out that having a 'helper' spring with high [low???] spring rates help tire grip when the primary springs are at high spring rates because it allows the system to deform to the road with[out] disturbing the chassis and causing unwanted load transfer. However, the stiff spring rates allow for proper rate of load transfer during maneuvers that occur in high performance situations so the helper springs are only allowed a few mm of travel...

Assuming you meant "low"-rate for the helper springs, then this is what I was suggesting earlier. The spring-rate curve for the wheel, from full droop to full bump, has a soft (= low-rate) zone in the middle, about +/- 1 cm either side of ride height. This means you only need quite low damping in this zone to snub out any unwanted oscillations. During high-G cornering the suspension is in the stiffer (= higher rate) zones, and these rates determine LLTD.

This multi-rate springing can be achieved with two conventional coilsprings in series, with one becoming "solid" (= coil-bound) at the changeover point from soft to harder. Or you simply use appropriate bump rubbers. And, of course, the definition of "full bump and droop" implies that the spring-rate becomes VERY MUCH stiffer there. Nothing in the real world stays "linear" for very long.

The benefits of this sort of multi-rate springing are rediscovered every few years, for as long as I can remember. But interconnected springing works better, and it can (and should) also be multi-rate.
~~~~~o0o~~~~~

Buckingham,


How significant is 'significant' ??
...
Was there a lot of Jacking.. yup. Did we figure that out ahead of time.... nope. Was the car super quick in slalomns... yup. Could the car handle rough bumps... like a champ. Was the front outer wheel cambered the wrong way during cornering because of jacking, short swing arm, and soft springs... yup. Did the car win the skidpad event anyway... yup. Pretty sporty in AutoX and Enduro.

Not something I would do for a smooth test track, but if I was going back to Pontiac it would be a good starting point.

I agree! Something is "significantly bad" when it costs you "significantly". Your high RCs did not seem to cost you much at all, so NOT a "significant" problem! Which is why I proposed, sketched, and recommended the "Swing-Axle" suspension (quite a few pages back). This seems to be functionally very similar to your short-FVSA, highish-RC, suspension, although if yours was with double-wishbones, then too complicated! :)
~~~~~o0o~~~~~

I would like to restress to students that (in my very "non-expert" opinion) ANY of the suspension types covered in this thread can be used on an FSAE car capable of winning the comp. The questions that are asked of the suspension by the currently very smooth FSAE tracks, are so simple that even the "stupidest" suspension can get a pass mark. (Well, the suspension can be "kinematically and elastically stupid", but it should be structurally strong and stiff.)

It is only when the organisers start to put a lot of real bumps in the tracks that you students will have to start thinking about "cleverer" suspensions. As Buckingham noted, that means softening the springs. But, I suggest, you should think of the "right" sort of softness to add ... :)

Z

mdavis
03-30-2014, 10:10 PM
This means you only need quite low damping in this zone to snub out any unwanted oscillations. During high-G cornering the suspension is in the stiffer (= higher rate) zones, and these rates determine LLTD.

Z

Z,

How would you propose having the variable displacement damping?

We (Bilstein) have a system that would work (it came from a necessity to pass a certain European standard crash avoidance test), but it doesn't seem like it would be very well when applied to a "typical" FSAE car. If the car had direct acting dampers, then it would be possible (the Bilstein system adds a lot of dead length to the damper, which eats travel for a given length), but is still somewhat difficult to use, and would not be easily adjustable (dampers would need to be rebuilt to change valving, which is rather time intensive with that product).

-Matt

Z
03-31-2014, 06:54 AM
Matt,

If you mean by "variable displacement damping" that the damping rate (ie. force/velocity) varies according to position along the damper's stroke, then this is very common on off-road racing dampers.

Briefly, these "monotube" dampers have a number of holes along the working length of the cylinder, and these holes are interconnected via external bypass tubes that allow different amounts of fluid to flow past the piston, at different piston positions along the stroke. The damper valving is actually very easy to adjust or change on these systems, because most of the valving (mainly for low-speed) is fitted to the external bypass tubes. From memory (?) the piston itself still carries the main high-speed blow-off valves.

But I was suggesting that a soft spring-rate zone in the centre of the suspension stroke allows a lower than usual, but CONSTANT DAMPING-RATE, to suppress unwanted oscillations.

Again briefly, if a "stiff" spring-rate suspension has dampers that give, say, only 0.2 x critical-damping, then you can expect several cycles of oscillation after any "event". These oscillations can be of the wheel (= hop), or car-body (= heave, pitch, or roll). To keep the oscillations down to about half-a-cycle you need more damping. In practice about 0.7 x critical is enough. But the nasty truth is that more damping means less grip on bumpy surfaces (ie. more damping = more Fz variation = less Fy capability).

Putting a soft spring-rate zone in the middle of the stroke means that exactly the same dampers as above now have, perhaps, 2 x critical-damping in the soft zone. So any oscillation movement that passes through this soft zone is, in effect, very heavily damped.

Note that having the soft spring-rate for the full range of suspension travel, and the same level of damping as above, gives a very highly damped suspension with no chance of oscillations. But the suspension springs are now so soft that the car goes onto its bump or droop stops, in heave, pitch, or roll, during any sustained high-G manoeuvres.

So add extra spring-rate, but only towards each end of the suspension stroke.

Z

Z
07-15-2014, 01:03 AM
I am posting this here because this thread already has several mentions of the Citroen 2CV.

My Citroen-Mechanic/Sportscar-Wheeler-Dealer mate is nearing the end of a restoration of a Porsche 911, so is looking for the next project. He recently became aware of the following kit that can be bought from the UK. He has acquired a very cheap 2CV body and chassis and is preparing for the upgrade.

Sparrow Automotive Citroen-2CV+BMW-flat-twin-engine website. (http://www.sparrowautomotive.co.uk/citroen2cvbmw.html)

From that webpage is this YouTube video of the car at Mallory Park.

Sparrow Automotive 2CV-BMW at Mallory Park. (http://www.youtube.com/watch?v=hAocmae9vyE)

Ok, so it is only a "track day", and the other cars are NOT super-high-performance racecars (only "hot hatches", etc.), and are probably being driven by amateurs.

But noteworthy is that the engine is only a medium performance bike engine with ~95 hp, and the 2CV has a chassis and suspension intended for "the world's cheapest car", suitable only for peasant French farmers (and occasionally for comic relief in James Bond movies...).

Nevertheless, the video shows this crude-as-possible package blowing away a lot of supposedly (?) much higher performance cars.

So, please, NEVER, EVER say that a Leading-and-Trailing-Arm suspension "WILL NOT WORK" on a race track!

(Interesting is the driver's very relaxed "truck driver's" steering style. For example, see the "Samurai sword grip" at 7:00 and 8:02! :))

Z

Big Bird
07-16-2014, 09:40 PM
Pantsing sports cars in a flat black 2CV - now that is my idea of a fun afternoon!

Nice find, Z

theTTshark
07-17-2014, 08:42 AM
So, please, NEVER, EVER say that a Leading-and-Trailing-Arm suspension "WILL NOT WORK" on a race track!

Z

You may not be able to make a pig into a race horse, but you can make a damn fast pig.

mdavis
07-17-2014, 10:18 PM
You may not be able to make a pig into a race horse, but you can make a damn fast pig.

We used this line many many many times to describe our 2013 car.

-Matt

stever95
12-09-2014, 07:41 PM
Hi all. I graduated and am no longer a part of blue hen racing's formula team, but I've been reading more about vehicle dynamics.

In "Chassis Engineering," Captain Herb makes a couple of statements that have really thrown me off. The first concept I've been struggling with (for too long...) is the following: "When a car rolls due to the cornering force, the tires usually roll with the car and develop a positive camber angle to the ground Combine that with this: "when...camber gain is over 3/4 degree per degree of body roll...This means if the car rolls at a 4-degree-angle, the outside tire will decamber at 3 degrees, so the outside tire will lose 1 degree of camber in relation to the track."

My opinion is (or was) that a tire does not gain positive camber due to roll. In fact, camber should change however your suspension (and compliances / deflections) geometrically prescribe. What Herb seems to be saying is that when a car rolls, it acts as if it's at the limit of tipping over (ie ROLLING OVER) with infinitely stiff springs, which would cause a positive increase in camber.

I haven't read anything that clears this up in RCVD, Valkenburgh's Racecar Engineering and Mechanics, or Carroll Smith's stuff. And I've drawn more FBD's than I care to admit.

Help greatly appreciated.

Thanks!!

(edit: speeling)

Pete Marsh
12-09-2014, 08:59 PM
I think this is just a terminology definition issue.
The more correct term for what you are reading about is "Inclination Angle"

That is the angle the wheel plane makes to the road

You may be confusing this with the "Camber Angle", the angle the wheel plane makes to the vehicle body reference plane. When playing with kinematics, and saying 'I have xx camber gain" this is relative to the body, not the road, so if the gain is less than 1, you will in fact loose inclination angle in roll. If your roll centres are above the ground, jacking will cause a further loss.

Pete

mech5496
12-10-2014, 07:22 AM
What Pete said! It is a common thing here, especially between new team members, as they cannot get their heads around "how it is possible to lose camber WRT road when the kinematics program says I have 0.5deg/deg camber gain?".

stever95
12-12-2014, 01:48 PM
Pete and mech,

Thanks for you responses. I'm still caught up on defining body roll though. I often try to relate vehicle parameter changes to two different vehicles, to see the effect on weight transfer. Let me explain.

Assume the 4 wheels of the vehicle are fixed to walls of your shop so that there is no change in inclination angle with lateral force on the CG, and deflections in sus components are negligible.

Case 1: With an inelastically sprung vehicle (go kart or similar), imposing some lateral force on the CG of the vehicle will cause weight transfer, since the CG is above the ground. No roll, simple weight transfer calcs. No camber gained because no suspension travel.

Case 2: With a sprung vehicle, a lateral force at the CG will cause the suspended mass to roll (about its roll center?), compressing the outboard springs and expanding the inboard springs. Roll is proportional F.lateral and roll stiffness (another question for another time). But as you know, weight transfer is NOT a function of roll stiffness, which means this spring compression does not, in a steady state, change the Fz on each tire. Still not 100% confident in how that works, but I bet a good FBD will clear things up.

I suspect, then, that the only reasons to reduce body roll are to decrease turn in response (time it takes for car to "take a set"), decrease ground clearance variations (highly aero dependent undertrays, high downforce aero, etc), OR if body roll is so excessive that you run out of shock travel and lift up the inboard wheel(s). Obviously the last one would require you to not fix the wheels.... but you get what I'm saying.

What really mucks up this analysis is if you allow an inclination angle, but I am not convinced that if you unbolted your wheels from the walls, you would end up with anything different! Why would the wheel have a tendency to change its orientation to the flat road (read: pick up the inboard wheels if it must use up all available shock travel before doing so?? If inclination angle is truly how you have described it, and you must depend on camber gain to counteract it, how does a go-kart's wheels seem to remain at 0 degrees inclination?

Sorry for long post... just trying to get my whole thought process out there!

mech5496
12-12-2014, 02:02 PM
... But as you know, weight transfer is NOT a function of roll stiffness, which means this spring compression does not, in a steady state, change the Fz on each tire...

TOTAL weight transfer is not...but think what will happen if you have an infinitely stiff spring on the rear and a super-soft spring on the front. How much of that weight transfer will be taken by the front and how much by the rear (i.e. what your LLTD is going to be?) Tip: LLTD is one of the main tools to tune US/OS in a car...

Reasons to limit roll, besides the ones you mentioned, include camber LOSS by suspension geometry. As Pete mentioned if your camber gain is below 1, your wheels lose camber WRT road (kinematically that is, as you also lose camber due to compliances). Karts do not roll (other than tire squash), so their kinematic camber is constant WRT road, compliances removed. Same holds true with a beam axle...

SomeOldGuy
12-12-2014, 02:12 PM
Not a suspension guy, but I believe you are not considering the other 4 springs in the system...

The tires will also provide some roll to the overall system and that is why the angle of the wheel plane will change, even when you do not have camber change due to kingpin and castor.

stever95
12-12-2014, 04:38 PM
Mech,

Thanks again for chasing this issue with me... I understand what you say about LLTD, it has come up several times in several books. Herb makes it abundantly clear with a couple of examples around page 15, from memory. But the big picture that I still don't understand from a physics standpoint is why a sprung vehicle has a tendency to tip over (unfavorably change inclination angle) in cornering. You sound very sure of this - if there's a reference you know of somewhere explaining it I'd be deeply appreciative.


Not a suspension guy, but I believe you are not considering the other 4 springs in the system...

The tires will also provide some roll to the overall system and that is why the angle of the wheel plane will change, even when you do not have camber change due to kingpin and castor.

Are you referring to sidewall deflection, or the wheel rate being affected by tire spring stiffness? I guess the former would sort of cause a change in inclination angle, but I don't understand how its spring stiffness would come into play. Can you elaborate?

BillCobb
12-12-2014, 10:52 PM
During lateral forced maneuvering, tire tread migration (or more correctly wheel migration away from the tread) due to sidewall deflections induces moments about the spindle that are reacted by the suspension. These moments can be quite large. There is an induced body roll reaction to these
'overturning' moments that can be quite substantial and amplified/attenuated via the 'roll by camber' SDFs of the axle. It goes without saying that spindle and wheel bending is also in play and taking you further away from the performance characteristics of suspension members made of unobtanium. The range of roll camber SDFs in typical vehicles is surprisingly large and has both signs. This affects the TLLTD and thus the vehicle's understeer and roll gradients (deg/g). A weak total roll stiffness and strong roll camber coefficients can induce a disturbing amount of roll velocity and its corresponding treachery. Because of the way tires behave, (as weakening force components, just the right recipe of tires, SDFs and dampers can cause a vehicle rollover at low speed no matter what its cg height, in the hands of a skilled driver and now even more fun with sophisticated driver robots which sense peak roll velocity. Tire relaxation (response time) is an interesting player here, too. Camera views of tire carcass torture during heavy steady state and transient cornering maneuvers are fascinating. I posted a video some time ago on YouTube of Calspan tests of a tire receiving a reverse steer type MX test. It ain't pretty. People are surprised that the tire doesn't air-out, but when it does, the MX change followed by the metal rim digging into the pavement can produce a decent 360 and some high "Dancing with the Stars" scores. This was the Explorer problem. Lowered tire pressures from a ride improvement process coupled with the normal air permeability (leakage with time) rate even got the attention of the US Congress. But, I diverge...

Unanticipated overturning moment loads usually result in suspension component fatigue and stress fractures that can be embarrassing for all involved, too. "I hate it when it does that" has a familiar ring...

stever95
12-18-2014, 01:05 PM
Bill,

Thanks for your reply. I have to say, I'm pretty perplexed by a lot of what's in your comment. I still have a fundamental mis-understanding of "body roll," but certainly you aren't saying that it is completely due to tire tread migration. So the answer is still out there..

Nevertheless!

When you say that sidewall deflection induces moments about the steering, are you referring to lateral Fz migration relative to the upright?

What is the 'roll by camber' SDF you speak of? Why would this play with TLLTD? Roll velocity? I am behind on the SDF acronym :)

Z
12-18-2014, 11:37 PM
Stever95,

I have been away for a while, so missed much of this. You ask,


Body roll definition ... have I been wrong all along?
=======================================
...
In "Chassis Engineering," Captain Herb makes a couple of statements that have really thrown me off...
...
... camber gain ...
...
I haven't read anything that clears this up in RCVD, Valkenburgh's Racecar Engineering and Mechanics, or Carroll Smith's stuff. And I've drawn more FBD's than I care to admit...
...
[then in later post]
... Assume the 4 wheels of the vehicle are fixed to walls of your shop so that there is no change in inclination angle with lateral force on the CG...
...
What really mucks up this analysis is if you allow an inclination angle, but I am not convinced that if you unbolted your wheels from the walls, you would end up with anything different!...

Firstly, I think this could all be explained very quickly if we had a simple sketching facility here on this Forum. But none, so I'll try with words.

A crucial point is that this whole issue of "Camber-Gain" (aka "Camber-Compensation", "Camber-Recovery", and many other terms) is really very simple.

1. This is purely a matter of the Suspension Kinematics. So,
NO FORCES (ie. no "Statics" FBDs),
NO DYNAMICS (ie. no F = P-dot),
NO COMPLIANCES (ie. no structural flexing of anything),
NO TYRE MODELS (ie. no rubbery stuff, just wheels that are dumbed-down to rigid discs),
and NO OTHER CONFUSING REAL-WORLD COMPLICATIONS.

2. Furthermore, this is purely a 2-D KINEMATICS issue, as seen in end-view of the car (ie. either front or rear-view).
~o0o~

So, sketch an end-view of a simplified Car-Body. Add simplified 2-D Suspension attached to Car-Body at its inboard side, and attached to simplified Tall-and-Narrow-Wheels with rounded tops and bottoms at its outboard side. Add a simplified flat Ground for all this to sit on.

Define "Wheel-Camber-Angle" as the relative angle between Wheel-reference-frame-vertical and Car-Body-reference-frame-vertical. Quite easy to measure, because all in 2-D. For example, if both "verticals" are parallel, then WCA = 0 degrees. Worry about directions for +/- later. (Usually Wheel-top leaning away from Car-Body means WCA = positive.)

Define "Body-Roll-Angle" as the relative angle between Car-Body-vertical and Ground-vertical. Worry about directions for +/- if you have to write a VD-sim-program...

Define "Wheel-Inclination-Angle" as the relative angle between Wheel-vertical and Ground-vertical. Again, worry about +/- when you have to get real numbers.

Next, and IMPORTANTLY, assume the Wheelprints (ie. the bottom surfaces of the Wheels) are always in contact with the Ground. In fact, you might define "Ground-vertical" as being perpendicular to the line through the two Wheelprints. Also, DO NOT ASSUME that the Wheels are "fixed to walls of your shop". And ignore any Heaving motion of the Car-Body, so assume that its CG is always at the same height above Ground.

Finally, come up with some sort of rigorous definition of "Camber-Gain" such that whenever there is finite Body-Roll-Angle but the Wheel-Inclination-Angle remains unchanged from when the car's BRA = 0, then "Camber-Gain = 100%"...
~o0o~

On the other hand, if you want to make a career of this, perhaps by writing books or giving seminars, then standard practice is NOT to give any definitions at all. Instead, introduce as many different buzz-words and phrases as you can dream up, never do any sketches, spend as much time as possible talking about "tyre-curves", and "magic numbers", and miscellaneous "centres", and try to make everything appear as complicated as possible. Much, much easier for you... :)

If the above still doesn't make any sense, then please post some sketches explaining your thinking, and where you are having difficulty.

Z

stever95
12-19-2014, 08:28 AM
Thanks for laying things out, Z. I was looking forward to your response. I'll get to work on some more sketches. Or a cardboard model, or something.

ChristianChalliner
12-19-2014, 08:43 AM
Stever95,

Cardboard models are great for everything, likewise lego, it makes understanding movements much simpler and is good for testing various ideas quickly before you run with them. For example, yesterday myself and our chassis guy had a three hour long conversation with a faculty advisor about how both front and rear suspension systems worked, trying to explain with words and drawings was difficult but once a model was made from some lego he immediately understood. By a similar notion it is very good for force visualisation as thin pieces will deform giving you instant results, much faster than building an ANSYS sim, more fun and more reliable too!

Which reminds me, i intend to update my build thread over the next few days, you might be interested :)

Thanks,

Christian

BillCobb
12-20-2014, 06:30 PM
The roll camber (S)uspension (D)esign (F)actor is one of about 18 key ingredients to the recipe that defines the handling characteristics of any vehicle. This particular one affects the tire lateral load transfer distribution (TLLTD) because the change in inclination angle attenuates the lateral force generator via the tire's camber stiffness function. Since race tires are bias belted tires and bias belted tires usually have large camber stiffnesses, the lateral force balance of the vehicle can be greatly affected by this SDF. Since a race prepped vehicle operates at high lateral force levels and race tires are still 'listening' to inclination angle inputs, the balance of the car is affected because the understeer at these g-levels is affected, hence its maximum lateral force capability.

While we are on the subject, the roll camber SDF is one of 18 or more important measurements made on a K&C machine and/or evaluated using finite element analysis programs. for typical production passenger cars, we can guess the following mean values taken from the normal distributions of a thousand or so such K&C tests. Now before you get all edgy about the comparisons between production cars and FSAE or other Rice cars (using my best accent) , I feel the need to say that sure, the rice cars don't have rubber bushes growing at the ends of suspension attachments, etc. and maybe don't even have power assisted steering, BUT (and that's a BIG BUTT), they run at about twice the lateral force levels, so they actually come out pretty close if they are any good. That being said, some rice cars are so spongy, they are actually WORSE that an engineered production car which has rubber or plastic bushings at the chassis attachments and power steering. FSAE car are no different.

Here ya go. mateys:

Kinematics:
Roll centers (force based) 100 mm front, 150 mm rear.
Roll steers (steer by roll) .05 deg/deg front, .025 deg/deg rear.
Roll Cambers (camber by roll) 0.75 deg/deg front, -0.7 or -0.1 rear (bi-modal)

Elastics:
Roll stiffness Nm/deg 1200 front, 700 rear.
Lateral force steer (steer due to lateral force) 0.1 deg/1000N front, 0.03 rear
Lateral foce camber (camber due to lateral force ) 0.25 deg/1000N front, -0.3 rear
Aligning moment induced steer (deg/100Nm) 0.8 front, -0.08 rear.
Aligning moment induced camber (deg/100Nm) 0.04 front, 0.01 rear.
Lateral stiffness (mm/1000N) 0.45 front, 0.05 rear.

Signs here adhere to the notion that + is an understeer effect, - is an oversteer effect.

These factors add a significant amount of understeer to any vehicle's handling characteristics, especially if certain types of tires are used or avoided. Anyone measuring understeer in a road test will be disappointed by the lack of fidelity to computer models of handling if they are omitted. There are some individuals (one has a Post Hole Digger and authored a book) who has completely ignored to this day these SDFS.

It should also be pointed out that correlation of road test measurements to handling models can be VERY high if K&C and tire measurements are made of any vehicle. And, that any vehicle which has had extensive dis-assembly and re-assembly is likely to suffer the slings and arrows of outrageous carelessness.

Finally, they are probably not really 'Design' factors, but parameters which are carefully monitored and adjusted to make sure they don't get out of hand or range and are symmetric, side to side. This is how a 'neutral steer' car, based on equal weights and the same tire construction all around , winds up with 3.5 deg/g of linear range understeer and a 30 deg/g understeer reading at its max lateral g performance.

Just sayin' .....

Zac C
01-02-2015, 04:58 PM
If only there was someone out there that has done multiple K&C tests on event winning cars...

If I get some free time in the next week or so I'll run through my database and post some hard numbers (names will be changed to protect the innocent). Bill's elastic numbers seem low (stiff) compared to what teams have achieved in the past but it's been a while since I looked at any of the data. I've noticed that a lot of FSAE teams have traditionally not done a very good job with bearing selection and hub design.

BillCobb
01-02-2015, 08:03 PM
These are production only, street car mean values. Distributions are reasonably Normal (unless otherwise noted). Use of pasta in FSAE car steering systems may show in your results. This could explain the observed, lower than expected steering gain (hence understeer indictment) that these cars exhibit in spite of their rear weight bias.

Actually there is at least one team with multiple tests. They just hired a new Coach, eh ?