UTAS seat & driver position design

[Jonny Rochester]

On our spec sheet from last year it says 180mm for CoG. Obviously just a made up number. I decided to tip the car up to balance point with a driver in it, and got to 70.5 degrees, or 19.5 degrees from vertical. (Driver was both amused and concerned). Noting the car was sitting on the tyre sidewall I added 20mm and calculated our center of mass was about 345mm from ground with current setup. (We have more ground clearance than we need). Since we are reducing the tracks this year we should also reduce CoG, but just as important I want the driver looking good, not obviously sitting too high. I note the drivers weight could be 25% of the car, so drivers CoG has a significant contribution so I want to push the driver down a bit. Thinking how to design a new car, I came up with this:

Flow chart to establish low driver position and design chassis cockpit.

Establish a floor or reference plane as the lowest point possible.
Draw rollhoops 800mm apart like our old car for a starting point.
Find the steering rack height, used to define heal position. (Our rack is at the bottom, but just above the lower wishbone).
(Heels maybe 100mm above bum).
Set minimum front rollhoop height for 350mm template rule and to clear steering rack.
Set seatback angle (try 45deg, 2014 car was 51deg) or until a 175cm persons visor is above the front hoop.
Set steering column height and angle by drawing a line from toes to chin.
Steering wheel distance ~340mm from chest, or elbows at 90deg.
Set main hoop height using Percy template (95%ile male) or tallest driver, 2“ rule. Allow extra height.
(Shortest driver will need foam to see over front hoop).
Pedal box adjustment is XXmm (5%ile to 95%ile).
Set harness bar height from rules.
Check steering wheel is not higher than front hoop.

Also, I am fishing for comments on my current theory that the steering column should be on a line drawn from the toes to the chin, or near enough, regardless if your driving a F1 or a truck. The theory holds for the sedans I drive.

[mech5496]

Jonny, just a few comments on the CoG height measurement procedure:

1) Put an iron angle on the side of the tires, so that you have a sharp edge to rotate the car about
2) Be extremely cautious when measuring the angle. Calculated height deviates significantly even with small angles deviation (do a spreadsheet and play around a bit)
3) One possible method to overcome (2) is to do a lot of different measurements: both sides and nose-to-tail pitch
4) A possibly better way would be to put 2 scales under the tires on one side (or end) of the car and gradually lift the other side (or end) to predefined angles/heights (the latter is more accurate). Then use weight transfer to calculate CoG height.
5) Remember to lock the suspension. Using solid rods instead of spring/damper units is the easiest way.

[Jonny Rochester]

We had zero time to lock the suspension or change the car in any way, but that is a good idea, and we have solid rods. I just came up with this very fast way to calculate something rather than have nothing to go on. It was a new concept to the team members present so they didn't feel the need to spend time with accuracy. I even had to ask someone to look at the tyre sidewall, as I was busy holding the car. It took several attempts just to explain what I wanted them to look at!

[Jonny Rochester]

I also did it without driver. ~278mm (balanced tilt at 24 degrees from vertical)

[Claude Rouelle]

Jonny,

Here is a few advice

1. Make the steering column axis "hitting" the driver face just between the eyes. Honestly I have no theory or documentation for that, I heard from an older guy when I was still as student but it seems to be working with the small race cars I drew and manufactured many years ago

2. Consider the compromise of CG height (driver leaning low) and the yaw inertia (driver seating up straight)

3. Most students design their car with a Solidwork or Catia driver but forget that the driver needs to turn the steering wheel and there is issue with the elbows hitting the side of the cockpit in corner

4. Make sure that there is enough space between the steering wheels and the leg for the driver inside hand (I mean inside side of the corner) when he steers more than about 60 degrees

5. In my mind there is no need for a seat. Do not waist your money and time and just use the usual expandable foam (2 components) in a big plastic trash bag and make one seat from each driver. Finish it with a nice, light velour that you glue (there are glue that you can spay from a pressurized can)

6. Make sure you have should good shoulder support, ribs and hips support. That can make a difference in the driver ability to feel the car. You "wear" a cockpit like you wear a suit and good tailors design a specific suit for each customer.

7. Pay attention to heels support and side feet support, for the same reason explain in 6. Imagine that the lateral G and longitudinal G will make your foot move and you won't always have enough muscle strengths or reflex to keep your feet where you want or need them to be.

8. The biggest issue I have sees in FSAE and FS cars as far as driver comfort is concerned is the steering torque unusual high efforts; That could screw all the attention and fine work you made on all other parts of the cockpit and seat design. Try to limit your steering to 10 Nm (a passenger car steering torque is about 3 to 5 Nm). You can calculate that torque with both front tire Mz, your estimate tires Fx and Fy (or combined), your caster trail and scrub radius, the toe base (or upright steering arm) and the distance between the steering rack axis and the pinion axis). Here is the ultimate test: put your car on the ground in the workshop, tire at hot pressure, put your driver in the car with all gears: helmet, fire suit, gloves on, belt tight etc... ask him to turn the steering wheel from one side to the other until he reaches the steering rack maximum movement. He must do it 20 round trips (so for example fully left to fully right to fully left) in less than 30 seconds. When I ask this to students at FSAE / FS competitions about 80 % of the FSAE / FS drivers fails. Sometimes because they do not have enough arm muscle or worse they smoke a lot but most of the time it is because they did not care about steering torque numbers.


There is also a lot to be said about steering rack design (or choice) and steering column design and support and their installation but that is another story.

[Claude Rouelle]

Harry,

"A possibly better way would be to put 2 scales under the tires on one side (or end) of the car and gradually lift the other side (or end) to predefined angles/heights (the latter is more accurate). Then use weight transfer to calculate CoG height."

Yeah I know; it is in the Milliken book. Been there, down that with many different cars. Theoretically it should work but practically it doesn't. Simply calculate how much error there is in in your CG height calculations if your scale has a 0.5 kg error if you lift your car on the side (Or by the way from the rear or the front) 30, 45 and 60 degrees. At 60 degrees the error will be smaller that at 30 degrees but it still will be many mm.

In any case, whatever method you use repeat the measurement several times. If all the measurements do no give you a CG height within 1 or 2 mm there is something wrong with the method.

[Jonny Rochester]

2. We are taking 200mm off the front of our car.

5. Expandable foam will be for the 5 percentile female. To see over the hoop.

6. I have looked at F1 seats as I don't think it's too different from what I want. Generous cutouts for the arms, but some shoulder support.

8. We have considered a power steering rack from the Caterham F1 team auction that is current... but seriously that would create more problems.

There was a discussion on the other UTAS thread that I read. Steering effort is a main concern. We have a fast rack. To be honest I'm not confident to calculate it, too many factors.

As head of Ergonomics, I have requested from the Suspension team some different angles. (How arrogant of me!)
I am asking for very small KPI and scrub, of maybe 1degree and 5mm. I don't care exactly, just small numbers for low effort. And caster of 6 degrees maybe acceptable to steer, or trail of 10mm if that is fair. And steering arm will be increased from 70mm of last year to 75 or 80mm if that fits.

For 2014 we had a steering wheel angle of about 100-110 degrees or 215 degrees total, with the wheels turning about 30 degrees. I'm not planing to change this, but we are buying a rack so there is opportunity to.

I got in our 2014 car and checked the steering effort. It is about 19 or 20Nm with my very old torque wrench on the steering wheel. Steering is heavy. The current specs of this are
KPI 12 degrees
scrub ~27mm
caster 6 degrees
camber -1 degree

[ChristianChalliner]

Don't forget to check for assembly errors, I noticed that much of the steering effort in our car comes from crap assembly more than anything, UJ binding + rack bar scrubbing on components + bolts for tie rods in rack clevis's catching on the frame = substantially increased steering effort!

Drop the UJ if you can, I noticed it creates some interesting non-linear feeling but that might just have been the poor install causing this? although it's worth remembering a UJ isn't a CV joint.

There's more but it's basically already covered I think.

Also a fast rack gives the chance to use large steer arms which is a plus! but one can cancel the other in terms of steering effort if you see what I mean.

[Claude Rouelle]

Jonny,

"To be honest I'm not confident to calculate it, too many factors"

Oops. Come on! isn't that what this competition is about? If you don't even try you won't even realize how simple it is. It takes a bit of research on the internet then 5 minutes to build it in simple Excel spreadsheet,

20 Nm steering torque is definitely way too much. The number you give are in the ball park so the issue is somewhere else.

What is your steering rack trail?

Also as Christian suggest try to locate any unusual friction? if you steering torque is nearly the same with your car of or on the ground the issue does not come from the tire - ground friction. Disconnect the left toe link an measure the steering torque. Reconnect the left toe link and disconnect the right one and measure again. Release the torque in the LF top wishbone ball joint and measure again....etc...

Too many FS cars have too many ball joint friction.

Remember that you should machine the cylinder in which you will put your spherical joint joint AFTER you have welded your wishbone. This cylinder will most probably be ovalized during the welding phase and create friction in your spherical joint. That means you need to create a jig with the ball joint cylinder a bit smaller that the the real ball joint outside diameter.

[DougMilliken]

Yeah I know; it is in the Milliken book. Been there, down that with many different cars. Theoretically it should work but practically it doesn't.
Yes, this is a difficult measurement to make with ordinary scales. In our defense, 20 years ago we wrote (page 671):

"Because the CG is often close to the height of the wheel centers (RsubLF and RsubLR) the change in load as the car is jacked is small. To get the best results from this test it is best to take data at several angles and average them."

Jonny -- as long as you have a defined fulcrum, tipping the car up to a balance point is sensible...but this is much harder to do with a full-sized car! I've also seen cars hung from a crane (two cables or chains) to determine CG height. Requires some kind of surveying equipment to extend the lines of the cables "through" the car.

[Kevin Hayward]

Yes, this is a difficult measurement to make with ordinary scales. In our defense, 20 years ago we wrote (page 671):

"Because the CG is often close to the height of the wheel centers (RsubLF and RsubLR) the change in load as the car is jacked is small. To get the best results from this test it is best to take data at several angles and average them."

Jonny -- as long as you have a defined fulcrum, tipping the car up to a balance point is sensible...but this is much harder to do with a full-sized car! I've also seen cars hung from a crane (two cables or chains) to determine CG height. Requires some kind of surveying equipment to extend the lines of the cables "through" the car.

Doug,

While hanging a full size car from cranes may be a little dicey for students it works great for getting the COG of some other difficult components. Hanging the engine at three different points, taking photos with a drop line is a great way of determining its COG.

I would say that probably the best way to get a good number these days is to model everything. CAD models should be in the 95%+ mass accurate to the actual vehicle. Plenty of data exists for COG locations of people. The engine is the only real "difficult part" in terms of measurement.

Bonus is once you have this modelled you also have all the inertias, which is harder to measure. Another big advantage is you have a rough idea of what your COG will be very early in the design phase

The test then ends up being confirmation.

Kev

[Jonny Rochester]

I don't think we have excessive steering friction. With the front wheels off the ground the steering is very light, as it should be. There are some suspect bearings and it could be better, not the main problem.

The heavy steering effort is when I am in the car. It is from a fast rack (Stiletto fast ratio) (~220 degrees lock to lock), and also from jacking moving the nose up related to caster and excessive KPI, plus too much scrub radius I believe.

Another factor (which is why I'm reluctant to calculate) is that the workshop guys kindly painted the floor while we where at competition. What is the exact coefficient of friction between a painted concrete surface and an old Hoosier?

I don't know what steering rack trail is, but I will go away and see if it's possible to calculate these things. (I know it's possible, I'm just worried the effort is meaningless if I have bearing friction and other weird stuff going on).

We have a double uni joint which is semi-well supported and at 65 degrees. I'm going to change this to 45 degrees in the new car as there will be less force on the support.

I'll tell the Suspension Team to calculate all this, but I should be it myself for my own learning.

[CWA]

Hi Jonny,

I seem to remember commenting on your UTAS build thread about excessive steering effort. I described what I thought was a useful way of determining which aspect of the steering system design was giving contributing to excessive steering effort.

Make sure you understand the limitations of the quick workshop test that Claude has described for assessing steering effort. Claude has suggested it as a 'rule of thumb' test, but it is not as robust a test as it may need to be, it may not account for all combinations of steering parameter values. For example if your system has reasonably low weight jacking but excessive amounts of mechanical trail, Claude's test will make the steering seem lighter in the workshop than what it will actually be out on track. And as you have suggested, twisting the contact patch rubber when the car is static (caused by a lack of scrub radius) will be introducing torques into the steering that aren't actually seen on track.

Your best bet if possible is to assess the steering torque out on the track. Either rig up some instrumentation on the column / custom wheel boss and collect some data, or use the natural capability of your muscles / brain to assess the level of torque during a test session. As Claude has said, you will know when it is too much.

PS - I've interpreted what Claude calls 'steering rack trail' to mean mechanical trail

[Claude Rouelle]

Jonny, (and CWA for the steering rack trail)

You make a mountain of something that does not need to be calculated but simply measured. Come back to earth.

What I call the steering rack trail is the distance between the axis of the steering rack and the axis of the steering column in plane perpendicular to the steering column passing through the steering rack axis

I am going to help you even more:

Steering torque at the steering wheel from one tire = [ (Tire Mz self alignment torque + (FY tire lateral force * Mechanical trail) ) / (Upright steering arm ) * Steering rack trail

That simplified calculation assumes that the toe link is and remains roughly perpendicular to the chassis longitudinal axis.

Also in this equation there is no Fx and scrub radius but nothing prevent you to input these parameters too.

That is for one wheel. You will have to add the effect of the other wheel too

You can get the Fy and Mz from your tire data or you can assume some reasonable numbers

Of course this simplified calculation does not take into account any possible "stiction" (a composite slang for sticky and friction) or hysteresis in any ball joint and there will necessarily be some (in fact more than students usually think)

Basic, not perfect but useful.

Hope this helps

Also "The heavy steering effort is when I am in the car" Are you the only driver?

If the steering is easy at 0 km/h and heavy at speed, I suspect you have major compliance (unless you have tons of aero downforce, which I doubt) either in your suspension elements or on the steering rack attachment on the chassis. or on the steering column bearings bracket on the chassis.

[ChristianChalliner]

Claude, I know that wasn't directed at me but it was extremely helpful to me also, thank you!

[Claude Rouelle]

Christian, My answer was prompted by Jonny comments/questions but I am happy if several other people can benefit from this information. I am sure other people like Bill Cobb could or will elaborate on this and I am also sure everybody will learn again from their thread. And I fine if they disagree with me (providing they do it in civil way) I don't know and I will never know everything. I will never have everything 100 right. That what a forum is about.

Try to attend the OptimumG seminar one day. Withe the design and thinking you already went through, you will learn a lot.

[CWA]

Thanks for the clarification on steering rack trail Claude


Jonny, (and CWA for the steering rack trail)
If the steering is easy at 0 km/h and heavy at speed, I suspect you have major compliance (unless you have tons of aero downforce, which I doubt) either in your suspension elements or on the steering rack attachment on the chassis. or on the steering column bearings bracket on the chassis.

Can you shed a bit more light on this mechanism, I'm having trouble picturing it. By compliance do you mean backlash, or flex?

[Claude Rouelle]

I keep saying in seminars for FS / FSAE participants that the 2 things that students do not understand is transient behavior and compliance. It seems that you just proved it by your question about compliance so I and other readers of this post will be glad that you asked

By compliance I mean that no elements of any suspension or chassis is rigid and there will be what you call flex (and flex with hysteresis) but there will also be play or "sticktion" in all ball joints.

Have a look at http://www.morsemeasurements.com/what-is-kc-testing/

The examples in http://www.morsemeasurements.com/kc-case-study-test-results/ are quite educating. Look at the bloody hysteresis! and think about what the driver will feel (or not) and how confident he will be (or will not be)

Too bad that there isn't toe compliance Vs lateral and/or longitudinal graph here; that is the most important.

By reading this you will understand one of the main reasons why the force based roll center (or pitch center) is not the one given by your kinematics software or your CAD. And the force based one is closer to the reality (but not yet the reality)

Many FS / FSAE teams do now spend at least 1 day on a K&C test. Some have built their (basic) own. Very useful if they know what input to use on the K&C test rig and how to interpret the data.

[CWA]

By compliance I mean that no elements of any suspension or chassis is rigid and there will be what you call flex (and flex with hysteresis) but there will also be play or "sticktion" in all ball joints.

Claude I am aware that metal parts are not rigid and displace with force, and that this is usually called 'compliance' when this mechanism influences a vehicle's handling in some way.

The reason I asked for clarification is because I could not understand how you made this deduction: "If the steering is easy at 0 km/h and heavy at speed, I suspect you have major compliance".

I'm still not sure what you mean by this. How can compliance make the steering heavy with speed (or more accurately, lat acc?) ? Have I misunderstood your claim? If not, I cannot picture this, if you could provide some clarification I would be grateful.

[Claude Rouelle]

CWA, OK I misunderstood your question. Soory if you think I thought you did not know wat compliance is. In any case I am sure a few readers will learn from my previous thread of this post. What I tried to say is that once you are at speed and you have tire slip angle and slip ratio, the tires forces and moments will put on the suspension linkages and ball joints efforts and deformations that are not necessarily existing and surely not with the same intensity when you turn the steering wheel in the workshop at 0 km/h. That could explain why the steering is "light" at 0 km/h and "heavy" at speed. Just basic qualitative not scientific or quantitative approach. Example: you have asymmetric camber or toe. You will probably not feel a steering torque difference at 0 km/h when you turn left or right but you will feel it more at speed.

[DougMilliken]

By compliance do you mean backlash, or flex?
See RCVD Chapter 23. Stiffness is defined as force/deflection and compliance as deflection/force.

Olley recognized compliance steer as a problem very early on, see the index in "Chassis Design" under his terminology, "deflection steer" -- first use is bottom of p.161.

[CWA]

CWA, OK I misunderstood your question. Soory if you think I thought you did not know wat compliance is.

I've just re-read my initial post and can now see that what I typed is misleading! So no apology necessary Claude.


What I tried to say is that once you are at speed and you have tire slip angle and slip ratio, the tires forces and moments will put on the suspension linkages and ball joints efforts and deformations that are not necessarily existing and surely not with the same intensity when you turn the steering wheel in the workshop at 0 km/h. That could explain why the steering is "light" at 0 km/h and "heavy" at speed. Just basic qualitative not scientific or quantitative approach. Example: you have asymmetric camber or toe. You will probably not feel a steering torque difference at 0 km/h when you turn left or right but you will feel it more at speed.

I think I understand what you are trying to describe Claude. The message I take from this is that compliance can alter the actual steering geometry during cornering from what has been designed at static. So the steering axis could migrate / change position and orientation with lots of cornering force and control arm compliance, changing how much torque ends up going through the hand wheel during cornering. This is something I had not considered before.

[Jay Lawrence]

CWA, as an example: in 2007 or 8 I was watching a car on the practice track at FSAE-A, and noticed that whenever the driver braked there was significant dive combined with excessive toe variation, no doubt unintentional. I imagine this would be hard to see at 0 m/s but would surely have felt terrible through the wheel whilst under motion.

[CWA]

CWA, as an example: in 2007 or 8 I was watching a car on the practice track at FSAE-A, and noticed that whenever the driver braked there was significant dive combined with excessive toe variation, no doubt unintentional. I imagine this would be hard to see at 0 m/s but would surely have felt terrible through the wheel whilst under motion.

Interesting anecdote Jay. I have no doubt that what you saw was unintentional (I would like to have seen it myself!), and agree that this behaviour is not ideal on a race car. I have seen cases similar to this before, although where a rear axle is the cause. I'm sure the toe variation you saw wasn't doing the driver any favours when considering his task of 'controlling' the vehicle. But I feel it is important to clarify that this is not what we are talking about in Jonny's case. We are just talking about straight-up too much steering effort, unless I am mistaken.

Can the observations you've made Jay ever be a cause of excessive levels of steering effort? I can't see how. I don't see that solely toe displacements during braking, due to compliance, will increase steering effort levels higher than what that same car would experience during max lat acc. When you are toeing the wheel, all you are doing is creating a slip angle, and some FY, which in turn is translated into steer torque based on your level of mech trail. Assuming mech trail remained constant on this car you saw (ruling out what Claude has described for our consideration) the toe variation behaviour you described is not going to be introducing any FY's around the steer axis greater than what you would see during max lat acc. For me this is the important distinction that has to be made; again, the toe changes are going to be varying the steer torque levels felt by the driver in a very unintuitive way, sure, similarly there are probably going to be generating unintuitive / unrequested yaw moment changes on the vehicle when these toe changes occur. But this is just a 'control task' issue. Unless I've misunderstood your point Jay, I don't see it as relevant in Jonny's case.

Jay what you've described doesn't really follow on from the mechanism that I think Claude was describing, either. Here is an example of a mechanism I can imagine Claude is trying to convey. I picture: during cornering, let's say SS max lat acc, my car's chassis has a soft 'front lower control arm' leading (inner) mount. The compressive cornering forces cause the outside lower front CA leading inner joint to displace inwards, causing the CA to 'pivot' about the rear joint, and shift the outer lower balljoint forwards (relative to the rest of the car), causing the caster angle to significantly increase, causing mech trail to significantly increase, which when coupled with the tyre cornering forces, also significantly increase the steer torque. This, as a hypothetical example, could increase dynamic steer torque to levels far greater than may have been expected when I was designing the car's 'static steering geo'. Let's say I had designed in 2 degrees caster, which gives 10mm static mech trail, which gives 10Nm steer torque at Aymax. But this compliance issue causes the caster to double to 4 deg, mech trail to 20mm, and steer torque to 20Nm at Aymax - seems viable to me.

Any toe change seen during such a hypothetical situation would crucially not be the cause, it would be an effect, and just coincidence. The above example situation Claude has lead me to perhaps could even occur without any toe change. The actual cause of the increase in steer torque in this case will be due to a mechanical trail increase due to a steer axis inclination change, due to compliance. Perhaps what you saw at FSAE-A Jay was actually caused by a mechanism similar to this, again I do see it as viable. But the way your message is delivered; essentially saying 'toe variation can cause excessive steering torque levels' is misleading without delving deeper and describing the mechanism's causes and effects with more precision, IMO.

I feel I should also make my views on Claude's previous statement clear. Even though I now understand it as a mechanism, I do not feel entirely comfortable with the deduction; "If the steering is easy at 0 km/h and heavy at speed, I suspect you have major compliance". Mainly because "If the steering is easy at 0 km/h and heavy at speed" (where I believe Claude really means "If the steering is easy at 0 km/h and heavy WITH LAT ACC") could simply indicate that mechanical trail is the main contributor to steering torque on this car. If I have a car with a pretty vertical SA (minimal weight jacking with steer) but lots of mech trail, of course steering will be light with no speed (no lat acc) and noticeably heavier with some speed (lat acc). If this was my car, and when I tested it I had steering that was 'easy at 0 but heavy with lat acc' as described, I would first be looking to verify whether this is an 'expected amount of heaviness', purely because of the amount of mechanical trail I had designed into the system / had resulted from build. Initial investigation down this route might show that my mech trail is double what I had expected / designed because someone cocked up the build and I now have twice the mech trail that I wanted. This could be the case for Jonny. Immediately assuming that there is a compliance issue and that my steering axis is migrating all over the place under cornering forces and not dynamically giving the geometry I wanted it to have / not giving the geometry it has when static seems like jumping the gun to to me. Maybe the steering being easy at 0 km/h and heavy at speed was always what my chosen steering geo was going to give me (whether I wanted it to / whether it is good this way or not). Claude perhaps I've misinterpreted you again.

On this note Jonny, you described some steering specs for your 2014 car earlier but did not say how much mechanical trail it is supposed to have. Out of interest, what is this figure?

[Claude Rouelle]

CWA,

I hope to find more time to give you more perspectives and comments later but for now I would simply give you one perspective and this one is about the rear toe which by the way and as it has been mentioned before (lately, again, by Tim Wright) is one of the biggest component of stability and driver ability to go fast into a corner.

I have been working with professional drivers and professional teams in GT and or WEC LMP1 and LMP2. When you simply ask the driver(s) or their engineer how much do you need to change the rear toe (per wheel on each wheel / no asymmetrical trick here) for the driver to feel a difference (whether the car is better or worse is irrelevant here; we just want to have an idea of the driver "ass sensor" resolution) the answer is 0.25 mm for the good driver and 0.50 mm for the not so good driver.

Wait a minute... the toe is measured with a fishing string and the rim diameter is 18" . Atan( 0.25 /( 18 * 25.4) ) = 0.03 degree. If it was 0.5 mm it would be 0.06 degree. If you had an amateur driver and you would have to change the rear toe 2 mm for the driver to feel a difference that would still only make 1/4 of a degree of rear toe change. Pretty small number for a tire which has its peak Fy at about 5 degree slip angle and its peak Mz at about 3 degree.

What are the conclusions?
1. You need to be pretty accurate when you measure the toe. That is why professional teams do not use a fishing string; they use lasers. And dummy, flat wheels to avoid even slightly twisted rims
2. You need to always have the same guy (mechanic) making this operation. You imagine the effect on toe when the guy tights one of the rod end counter nut? That is why by the way whenever possible I prefer toe adjustment with shims.
3. IF 0.25 MM =0.03 DEGREE) MAKES SUCH A DIFFERENCE DO YOU REALIZE HOW DRAMATIC THE EFFECT OF REAR TOE COMPLIANCE CAN BE?

Now I have to say the rear toe is THE adjustment having the biggest effect on car stability. And your classical Hoosier FSAE tire is a bit (but just a bit) less slip angle sensitive than a good race tire.


Now.do you understand why some design judges become agitated when they see the toe link pickup point right in the middle of one of the chassis tube?


By the way the camber is important but not as important as the slip angle. In fact you should see this on your tire data / graph. In fact you should ask your self this simple question: how much camber or tire pressure or vertical load to I need to have the same effect as lets' say 0.1 degree of toe in terms of Fy, Fx, Mz, Mx. Useful parametric analysis......

[Claude Rouelle]

CWA

"....where I believe Claude really means "If the steering is easy at 0 km/h and heavy WITH LAT ACC"

No, not necessarily. You can need 5 Nm to go from 0 to 20 degree of steering wheel at rest and 10 Nn for the same steering wheel movement at 50 km/h. That is why some good power steering control loop of passenger cars do include the speed as one of the inputs.

Claude

[Z]

... for now I would simply give you one perspective ... the rear toe ... is one of the biggest component of stability and driver ability to go fast into a corner.
...
What are the conclusions?
...
3. IF 0.25 MM (=0.03 DEGREE) MAKES SUCH A DIFFERENCE DO YOU REALIZE HOW DRAMATIC THE EFFECT OF REAR TOE COMPLIANCE CAN BE?

Claude,

[Drum Roll .....] I agree 100% with all of your above post!!! (Have been saying it myself for a long time too...)

To repeat the message - REAR-TOE-ANGLES ARE VERY IMPORTANT!!!
~o0o~

But wait! How can this be, that Z is in agreement with Claude??? Dammit ... must fix things ... must set the World right again...

OK, one disagreement.

IMO it is acceptable, in fact, preferable, for FS/FSAE students to check their toe-angles with "fishing string". I have suffered too often the consequences of "gee-whiz laser tools" that have had a small bump and been knocked out of whack. The damage is INVISIBLE! A string-line may be crude, and it can move around on a windy day, but its crudity gives it a high degree of trustworthyness.

Most important is that the person using the string-line is well trained and uses it correctly. I suggest two lateral "toe-setup" pipes (can be SHS or RHS) that are clamped to F&R of the car in a way that guarantees the same position everytime. Then run some "fishing tracer-line" (= very thin plastic-coated stainless-steel cable) down the L&R sides of car. Locate these lines in V-notches in the toe-setup pipes so the lines are just outboard of wheels, and stretch them very tight. Use a steel-ruler to measure gap between line and wheel-rim 3 o'clock and 9 o'clock, then roll car forward half a wheel revolution and measure again. A good eye should be able to see ~0.2 mm differences.

Do above measurements before EVERY test or event, and most certainly after any other suspension adjustments!

Z

[Claude Rouelle]

Every force and moment induced by tire vertical load, slip angle and slip ratio will generates forces and moments on each suspension elements and the chassis pickup points. Nothing is 100 % rigid. You will have deflections. Hopefully only a fraction of a mm or a fraction of a degree. But in reality many FSAE / FS cars have pickup points movements, camber and toe change visible with a naked eye by just applying with your own hands / arms / knees a force and or a torque on a tire which in fact is a fraction of what the tire force and/or torque can generated when the car is at reasonable speed on track.

You would hope that "one into the other" the compression here will be compensated by the tension there. But in engineering and even more in race car engineering the Gods are not with you: ERRORS WILL KEEP ADDING. If there was maybe only one flexing suspension pick up point, or only one little part of the upright or the rim or the suspension elements maybe that would not be that bad but the WHOLE bloody assembly is flexing everywhere. And there is worse. the amount of deflection won't be the same when you increase the force and when you decrease it: there will be HYSTERESIS.

There is already a huge amount of Fy, Mz etc hysteresis in the tire. If you add many more deflections and hysteresis in the suspensions and the chassis, how do you want the driver to feel the car and be confident with something so unpredictable?

Imagine the pilot of a jet fighter moving the stick left and it takes a fraction of a second for the ailerons to move and another fraction when he moves the stick back to the right? Will he feel confident to land on an aircraft carrier? It is not different for a race car.

Just imagine the number of springs, dampers and hysteresis there is between the steering wheel and the 2 front tire contact patches:
- the steering wheel itself
- the spherical joint and the bracket which hold the spherical joint of the steering wheel on the cockpit front bulkhead
- the steering column itself
- the spherical joint(s) and the brackets which hold the spherical joint(s) of the steering column on the chassis
- the steering rack shaft
- the steering rack housing
- the attachment of the steering rack on the chassis
- the toe links bending, buckling, compression, tension
- the upright deflection
- the upright - hub bearing deflections
- the hub itself
- the rim (biiig one)
- and finally (but that one is unavoidable) the tire with its relaxation time (or length)
AND
- all the sticktion" (force you need to apply to have a beginning a movement, a bit like a spring pre-load), the friction (force to keep a given movement), the damping and the hysteresis that you have
- in suspension spherical joints
- and rod ends
- and the steering rack shaft inside its steering rack housing....
that are themselves
- force
- speed
- (I mean speed of the friction)
- temperature,
- bolt torque number
sensitive

All these frictions and compliance are "buffers" between the driver and the tires especially the ones he has from the steering wheel. It will decrease the driver feedback, feeling, confidence and response .


OK I guess you get the message now.

Fight compliance! Or go fishing!

*************************************

The first thing to do to fight compliance is not to make the parts stiffer: that is the second thing to do.

The first thing to do is to decrease the forces acting on them. And how do you do that? 2 ways
1 Decrease the forces by decreasing the car mass.
- If a car weight 200 kg (with driver) and takes 1.5 G of lateral acceleration, the sum of the 4 FY on the tires will be 3000 N
- If a car weight 300 kg (with driver) and takes 1.5 G of lateral acceleration, the sum of the 4 FY on the tires will be 4500 N. If you want the same minimal, acceptable deflection of 0.x mm at a given point of your chassis or suspension, well ... you can be the king of the FEA, once you have optimize your design the only way to minimize deflection with a heavier car is to add material: larger and/or thicker suspension and chassis tubes, thicker upright fabricated metal sheet or thicker CNC aluminium upright ribs......that will add weight.... that will add forces..... (again f=ma no?) ... that will increase compliance...and the crescendo towards the disaster continues.

THE LIGHTER THE CAR IS THE LIGHTER IT CAN BE. THE HEAVIER IT IS THE HEAVIER IT WILL HAVE TO BE.

THE GOAL IS NOT TO DECREASE LAST YEAR CAR WEIGHT BY X KG : THE GOAL IS TO START FROM ZERO AND DURING THE CAR CONCEPT / DESIGN TO INCREASE THE CAR WEIGHT A MINIMUM

2. Intelligent design. Just as an example think about how the shape of a wishbone will influence the forces in its links. A wishbone with a forward leg perpendicular to the chassis and a reward leg at 45 degrees to the chassis will see all a given lateral tire load going in the forward leg and none in the reward leg, while the braking force will entirely goes in the rearward leg and not in the forward leg. for each lateral and longitudinal load case you need big tubes and rod ends. While with a A shape wishbone both leg will participate to the recuperation of the lateral and longitudinal loads....and do not need to be that heavy.

Again it is possible to make any car part light and stiff and cheap and easy to manufacture. That is one of the wonderful educative challenge of FSAE / FS competitions

[slicktop]

In addition to Z's advice on a string setup. It's essential to have the system attach to the car, as he suggested. Makes it much easier.
Google "smart strings" for some more inspiration. Plus, the manual lays out the procedure.

[Jay Lawrence]

CWA,

I've been a bit casual with my post and you've called me out :( I guess I should have elaborated. Basically I was just pointing out a situation where the front wheels did not seem to be doing as the driver asked them (and me being a noob only really noticed the toe at the time). I honestly have no idea how this affected steering effort because I don't know enough about the car (and, admittedly, about steering geometry), but clearly there was a difference between 0 m/s and >0 m/s, which I imagine would have an effect on the driver. Perhaps it was geometry related and they had some out of plane axes going on, or perhaps it was compliance related? I imagine it was a combination but I would think that compliance could definitely cause increased steering loads (as per your example).

[ChristianChalliner]

As a little bit of a hijack but still very related.

How do you set rack pre-load? Because looking on the interwebz it says to do the pre-load adjustment up until the rack bar has no play rotationally but if I do that then the rack is as good as locked solid? Yet with the adjuster out of the rack the rack is extremely low friction and feels really nice! Likewise doing up the adjuster nut seems to make as good as zero difference to the play of the input shaft, only the rack bar travel stiffness.

Any ideas?

[DougMilliken]

It's essential to have the system attach to the car, as he suggested. Makes it much easier.

Google "smart strings" for some more inspiration. Plus, the manual lays out the procedure.
One of our projects in the 1980's required re-aligning a Corvette several times a day (long story). I drilled tiny holes in the bumpers and used them to locate tubes across the car, front and rear. With some carefully filed notches in the tubes the parallel strings went on the same way every time. No one ever complained about the tiny holes...

Looking through the Smart Strings manual reminded me of a very complete treatment of alignment -- I thought it was long out of print, but it's now available again through print-on-demand:
http://www.barnesandnoble.com/w/a-chassis-alignment-procedure-michael-w-velten/1115530973?ean=9781451559286
https://www.createspace.com/3441188
"A Chassis Alignment Procedure For Formula Cars" by Michael W Velten

[Claude Rouelle]

Christian,

Don't you have a little adjustment with a spring that you can preload either with shims or with a nut you can turn.....which finally creates to pre-friction on the shaft ?

https://www.google.com/search?q=steering+rack+preload+adjustment&es_sm=93&tbm=isch&tbo=u&source=univ&sa=X&ei=w-YCVdrdLpChyAT7rIGACw&ved=0CDEQsAQ&biw=1920&bih=947

http://www.bobistheoilguy.com/forums/ubbthreads.php?ubb=showflat&Number=559692

I mentioned before that one of the most useful but most ignored sensors (by the students and by many professional race engineers) is a steering torque sensor.

You would learn a lot by looking at steering torque Vs steering angle and particularly the shape of its hysteresis. If there is preload you will see some steering torque Nm at 0 steering angle up and down depending which way you steer. The average line will give you your steering sensitivity (Nm/deg) You can also look ta the deadband: your hysteresis curve will cross the zero Nm line twice; the distance between these 2 ponts is the deadband

Also worth to look at steering torque Vs lateral acceleration (the average line is a torque gradient) and the hysteresis of this curve, the steering angle Vs lateral acceleration and the hysteresis of this curve. Same for the yaw rate Vs steering angle and look at the off center and the on center gain. That will five you a good idea of the driver feedback linearity.

Look at all this curve at different speed; you will learn a lot

Worth to look at http://www.raetech.com/Instrumentation/Steering_Torque_Sensor.php

[ChristianChalliner]

Claude,

Yeah it's a nut that's at 90 degrees to the pinion gear, basically you tighten the nut which presses a brass bush up against the back of the rack (there's an intermediate spring between the bush and the nut) but the thing is that you're pressing a flat bush up against a round bar so in order to sufficiently restrain the rotation of the rack bar you have to run a massive amount of pre-load which makes the rack incredibly stiff. We have another old rack which uses a similar method but the section of the rack bar which the bush presses against has been machined flat, this is a much better way of reducing the rotation and allows you to run much less pre-load so I'm tempted to modify the other rack to be the same.

My question though is why is this even a problem? I can see how you might get premature wear of the gears without the pre-load but I don't see why it's a problem, this rack sees barely any time in service so if it wears quickly it's not really a problem? and the adjustment seems to make negligible difference to the amount of play at the pinion.

The idea of a torque measurement is particularly interesting. I play a decent amount of racing games and something I play around with a lot is the steering torque in order to find the sweet spot, it's a bit artificial I know but given that the feel through the wheel and the screen are the only two feedback mechanisms in this case it's extremely important to get this right and get a good balance between feel and usability. Heavy steering seems nice at first but after a 3 hour race your arms are falling off and couple that with the lack of feel for very subtle inputs at the tyre and you realize you're losing a lot of valuable information because all your effort is going into turning the wheel rather than feeling what is coming from it.

Likewise, it sounds that the deadband is similar to deadzone and minimum force settings (for games) if it is indeed akin then it seems important to get this as low as you can get away with as a weird feeling around center isn't fun.

Also, sent you a PM Claude.