Vehicle Dynamics starting points and design process

[ChristianChalliner]

Z - Single wheel bump would be a fairly major road anomaly (pothole) I suppose or a kerb, neither of which exist in FSAE.

In other terms, I think the idea of having major design work only starting in December is far too ambitious, you assume people are like robots and will work on concepts day in day out and lets be honest people are just lazy and wont do that!

One thing I did read which leaves me at somewhat of an issue is that RCVD says that the RC isn't completely linked to the kinematic and elastic weight transfer distribution in the case of a beam axle, in which case how does one determine the split and its relationship to the linkages used?

With regards to tips on the braking system, It's fair to say it's most likely going to be me spec'ing everything about them, our calipers are fixed as the current AP Racing 4 pots (too expensive to change) (also thanks BCU for lending them to us!) but master cylinder sizing, etc is up in the air.

Anyway, onto sketches and a few ideas!

WB=1530mm
CG height (with current engine spec) = 297mm
CG height (with dry sump) ~ 250mm
F/R WD = 35/65
Ground clearance = 30mm

Basically the big square is the engine and the crossed square represents the size of our current diff sprocket, biggest circles are wheels, I've also included percy along with arms to get a feel of the steering reach required. Furthest forward point is the bulkhead/IA plate, next up is pedal face, etc (big cross line represents top of side impact structure).



Main assumptions are that the two largest masses drive the WD (driver + engine) I appreciate this will change substantially once the other parts are added in but that will probably move it forward 5% or so (I've also just realised the drivers CofG is probably slightly further forward given the leg position but I'll rectify that on MK2.

Slightly disappointed no one had anything to say about my thoughts on caster and hanging suspension hardpoints from the IA bulkhead

[Z]

Christian,

Very briefly today...

* Yes, no "bumpy tracks" in FS. For "kinematic camber changes" it is enough to think of the wheels sitting "stationary" on a flat road, and the car-body moving in heave, pitch, and roll.

* I was suggesting that the early stages of design, say November, are very "fluid", with lots of free-hand sketches (like your underfloor-MC brakes <- good!) but not much accurate dimensioning. Then later, say December, each team member starts "filling in the details".

* Beam and independent suspensions are very simple mechanisms. They are best understood using the age-old principles of Classical Mechanics (= Statics + Kinematics + Dynamics). Anything you read in modern, dedicated "VD" books should be taken with a very large grain of salt. (More detailed explanations later, if you want...)

* Yes, driver's CG is forward/above their navel/belt-buckle. Quite heavy IA/front-bulkhead, which is long way in front of front-axle also makes a difference (because nothing similar behind rear-axle). Also here I would suggest fitting the radiator just behind the engine. This "close-coupling" simplifies plumbing and allows thermosiphon.

* Keep in mind that a lowered engine will allow you to shorten the car. But at the moment you are at the minimum limit... So, perhaps, move front-wheels 100 mm forward relative to driver and Front-Bulkhead, for wheelbase = 1630 mm. Then if you manage to lower the engine you can push it and the rear-wheels forward to get back to the minimum wheelbase again.

* Castor, BY ITSELF (and with narrowish tyres), DOES NOT JACK (err..., not much, and depends on wheel radius...).
Castor + Offset DOES JACK, potentially quite a lot!
Big subject, better left til later, because not important now. (However, if a Team intended to run a spool-diff, then this would be important to think about now.)

* Yes, IA/Front-Bulkhead is a VERY GOOD PLACE to hang suspension from! Unfortunately, most Teams have their IA/FB waaay too far forward, because of failure of good thinking at this big-picture level.

Z

[ChristianChalliner]

Sketches - That's possible but I don't trust people enough to let them design nothing until December as currently a lot of people have come back with nothing, hence why I was drawing the pedal box ideas.

Beams - True, they are at least much simpler to analyse in terms of forces than the double wishbones but with the exception of panhard bar constrained setups I'm struggling to understand where the location of the RC actually is but I'll keep going and see where I get first. More detailed explanations would be nice but I will try myself first and post the results

Radiator - Difficult for now, personally placing it on the car centerline seems a good idea for L/R WD but unless it was to sit behind our diff it might be difficult to do that. Currently it sits on the left with the exhaust on the right but I think the radiator when full is probably heavier (but I will weigh it and see)

Castor - I think I understand somewhat... would I be on the right track to think it causes an inside rear wheel to lift? (Quite prepared to be told I'm wrong!)

Well, I think it's possible to do, our tape mockup went well although some members were in dis-belief at how short the car could actually be! We're going to make a wooden mockup tomorrow so I'll post pictures once we've done that. I also started messing around with designs for the beams, actually a bit trickier than I thought but again, once I have pictures (especially of front steering) I will post them, looks like an R&P makes life more difficult than necessary though...

[Jay Lawrence]

Christian,

The side pod is the go-to place to mount your engine heat exchanger. It balances the exhaust out (like you've mentioned) and can keep your yaw inertia (and CG) down. Just don't try to get fancy with 'optimising' your sidepod design. A big opening with a well sealed fan on the back is a good way to go.

As for castor, take a look at a go-kart. Turn the steering hard one way and feel what happens to the inside rear wheel, remembering that karts have spools.

[mech5496]

Christian, I regarding RC's I would really suggest to take a look at Z's posts regarding n-lines, they will help you understand a TON about linkages in general.

[Menisk]

* Yes, IA/Front-Bulkhead is a VERY GOOD PLACE to hang suspension from! Unfortunately, most Teams have their IA/FB waaay too far forward, because of failure of good thinking at this big-picture level.
Z

I don't think it's a failure of big picture thinking. I think it's more the pesky problem of drivers and percy having legs. Ultimately your rear wheels end up as far forward as your engine and driveline packaging will allow, you place your driver as close to the engine as you can and your front bulkhead gets defined by leg length of your ergo. Unless you want to extend your wheelbase beyond the minimum defined, but that's going to hurt you on our tracks. Ultimately we attempt to get everything in the car as close the rear wheels as possible and the front draws itself based on the position of the driver and his seating angle.

[MCoach]

Typically at these levels, Rear axle center line is defined by how far forward the differential can be placed without becoming a sawblade applied to engine. The front is defined by whatever it takes to reach the desired F/R weight balance. This implies that as much weight as possible is already shoved to the back, allowing the car to achieve close to the minimum wheelbase with the desired balance. However, the front bulkhead is still hanging way out there because of drivers' twiddly bits. To push the front track further towards the bulkhead means compromise of the weight balance and tire selection.

Some solution to this seems to not have the driver and the Percy sit in the same respective style. A taller main roll hoop means a shorter required distance to front bulkhead. However, this means that either your driver is sitting straight up and down with a mega tall main roll hoop, or the cockpit is designed to accept both a comfortably seated, low driver, and a straight Percy. Demonstration of this concept is the entry from ETS this year.

Big picture level led me to that conclusion. Also, hitting vehicle dynamics targets are higher on my list than 'optimal' loading and minimizing some silly tubes.

[Z]

Groooaaann..., excuses, excuses...

Minimum wheelbase,
+ very low CG,
+ very low Yaw MoI,
+ best F:R% for RWD (~40:60+),
+ driver's CG close to car CG (for consistent F:R%, and implying driver entirely WITHIN wheelbase),
+ narrow fuselage for good aeroflows,
are ALL possible, and all VERY EASY, because...
= BROWN GO-KART!!!!!

Doing this simply requires you to STOP COPYING EACH OTHER, and START THINKING BIG-PICTURE!

(Hint (for thousandth time...): The engine-drivetrain is merely "the necessary ballast". As such, it can be squashed into whatever shape best suits the overall plan. And since the rest of the car is SO SIMPLE to build (remember, "brown go-kart"!!!), there is plenty of time and resources available to squash the engine-drivetrain into whatever shape you want.)

Stop copying everyone else, stop being so lazy, and DO SOME ENGINEERING!

... like trying to put brains into statues...

Z

[Menisk]

Groooaaann..., excuses, excuses...

Minimum wheelbase,
+ very low CG,
+ very low Yaw MoI,
+ best F:R% for RWD (~40:60+),
+ driver's CG close to car CG (for consistent F:R%, and implying driver entirely WITHIN wheelbase),
+ narrow fuselage for good aeroflows,
are ALL possible, and all VERY EASY, because...
= BROWN GO-KART!!!!!

Doing this simply requires you to STOP COPYING EACH OTHER, and START THINKING BIG-PICTURE!

(Hint (for thousandth time...): The engine-drivetrain is merely "the necessary ballast". As such, it can be squashed into whatever shape best suits the overall plan. And since the rest of the car is SO SIMPLE to build (remember, "brown go-kart"!!!), there is plenty of time and resources available to squash the engine-drivetrain into whatever shape you want.)

Stop copying everyone else, stop being so lazy, and DO SOME ENGINEERING!

... like trying to put brains into statues...

Z

Very low CG means lying your driver (the heaviest component of your car) flat, which means his heavy legs stick out forward, increasing your yaw inertia. This also moves your bulkhead forward, so if you want him entirely within the wheelbase, you're not going to have a minimum wheelbase. From the sole of my foot to the back of my bum is 1100mm while I'm sitting bolt upright. No one sits bolt upright is an SAE car because you raise the CG way too high. You're asking a lot to fit the driver entirely within the wheelbase and be minimum wheelbase as the driveline and CV angles limit the location of your rear axle. Literally the only way to achieve that is to turn the engine around a have a mid driveline and dangle the engine off the back, something interesting I'd like to have a look at.

Also last time I check engines aren't squishy. They're pretty damn rigid and the have a set size. You cram everything as tight as you can to it but you can't just magically make the gearbox disappear and the engine shorten. I'm sure with a single you can probably achieve a 40:60 bias with the conventional layout of driveline behind engine, but no chance with an inline 4 with low CG. With a single you're not going to have to worry about putting power down, you don't have any (not necessarily a problem). At this point the rear weight is only good for evening out wheel loads under braking. Long and short of it is you can't have all the things you want. They're all trade offs.

I can only assume your legs must end at the knee, the rest of us have to deal with the problems normal legs cause. Please stop chanting that people are mindless idiots on this forum. There are annoying realities caused by rules and the physical size of drivers that you completely ignore.

[mdavis]

Typically at these levels, Rear axle center line is defined by how far forward the differential can be placed without becoming a sawblade applied to engine. The front is defined by whatever it takes to reach the desired F/R weight balance. This implies that as much weight as possible is already shoved to the back, allowing the car to achieve close to the minimum wheelbase with the desired balance.

The way we did it was start with the rear wheels/tires in a spot, the place the diff as far behind as we were willing to. From there, but the engine against the diff (as close as possible). Then place driver. This starts by moving everything as far back as possible, where your method could have the diff pretty far in front of the rear wheel centerline, which moves CG forward. We ended up something like 45-46% front with 160lb driver.

Groooaaann..., excuses, excuses...

Minimum wheelbase,
+ very low CG,
+ very low Yaw MoI,
+ best F:R% for RWD (~40:60+),
+ driver's CG close to car CG (for consistent F:R%, and implying driver entirely WITHIN wheelbase),
+ narrow fuselage for good aeroflows,
are ALL possible, and all VERY EASY, because...
= BROWN GO-KART!!!!!

Doing this simply requires you to STOP COPYING EACH OTHER, and START THINKING BIG-PICTURE!

(Hint (for thousandth time...): The engine-drivetrain is merely "the necessary ballast". As such, it can be squashed into whatever shape best suits the overall plan. And since the rest of the car is SO SIMPLE to build (remember, "brown go-kart"!!!), there is plenty of time and resources available to squash the engine-drivetrain into whatever shape you want.)

Z

I don't think any of the current popular engine setups are going to give this result. Do I think it's possible to put together a brown go-kart? Yes. With the current popular line of thinking? Probably not. Newer Yamaha dirt bike engine would give the best bet (rearward tilted cylinder) of the currently used options, but I still don't think it's enough. Someone is going to have to think outside the box to get this to happen, if they really want to do it, unfortunately. ETS is probably the closest to this package, but they still have a lot of unnecessary bits on the car.

From some layouts I did, I think a 72" wheelbase would be about perfect to have suspension points hanging on the front bulkhead with a much more traditional rear end layout (4 cylinder, chain drive, nearly straight halfshafts, etc.) and provide 60+% rear weight bias. This would be a really fun A-Modified project for SCCA autocross, if I had someone to bank roll it, and could drive at a level to do the car justice.

-Matt