Wavetrac Differential

[CWA]

I think some of the focus shifted on to figuring out the suspension issue. Which while I appreciate it, is not exactly why we need a different differential. I do not think it is a "band-aid fix". While I see the point that the suspension is causing an issue, I don't see how a differential that will handicap the car in odd situations is acceptable. A different differential isn't the solution to this problem, its a solution to a few different problems. One of which, is the diff is currently untuneable.

Well I don't see how you can justify spending x-hundred of your sponsor's hard earned dollars when you: a) haven't even attempted to determine and address the root cause of the issue, and b) can't quantify (or even make estimates of) the gains fixing this 'issue' will bring you.

Your chassis setup causes the 'unloading of your rear inside wheel'. You described the 'unloading of your rear inside wheel' as an issue and the main reason for wanting to buy a new diff in your very first post. We are trying to help you cure this issue, but you seem to have no interest in this, and have it in your mind that you just have to have a new diff. If you are not interested in curing the issue, and are actually only interested in having a tuneable diff you can mess about with, or you just want to piss away money, don't try and gain support for this approach under the pretense of trying to objectively solve a problem.

I have a few hunches to our wheel lifting issues. One of which is an overall roll gradient vs wheel travel issue. The car was allowed to roll quite a bit in the last cars, and I think it rolls more than calculated. Which is easy to prove in this car, but I haven't had a chance to yet." - Suspension guy 2

Curious reasoning. I don't see how excess body roll in isolation can be the root cause of lifting a wheel. My VW Polo rolls a shed-load but doesn't jack a wheel. Therefore roll gradient alone is not the metric that will explain this behaviour.

We have scales and the car is on them frequently for testing. I don't have a data sheet with me to record actual corner weights.

????? Why post this?? Don't you have budget for pen and paper??

I guess I will have to track down the rest of those numbers myself when I find the time.

????

Steering Geometry: Look at the design tech sheet. Those are all "designed" numbers

?????

Ignore the ARBs and do not consider them into wheel rates.

Just how exactly should we be quantifiably considering the varying stiffnesses of ARB's, then???

Wheel Rate: 20n/mm seems low, maybe a conversion error? Check the tech sheet we turned in for lincoln, that will give you a closer estimate.

"Wheel rates are about 112 lbs/in on all corners" is what you posted on Page 1. Why don't you check the 'conversion error'??

CG height: 10.2 measured without wings, 10.5" with wings
said the cg raises about 1.5 in with wings

There is inconsistency between what your 2 suspension guys are saying. Why aren't they aligned?? Are they not on the same team?

"I think our actual weight dist. was 49%, -Suspension guy 1
Static weight distribution: 45% front actual - Suspension guy 2

I appreciate the diligence in relaying the info as you receive it Tyler, but come on! Weight distribution is what it is, it is not open to interpretation, this value shouldn't vary depending on source! Measure it!
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Some constructive notes:

Also we haven't explored 100% of the bar options, there are some left which could likely fix the problem by adding a lot of roll stiffness, but the car would probably understeer a ton (since we'd be adding most of that to the front).

The theory implies that more front bar/springs than rear gives more understeer. The theory more specifically says that a lateral load transfer distribution is what adjusts your balance. Lots of practical issues can interfere here, so that adjusting springs and bar do not actually change your LLTD as you might expect. These practical issues can be extremely significant, to the point you may not believe. Understanding what the theory should tell you is one thing, but don't be blind to what you are actually seeing, even if it disagrees with your current level of understanding of the theory.

As many have already stated, compliance if a practical consideration that can make basic spring/bar LLTD predictions redundant. If your chassis is too soft, stiffening front springs / bars won't actually adjust your lateral load transfer distribution. Have you verified that adding lots of front spring and front bar actually gives your car excessive terminal understeer? Try it. If it doesn't, your chassis is too soft, and all those LLTD predictions you are taking from Milliken equations under the assumption that your chassis is rigid are meaningless and far from the truth.

The suspension does not have bushings and does not carry much friction at all. Essentially is negligible for most calculations we do

More friction on one axle than the other will also take your car far away from the LLTD predictions you have based solely on spring/bar stiffness. I am curious to know how you are so confident that you have negligible levels of friction. I don't just mean control arm friction when unloaded, I mean net system friction when loaded, including your damper seals. Total system levels can add up to be quite significant. Even if you can see your suspension actuating, your friction levels may be considerably higher on one axle than another, contributing to an unexpected balance difference.

From what I've been learning, I'm pretty suspicious of our roll centers and how they work with the rest of the car

Yes. This is as Tim said, and is a possible cause. Why not explore it.

how the travel available is mostly for compression.

As Z suggested to you, be careful you are not hitting rebound stops (over-extending your damper). Once you reach the level of lat acc that causes on one of your inside dampers to full extend, you will see full, immediate load transfer across this axle. If this is happening on the rear axle of your car, not only will it completely reverse the base chassis 'understeer' balance you believe you have, but it could also very well contribute to lifting a wheel..

I don't remember our exact lateral g's but our best skidpad time was 5.381s. From our data acquisition we have seen up to 2.1g's during Endurance. I drove the skidpad event at Formula North and from what I could tell (I'm by no means a professional driver) the car wanted to over-rotate at the limit. The car doesn't appear to be jacking at all during steady-state. The time we noticed the problem most is the hairpin at the end of the slalom at the FSAE Lincoln Endurance course.

If you reach 2.1 g then your issue is not big. What kind of % reduction in lap time do you really believe curing this issue will bring to the team? Will that % be worth the amount of money you seem set on spending?

If you depart with oversteer on the skidpad, adding more torque to your outside wheel will not increase cornering capacity, so again a new diff will not benefit you here. Also, check that the terminal oversteer you appear to be seeing actually aligns with your predictions for steady-state behaviour. Why don't you have limit understeer? Is what you see versus what you expect acceptable?

Hairpins are usually the most steady-state of all the FSAE endurance event corners. What exactly do you think it is about the hairpin that is different to the skidpad that might help to explain why you see this inside wheel spin at the hairpin but not at the skidpad? Does the jacking only occur during turn-in at the hairpin? If so, why are you worried about getting torque down here, again, consider how you will actually be faster by removing this symptom.