Wavetrac Differential

[CWA]

"Man that is a lot of questions, I don't have access to all those numbers at the moment and I might hesitate a little bit to put that much info on the forum (probably not a big deal, but he's asking for almost our whole design haha). I can answer all of those though, and it sounds like he is wondering whether the front wheel motion in steering is jacking the inside rear too much, and also if our bar setup is lifting it. I wouldn't suspect too much jacking from the front (although I wouldn't totally rule it out) since we have 3 deg of caster and about 0.8 inches of scrub radius, which doesn't displace the wheel downward very much (a lot of caster would be more like 5-7 deg). Wheel rates are about 112 lbs/in on all corners, and since we've tried almost all the bar combinations I don't think he can help us too much otherwise. It's got low friction and it's good on the scales, so we can pretty safely rule those things out."

First, there is no magic formula that your suspension guy might have happened upon, so don't be so worried in sharing your setup details. Even if your setup did give the best handling car in existence (which by the sounds of it, no offence; it does not), true value is found in the understanding of the relevant mechanisms and knowing exactly why the setup gives good performance. It is entirely possible that the setup that might be golden on one car would be considerably less effective on another car. But knowing how to adjust the setup slightly depending on your situation is where the real skill lies. Point being, your setup details are pretty arbitrary - you should have no qualms with sharing this info.

Second, one way or another your car is lifting a rear inside wheel. This is truth, this is reality, so you say. Don't forget that THIS is the root cause of your issue, and replacing the diff that doesn't fully lock is effectively a band-aid fix. In my opinion, until you've exhausted all of your options in chassis tuning, you should not be spending more money on hardware. Even if you have tuning limitations on your own chassis, I'd say you have a duty to figure out the mechanism causing this behaviour so that you can: a) justify to your budget providers why exactly you need a new diff, and b) pass on this knowledge to next year's team as a warning. If you guys are designing the next car, this is an even bigger reason to want to figure this out. Don't forget, plenty of teams do not lift their I/S/R wheels round corners, and so manage to make do with lesser diffs. You should strive to find out why that is, even if it turns out you can do nothing about it.

Now, I am 90% sure that real answers to all of the questions I asked will expose the cause of this behaviour (by 'real answers' I mean actual, measured values, rather than nominal design-intent values. The car certainly won't have turned out exactly as the ideal spreadsheet solution showed you it should - friction for one can have significant effects). There may be some obscure mechanism causing your I/S/R wheel to lift that isn't related to control arm jacking ("RCHs"), steering system jacking, or roll moment / lateral load transfer / static weight distribution, but I will be surprised if this is so. As said, there are plenty of cars out there that do not jack their inside wheel; it should not be difficult to determine why your car is different, it abides by the same rules of physics after all.

See if you can post back with the vales you haven't answered: Static Weight Distribution, estimates of F&R Roll Centre Heights and CG height. Also do you know what your chassis torsional stiffness might be? For starters, if this is too low, it will explain why bar tuning has not been as effective as you were perhaps expecting. Do you really have just 20N/mm wheel rates front and rear? To confirm this, please don't consider me patronising in checking; what are your spring rates and what motion ratio do you have? How much 'stiffer' does your range of bar combinations supposedly make your wheel rate in roll (a number would be useful, for you and us both)? Also, more out of curiosity than much else, how much does your car actually weigh?

Yes as your suspension guy recognised, I was trying to determine whether steer jacking might be the main contributor to your issue. You've suggested your caster angle and scrub radius are within a reasonable range, but don't forget that these aren't the only contributors to steer jacking. Your KPI and trail contribute to this too - what are these values? Also, the stiffness of your tyres / suspension / bars / chassis contribute to this mechanism - an infinitely stiff car will theoretically lift its inside wheel with far less caster and scrub than you have mentioned. Your quoted wheel rates do not seem too stiff if they are accurate, but you need to be sure that you don't have too much friction that acts to ruin these rates - the friction will act as a false suspension stiffness, at least in that it will serve to increase the effects of steer jacking. Also, your 'scrub radius' in CAD is a hypothetically ideal value, which technically isn't even a kinematic dimension (it should be based on the contact patch centre of pressure, not just the centre line of your wheel). As can be imagined, in real life, exactly where this centre of pressure is in relation to your steering axis ground level intersect depends for starters on the width of your wheel, it's camber angle with the road (with steer and roll), and tyre sidewall stiffness's.

So can you also post up: static front camber, camber gain of your front suspension, wheel widths, tyre type and sidewall sizes? Do you have your own scales? If so, you might as well get the car on them, record and then post here: a) your static corner loads with driver and steering straight ahead, and b) your static corner loads with driver and max steering angle in any direction (might as well do both directions to assess asymmetry). I believe this would be a great starting point.