Wavetrac Differential

[Tyler Jones]

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.

The initial purpose of the thread was just to obtain some technical information about the Wavetrac, not to ask for suspension advice. This is why I'm appearing biased, I was already decided on switching to the Wavetrac if I could obtain the necessary drawings and info. I'm also not very knowledgeable about vehicle dynamics, so I can't comment much myself. I will try to get my suspension guys to respond more directly to your questions, as they are the ones against keeping our current differential.

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

We are not all in direct proximity to the car at all times, I will take the time to try to get corner weights for the suggested scenarios this weekend.

????

My suspension guys apparently don't have the time to get some of the numbers you want so I will have to try and find some of them myself, most should be on spreadsheets on our network harddrive.

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

I'm not sure they fully understand how ARB's contribute to overall wheel rates.

"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'??

According to that number, 20n/mm is accurate. Not sure where he is coming from.

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

They don't work directly together as much as I think they should. This is more a team organization issue which I will try to resolve this coming competition year.

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!

I stated before that before Formula North the car weighed in at 45% front and at Lincoln the car weighed in at 49% front. Ride heights and aero were adjusted between those two competitions so that may be contributing to the difference. Again, they don't work directly together as often as they should. The whole team has some communication issues I'm trying to resolve.

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Some constructive notes:



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.

We have experimented with somewhere around 150-225lb/in but have springs up to 400lb/in. I will try to verify when we get a chance to test, which may take at least a week before we have the opportunity. I would hope our chassis is reasonably stiff as it weighs ~83lbs with powdercoat and the whole car weighs 405lbs without driver, but I understand that may not be the case.

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.

Do you have a suggestion on how we can verify levels of friction from one axle to the other? The control arms pivot about sphericals and the rockers/bellcranks pivot about needle bearings so they shouldn't hold much friction. I don't see this being a major contributor to the overall issue, but feel free to correct me.

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

What should I tell my suspension guys to look into specifically? Are the roll centers too low or too high? Again, I'm the wrong person to be answering these questions. Is there a good resource to familiarize myself with vehicle dynamics on a basic level? (I've tried to read Milliken's RCVD but it is way too long and dry for me to get very far, I start falling asleep at slip angles)

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..

How do we resolve this, if it is happening?

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.

Personally I think we perform very well as is, it is my suspension guys that wanted to get a new differential in the first place. The Wavetrac is the most economical differential that solves the issue of an unloaded wheel. I'm more or less trying to facilitate the switch should the rest of the team decide that it is worth the extra $200 and time spent. As is, myself and the Technical Director think we should stick with the Torsen unless I can get those Wavetrac internals drawings before the school year begins. I'm still not certain why everyone in this thread is so against the switch for financial reasons when $200 is next to nothing compared to the ~$20,000 we raise each year to spend on the car. I actually reduced the cost of the 2014-2015 car by $600 compared to the 2013-2014 car by switching away from a Tilton bias bar to one constructed with McMaster hardware, so you could almost say I covered the cost of a different differential already.

I guess I didn't consider that a hairpin is steady-state. I feel as though the extra speed carried followed by hard braking before turn in might contribute to more load transfer/roll but as I said before, I'm not a professional driver nor am I an expert on Vehicle Dynamics. The jacking is only really noticeable in the hairpin, but when it is noticeable it is really noticeable. The inside rear wheel actually completely lifts by 1-2" off the ground. This makes me think that the issue might be related to over-extending the damper as you said. Let me know what you think.