Braking wobble: PLEASE HELP

[jpusb]

OK so our new car is up and running. We have not seen a dry day since the car is running, so we have NOT tested in dry conditions. On the other side, till this week, the last 2 years were very dry and we hadn't run in the wet for almost 2 years, so this means maybe I don't remember that our 2008 prototype (the last one I drove in the rain) also did this. Did it or did it not, I don't think it's acceptable.

Here is the problem. You are approaching a braking point, and as soon as you brake hard the rear end starts to sound clack clack clack real hard, the whole car starts to shake real hard. Rear tires lock up and then get loose and lock up and get loose.
Nothing of this happens if you use the clutch (obvious, just wanted to clear that out).
The outer CV joint (Taylor race) has some freeplay in the hub (in our design, the CV joint couples into the hub itself), but the 2008 prototype (which we tested a month ago in the DRY and showed NO signs of this wobble, none of our cars have ever showed this, you can see the video searching for F-SAE MARZO on youtube) has a lot more free play (LOT of miles on that car) and has never shown this, neither did the 2009 prototype. The braking disk (2 piston caliper with floating disk) has the same freeplay in the pins as it did in previous prototypes (the freeplay recommended in Tune to Win or something like that). The hub has no play at all in any direction, and the hub-wheel coupling is very tight. Caliper mounts are the practically the same as in 2007, 2008, and 2009 and they are TFT so that is not the problem.
If you turn the engine off, stop the car and shift to 1st gear, and start to push the car (normally it would stop wheels blocked by the gearbox) the wheels will do just what I explained before, try to turn, then block and slip, then try and turn again, and then block, all of this with the axles not rotating at all.
In normal braking situations we have always shown a slight clack clack clack in our previous cars, when downshifting, but with no problems at all, and we think it's the combined movement of the chain and gearbox free play, or something like this in the drive train, but nothing like this, I mean our 2008 and 2009 prototypes are pretty good cars both finishing in the top ten in AutoX in MIS.
We have tried various brake balances and even increased the rear end wheel travel in rebound, but the problem stays the same. I'm starting to think it is just the grip conditions in the wet and the slip ratio admittance (in wet conditions you loose grip really fast when slip ratio decreases, when you start to block the wheel), and I'm praying to the racing gods that we don't see that in the dry, hopefully tomorrow morning. So, I wanted to know if any of you guys have felt this wobble in your car, and if luckily for us you did, how did you solve it. I wanted to know if you have any suggestions or theories of what it could be, anything would help.

Thanks in advance and if you have any additional questions about the problem I can answer them all.

[Mike Cook]

How much anti-squat does this car have? Have you played with your high speed compression? I have seen this problem before, and although I never pin pointed it, it seemed like it had to do with a single rear disc braking through the diff, or major compliance somewhere in the system.

[Kirk Feldkamp]

You car has a diff-mounted brake, right? I vaguely remember there are some *really* old threads on this forum that talk about the diff "ratcheting" during heavy braking events. You should see if you can dig them up.

-Kirk

[Adambomb]

Yeah, we had somewhat similar problems in the past where the inside rear tire would unload and actually spin backwards (torsen diff), which is why we haven't run diff-mounted brakes since 2005.

[BillCobb]

Is it my imagination, or does it only chatter when entering right handed turns?

Set one of the rear tire pressure down 20% and determine whether its better or worse. If worse, set the other tire pressure down instead. If better, figure out why the SLR is affecting the diff so much. Then run the diff open to see if its actually the diff. A video of the left or right wheel during this 'wobble' would also be informative.

[exFSAE]

As a quick and dirty bandaid on the problem.. since you don't exactly have time to change the design.. have you tried braking with a little bit of load on the throttle? Just enough to unload some of the engine braking. Likewise does a noticeably taller final reduction ratio make any difference?

Rear end 'chatter' under the brakes or when shifting is a recurring theme with FSAE cars. Some is driver related. Some is design related.. though at the moment I'm not sure what specifically does it.

[Fred G]

Could you be out braking your engine causing chain chatter? Dial brake bias towards the front and see how severe the 'wobbling' is.

Slipper clutch?

http://fsae.com/eve/forums/a/t...48/m/33110150631/p/1

Fred

[Hector]

We have had this problem for several years. Finally fixed it this year for the first time. We've been testing for a few months and no recurrence of the problem.

1) How stiff is your drivetrain? By this, I mean all the components. Sounds like you're running TRE stuff (which is plenty stiff) but how stiff are your diff mounts? We traced some of the problem back to the entire drivetrain winding up (shafts twisting, mounts flexing, control arms flexing, etc) then suddenly unwiding, making the snapping, clacking noise.

2)Have you tried hard braking and clutching? We found that clutching the car last year under hard braking eliminated the problem completely. This is one of the ways we were able to pinpoint drivetrain stiffness as a problem.

3)If clutching while braking solves the problem, you may want to consider a slipper clutch as a fix. It dramatically reduces the amount of engine braking that occurs by allowing the clutch to slip in the opposite direction. Best case, it fixes your chatter issue. Worst case, you get more consistent braking and less fatigue on drivetrain due to lower reversal loads.


Good luck.

EDIT: Definitely not limited to single rear brake or inboard braking setups. I've personally seen this occur with both dual inboard and dual outboard brakes, thought the outboards seemed to be worse.

[Mike Cook]

Yes, I have noticed the problem in a car where the diff was not very stiffly mounted. As everything twisted up, it could have even been the deformation was causing the brake pad to unload itself from the rotor. I'm not 100% sure. As an easy fix, I might just dial the brake bias way to the front.

Mike

[Gaanja]

Would it be possible to put up a few pics of the drivetrain and the hubs?? would get a better idea of what could be wrong