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 (http://fsae.com/eve/forums/a/tpc/f/125607348/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

[BuckeyeEngines]

Setting the brake bias more forward will help this issue. Whats happening is the rear wheels are slowing down faster than the engine can slow down, the result is rear brake chatter/ rear tires hopping when it is really bad. The easiest and best way to get rid of this is with a slipper clutch. You can research how they work on your own, they work great, the only negative thing is you will wear clutch plates faster and they run about 1000 dollars. Well worth the investment, we were able to lower our lap times by adding the new clutch, due to better braking.

[The AFX Master]

I´m a former drivetrain leader from F-SAE USB, The 2008 diff mounting design is practically the same since 2008 without stiffness problems. But i don´t know how stiff are the chain tensioner mounts on the new car.

For clarification, we have both rear disks since 2007 without the chatter described above.

I´m for some compliance on the diff mounts. Could the torsen t1 cause those types of sounds with a dual rotor setup?, i don´t think so.

[woodstock]

We got this problem after we switched from a single brake on the diff to two outboard ones. We ran a T1 torsen and our thinking is that it's the diff's fault but could be wrong. Our mounting is not particularly stiff but not that much different than other years.

Changing diff this year anyway but mainly for other reasons.

[jpusb]

Thanks for all your replies guys, the thread has come to be a very good discussion.
Let me summarize a little our progress through the problem, and re-explain our configuration.
We run TRE axles and CV joints, outer CV joint housing is the hub itself. Torsen T1 diff. Two outboard rear discs.
The ONLY difference with previous years' configurations is that axles are longer (I did thought about system stiffness because it is a lot easier to twist this longer axle).
I DID tried braking under a little bit of throttle and the problem was GONE.
As I said before, if you brake with the clutch engaged, there is no chatter either.

Finally we got two dry days and as expected, the problem was dramatically reduced. However, personally I thought it was still a bigger chatter than our previous cars showed. On Saturday, we stopped the car to finish the header (were racing with 2007 header) and other stuff and found the rear right caliper piston was stuck. We make our own calipers and sometimes we have a problem here and there when the brakes are new, about a piston not returning or a broken seal. We knew that tire had a little more brake drag than usual but nothing really really bad, but the problem seemed to help a lot in locking that wheel way before the other. When you lock just one rear wheel then all the free plays in the powertrain (specially the diff) go amplified to the other wheel, and I THINK that's what's happening.
In the mean time, drivers are getting used to the brake with throttle technique because not only it eliminates chatter but it also helps to make faster downshifts so I think we're sticking to it so far.

I will try to get you pics of the powertrain setup if I can.

Slipper clutch is not an option for us here at Venezuela, you have no idea how difficult and expensive it is for us to use dollars $$ (government issues).

[Wesley]

What does your timing map look like at 0 percent throttle and high RPM?

A lot of people just ignore those cells, and on FSAE engines when you tend to have fuel puddling, sometimes you can have some bucking problems when you let off at high RPM (amplified going into braking) and that extra fuel is suddenly vaporized in the low pressure. Try bumping the timing up in those cells.

[jpusb]

Well I don't think timing would be a problem, we have a lot of advance near those cells, I know 'cause I tuned it myself. Besides, right now we are racing with the same map as in 2009 and that prototype didn't show this.
Today we have a dry day and we're going to do some autoXs, I'm going to try and play around more with the brake bias and shock damping, and I'll let you know.

Thanks again

[jpusb]

Well today I set the brake balance (in the dry) as I never did before (really took some time into it) and everything looked better, the car still behaves different from previous models but I'm really loving the slight and short throttle input when downshifting, which completely eliminates the chatter.

Thanks to all of you for your help and ideas, now there is another good post on the forum for future guys to read when in a confusing situation like this one.

[barba_p]

You don't have enough rear downforce
try putting big rear wing

[Mike Cook]

lol.

[2BWise]

A lot of good information here. Similar to powerhop, brakehop occurs when you reach the resonant frequency of all the components in the driveline. The total stiffness of the entire driveline (mounts included) will give you the frequency.
Sounds like you've made some gains to reduce it, but ultimately compromising your setup and adding extra driver inputs is not ideal. If you make some component changes you may be able to move the frequency enough to significantly reduce the problem. One is to use different stiffness axles, or modify the mounts.

[atm92484]

You are probably causing the engine to approach a stall. Get in a manual car, take it up to 60, hit the brakes, and don't downshift. You will get a similar sensation once the engine RPMs drop below idle.

Its much more harsh on an FSAE car since they're lighter, small displacement engines have less torque, and everything is hard mounted.