My wiggly rear end

[paradocs98]

lucid, Gearhead, rldzhao, trackrat, all you usual track guys...

Yesterday at Lime Rock I noticed that under heavy braking the back of the car felt a bit loose and wiggly, dancing around a bit. I was able to gently play with trail braking and rotation a bit, but I was a little concerned with how loose the rear end felt under heavy straight-line braking. Have you guys experienced this as well? How can it be lessened? Tires? Suspension changes? Thanks...

[Gearhead999s]

Originally Posted by paradocs98

lucid, Gearhead, rldzhao, trackrat, all you usual track guys...

Yesterday at Lime Rock I noticed that under heavy braking the back of the car felt a bit loose and wiggly, dancing around a bit. I was able to gently play with trail braking and rotation a bit, but I was a little concerned with how loose the rear end felt under heavy straight-line braking. Have you guys experienced this as well? How can it be lessened? Tires? Suspension changes? Thanks...

I have not noticed it to any great degree but I also run stock alignment settings.My 11 with the ZCP does seem more balanced than my 08 and the braking has much less drama with the the Stopteck 380mm BBK than I had with my stock 08.

[LiM3y]

Originally Posted by paradocs98

lucid, Gearhead, rldzhao, trackrat, all you usual track guys...

Yesterday at Lime Rock I noticed that under heavy braking the back of the car felt a bit loose and wiggly, dancing around a bit. I was able to gently play with trail braking and rotation a bit, but I was a little concerned with how loose the rear end felt under heavy straight-line braking. Have you guys experienced this as well? How can it be lessened? Tires? Suspension changes? Thanks...

How are your rotors today? Were you running stock pads?

[lucid]

I have not had any significant issues with straight line braking stability with the E92 M3. What is your front and rear toe set to? That question does not address dynamic toe changes under compression; I don't know what that curve looks like for the car, but I doubt that the dynamic changes are an issue in general as we have not heard of any negative reports about it here. However, a bumpy braking zone can bring such things into play. Where exactly on LRP was it happening? Also, are you sure your wheel was straight (I know that sounds obvious, but sometimes people don't notice that they actually have a little bit of steering input). Finally, are you running the same pad on all 4 corners?

On a somewhat similar yet tangential note, I have been having serious instability issues with my E30 M3's straight line braking, and we have not been able to pin it down despite checking and realigning the suspension, and checking the operation of the caliper pistons. Maybe the bias proportioning valve or ABS is failing, but that is 25 year old car, so those shouldn't be issues in a new car. The car rips through the field in DEs for the most part otherwise, but I have to brake early to avoid having the car drive itself off the road in the braking zone. Really annoying...

[Martin_D]

make sure the steering wheel is dead straight, and this should not happen. happens a lot when the motor is behind you thoough.

get a more aggressive alignment for the rear, if you have not already...

[RickyBobby]

Can you tell us more about your setup? Brake pads, suspension, tires, alignment settings would be helpful in troubleshooting.

Two things that come to mind arw bias and alignment.
Too much rear bias will cause instability. When my vendor accidentally gave me ht10's instead of dtc 70's I had EXACTLY what you are describing because my rear dtc 70's were overpowering the fronts.
If you are using stock brakes, you may have overheated the fronts but not the rears which would also lead to rear bias.

Rear toe out will also cause instability.

[lucid]

My thinking is that increased rear bias should actually increase stability on a straight line as long as there is rear grip to be taken advantage of. If there is no rear grip left on the table, then you can get into trouble, but my understanding is that most OEM systems leave a good amount of rear grip untouched for max. deceleration scenarios.

[Richbot]

Might need a little more rear toe-in. I was surprised how little the car has from the factory when I checked mine. I bet it toes out in droop which would make small changes in road surface under braking more influential on where the rearend tries to point itself

Have you put sphericals in the back of the E30? Sounds like the same problem just more pronounced, wondering if a bushing is allowing too much compliance and/or you're bottoming out a spherical and binding something in droop. Behavior when the car is moving is tough to find sometimes, especially stuff that's happening when the suspension is in droop since it's a lot easier to compress than extend when it's on jackstands

Wild idea - do you have a magnet mount you could stick to the spare tire well and point a gopro at the rear upright?

[paradocs98]

Thanks for the replies so far, guys. Some details to help in diagnosis:

E90 ZCP

Dinan Stage III suspension--alignment done to Dinan specs. Not sure of degree of toe, but front camber -1.8, rear camber -1.7

StopTech BBK--ST60 6-piston/380mm front, ST40 4-piston/355mm rear with the Street Performance pads that came with the kit

Nitto NT01 275/35-18 tires all around

I didn't notice the rear end wiggle at Watkins Glen the prior week, but a couple of variables were different there--it was damp/wet, and I was running on the stock Pirellis. So I would assume I was not braking nearly as hard at Watkins Glen and therefore not bringing out this characteristic.

Now that you guys mention it, though, the places at Lime Rock that I noticed it were at the end of the front straight and on the downhill. At the end of the front straight I might have been transitioning to turn 1 early while still under braking--not braking truly straight as you guys said. On the downhill I was probably on the brakes too much until I built confidence to just brush the brakes a bit and let the car track thru the turn onto the straight. I would imagine that hard braking while going DOWNHILL would be enough to unsettle the rear.

Interesting thought about brake balance. Although supposedly StopTech prides themselves on "balanced brake upgrades" that are designed specifically for each application to maintain correct front-rear proportioning.

[lucid]

Originally Posted by Richbot

Have you put sphericals in the back of the E30? Sounds like the same problem just more pronounced, wondering if a bushing is allowing too much compliance and/or you're bottoming out a spherical and binding something in droop. Behavior when the car is moving is tough to find sometimes, especially stuff that's happening when the suspension is in droop since it's a lot easier to compress than extend when it's on jackstands

Wild idea - do you have a magnet mount you could stick to the spare tire well and point a gopro at the rear upright?

Yeah, I have GC spherical shock mounts in the rear (and GC camber plates in the front with spherical bearings). It's running a fairly stiff setup; GruppeN springs and shocks (800lb/in F and 1150lb/in R), and doesn't weigh much at ~2400lb with no fuel&driver, so I can't imagine travel would be significant under braking, but not a bad idea to try to use a camera to see if it is binding. ~1/8" toe out front, and 1/8" toe in rear. It is extremely planted elsewhere on the track.

I am about to swap in Wilwood calipers on all 4 corners with the same size discs all around, which should move the bias to the rear a little (a setup proven to be effective in other cars), and see if that settles things down. I am also wondering if fluid lines are unobstructed to all 4 corners--something to consider with an old car.

[lucid]

Originally Posted by paradocs98

Dinan Stage III suspension--alignment done to Dinan specs. Not sure of degree of toe, but front camber -1.8, rear camber -1.7

Sounds like you had your suspension realigned because those are not the stock specs. Toe settings will affect straight line stability, so you need to know those. Did they give you a printout from the machine after the alignment? If the issue is severe, I would take it back in and have them verify the rear toe specs. What are the Dinan specs?

Stoptech does a good job in getting the bias right, so I wouldn't worry about that.

On a side note, yes, you don't need to brake that hard downhill at LRP, but you should do what you feel comfortable with for the time being. On a stock suspension in your car, a lift and a slight tap on the brakes to settle the car before turn in should suffice there, but I don't mean to say you should go out and try that the first lap the next time you are out there or anything. That's an apex you don't want to overshoot because if you go off line at that fast sweeper, you'll find lots to debris and rubber at high speed, which is never fun.

[Richbot]

I'm more worried about suspension pickup points than shock mounts, so you're still using rubber in the rear aside from the shock mounts?

The spring rates on the front might resist dive but you'll still be in droop in the rear somewhat under braking relative to the static ride height, so something weird might be happening in droop if you have rubber in the suspension, and that's a lot of spring for a suspension with rubber in it anyway, just a thought

And air in one caliper but not the other might do it too, lots of room for failing miserably at getting air out of the system been there...

[lucid]

No stock rubber left in the suspension; all bushings are polyurethane. Right, I wouldn't run the GruppeN springs on stock rubber. The bushing stiffnesses should be close to GruppeN specs as well, so they should work together.

I wonder if there might be air trapped in the ABS module. The master cylinder was done a few years ago, and not sure if they bled that part of the system well...

[RickyBobby]

Originally Posted by paradocs98

Thanks for the replies so far, guys. Some details to help in diagnosis:

E90 ZCP

Dinan Stage III suspension--alignment done to Dinan specs. Not sure of degree of toe, but front camber -1.8, rear camber -1.7

Measure your toe, that is the important value for stability. You'd be surprised what even a bit of toe-out will do to a car's handling.
If you're going to be doing a lot of track days invest in a set of toe plates (or make your own). They're cheap and a good way to spot check your alignment.

[Richbot]

Ugh.

<- glad he doesn't have to deal with troubleshooting track car ABS systems

[lucid]

You know what, this whole thing wants me want to rip it out. I know people who got rid of their boosters and ABS modules and are doing fine except for a few flat spots here and there.

[rzm3]

1. Dinan's S3 spec calls for 0.17+/-0.08 (deg) on the rear toe. Which is the same as BMW's spec. So let's assume that the car is aligned properly.

2. When your rear end gets loose, that means your rear tires have lost traction before the front tires. Mechanically, this could be due to

a) too much rear brake bias (rear tires lock up)
b) too much rebound in the rear shocks (during braking, your rear wheels don't "drop" onto the ground quickly enough)
c) out of spec rear toe, perhaps due to wear/damage, as some have mentioned

But since you have a pretty new car, and that your modifications are sound, I doubt it would be a, b, or c.

Which leaves us to, driver input. As you mentioned, when you brake heavily going downhill, your car dives forward and transfers a lot of weight onto the front axle, which leaves very little traction left on the rear tires.

On top of that, if you have steering input, the front end will try to rotate the car, and if there is no traction on the rear tires, your rear end gets loose.

Just my 2 cents.

[rzm3]

And here's what I've been taught to trail brake.

The initial braking should be very deliberate. This should by no means be a 'panic' stop, but rather a forceful, meaningful attempt.

After the initial braking scrubs off most of the speed, you start to lift up on the brake pedal. This allows the car to transfer weight back to the rear end.

In the mean time, you start to turn the wheel. Because the front of the car still has more traction than the rear, your rear tires will gently 'slip' to rotate the car.

This is when you want to transition back onto the gas, which will transfer weight back onto the rear axle and stop the rotation.

Finally, when you feel the car has enough traction in the back, power out of the corner.

[LiM3y]

Originally Posted by rldzhao

And here's what I've been taught to trail brake.

The initial braking should be very deliberate. This should by no means be a 'panic' stop, but rather a forceful, meaningful attempt.

After the initial braking scrubs off most of the speed, you start to lift up on the brake pedal. This allows the car to transfer weight back to the rear end.

In the mean time, you start to turn the wheel. Because the front of the car still has more traction than the rear, your rear tires will gently 'slip' to rotate the car.

This is when you want to transition back onto the gas, which will transfer weight back onto the rear axle and stop the rotation.

Finally, when you feel the car has enough traction in the back, power out of the corner.

I have posted this on my air bag for VIR!

Is that oversteer and correction at the end of the trace?

[rzm3]

Originally Posted by LiM3y

I have posted this on my air bag for VIR!

Is that oversteer and correction at the end of the trace?

Yup. And the whole point of my above post is that keep the wheel straight during the initial heavy braking

[R A W L S]

You are obviously in good hands here and I can't comment on your suspension or brake setup but I did notice you have a square wheel/tire setup and my experience with this is that there is more grip up front than with a stock or narrower front setup (obviously). This front grip plus steering wheel slightly turned plus hard braking seems like it could unsettle the rear end a bit.

[dcstep]

Originally Posted by R A W L S

...but I did notice you have a square wheel/tire setup and my experience with this is that there is more grip up front than with a stock or narrower front setup (obviously). This front grip plus steering wheel slightly turned plus hard braking seems like it could unsettle the rear end a bit.

Exactly what I was thinking. This is my autocross setup and it DOES increase the propensity for the car to rotate. That's a big plus for AX and may or may not be on a road course, depending on whether you like a "loose" car or not.

BTW, rldzhao's trail-brake diagram is classic. His written explaination was good, but the diagram is worth 1000-words. BTW, for AX you can risk unsettling the car much more, such that you can brake way later and start your steering input sooner than on this particular graph. Given the propensity for AX course designers to throw in many 180-degree corners, this trail-brake/turn-in/rotate/throttle-out scenario is THE key to fast AX times, IME. Be sure to roll onto the throttle smoothly as you stop the rotation when you're headed in the right direction and begin accelerating, all in one move. (For some reason, I've had a large percentage of students think that they should "jump" on the throttle to stop the rotation).

Dave