Braking instability on new track build - Solutions?

[fsmtnbiker]

Stable under most conditions, except braking - I think you're on the right track so far. I'm still inclined to try a big rear rebound adjustment just to see how it affects the car (like 4-6 clicks open/reduced), but I suspect you may be fighting something else in the brake system.

Just to be sure: ABS is still functional and working? And all the sensors for the system are still installed and properly calibrated? No codes?

[tsk94]

Originally Posted by fsmtnbiker

Stable under most conditions, except braking - I think you're on the right track so far. I'm still inclined to try a big rear rebound adjustment just to see how it affects the car (like 4-6 clicks open/reduced), but I suspect you may be fighting something else in the brake system.

Just to be sure: ABS is still functional and working? And all the sensors for the system are still installed and properly calibrated? No codes?

Haha, well regarding the ABS.. We believe it is still functional. The DSC module was removed, so stability control is permanent disabled. With that being said there ARE ABS and DSC codes - I believe these are from removing the module itself. I spoke to Randy at EPIC and said this is to be expected. I believe the ABS is still functional as we tested to see if we could cause the tires to lockup and weren’t able to - so we assume it’s still working. The only way to test it would be to put the module back on and test if the brakes react any different I suppose.

[Richbot]

Does what you did with the stability systems impact active brake force distribution at all? I don't recall which module/computer that runs through and how it can be disabled

This is one of those "my kingdom for line pressure sensors" situations but maybe you could pull channels from somewhere to get an idea of what the car is doing without your explicit consent?

[tsk94]

Originally Posted by Richbot

Does what you did with the stability systems impact active brake force distribution at all? I don't recall which module/computer that runs through and how it can be disabled

This is one of those "my kingdom for line pressure sensors" situations but maybe you could pull channels from somewhere to get an idea of what the car is doing without your explicit consent?

Honestly I’m not sure. I’d have to look into that. I’m thinking of putting the DSC module back in and seeing if that makes any difference.

EDIT: It looks like it is part of the DSC module I removed. Do you think this could be part of the cause?

[SYT_Shadow]

I see I'm late to the party

You'll find that the more serious BBK setups move bias to the rear of the car to aid stability under braking. In those you can use the same pad compound front and rear and allow the natural bias in the system to add stability.

I've done a lot of tracking in the E90 which has a front and rear PFC BBK, the E92 which has a front-only PFC BBK and now in the F80 CS I've used a front PFC BBK plus a rear AP Radical*.

In all the cases I use the same pad front and rear (PFC11). Any BBK is going to be much more consistent than the stock system.

In all the systems so far I've found that 'shocking' the brakes/tires results in poor brake performance. This is easy to understand but hard to do, especially when you're pushing braking zones and you feel like you are about to die.
The critical part there is the initial application of force to the pedal.

I don't think you will be able to get away from getting a full BBK. If I were starting over again I'd get the PFC kit all over as you cannot improve on perfection, however, the AP Radical 'big boy' kit is likely comparable.


*PFC does not build brackets for the Z54/Z45 BBK from the E9X to go onto an F80 so I spent a couple months having the front adaptor built so that I could use the Z54 caliper on the F80. For the rear I didn't want to deal with the parking brake diameter difference and spend 2 months on it so I got the AP Radical rear BBK

[OG Shark]

This happens in my M3 under hard braking from speed. I believe it is aero related in my situation. I have the Essex / AP Racing BBKs front & rear - this did not happen before I installed the GT4 lip (no rear wing). Now on hard stops - noticeably from 100+ down to 40ish - the rear end wants to dance a little bit. Also the stickier the tire the more noticeable it is. Even though the lip isn't that big I feel like it it provides enough of an imbalance without a rear wing to cause this rear end wiggle during these hard high speed stops. I still am getting excellent braking performance - just that little bit of instability that I would also like to dial out. On my setup I feel like the answer is a rear wing - which will be on before the next time my car hits the track.

[tsk94]

Originally Posted by OG Shark

This happens in my M3 under hard braking from speed. I believe it is aero related in my situation. I have the Essex / AP Racing BBKs front & rear - this did not happen before I installed the GT4 lip (no rear wing). Now on hard stops - noticeably from 100+ down to 40ish - the rear end wants to dance a little bit. Also the stickier the tire the more noticeable it is. Even though the lip isn't that big I feel like it it provides enough of an imbalance without a rear wing to cause this rear end wiggle during these hard high speed stops. I still am getting excellent braking performance - just that little bit of instability that I would also like to dial out. On my setup I feel like the answer is a rear wing - which will be on before the next time my car hits the track.

Very interesting, thanks for your input. We are running almost the same aero setup currently, only difference is the brakes - interesting note that even with the front and rear BBK you still notice it.

[OG Shark]

Originally Posted by tsk94

Very interesting, thanks for your input. We are running almost the same aero setup currently, only difference is the brakes - interesting note that even with the front and rear BBK you still notice it.

Yeah I think that could possibly be a factor. Mine didn't appear until after the lip went on and it sounds a lot like what you are describing. Not enough to upset the car, but enough that you would rather not have it? I didn't try to chase it down knowing that my car was about to undergo major changes - figured if still had it after its done then would problem solve. Will be interesting to see what you come up with.

Does the rear of your car feel pretty stable during higher speed sweepers? That is a weakness in my car - which also makes me think I need the rear wing.

[Redd]

Originally Posted by SYT_Shadow

In all the systems so far I've found that 'shocking' the brakes/tires results in poor brake performance. This is easy to understand but hard to do, especially when you're pushing braking zones and you feel like you are about to die.
The critical part there is the initial application of force to the pedal.

Can you explain more about 'shocking the brakes' please?

[tsk94]

Originally Posted by OG Shark

Yeah I think that could possibly be a factor. Mine didn't appear until after the lip went on and it sounds a lot like what you are describing. Not enough to upset the car, but enough that you would rather not have it? I didn't try to chase it down knowing that my car was about to undergo major changes - figured if still had it after its done then would problem solve. Will be interesting to see what you come up with.

Does the rear of your car feel pretty stable during higher speed sweepers? That is a weakness in my car - which also makes me think I need the rear wing.

Ya what you're describing feels very similar to what I'm experiencing. fsmtnbiker also mentioned he thinks it may be a aero imbalance causing issues as well.

At my local track, and the only track the cars been driven on since being completed, the fastest sweeper is ~80mph (relatively slow, tight track). The car feels pretty good there, but the though has passed that 'a rear wing for turn X would be nice...'.

[SYT_Shadow]

Originally Posted by Redd

Can you explain more about 'shocking the brakes' please?

Not that the brakes really care, but if you apply the brakes too fast the tires are unable to grip and start sliding around.

You should apply the brakes in a way that allows the front tires to load with weight transfer, then continue applying pressure and this way the car will take way more braking force.
I forget the number, it's supposed to be like half a second from initial application to full braking force. Any kind of data acquisition system like a solodl will show you how fast you go from no brake to full brake.

You find a similar situation if you run slicks on stock suspension: every time it hits the end of the bump stops the car misbehaves as the tires can't take the delta in grip you are requesting

[Redd]

Thanks!

[fsmtnbiker]

Originally Posted by tsk94

Honestly I’m not sure. I’d have to look into that. I’m thinking of putting the DSC module back in and seeing if that makes any difference.

EDIT: It looks like it is part of the DSC module I removed. Do you think this could be part of the cause?

This is something that Randy could probably answer, but if it affects the brake force distribution (active bias) then it can definitely cause what you are feeling. All of the brake bias control is done by the ABS. Definitely worth a little more investigation.

[tsk94]

Originally Posted by fsmtnbiker

This is something that Randy could probably answer, but if it affects the brake force distribution (active bias) then it can definitely cause what you are feeling. All of the brake bias control is done by the ABS. Definitely worth a little more investigation.

Ya I'm wondering how much this is impacting it. So I've decided to put this module back in, took a bit of wiring hunting but it wasn't too bad. I'll see if this makes a difference at all, if not I'll start going down the list of playing with damper changes and so on.

Also looking into a rear wing right now to balance out the aero on the car.

[OG Shark]

What is the purpose of removing the DSC module? To insure that you do not leave it on by accident?

9 Lives Racing wing - if you haven't seen this one might check it out. Appears to have pretty favorable data compared to the standards (hopefully its not BS). Have one waiting to go on my car once its ready.

[tsk94]

Originally Posted by OG Shark

What is the purpose of removing the DSC module? To insure that you do not leave it on by accident?

9 Lives Racing wing - if you haven't seen this one might check it out. Appears to have pretty favorable data compared to the standards (hopefully its not BS). Have one waiting to go on my car once its ready.

For the most part yes. Assuming the module wasn’t needed, the convenience of not having the manually disable stability control every time you get in the car. Also to remove the module and associated wiring for weight and cleanliness.

I’ll take a look at that wing. Was looking at the APR and Bimmerworld ones but always open to other options.

[ThunderMoose]

Originally Posted by tsk94

For the most part yes. Assuming the module wasn’t needed, the convenience of not having the manually disable stability control every time you get in the car. Also to remove the module and associated wiring for weight and cleanliness.

I’ll take a look at that wing. Was looking at the APR and Bimmerworld ones but always open to other options.

In the E46, the DSC module is also the ABS module. I just leave the yaw sensor unplugged to leave DSC off whilst still keeping ABS.

[tsk94]

Originally Posted by ThunderMoose

In the E46, the DSC module is also the ABS module. I just leave the yaw sensor unplugged to leave DSC on whilst still keeping ABS.

Could very likely be the same for the E9X.

Thanks everyone for their responses trying to help me figure out this issue.

The DSC module is being put back into the car, and a rear wing has been ordered (https://www.bimmerworld.com/Body-Aer...Race-Wing.html) - which hopefully will arrive and be installed in time for the next track day. I'll post an update after the next track day (Aug 11th) to see the if these changes made any improvement.

[Kelse92]

My first instinct says aero. Looking forward to you letting us know how the wing does

[dogbone]

I’m happy to stir the pot with my opinion!

If it were my car, I would hold off on installing the wing for at least one more event. Try the smaller things that have been discussed for one track day. See if you can reduce the problem with setup change. For me, finding out whether I could dial the problem out with setup change would be valuable knowledge.

I believe the wing is a good step for overall performance—-it will make you faster overall, but is it necessary, or the only thing that can fix unstable braking? In my quest for learning, I would run one more day without it to try to find out.

When I gutted and caged my car, it did not have aero. I didn’t have braking issues. But, the rear wanted to step out like crazy because the backend was so light. Aero fixed the crazy backend, but I still needed to be careful at slow speeds when the aero does nothing. I experimented with rear spring rates to feel how the backend would respond to the throttle at low speeds. Tried 800, 900, 1000......settled on 900.

I’ve watched race teams show up at a track with a bunch of springs and an alignment rack and go crazy experimenting with setups. That’s what it takes.

(Of course, if this is your last track day of the season before a long Canadian winter, then I guess you just slap the wing on there and be done with it..... )

Anyway, just tossing two more pennies of opinion into the fountain.

[ajsmithvmi]

Adding a rear wing to my car definitely got rid of the weird rear end sensations under braking, although it's a 135i so not a perfect comparison.

I have modest spring rates too, 370/670.

[tsk94]

Originally Posted by dogbone

I’m happy to stir the pot with my opinion!

If it were my car, I would hold off on installing the wing for at least one more event. Try the smaller things that have been discussed for one track day. See if you can reduce the problem with setup change. For me, finding out whether I could dial the problem out with setup change would be valuable knowledge.

I believe the wing is a good step for overall performance—-it will make you faster overall, but is it necessary, or the only thing that can fix unstable braking? In my quest for learning, I would run one more day without it to try to find out.

When I gutted and caged my car, it did not have aero. I didn’t have braking issues. But, the rear wanted to step out like crazy because the backend was so light. Aero fixed the crazy backend, but I still needed to be careful at slow speeds when the aero does nothing. I experimented with rear spring rates to feel how the backend would respond to the throttle at low speeds. Tried 800, 900, 1000......settled on 900.

I’ve watched race teams show up at a track with a bunch of springs and an alignment rack and go crazy experimenting with setups. That’s what it takes.

(Of course, if this is your last track day of the season before a long Canadian winter, then I guess you just slap the wing on there and be done with it..... )

Anyway, just tossing two more pennies of opinion into the fountain.

Was thinking the same thing actually. Might hold off on installing the wing and just try to see if it was ABS related issue without the module. But the wing has been ordered so later this summer we'll drive it with the wing and see how it does.

Still got 5-6 track days left so lots of time to continue testing this year

Originally Posted by ajsmithvmi

Adding a rear wing to my car definitely got rid of the weird rear end sensations under braking, although it's a 135i so not a perfect comparison.

I have modest spring rates too, 370/670.

Good to know anyways, thanks!