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      06-15-2019, 12:15 AM   #1
AndyW
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Brake feel question

Since last year I have had a symptom that i thought was RAB but was on the track recently with RAB disabled (using Protool) and had the same symptom so am here to see if it is simply a normal thing for hard braking or if there is something I should troubleshoot. Any advice appreciated.

Here is what happens, typically at hard braking zones at the end of straights on the track.

- Hard initial application of brakes - they bite and slow the car aggressively. Pedal feel is good, depresses probably 1/2 of the way but definitely stops moving.

- As the car slows and I approach the end-of-braking and turn-in, I slowly release the brake pedal. Within about a second of letting off full pressure on, but not having fully released, the pedal, it changes firmness quickly- tightening up to my foot noticeably. There is no effect on the actual braking pressure or car control. It is just like something kicked in to increase pressure in the system.

This has consistently happened over the last year with the stock brakes, stock brakes with RB pistons, with the new Stoptechs this year. This has also been consistent through multiple brake fluid changes/bleeding.

I had assumed this was what RAB felt like so just ignored it as I still had consistent, good stopping power and it didn't appear to be fade or boiling (I use motul 660, for reference).

So, I am puzzled now that it does it with RAB disabled...is this just normal pad release from hard braking that I mis-attributed to RAB? Or is there something wrong I should look into? Maybe related to the brake booster or something? Anyone else experience this?

I also turned off: FBS and FLR. But those shouldn't matter and the effect above was both before and after the coding.
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      06-15-2019, 05:43 PM   #2
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It could possibly be the thermal expansion of the pads forcing fluid back to the pedal boosting pressure after a large braking event.
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      06-15-2019, 11:42 PM   #3
AndyW
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Quote:
Originally Posted by chris_flies View Post
It could possibly be the thermal expansion of the pads forcing fluid back to the pedal boosting pressure after a large braking event.
This may be it...I have only ever tracked this car so have no others to compare to so it may just be a normal thing.
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      06-16-2019, 09:43 AM   #4
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Assuming you are running DSC and TCS off? Mine will do some weird stuff as well, so far has cutting power if I divebomb a corner on a late point by. What is RAB?

Ok, I used google. Interesting about RAB. I donít have it coded out but feel it when Iím threshold braking. SRF, DTC60c stock calipers, no baking plates and Porsche brake scoops.

Following. Maybe call Tyspeed in NJ.
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      06-16-2019, 11:02 AM   #5
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Quote:
Originally Posted by Relegate View Post
Assuming you are running DSC and TCS off? Mine will do some weird stuff as well, so far has cutting power if I divebomb a corner on a late point by. What is RAB?

Ok, I used google. Interesting about RAB. I donít have it coded out but feel it when Iím threshold braking. SRF, DTC60c stock calipers, no baking plates and Porsche brake scoops.

Following. Maybe call Tyspeed in NJ.
No, BMW HPDE rules here require us to leave both DTC and TCS on. I have never had power cut drastically due to braking. I can feel a little power cutting when DSC engages sometimes coming out of a corner when the DSC light blinks, but it isn't too horrible.
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      06-17-2019, 08:41 AM   #6
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Quote:
Originally Posted by AndyW View Post
Since last year I have had a symptom that i thought was RAB but was on the track recently with RAB disabled (using Protool) and had the same symptom so am here to see if it is simply a normal thing for hard braking or if there is something I should troubleshoot. Any advice appreciated.

Here is what happens, typically at hard braking zones at the end of straights on the track.

- Hard initial application of brakes - they bite and slow the car aggressively. Pedal feel is good, depresses probably 1/2 of the way but definitely stops moving.

- As the car slows and I approach the end-of-braking and turn-in, I slowly release the brake pedal. Within about a second of letting off full pressure on, but not having fully released, the pedal, it changes firmness quickly- tightening up to my foot noticeably. There is no effect on the actual braking pressure or car control. It is just like something kicked in to increase pressure in the system.

This has consistently happened over the last year with the stock brakes, stock brakes with RB pistons, with the new Stoptechs this year. This has also been consistent through multiple brake fluid changes/bleeding.

I had assumed this was what RAB felt like so just ignored it as I still had consistent, good stopping power and it didn't appear to be fade or boiling (I use motul 660, for reference).

So, I am puzzled now that it does it with RAB disabled...is this just normal pad release from hard braking that I mis-attributed to RAB? Or is there something wrong I should look into? Maybe related to the brake booster or something? Anyone else experience this?

I also turned off: FBS and FLR. But those shouldn't matter and the effect above was both before and after the coding.
Just a quick thought... if someone can confirm/deny the below might be possible I'd appreciate it...

I think it might be related to ABS. I have "pre-tension" brakes disabled, but, I have a feeling the system is kind of designed to have some of this stuff left on, you know? Sometimes if I mash the brake hard (similar to what BMW probably programmed as a "panic" situation on the street) it will just go to the floor... If pre-tension brakes had been enabled then I would've been met with a stiff pedal but instead it's almost as if the system is still trying to run the brake pre-tension and nothing happens instead since it's coded out (and you get a scary soft pedal that then firms up when you lift off, as you described). RAB specifically notes a delay in pressure building that RAB was implemented on a software level to overcome in panic situations.

Again, just a thought... you may be feeling something completely different.

1M has ready brake alert and brake fade compensation enabled as does the M3.

What happens if you pre-tension the brakes yourself? Give them a quick tap before rolling into the brake hard before a corner and see if you still get the same pedal behavior?
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Last edited by bbnks2; 06-17-2019 at 09:00 AM..
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      06-17-2019, 06:50 PM   #7
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Quote:
Originally Posted by bbnks2 View Post
Just a quick thought... if someone can confirm/deny the below might be possible I'd appreciate it...

I think it might be related to ABS. I have "pre-tension" brakes disabled, but, I have a feeling the system is kind of designed to have some of this stuff left on, you know? Sometimes if I mash the brake hard (similar to what BMW probably programmed as a "panic" situation on the street) it will just go to the floor... If pre-tension brakes had been enabled then I would've been met with a stiff pedal but instead it's almost as if the system is still trying to run the brake pre-tension and nothing happens instead since it's coded out (and you get a scary soft pedal that then firms up when you lift off, as you described). RAB specifically notes a delay in pressure building that RAB was implemented on a software level to overcome in panic situations.

Again, just a thought... you may be feeling something completely different.

1M has ready brake alert and brake fade compensation enabled as does the M3.

What happens if you pre-tension the brakes yourself? Give them a quick tap before rolling into the brake hard before a corner and see if you still get the same pedal behavior?
Pretty sure "pre-tension" is RAB for the 135i and I have felt the effect above with RAB enabled and disabled.

The feel is almost like you mentioned, but I don't feel the brakes "go to the floor" or anything like that.

I need to list all the DSC parameters from Protool and hopefully generate a discussion...for example, "ASL', which is trailer stabilization is Active by default on my car -

I will try the manual pre-tension...that is a good idea!
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      06-22-2019, 10:18 AM   #8
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Update: Was at the track yesterday, tried the manual pre-tensioning. Good news: If I tap the brakes right before hard braking, it doesn't do it. So I'm not going to worry about it. Brakes work great and the effect doesn't keep me from effectively modulating the brakes.
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      06-23-2019, 06:48 PM   #9
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Haven't used protool but when I coded out my braking assists with the BMW tools there were more things to turn off than you list. I forget the names though.

Quote:
Originally Posted by AndyW View Post
No, BMW HPDE rules here require us to leave both DTC and TCS on.
what the fuck
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