[hiplanezDrifter]

Just got back from VIR and my brake pedal traveled much more than I was comfortable with. I basically had to pump my pedal once and again to get it to be ready to bite at the normal travel spot. My brakes did not 'fade' as they were still there and adequate. My instructor thought it could be 'knock back'.

What can I do to fix this?

I'm on stock calipers and rotors and track pads.

Thanks!

[K8_M235i]

How new are the pads? What you are describing is exactly what pad knock back is. It is also more likely to happen with thin pads

[detroitm2]

Did you flush/bleed your brake fluid? Could also be bubbles in the system.

[5.M0NSTER]

What detroitm2 said. If you're on stock fluid you will come close to boiling it, and the pedal gets super long. Mine got too warm at Hockenheimring. Switched to Castrol SRF and no issues.

[detroitm2]

Originally Posted by 5.M0NSTER

What detroitm2 said. If you're on stock fluid you will come close to boiling it, and the pedal gets super long. Mine got too warm at Hockenheimring. Switched to Castrol SRF and no issues.

Also have had this happen to me even with RBF600. Too many hotlaps in quick succession without proper cool down.

[richard in NC]

Was that a rare occurrence or repeatedly during early and later sessions?
Was it more common on specific corners?

I don't recall that happening on track, if so rarely, but it did happen repeatedly on a skid pad during big spins, meaning long pedal travel to get the car stopped. I couldn't put my finger on a cause or repeatability. From what I just read, high G forces can lead to pad knockback on some cars.

PS: I've used Castrol SRF fluid in my otherwise stock brakes and have not had an issue with fade, even after 2 hard days at VIR or CMP.

[hiplanezDrifter]

Thanks for all the responses!!

Pads are aftermarket, but unsure as previous owner did them. I've had them for a year at 5+ events, and there is still about 1/2" left on them.

BF was new last month before I went to RoadATL. This happened there, but not as pronounced.

It seemed to happen at the top of the uphill esses at the slight braking zone for the left turn that heads to oak tree, and going into turn 1 & 3.

[KHAP13]

Originally Posted by detroitm2

Also have had this happen to me even with RBF600. Too many hotlaps in quick succession without proper cool down.

Agree. If you are running a fast pace on a hard braking track, even with upgraded fluid/pads, you will overwhelm the stock system's ability to dissipate heat and get a long brake pedal after a few hard laps.

[TUNEDM3]

Sounds like pad knockback. Happens to me all the time. On the straights use your left foot to tap the brake pedal a few times.

[dvas]

I have probably participated in over 100 track events and have never driven a car on track that had a super consistent brake pedal feel - at least not since I was beginner/intermediate and not pushing hard. I think any time you are really braking hard on a car, it will create some inconsistencies. It could be heat related, knock back, etc.

What I personally watch out for is whether pedal feel gets progressively worse, or softer. That would cause alarms. Otherwise, if the pedal seems really hard for one corner, then has a relatively long travel on the next corner, and back to hard, I would attribute that to hard track driving.

Having said that, I would love to have a consistently hard brake pedal. If anyone is running aggressive advanced/instructor pace and has managed to get a consistently hard pedal, I'm all ears on how to get that for myself.

[jim1013yyf]

Originally Posted by dvas

I have probably participated in over 100 track events and have never driven a car on track that had a super consistent brake pedal feel - at least not since I was beginner/intermediate and not pushing hard. I think any time you are really braking hard on a car, it will create some inconsistencies. It could be heat related, knock back, etc.

What I personally watch out for is whether pedal feel gets progressively worse, or softer. That would cause alarms. Otherwise, if the pedal seems really hard for one corner, then has a relatively long travel on the next corner, and back to hard, I would attribute that to hard track driving.

Having said that, I would love to have a consistently hard brake pedal. If anyone is running aggressive advanced/instructor pace and has managed to get a consistently hard pedal, I'm all ears on how to get that for myself.

I'm also interested. I've been struggling to get better brake feel on the F8x. Even with my BBK (stoptech trophy up front) the brake feel is still inconsistent. I think it might has something to do with BMWs dynamic brake control software.

[pal]

It sounds like you're getting knock back, which gets more pronounced with worn pads. Looks like your pads have many track days on them and are likely quite a bit worn down. I first experienced knock back many years ago on my Z4MC on StopTech ST-40s going into T4 at VIR and had a big tank slapper moment. T1-T3 are longish corners that do a right/left/left and are enough to push the pistons back in. Top of the esses would do the same.

Since then, I've learned to left foot tap the brake after corner sequences or on straights before braking zones. This works really well once you're left foot is trained to do a light tap and not get into the brakes. This has allowed for a consistent pedal feel for the most part on the Z4M, subsequent TT RS with Brembos and now the M2.

[kthor703]

So I just took delivery of my M2C at the performance center. During the download of all the ABS/DSC tech, I recall the instructor saying there is a built in program where the brake system will automatically apply light pressure to the brakes to "prep" the vehicle for braking. It is triggered by abrupt lifts of the throttle. It likely for emergency braking, but I presume it doubles as an anti-knockback measure by extension.

[richard in NC]

Originally Posted by kthor703

So I just took delivery of my M2C at the performance center. During the download of all the ABS/DSC tech, I recall the instructor saying there is a built in program where the brake system will automatically apply light pressure to the brakes to "prep" the vehicle for braking. It is triggered by abrupt lifts of the throttle. It likely for emergency braking, but I presume it doubles as an anti-knockback measure by extension.

Correct. It may help prevent (resolve) a knockback situation but the sudden lift of throttle required to activate it may be more upsetting to the car than the gain in braking feel.

But the pre-loading does seem to cause an inconsistency in braking effort for some on track. I learned that in skid pad training. As I would spin and "go 2 feet in" to stop the car, sometimes I'd need easy effort to woah the car, other times a deep hard push on the brake. That really bugged me.

[Dean_Clevername]

Agree it sounds like knock back. Not something people talk about enough, but something everyone should be prepared for. How much track time you have on your wheel bearings? Wheel bearing flex during cornering or banging off curbs will also cause your rotors to move a little bit since they rotate on that axis. The brake caliper isn't subject to that and stays put. These two moving independently can push your pads back into the rotors. Basically like a pad spreader. Other stuff can cause this too, or just the general forces cars experience on track in general. But maybe you're suddenly experiencing this because you have a wheel bearing that is allowing more flex than it used to?

As others have said, giving a light left foot tap can get the pistons back where the need to before your next braking zone. Run the best fluid you can find so you don't have two weird things happening at once. I'm a big fan of AP's R4 since trying it this spring.

[nando_514]

Originally Posted by dvas

I have probably participated in over 100 track events and have never driven a car on track that had a super consistent brake pedal feel - at least not since I was beginner/intermediate and not pushing hard. I think any time you are really braking hard on a car, it will create some inconsistencies. It could be heat related, knock back, etc.

What I personally watch out for is whether pedal feel gets progressively worse, or softer. That would cause alarms. Otherwise, if the pedal seems really hard for one corner, then has a relatively long travel on the next corner, and back to hard, I would attribute that to hard track driving.

Having said that, I would love to have a consistently hard brake pedal. If anyone is running aggressive advanced/instructor pace and has managed to get a consistently hard pedal, I'm all ears on how to get that for myself.

I'm working on becoming an instructor at the moment!

But you bring up a good point about being vigilant during each braking phase-- reflecting afterwards and keeping a tab on how things progress throughout the session. That's definitely a good habit novice drivers should pick up! Thanks for that!

It isn't until you get to the upper echelon of "braking technology" that consistency in pedal travel becomes less of a worry. I haven't done too many track days with the M2 yet, but I'm no stranger the paddock. From the little experience I have with this chassis, in its stock form, the M2 is tremendously capable and consistent, but it's also track dependent. I haven't yet seen a need to upgrade beyond the OE system. On a course like Lime Rock where you only have one REAL braking zone, consistency is almost a non-issue during the sessions, allowing you to pound away lap after lap. However, some courses like Watkins Glen or even Road ATL (to be extreme) consistency can become a factor. As long as you have a good quality, purposeful pad, stainless steel lines and decent fluid, you should navigate a traditional 20 min session with ease. Don't be afraid to lay into that pedal! Is it something you're ALWAYS running into, or does it happen seldomly?

Remember-- inconsistencies feel different to each other. A dying/fading pad will feel different than a boiling fluid, will feel different than knock back. Remaining sensitive and perceptive to what the car is telling you is key!