Pad Knockback - wheel bearing upgrade

[wdb]

I can't help with the math but can you install a brake bias valve or somesuch to make it a bit easier to experiment?

[dre99gsx]

Minor update: the E46 MK20 master cylinder does not fit on the z4 brake booster. It is a bit too long, the diameter is 2mm wider, and the bolt holes are further apart.

[ctdriver86]

Man just ready quickly thru your trial and error. What a pain. There's someone here I follow on IG who tracks their z4m a lot. I asked about the trophy kit they put on and he was very unsatisfied (bad knockback) and sold it for an AP kit. No issues since. I believe he preferred the PFC BBK but they discontinued it.

[dre99gsx]

That's interesting to hear. Having tested another caliper and finding the same issue, but in that vehicle the brakes work exceptionally well (Audi RS4), I realized that there was something else going on.

The RS4 has a 25.4mm MC piston, similar to the updated MC I added to my Z4. Once going larger on the piston size, my issue with the pedal drop went away. That suggests to me that there is a certain limit of internal pressure the stoptech kit can withstand before it potentially weeps past the caliper seals. If you get to close to that threshold, it's possible the issues start.

It's a great looking kit and when it works, it works, but I'm looking at alternatives now that I'm hearing rumors the company Stoptech may not be around able to keep up with future business anymore (see my other thread on this).

I may have this kit up for sale soon with Z4M and/or non-M adapters if anyone will want it for a good price.

[Michael9218]

So did the MK60 MC solve the knockback issue or just the pedal droop?

[dre99gsx]

It solved the slow pedal droop issue 100% - I can stand on it and it won't droop at all, and to some degree, lessened the knockback issue.

As I got faster and more confident, I still noticed slightly lower brake pedal activation on some turns, and I began going back to my old habit of 2-3 pumps before every turn.

Time has passed but as I think more about it, it's possible that my brake pedal normally activates at like 60% from the top? I wonder if, in my habit of pumping normally brings it up to 90% from the top, but in doing so, my muscle memory goes out the door as I don't have a consistent activation point on every turn. I either pump before every turn, or I don't and get used to 60% travel before activation. It still brakes hard, I just get nervous...

It is much better though, so this was a step in the right direction. I'm looking at moving the bias further back to see if just a bit more does the trick. So, I need smaller pistons up front, or larger in the rear to accomplish this.

[dre99gsx]

UPDATE: The saga isn't over..

Things were looking good until I went to Watkins Glen this weekend. Pad knockback was an issue again (long highspeed high-G corners) but even worse, I experienced brake fade going into the toe near the end of each 20 min session. I clocked myself braking from 160mph to 80mph, 150 to 70 and 145 to 70 from 3 long straights to tight turns, within each lap - that's probably not enough time to cool down the brakes...

I had to resort back to pumping the brakes 3-4 times before every turn to bring the pedal back up to firm height. It's very difficult to establish braking muscle memory and correct heel-toe when the brake pedal height is always changing.

So, to recap, I'm on:
- fairly fresh front wheel bearings torqued to spec (no noise, no play)
- 3" brake cooling ducts right into center of front caliper hubs
- 355mm rotor, full floating hardware, STR-40 caliper with 4lb knockback springs (measured runout of discs at <.001 - they are straight)
- Z4M master cylinder
- 135i rear caliper (42mm piston) and Z4M rear rotors
- fairly fresh rear wheel bearings, no noise, no play (yeah, I'm on my 2nd rear-driver bearing for some odd reason. *** bearing too)
- brand new PFC08 front pads, brand new Carbotech XP08 rears

I am running some old anti-squeal metal shims behind the front pads. I'm not sure this is contributing to anything, but I plan on removing them next outing.

I'm also going to M4 rear caliper that has 44mm piston vs. the current 42mm 135i. If I can move my brake bias further back, perhaps this will help reduce some of the knockback effects.

I did further research on why a guy there with an E46 M3 on similar brake setup has no issues with braking.
- Are my hubs weak? I compared part numbers and Z4 3.0si hubs are almost identical to E46 M3 hubs, but I'm not sure if the hub stub is more stout. I don't think I can swap this over. The Z4M hub is completely different - cannot use it.
- Wheel bearings weaker? The part number is different than E46 M3 but looks like the same dimensions.
- Could my steering tie-rod assembly OR lower ball joint be putting pressure on my brake heat shield, then on the rotor on high-G turns? It's very close, probably 1-2mm distance. I may trim the heatshield near those areas, but unfortunately this would expose the lower control ball joint and tie-rod ball joint to the high heat of the rotor disc.
- Could my rear hybrid brake setup be contributing to pad knockback? I do have the 3.0si rear suspension with Z4M rotors/135i calipers (with custom bracket).

Looking for any other feedback - starting to grasp at straws here

[3002 tii]

Jeeze 160mph at the Glen is awesome. What does that translate into for lap times?

[dre99gsx]

Doesn't gain me much when my braking is impossible to nail down. I end up braking so early that I coast before the apex. I have run a few 2:07.5s, first time there too. Nothing to brag about. My legs start to shake from fear going up the esses at 125mph while staring at that wall! I just started drag racing 911 cup cars on the back straight after giving them the point by because I can be a bit of a douche.

[3002 tii]

Originally Posted by dre99gsx

Doesn't gain me much when my braking is impossible to nail down. I end up braking so early that I coast before the apex. I have run a few 2:07.5s, first time there too. Nothing to brag about. My legs start to shake from fear going up the esses at 125mph while staring at that wall! I just started drag racing 911 cup cars on the back straight after giving them the point by because I can be a bit of a douche.

2:07 is still really really good. You measure how fast you go through the bus stop?


Sucks to hear about your braking woes but honestly for all that speed you’re carrying not surprising to hear your brakes succumb to fading. Maybe less weight more aero and you can get away with less braking?

I’m sure we’ll run into each other soon. Been spending last 3 months getting my car track ready. Feels like it’s been ages since I really drove, so much relearning I need to do.

[dre99gsx]

Get that thing going already

I enter the bustop at 81mph, clip the 2nd left-curb at 80, then the exit right curb at 86. Some guys were telling me to go all 4 over the first entry right-curbing, and again all 4 over the exit curb on the right side. The car vibrates so violently over those ripples, I couldn't do it. I got some photos of wheels off the ground from coming in too hot and jumping that curb.

I've seen a 2:01 E46 M3 run it at 97 in, 93 2nd curb and 90 out. That guy is almost 20mph faster than me on T10. I have a long way to go - the speed/power is definitely masking my abilities

[3002 tii]

Originally Posted by dre99gsx

Get that thing going already

I enter the bustop at 81mph, clip the 2nd left-curb at 80, then the exit right curb at 86. Some guys were telling me to go all 4 over the first entry right-curbing, and again all 4 over the exit curb on the right side. The car vibrates so violently over those ripples, I couldn't do it. I got some photos of wheels off the ground from coming in too hot and jumping that curb.

I've seen a 2:01 E46 M3 run it at 97 in, 93 2nd curb and 90 out. That guy is almost 20mph faster than me on T10. I have a long way to go - the speed/power is definitely masking my abilities

That’s exactly why I ask. You make up (and lose) so much time there. Ideally you can hit the curbs at 95+ there but you need really good suspension to absorb the bumps.

[dre99gsx]

Just took my front rotors out for inspection. Lots of hairline cracks (normal) but I found 3 cracks going through the inner edge on the rear. I wouldn't have seen these if I didn't take the rotors off.

Last set of 355 rotors lasted me 14 HPDE events. This one was at 8. I'm considering moving up to 380mm and keeping my STR-40 calipers. This is probably against all conventions - larger diameter = more knockback, 355 to 380mm is 3lbs more weight.

Miatas have it so much easier lol

[dre99gsx]

Been working through lots of math to try more things, and I'm going to do something that is against the grain.

My current brake bias is at 61.3% front / 38.8% rear (all things considered). The same math nets 57.2% front / 42.8% rear for a stock Z4M. So, I think by moving my bias rearward some, I could ease the front brake's workload to help reduce tendency to fade (this is theoretical). But, I also want to get this back to Z4M/Stock bias if possible.

The reason my bias is further forward is that although I have the Z4M Stoptech 355mm kit up front and the Z4M master cylinder, I don't have the Z4M 46mm caliper. I'm on the 42mm 135i rear caliper.

To remedy this, I found Wilwood has a 4-piston 31.75mm caliper (120-13233) that moves my bias right back to Z4M bias. It is made for 320-340mm rotors at 20mm thickness, and allows me to use 20mm pads (thicker than 16mm standard).

I know Wilwood doesn't have a well known racing reputation in these corners, but I could not find anything else from Porsche/Brembo/OEM calipers from other BMWs, to get my bias correct. Very exhaustive search. Stoptech does have an option but that company is just in the sh*tlist right now and can't even source spare parts.

I will have to design a rear bracket for the caliper, but other than that, will report on how these behave. I'll be able to bring my 135i rear calipers/pads to the track just incase this doesn't pan out.

I'm also upgrading my stoptech 4lb knockback springs to 6lb. Trying everything I can to fix these two latest issues without breaking the bank.

[dre99gsx]

UPDATE: Just ran this set up on the track NJMP Lightning. Brakes did well, I felt the increase rear bias providing more stopping power with less brake pedal effort.

I was not putting as much leg pressure, and had more solid brake pedal feel at the top. The pedal feel was almost closer to an On/Off switch, still progressive, but more linear than the prior feel which felt somewhat spongy.

The main updates were:
- front knockback springs from 4lb to 6lb
- added new PFC11 pads all around (from PFC08s)
- wilwood rear calipers with 3.6lb springs
- old brake bias: 61.2%/38.8%, NEW brake bias: 57.5%/42.5%

I had only one section of the circuit cause knockback, and it was always the same amount and in the same location.

[Vanne]

Mate, been reading up over the last few post and i gotta say Andre, and i am surprised you haven't done so already.. dump the stock brake booster and the crap that goes with it and get yourself a professional pedal box and race brake setup, youll thank me later, as will your lap times. depending on how you plumb it, youll loose traction control and possibly ABS, but then you dont really need neither of those on track.