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      07-24-2019, 09:06 AM   #23
jns_e85
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Quote:
Originally Posted by 3002 tii View Post
I thought of the same thing and asked OP if he swapped front sway but said he's still on the stocker. But I had this same issue last year and would encounter no brakes every time I lined up to grid for a TT session. It was the worst feeling lmao... how to put in a fast time when you had no brakes 2 seconds ago while trying to line up!?!?
Hmmm, post 5 says H&R 27mm bar....
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      07-24-2019, 10:15 AM   #24
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Quote:
Originally Posted by jns_e85 View Post
Hmmm, post 5 says H&R 27mm bar....
Lol I didn’t check this thread since then but I swear he told me via PM few months before when he first had this issue he was running stock!
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      07-25-2019, 08:39 AM   #25
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Yes, 27mm up front.

I'm confused - I took a look at my setup, there is over 6" gap between the end link of the sway bar and the caliper. Even turning the steering wheel 3/4 (more than I ever turn at the track), I still see a large gap. How would it be possible for the sway bar to contact the caliper?

Keep in mind I do have Z4 3.0si, not the M. I'm not sure about the length of sway bars between models.

Enlighten me since I'm running out of options!
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      07-25-2019, 01:54 PM   #26
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Quote:
Originally Posted by dre99gsx View Post
Yes, 27mm up front.

I'm confused - I took a look at my setup, there is over 6" gap between the end link of the sway bar and the caliper. Even turning the steering wheel 3/4 (more than I ever turn at the track), I still see a large gap. How would it be possible for the sway bar to contact the caliper?

Keep in mind I do have Z4 3.0si, not the M. I'm not sure about the length of sway bars between models.

Enlighten me since I'm running out of options!
How it was explained to me was it would not be immediately evident when I had the car in the air, as the positioning of the sway/caliper unloaded isn't the same as loaded.

Really simple to test if there is interference, and how my indy showed me the issue. Go to an empty lot and make sure you have full brake pedal before starting. Take foot off brake and crank the wheel one way full lock and drive a small circle. Return to center and then check the brakes. If the brakes go low, something interfered with the caliper, and if so the likely culprit is sway bar.
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      09-11-2019, 10:01 AM   #27
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Pads too tight

Ok, so here's another idea for you:
Changed out my pads last week. I have found on my Trophy calipers that I have to grind a mill off the top and bottom of the backing plates to fit in the caliper. Otherwise I find that they're a tight fit. As I was doing this yet again, it occurred to me to mention this to you. Could this be the reason you're getting knockback and I'm not? Dunno...
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      09-11-2019, 10:44 AM   #28
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Can you provide more detail?
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      09-11-2019, 03:28 PM   #29
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I have found that both Hawk and PFC pads are too tight a fit. When you slip in the pads (with pistons pushed in so plenty of clearance from piston to rotor), have you felt that they were a tight fit? I'm talking about where the caliper has stainless steel inserts at the top and bottom of where the pads slip in. The pad backing plate should be designed to have some play. Maybe yours are binding/sticking. I grind off a about a millimeter at the top and bottom of the pad backing plate with a bench grinder. This allows the pads to slip in without effort.

Initially, I figured it was just an issue with Hawk pads, but now I've verified that PFC and Stoptech pads are also too large/tall to fit properly.
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      10-13-2019, 09:27 PM   #30
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Verified my pads are not 'tight', but snug. I can move them in/out without issue.

Any thoughts on mine being a 3.0si (330i E46) front spindle/mcpherson strut design vs the Z4M's E46 M3 spindle being stronger? I haven't seen many 3.0si's with BBKs commenting on knockback.

I also wonder - I use 7075 T6 Alum lug nuts. Could they be too compliant where the wheel and rotor actually move during high G turns? I suspect I'm the only one here on aluminum lug nuts. I can run steel next time, but my last resort is to install the 4lb springs into the caliper. I have an event in 4 weeks to test this out.
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      10-14-2019, 08:18 PM   #31
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Quote:
Originally Posted by dre99gsx View Post
Any thoughts on mine being a 3.0si (330i E46) front spindle/mcpherson strut design vs the Z4M's E46 M3 spindle being stronger? I haven't seen many 3.0si's with BBKs commenting on knockback.
You're in uncharted territory here...

The Z4m has E36 knuckles.

Quote:
Originally Posted by dre99gsx View Post
I also wonder - I use 7075 T6 Alum lug nuts. Could they be too compliant where the wheel and rotor actually move during high G turns? I suspect I'm the only one here on aluminum lug nuts. I can run steel next time, but my last resort is to install the 4lb springs into the caliper. I have an event in 4 weeks to test this out.
Haven't seen anyone using aluminum wheel nuts. Doesn't sound like a good idea. Definitely an application for stronger steel.
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      11-14-2019, 03:14 PM   #32
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Installed 4lb anti knockback springs in front, problem solved. Happy camper!
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      11-12-2020, 03:48 PM   #33
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And... I'm Back! Problem not solved.

I'm starting to suspect the Z4 3.0si knuckles may be 'weaker' and allowing more flex than normal. Anyone have any good comparison of the Z4M vs. Z4 3.0si knuckles?

Any idea if the z4m are beefier? If you say they are E36 knuckles (z4m), the part number on realoem doesn't say they match E36. The z43.0si knuckles are E46 330i.

Also, any aftermarket knuckles available for these cars?

ALSO - can anyone test: car at idle, pump brakes, press them firmly, does the pedal begin to slowly drop until it bottoms out? Or, does it stay put?

Latest track event I had knockback at a few corners, but I went to fresh RR tires (essentially slicks) with more G-force on corners.

Uncharted territory - I'm possibly the only one running a 355mm Stoptech Trophy on a non z4m in the world? lol

I may have to go back to my 335i floating caliper setup with Z4m rotor. That was the most solid pedal system I had and reliable...
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      11-12-2020, 10:37 PM   #34
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Hi, pedal *Must* stay put, otherwise you have air in system, or your pipes are expanding... or something else freaky deaky is going on.

Just to chime in about knockback, zero here.. absolutely none, and I was actually a little horrified you mentioned you have to pre pump the brakes before a corner...

That said, this was previously to me removing the whole brake actuator system, though I ran 3 seasons with the stock system and Brembo gt’s.

Did you run through the inpa abs procedure after flushing the lines? (Very long shot, as I am sure you would have done that)
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      11-12-2020, 10:56 PM   #35
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I just recently replaced the ABS pump so yes, did have to run through INPA. That didn't change anything anyway. I've bled this a gazillion times...
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      11-13-2020, 05:05 PM   #36
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If the pedal sinks there is a leak somewhere. It could even be the master cylinder, leaking past the seals and back into the reservoir. I'm with Vanne -- the pedal should stop going down at some point. Otherwise something is broken.
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      11-13-2020, 06:02 PM   #37
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Quote:
Originally Posted by wdb View Post
If the pedal sinks there is a leak somewhere. It could even be the master cylinder, leaking past the seals and back into the reservoir. I'm with Vanne -- the pedal should stop going down at some point. Otherwise something is broken.
Remember, with car off, pedal stays solid. It's when it is idling, I'll pump and get a firm pedal, but then within 5 seconds it slowly drops.

I have a new master cylinder going in and will report back. I suspect that could be the issue. I did replace the ABS pump and all the lines who no signs of leaking. I don't think this is related to my knockback issue but it's about the only thing left in the braking system I haven't replaced.
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      06-09-2021, 06:06 PM   #38
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UPDATE:

I just had an event. I have now replaced:
- Brake master cylinder, brand new
- both front lower control arms with new ball joints
- re-bled ABS w/INPA, tons more bleeding of brakes

To be more clear, the issue is this on track:
- If I pump at least 6-7 times on a long straight AFTER a high-G corner, I get solid pedal that bites up top
- If I pump 2 times or so, or none, I will get a mushy pedal that bites much lower than normal, but still get solid braking.

I'm trying to build muscle memory on these corners but every corner I get varying braking pedal points that I lose confidence on how much brake I am applying. Some corners the pedal is almost to the floor, others it's right up top. Can't win HPDE like this...

If I'm at idle and press the pedal, it goes down a bit, but slowly sinks and sinks till almost bottom. If I pump, it catches up top again, but will again slowly sink.

If I had air in the system, would that mean I would never get a solid pedal no matter how many times I pump?

I've also thought about pad taper, and this may be causing the pads to angle a bit after I press the pedal, thus the reason for it to sink.

Uncharted territory yes, but the odd thing too is when I had my 335i front floating brakes, there was none of this nonesense. Only after going to Stoptechs did this happen.

The only thing left is to move over to Z4M front end, but that will be costly on so many levels. I don't want to go back to floating caliper either.

Video below captures one of the issues I have:
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      06-09-2021, 06:52 PM   #39
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Quote:
Originally Posted by dre99gsx View Post
UPDATE:

Video below captures one of the issues I have:
I don't quite understand what the issue is.
Do you have a sagging pedal when the car is stationairy?
Then that has to be either air somewhere or leakage in the master cylinder.
I mean the brake fluid that goes towards the calipers (after the ABS pump) can't go anywhere else. So either air is compressed maybe in a solenoid in the ABS pump or there is leakage in the seals of the master cylinder.

But what youre saying here:
Quote:
To be more clear, the issue is this on track:
- If I pump at least 6-7 times on a long straight AFTER a high-G corner, I get solid pedal that bites up top
- If I pump 2 times or so, or none, I will get a mushy pedal that bites much lower than normal, but still get solid braking.

I'm trying to build muscle memory on these corners but every corner I get varying braking pedal points that I lose confidence on how much brake I am applying. Some corners the pedal is almost to the floor, others it's right up top. Can't win HPDE like this...
Seems to me that your thoughts go to either the knuckle or calipers (weak caliper bridge?) are bending, pushing the pads (and pistons in the caliper) back and then you have to pump the brakes again to make up for that space to get full pressure on the pads. Is that right?

I have no idea if that's a likely scenario (being cast iron etc), but calipers do bend under pressure (maybe your old calipers were stronger).

I can understand that you dont want to go z4m front end: new coilovers and new calipers/BBK are required
But have you ever tried to fit e46 M3 front knuckles?
Wheel alignment might be different (but thats maybe solvable), track width might be different (solvable with maybe different spacers if you use them)
But they look pretty much the same as the normal e46/z4 knuckles.
It might be slightly beefier.
Differences between M parts and normal parts are not always dimensional, sometimes they are just stronger (like z4m front control arms and z4 control arms; dimensions are the same, it's just beefier)
Of course caliper offset might be different, but caliper size (so what is normally adjusted with the caliper carrier is I think the same as the normal e46 (M3 has 2 carriers: 325mm and 345mm (competition package, same carrier as the z4m)
So if you can get a used e46 M3 knuckle for not so much money it might be interesting to try and fit that and see how it measures out. I don't think it has been done before.

Z4 knuckle:




E46 M3 knuckle:



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      06-09-2021, 09:17 PM   #40
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That's correct - as long as I pump (enough times before a braking zone), braking is strong and catches up top of pedal travel.

It feels so much like pad knockback, except I've done everything to mitigate. 4lb springs in the calipers (these are the "top of the line" Stoptech Trophies btw), converted the Stoptech calipers to full floating with flat washers, new wheel bearings with proper nut torque, freshened up mostly all of the front suspension parts, etc.

The old calipers (335i stock units) were floating on brass pin upgrade, so because I didn't have this issue then, it further leads me to believe something is flexing in the front end.

For shits, I just tested my brake boost. Holds 30psi (check valve works), and holds 15in/hg vacuum for hour (no leaks in booster). I read stories of boosters acting funky.

If it was air trapped in the system, I don't see how I could build up pressure by pumping, it would always feel mushy no? The fact that I can usually get a solid pedal (sometimes I would forget to pump the brakes before a turn and surprisingly had a strong pedal) doesn't seem like air in the system.

Interesting thought about M3 knuckles... I may look into that further, worth a shot. Thanks for the photo references!

UPDATE: Ok - I ordered a used M3 front knuckle (cheap) with hub attached, will do some measurements and see what I can find. Wish someone designed a stronger version of the knuckle.
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      06-10-2021, 08:46 AM   #41
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If it's pad knockback, something would have to happen to knock the pads back.

I'm thinking out loud but could there be a test to see if the pedal holds once you've pumped it up, and removing the possibilty of pad knockback at the same time? I guess the simplest would be to pump it up at a full stop, then keep hitting pedal at various longer intervals to see if it sinks. In other words take pad knockback -- and only pad knockback -- out of the equation.
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      06-10-2021, 10:24 AM   #42
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Can anyone here confirm that they:

A- get a high and firm brake pedal when engine off
B- brake pedal slowly sinks to floor with engine on when you push and hold on it for ~10 seconds?

It seems this may be normal, and I want to rule out something else. I just replaced the Master Cylinder, so chances that I replaced it with a faulty one seems odd.

I tested this by having the engine off and applying vacuum directly to the booster (using a vacuum pump), and same thing happened. I suspect the booster is providing so much assist that I must be overpowering the master cylinder seals and that is what causes the pedal to slowly drop. Seems like this may be normal?

ALSO

Those of you with BBKs on Z4M, can you find out how much room you have between your rotor and the tie rod/lower ball joint/splash shield? On my setup, I have about 2mm. It's VERY close. I'm suspecting there may be a chance that under steering load, possibility that my tie rod/balljoint may be pushing on the splashshield, which may be pushing on the rotor causing it to go out of parallel with the caliper. It's just a hunch, but I may attempt to design 1mm spacer between rotor hat and hub, and also on the caliper mounting bracket.
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      06-12-2021, 06:46 PM   #43
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UPDATE:

I ended up finding a leaking brake fitting right on top of the ABS module - it would weep as I pushed down on the pedal with the vacuum pump on.

I fixed it, re-bled the system, and the issue still persists. I have a new theory.

On the track, I'm never more than 13in/hg vacuum on decel. With the vacuum pump, I'm at 25in/hg, which is probably providing more brake assist, thus probably why I'm able to have the brake pedal drop this much.

I dialed the vacuum pump back down to 13in/hg and it took more force and longer time for brake pedal to drop.

I checked every fitting, no additional leaks or weeping found. I did yet another INPA abs prime and powerbleed fluid, no change. I read that the N52 vacuum pump provides 25~30hg/mm vacuum at all times to the booster? I'm directly connected to the intake manifold thus it only pulls vacuum at idle/braking. Engine is cammed and has low vacuum at idle by nature (around 8hg/mm). At boost, the check valve on the booster blocks positive pressure (tested that as well).

I am now suspecting this may be part or most of my issue. I'm on the brakes hard for 3seconds? or so per corner, which then my pedal sinks a bit from just standing on it, and my pedal now is lower than before for engagement on the next corner REGARDLESS if I was turning or not. I can test this on the highway to confirm, but it makes sense.

If you guys know my pedal should not sink with engine on/vacuum applied, then the only thing I can thing of is the ABS module is leaking fluid internally, and needs to be replaced (I'm on my second one already from the trifecta-of-lights issue). I'm going to buy plugs for the master cylinder and isolate the system part by part, see if I can find something...

Master cylinders are now $5xx (was $3xx), not looking to replace a new one I just installed.
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      06-13-2021, 11:07 PM   #44
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Subbed.. interesting indeed... I wish I could offer more insight, but as I no longer have a brake booster/vacuum assist, I can't compare values anymore.

But from memory, turn 1 at DXB autosome is a pretty darn fast downhill right hander, and I sure as shit would have absolutely shit my pants had my brake pedal gone slowly to the floor. I CAN for certain say I've stood on my brakes /THAT/ hard that the back of the car felt like the tail of my old gsxr750r being in the air, and I can tell you for certain, there was absolutely zero pressure loss on the pedal.


I wish I could help more, as I know how annoying it is chasing something you can't put your finger on.

Last edited by Vanne; 06-13-2021 at 11:20 PM..
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