Pad Knockback - wheel bearing upgrade

[dre99gsx]

Looking for some input. I have Stoptech Trohpies on, and ever since installing them, I noticed severe pad knockback (I assume this is why I have to pump brakes after high-G turns).

I have 1yr old front wheel bearings on, *** units, and I don't notice any wear or movement from them when testing. But, curious if there are any higher quality "racing" bearings I can swap in that could take the loads?

The one thing I can mention - back when I ran the 335i front caliper/345mm z4m rotor, I never had this issue, and just as good braking power. Technically didn't need the upgrade, but I was looking to see if there was any improvement over a "stock" setup. Not much to date to be honest, other than ease of swapping pads

For you local guys, I get knockback on NJMP Lightning after T2-3-4 combo going into T5 (tight up-hill right), under the bridge going into T7? (after the long highspeed right), and going into T1 after the lightulb. Most are long highspeed banks.

I got into the habbit of tapping brakes before braking zones but feel this shouldn't be the default for an expensive brake upgrade. Anyone else have this issue on stoptechs?

[3002 tii]

Thought this was actually a “common” issue with Stoptech. Have you seen these?

https://www.zeckhausen.com/catalog/i...th=24_217_7351

[dre99gsx]

Yup. Ordered parts to upgrade the kit from sport to Race (floating rotor and knockback springs). Will report back on results next month!

[3002 tii]

Originally Posted by dre99gsx

Yup. Ordered parts to upgrade the kit from sport to Race (floating rotor and knockback springs). Will report back on results next month!

Just curios are you running stock front sway?

[dre99gsx]

No, H&R 27mm

[Michael9218]

Which Stoptech kit do you have? The Trophy series all come with the floating rotors and the gray calipers. The only difference between the "race" caliper kit and the "sport" caliper kit is the sports have dust boots and come with a sport pad compound. Both Trophy kits should have the Aerohat floating rotors that prevent pad knock back.

I have the Sport Trophy kit and have no knock back.

Unlikely to be a wheel bearing issue. Your wheel bearing would have to be pretty shot to move that much. The floating rotors should allow hub movement anyway.

[dre99gsx]

Trophy Sport kit.

Not what I was told..:

"StopTech® Trophy Race Big Brake Kits®
Designed specifically for motorsports competition and dedicated track cars. Unique to the Trophy Race Big Brake System is full-floating mounting hardware, which eliminates pad knock-back, and the elimination of the dust boots in the caliper. Together, these technologies optimize the brake system to provide improved performance and consistent pedal feedback under the unique combination of hard cornering and braking found on the track. Trophy Race Big Brake Kits are the ultimate in racing performance, with numerous wins and championships in World Challenge, Grand-Am, NASA and SCCA racing."

"StopTech® Trophy Sport Big Brake Kits®
Intended for ultra-high performance street use. They are designed for street driven production vehicles. The Trophy Sport Big Brake Kits use the same STR calipers, AeroRotors® and AeroHats® found in the full-race Trophy Kits, but add silicone dust boots for extended service intervals and anti-rattle rotor mounting hardware for quieter operation. They also include StopTech® Street Performance Brake Pads for high performance street and occasional track use. Trophy Sport Big Brake Kits are the ultimate street braking system.

Any StopTech® Brake Kit can be ordered as a Trophy Race or Trophy Sport(street) kit simply by specifying it on your order."

I was informed by Zeckhausen as well that doing springs and removing anti-rattle should reduce/eliminate pad knockback.

At the same time, I may be experiencing higher g-forces through these turns at this track than you? Just guessing here...

[Michael9218]

Well, don't know what to tell you. The full floating mounting hardware referenced in bold is the rotor ring to hat mounting system that is on both the race and sport versions. And I seriously doubt you're pulling more lateral G's on your car.

Definitely some confusing descriptions out there. This from the Bimmerworld site on the Trophy kits:
"Where Trophy kits become extra special is in the details: the STR calipers are 20% lighter thanks to extra machining, the rotors have 50% more air airflow with a special mounting system for the rotor hat, and pad knock-back is eliminated by using floating hardware between the hat and disc."

I do know that when I ordered my brake kit from Stoptech through my local shop, we talked to Stoptech tech support and they stated the only difference was the addition of silicone dust boots and anti-rattle clips on the sports and the different pad compounds. Can't see how an anti-rattle clip or the silicone dust boots would cause knock back. Mine don't.

Makes me think your issue is with the rotor to hat interface. It's the floating of the rotors that is what prevents the rotor from flexing with the hub. You shouldn't need springs. Piston springs don't prevent knock back, they react to knock back. They are available as a bandaid for the inferior street rotors that cause knock back. If you paid north of $3,000 for the front kit, than it should have the gold colored rotor hats and should definitely not have knock back.

I would pull off your calipers and inspect the rotor to hat mounting pins. My suspicion would be that they may have seized.

[dre99gsx]

I got this information from Dave, who is very knowledgeable all things Stoptech - From Zeckhausen:

https://www.zeckhausen.com/catalog/p...oducts_id=9780

"Floating rotor hat mounting kit for rotors and hats with 8mm thick mounting flanges. Full-float flat washers allow 0.4mm axial float, with no anti-rattle function, meaning these should only be used for dedicated race cars. The 8mm diameter drive pins allow for unconstrained radial float, as operating temperature varies.
Kit consists of ten drive pins, Allen bolts and flat washers. Includes thread locking compound and instruction sheet. For 332x32mm, 355x32mm, 355x35mm, 380x32mm, and 380x35mm AeroRotors. One kit required per rotor."

The conical washers used in the Sport kit are the anti-rattle feature, which prevent the rotor from moving freely within the hat. The Race kit is supposed to not have this, allowing more movement, conforming with the pads due to any flex in the system, thus reducing knockback. The Sport kit essentially reduces the "floating" nature to a degree.

I also read others with Trophy kits who have and don't have knockback issues, it seems to be a toss up. You mention you don't have issues with knockback - what tracks do you run, what tires, times? Curious...

I ordered the springs and floating hardware, will install and test. Only way to find out at this point...

[Michael9218]

Interesting. Keep us posted when you change out the pins. It looks like you could just replace the cone washers with a grade 8 flat washer. If I were you, I would hold off on the piston springs. You shouldn't need them, it's a lot of effort to install them, and I would be concerned they would drag the pads.

Maybe mine came with the flat washers. I guess I'll find out when I replace the rings. If they're not knocking, I ain't going to knock it...

As for setup, I'm running 275 Z214 Hankooks. Road Atlanta lap time of 1:36.2 which would be about a second faster than an IP car. For reference IP cars look to run about 1:12 at NJ Lightning. My AIM Solo2 logged peak lateral G of 1.95 (not sure I believe that, though).

[dre99gsx]

Agreed - I'll save them for last.

Not doubting your abilities or speed, just looking for a reference point on if you would be pushing to the point where Gs could induce knockback. Seems you are (pushing) Will update this post in about a month after I swap pins/washers.

For the record, I lap Lightning here 1:13s on R888R

[Michael9218]

Well, you're far more tolerant than I. I would've returned my brakes if they had knock back. Brakes at this price point simply should not have knock back. Serious design fault if they do, IMHO.

Anyway, can you measure the thickness, ID, and OD of those flat washers when you get them?

[dre99gsx]

Update: Knockback still exists just swapping rotor washers to flat washers. Next step is to go brand new *** wheel bearings up front (one is original, another is 2 years old and seen many track events). They feel fine and solid, but who knows...

[GuidoK]

Wow, this forum sure hates products from the highly esteemed Fischer Automatische Gussstahlkugelfabrik

What you also can do is measure around the disc with a micrometer if the disk has the same thickness everywhere (its a long shot and should be impossible as wear should solve this, but try and exclude all possible geometry deviations...) and measure with an indicating dial if your disk is still straight (might be difficult with a full floating setup, but maybe you can remove the floating hardware springs and fix the disc solid to the hub with a few small bolts).

How worn are your pads? Or does it occur with both worn and new pads?

As for detecting wear/play on your wheel bearings, jack up the car, and feel with your hand on the spring from the strut and give the wheel a spin.
If you feel vibrations then wear starts to kick in.

[Michael9218]

Originally Posted by dre99gsx

Update: Knockback still exists just swapping rotor washers to flat washers. Next step is to go brand new *** wheel bearings up front (one is original, another is 2 years old and seen many track events). They feel fine and solid, but who knows...

I feel for you. This is crazy. I have the same brakes and their performance is stellar.

Makes me wonder if something else is going on. Could it be a defective rotor hat that's flexing? I would think runout in the rotor ring would be obvious when braking. Replacing the hub is worth a shot.

It could be worse. I'm having chronic power steering issues. Last weekend I had to finish the weekend without power steering when my racks pinion seal puked. I think I have Popeye arms now...

[dre99gsx]

It's basically a brand new system - new pads too. Also, when I had the 335i floating caliper + z4m rotor, I felt the same thing going into a specific turn. With the Stoptech upgrade, I now feel it in more than one turn.

You may be right in that something else is going on. I spoke to a few other bmw guys (and Afrim on here) and essentially everyone gets pad knockback to some degree. It's just the nature of our tiny hubs. I do tend to trail brake heavily, which may make it worse.

They all left-foot brake before turns. I'm getting used to it at this point.

I don't feel any play in the bearings, but that doesn't mean they are ideal. It could be they are simply worn enough for deflection, but not enough for feeling loose upon inspection when "cold"?.

I have new bearings in front to install.

Any thoughts on the rear bearings contributing to this? Or is this usually just front?

[Michael9218]

Definitely check the rear bearings. On my previous race car (E36 M3) it was always the left rear bearing that wore out. Like every other season. I didn't have fixed calipers in the rear on that car so never felt knock back.

I would think that you would be able to feel the play in the hub bearings if they were bad enough to cause knock back.

[Michael9218]

Originally Posted by dre99gsx

I spoke to a few other bmw guys (and Afrim on here) and essentially everyone gets pad knockback to some degree. It's just the nature of our tiny hubs.

I don't agree with this. I know a lot of people tracking and racing BMW's. Knock back is not common. In fact, I don't know of anyone around here complaining about knock back.

If you even had knock back with floating calipers, something else is amiss. Could it be a master cylinder issue? ABS pump?

[dre99gsx]

Ok. Replaced both front bearings with new *** units, still have knockback. Last ditch effort is to go anti-knockback springs in the front. I know this is front BBK related since I didn't have any of this with my 335i front caliper/z4m front rotor setup before hand.

I may just go back to that setup in the end - it worked fantastic for what it was. No fade, solid pedal, etc.

[3002 tii]

Originally Posted by dre99gsx

Ok. Replaced both front bearings with new *** units, still have knockback. Last ditch effort is to go anti-knockback springs in the front. I know this is front BBK related since I didn't have any of this with my 335i front caliper/z4m front rotor setup before hand.

I may just go back to that setup in the end - it worked fantastic for what it was. No fade, solid pedal, etc.

that sucks man, sorry you've yet to chase this down.

[jns_e85]

Have you checked front sway interference with the caliper while turning?

I was having what I perceived as knock-back on my z4m. Checked hubs, changed pads, flushed system, etc. etc. Nothing worked and would occasionally get a super soft pedal going to the ground (and the corresponding "code brown" in iffy situations).

Finally broke down and took it to my indy and he found that the aftermarket sway I put on this year (H&R 30mm bar) actually makes contact with a front caliper under significant turning conditions and push it out of position. Apparently, H&R doesn't recommend this bar for the Z4M (though various sites like ECS say otherwise). Needless to say, i'm putting the stock bar back on. YMMV!

[3002 tii]

Originally Posted by jns_e85

Have you checked front sway interference with the caliper while turning?

I was having what I perceived as knock-back on my z4m. Checked hubs, changed pads, flushed system, etc. etc. Nothing worked and would occasionally get a super soft pedal going to the ground (and the corresponding "code brown" in iffy situations).

Finally broke down and took it to my indy and he found that the aftermarket sway I put on this year (H&R 30mm bar) actually makes contact with a front caliper under significant turning conditions and push it out of position. Apparently, H&R doesn't recommend this bar for the Z4M (though various sites like ECS say otherwise). Needless to say, i'm putting the stock bar back on. YMMV!

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!?!?