What Would E9XM Track Forum Do? STR40 front kit replacement/knockback dilemma

[Richbot]

My car has had a Stoptech Trophy front 355mm kit on it since 60,000 miles and 2014. It's older than one of my kids, and has more miles on it than two of my other entire cars (M3 is currently at 137k and cruising) It's had one (1) pad change from the ST sport pads it came with, to DS2500's. Still on original rotors that came with the kit. Probably partially thanks to no knockback springs

So it's made me lazy. But it also has the knockback typical of a lot of the 4pot Stoptech kits, and that's making me angry, despite the excellent durability of the kit overall.

The DS2500's probably have one track day left in them and then I imagine the rotors will be ready for new rings too. And that's starting to turn into real money.

Should I kill it with fire and get an AP372 or similar front kit, or look at anti-knockback springs for my STR40 calipers, or what? This is an almost exclusively fair-weather street and very occasional (like 3-4 sessions every two years) track day car.

My dilemma is A) like I said, these have been so trouble-free and cheap to run that I hate to spend much money on upgrading yet again and B) it's actually a very light front kit, and I have no need for a bigger rotor, I just need the knockback to go away and C) gold hats are cool. So I hate to bin the whole thing.

So...anybody successfully killed knockback on their ST40/STR40 front kit? How did it act on the street with knockback springs install? It looks like the low-drag springs are all backordered and so are the 355x35 ring replacement kits, so this isn't really a now problem.

[tsk94]

I'd just look into getting springs for the STR kit you already have. I believe berns fixed most (maybe all?) of his knockback issues after installing them on his STR kit - hopefully he can confirm.
For how much you track it, seems like a waste to swap it out entirely.

[derbo]

I remember m3number86 said he didn't get rid of all his knockback on his ST40 kit when he was still actively tracking.

That was what pushed me to AP Racing. I didn't want to add all this work to install springs to just minimize not eliminate the issue. With 3-4 days/year, I'm sure I could've worked around it but knowing it wasnt fixed bugged me more so I threw money at it. :lol

[berns]

Originally Posted by tsk94

I'd just look into getting springs for the STR kit you already have. I believe berns fixed most (maybe all?) of his knockback issues after installing them on his STR kit - hopefully he can confirm.
For how much you track it, seems like a waste to swap it out entirely.

New pads, new rotor rings, 6# kb springs, AND, run the top and bottom edges of the pads (the metal part, not the pad material) on a grinder to shave off enough material that'll let the pad slide into the caliper without any force. Typically the tolerances are so tight that the pad requires some wiggling and pushing to get it into place. Let that lil fella sit freely.

Following this, it's also helpful to think about WHERE you're getting knocback and what the car is doing at that time. Usually, you're experiencing high side-to-side g loads and dealing with a ton of weight transfer. Now ask yourself if there's a way you can cause the car to have less of this by a) adjusting your line or b) adjusting what your feet are doing, c) all of the above?

I found that by adjusting my line and finding the confidence and grip to stay on throttle more consistently, rather than lift off throttle and toss the car into the left kink leading to Bus Stop at Buttonwillow, the car was experiencing way less weight transfer. I was running much quicker AND my knockback was gone.

TLDR -- the STR40 trophy kit is awesome, and clearly has lasted you a while. For how little you're tracking, I don't see the point in dumping money into an AP kit. The kockback can be fixed and driven around, and you're not talking about a lot of "real money" because this kit hasn't costed you much at all if you've never changed rotors or pads... Even being older than your kid, it's costed you much less than the kid! Enjoy.

[Richbot]

So how do the knockback springs behave on the street? Transparent or noticeable difference? I don't get knockback on track, unless I really smash a curb and then I'm left footing the pedal before the next brake zone anyway out of habit from the better part of two decades of wheel to wheel club racing. It's always been parking lot speeds, since the kit was new, lots of turning the wheel that causes it.

[Redd]

Here's a way out there reply - Try brake residual pressure valves.

Here's my story. I have Brembo GT kit front rear and have been experiencing knockback as well as spongy pedal since Day 1. On every brake press, there is some initial dead travel before I feel the pads contact the discs. And then when I apply more pressure, there is a lot of travel before I get full braking. It's even worse at track. I had to get used to double-pumping the brake pedal each and every time just to get somewhat predictable brake performance. And even then I could not lock up the brakes.

I tried all solutions - Endless RF650 fluid, bled so many times my brake lines are shiny inside. Steel brake hoses, new master cylinder, replaced the DSC pump (it was leaking), Project Mu HC+ pads (which I know give good pedal feel). I tried to look for anti-knockback springs but Brembo did not make them for the GT calipers which were also not designed to accept springs. I had given up and just learned to drive around it, but I never fully trusted the brakes.

Then I found this thing called the brake residual pressure valve (RPV). It's a small valve which allows pressure to bleed out of the calipers when you release the brake pedal, but maintains a small amount of residual pressure so that the pistons can't retract all the way. It's used mainly in custom hotrods to prevent all the fluid from back flowing to the fluid reservoir when the reservoir is mounted lower than the calipers. A lesser known function is to prevent knockback. By keeping a small amount of pressure (2psi) within the calipers, it stops the pistons from retracting too much and prevents caliper/pad movement from pushing the pistons further in (knockback). Best of all it's cheap! The ones I used are from McGill Motorsport but you can get them from all kinds of motorsport shops and even on Amazon (I don't trust the cheap China ones though). I went with McGill for the metric fitting.

https://www.mcgillmotorsport.com/bra...psi-c-70-p-364

I mounted them at the end of the hard line before each brake caliper and had to get custom brake hoses made from the RPV to the caliper. So if you have only front bbk then you need 2 RPV only.

The difference was amazing. With the RPVs installed, there was near zero dead pedal travel, the moment you step on the brakes you can feel it contact the disc immediately. There is no more spongy feeling either. The brake pedal is nice and firm and consistent every brake press. Best of all, it somehow feels as if the pressure in the entire brake system is increased. I can now lockup the tires at 60-70% pedal travel, the braking is now tire limited and not pad/caliper limited. Modulation is still easy and it is not as on/off as the AP Racing Pro5000R kits I've tried.

As for downsides, I did not find any. There is no pad drag, the pistons still retract far enough to let the pads clear the discs. Shortly after I installed the RPVs I got new pads and discs and even at their thickest there was no problem. However, I have only tested this on Brembo GT. Ymmv. I think the RPV only works for fixed calipers, it may not work properly with sliding calipers like our stock brakes. I have also not tested it at track but I have done dozens of balls to the wall hill runs with tons of hard braking and zero issues.

Anyway thought I'd share this as it's taken me 7 years to find a solution to finally allow me to trust my brakes fully.

P/s: Had the work done by a local race shop and that's how they cable tie their brake lines. If you want you can custom fab a bracket but they said it was not necessary.

[tsk94]

Very cool - thanks for sharing ^

[RickyBobby]

I was in your situation after dealing with knockback for nearly 20 years on various cars with stoptech's. I have always installed springs in all of my Stoptech calipers and while it helps, it doesn't eliminate the knockback completely. I finally bit the bullet and got the AP's and the knockback was completely eliminated. I couldn't believe the drastic difference I felt.
I agree with you that the stoptech's served me well over the years. Pads, rotors, rebuild kits are plentiful, but the same looks to be true for the AP's.

[roastbeef]

that actually seems like a no-brainer for those without akb springs.... pretty cheap and a lot less hassle than upgrading.

[Richbot]

The 355x35 rings are actually kinda hard to find so I guess this decision will be made for me by whether or not I can order a set of rings when I want.

[turbo_joe]

I run 4lb AKB springs in my ST60 kit up front and have zero knockback issues since installing the springs. If you know the piston sizes I can probably get you a set. My distributor has 13 in stock right now for 26-32mm pistons

Shoot me a DM

[Richbot]

So, fun story, the 355x35mm rotor rings are *not* out of production. I just ordered some from Bimmerworld and they arrived yesterday. So I guess I'm back in business with my STR40 front kit. Hopefully I don't screw up the fasteners

[turbo_joe]

Originally Posted by Richbot

So, fun story, the 355x35mm rotor rings are *not* out of production. I just ordered some from Bimmerworld and they arrived yesterday. So I guess I'm back in business with my STR40 front kit. Hopefully I don't screw up the fasteners

The rings should come with new hardware

[bigjae1976]

Originally Posted by Richbot

So, fun story, the 355x35mm rotor rings are *not* out of production. I just ordered some from Bimmerworld and they arrived yesterday. So I guess I'm back in business with my STR40 front kit. Hopefully I don't screw up the fasteners

Just saw them on Amazon...not too far off from pre-cough-cough era. For awhile they were like $800 ea!

Did you resolve the knockback?

I've never had knockback with an ST40 kit. I also never had to force the pads in. They slide in relatively easy. Original slider things that go on the end.

My ST60 kit, where I have to pry the pads out, has had a couple instances of knockback.

[Richbot]

I haven't done it yet but I can't imagine it will be worse - thanks for inspiring me to get off my ass and order pads - the rings are 10! years old and have more than 82,000 miles on them, so there's a good lip inside and out by now. The min thickness is 33mm and when I put this pair of DS2500's in them (38,000 miles ago, and only the second set of pads I've used on this set of rings, they're down to about 5-6mm of material left from 18 new) they were a little over 34mm. So they have to be close to minimum now. Which just leaves more room for everything to wiggle around.

Obviously with that kind of life, even with a handful of light track days on 200tw tires, the 355x35 rotors are overkill city for a street car

[Richbot]

So, learnings:

1) aluminum hats and steel hubs do in fact experience galvanic corrosion. I had to do some cleanup after 82000 miles and 10 years of that. It was pretty mild and wire brush able on the hat. The hub would have to be replaced to get it all the way cleared off the hub face, but I was able to get rid of any high spots. I imagine this surface rust on the hub face would be present with the factory rotors too but I’ve never pulled OEM rotors off a high mileage E90M like mine so dunno

2) the stop tech hardware and pin holes and hat all held up great, except there is some pitting here and there from said corrosion. Everything just slid right out, nothing was wallowed out etc. So next time I have the hats off for some reason I’ll probably replace them. In 2050?

D) yes having new pads and new rotors does help with knockback but it’s too early to tell how much because all I’ve done is the bedding run so far

[Richbot]

Addendum - You still have a little knockback with extreme steering, but it's nowhere near as bad as it was with 5mm of pad, with lips on them, that were squeezing 10 year old, worn rings.

And I haven't even bled them yet, just slapped it all back together beacuse I like to bleed the fluid that's in the calipers during bedding.

So problem sorta solved, sorta not, but long story short if you're getting bad knockback and you're on old pads, I think it's safe to say new pads will make it a lot better on the STR40 caliper anyway

Not exactly a shocking revelation

[Redd]

Try the RPVs. It's cheap and you may be surprised how much braking power is actually possible.

[bigjae1976]

I recommend checking the hardware after bedding the pads in or the first heat cycle. Could have been installer error but I did have one get a little loose.

How tight are the pads when installing?

Originally Posted by Redd

Here's a way out there reply - Try brake residual pressure valves.

Your zip tie game is insane!

[Redd]

Originally Posted by bigjae1976

Your zip tie game is insane!

Innit lol. Race shop didn't even bat an eye lid. They just said don't worry about it.