435is sport brake calipers

[mrsuper2]

Hi all. I have seen several threads about various different BMW multi-piston brake caliper conversions onto the Z4 M coupe. I just wanted to contribute something interesting I have discovered. Having had some spare 435i 4 piston sport brake calipers kicking around and some spare time, I decided to do a test fit to see how it would turn out. first off, the 435 brake rotors don't fit regardless of what I tried. but with the standard Z4 M front rotors the 435 brake calipers bolted right up with a 6.0mm washer/spacer on the front knuckles. the front brake hoses also fit up just right, though a custom front brake hose that is longer would be preferred. the only thing that didn't fit was the front brake pads. they were just a bit deep towards the rotor hub and had some interference. after removing about 4mm of the backing plate & pad material, everything clears. after a quick brake bleed the initial road test impressions were good. pedal is firmer than standard with no issues so far (after about 50km of driving) I hope the pictures upload properly. this may be a good cheap upgrade for some folks out there. btw, the front caliper pistons measure at 36.0mm (x4). for those who want to do the piston area math.

[mrsuper2]

now I wonder if I can make the M4 calipers fit the same

[The HACK]

Originally Posted by mrsuper2

btw, the front caliper pistons measure at 36.0mm (x4). for those who want to do the piston area math.

Stock: 2,827.4333882308137 mm^2
These: 2,035.75203952618586 mm^2

If my math is correct.

I wouldn't touch it with a billion foot pole. JMO. That's nearly 30% decrease in surface area. If it's 5-10% it'll work. Not at 30%.

[crawlin]

The stock M coupe has some pretty damn good brakes from the factory, I don't think I would mess with them unless I was doing a complete aftermarket upgrade. Swapping around oem parts with other non M cars im not sure would yield results worth switching for.

Plus I would do some testing stock vs. these before I would say they were better. Going by feel it would be pretty hard to say, its like when you put an intake on and you think its faster until it goes on the dyno and it only added 1.5 hp, sometimes it feels better just because you changed something out or the other calipers were maybe just worn out.

[dc_wright]

Originally Posted by The HACK

Stock: 2,827.4333882308137 mm^2
These: 2,035.75203952618586 mm^2

If my math is correct.

I wouldn't touch it with a billion foot pole. JMO. That's nearly 30% decrease in surface area. If it's 5-10% it'll work. Not at 30%.

I believe the calipers he installed are 4 piston calipers so you'd need to multiply your 2,035 figure by 2X so looks like it's actually a 44% increase, not a 30% decrease..........

[The HACK]

Originally Posted by dc_wright

I believe the calipers he installed are 4 piston calipers so you'd need to multiply your 2,035 figure by 2X so looks like it's actually a 44% increase, not a 30% decrease..........

No. fixed calipers you only calculate calipers per side. Don't ask me why, it's just the way it is.

[dc_wright]

Originally Posted by The HACK

No. fixed calipers you only calculate calipers per side. Don't ask me why, it's just the way it is.

There are two pistons on each side of the caliper so even though the caliper body can't move the pistons are free to find their own space so the effect is the same as if the caliper was floating.

[mrsuper2]

hi all, I was not implying that this swap was going to better than a full BBK upgrade compared to a Alcon or AP racing kit. nor am I suggesting that everyone should be running out to the auto salvage to get these. just something interesting I discovered and wanted to share. my stock brake components are due for rebuild again after a year of track day session fun time (14 events this year) and wanted to see if I could still have my car mobile while I wait for the rebuild parts to arrive. though to my surprise the 30% less brake area at the front end has resulted in a very firm brake pedal and noticeably less front end dive while under heavy braking. recently tried some wet braking from 100-10kph on low traction street tires. ABS functions normally and car stopped without drama and with much less dive. . I personally prefer very firm brake pedal feel and a more rearward brake bias than the stock Z4M setup. my ideal set up would be about 15% less area in the front & 10% more area in the rear. this would lead to less work & fade on the front axle while distributing more braking load to the rear axle. and as I mentioned I just wanted to share something interesting I discovered that was easy to retrofit.

[The HACK]

Originally Posted by mrsuper2

hi all, I was not implying that this swap was going to better than a full BBK upgrade compared to a Alcon or AP racing kit. nor am I suggesting that everyone should be running out to the auto salvage to get these. just something interesting I discovered and wanted to share. my stock brake components are due for rebuild again after a year of track day session fun time (14 events this year) and wanted to see if I could still have my car mobile while I wait for the rebuild parts to arrive. though to my surprise the 30% less brake area at the front end has resulted in a very firm brake pedal and noticeably less front end dive while under heavy braking. recently tried some wet braking from 100-10kph on low traction street tires. ABS functions normally and car stopped without drama and with much less dive. . I personally prefer very firm brake pedal feel and a more rearward brake bias than the stock Z4M setup. my ideal set up would be about 15% less area in the front & 10% more area in the rear. this would lead to less work & fade on the front axle while distributing more braking load to the rear axle. and as I mentioned I just wanted to share something interesting I discovered that was easy to retrofit.

That is all fine and dandy. I'm merely pointing out that the 30% less front surface area creates a huge problem in real world performance applications.

The reason why you're not feeling any front dive, is because 30% of the fluid being pushed is now diverted to the REAR wheels. The front end is doing far less work than it used to. What that means, since under heavy braking the front end loads up and do the vast majority of the work to slow down and stop the car at threshold, you're now asking the rear to do significantly more than necessary.

Sure, the pedal is now rock solid, what you're feeling in ABS pulse is probably generated by the rear wheels, not the front. Car doesn't feel like it wants to swap ends because ABS keeps the rear wheel rolling. The real world problem now, is under emergency brake applications you're probably going to take much longer to stop than had you kept the OE caliper.

It would be interesting to see what the actual braking distance difference(s) would be between these two set-up, as I suspect having the rear ABS activate before the front could contribute to significantly elongated braking distances.

Like I said. *I* wouldn't touch it.

[GuidoK]

Originally Posted by dc_wright

There are two pistons on each side of the caliper so even though the caliper body can't move the pistons are free to find their own space so the effect is the same as if the caliper was floating.

You're forgetting Newton's third law
That explains why you should or shouldnt add the pistons of both sides (at least thats what I learned at physics)

With the 30% decrease you're shifting the brake bias more to the rear (pressure wise).
Whether thats a good or bad thing also depends on pads.

This caliper obviously has different size pads and maybe different material. Also with multiple piston the pressure gets spread out better which again changes how effective the caliper is.
Its not at all just about the surface area of the pistons, its the combined efford of a lot of factors (total pressure, pad size, pad material pressure distribution etc).
I dont think these calipers wouldn't be strong enough to force the wheel to the point of lock up, so that leaves only brake bias.
That is something you test on a quite road/private road by making emergency stops (if you can without abs) and see if the fronts lock up first.

A larger pad with more evenly spread clampingforce generally gives better control on the modulation of the brake up to the point of the brake locking up, so multiple piston calipers definately have a plus on that point.
Thats what you want for a road car, mainly front biassed so if there is a lockup, it will be the fronts. (rear lockup=spin if you're not careful)

Fitting a different caliper with different pistons and pad size is no different than fitting different, more agressive pads in just the front or the rear.
Both are things you have to test before using it in traffic.

Caliper wise it looks like a good match. the pads seem to grab all the way at the inner diameter (and als presumingly at the outer diameter). Detailed pictures of that would be nice

[mrsuper2]

yes the front end is doing less work at the SAME pressure applied compared to stock. the same braking force can be achieved with an additional 30% pedal effort (due to 30% less piston area, hence less hydraulic force multiplication). No more fluid is diverted to the rear brake circuit as they are separated. front master cylinder bore is 25mm, rear bore is 20mm. brake bias is already proportioned from the source. again, I am not arguing this to be the best upgrade. I just find discussions on brake system mods to be very interesting ill see if I took any pics of the outer pad contact to the front rotor. I did remember it to be same as the with the original caliper as when I tested it with my old rotors the pads fell into the worn out rotor grooves.

[mrsuper2]

the track that is local to me is heavy on brakes and I have been interested to see how others have been addressing the front brake fade issue (besides driving around slower) a BBK with larger/wider rotors will help but will ultimately reach a thermal limit too. I have "opened" the brake rotor backing plate and installed lower control arm ducts to channel additional airflow to the rotors. my brake pad of choice are the PFC 01,or 08's. I have noticed that my brake component destruction is limited to only the front brakes. correct me if I am wrong but I am assuming that if less work is being done on the front and the rear is contributing more. more laps can be completed (and more fun) can be had per session before having to retire again due to brake fade.

[GuidoK]

Originally Posted by mrsuper2

No more fluid is diverted to the rear brake circuit as they are separated. front master cylinder bore is 25mm, rear bore is 20mm. brake bias is already proportioned from the source.

Are you sure that the z4 doesnt have a diagonally split system? Or is that only on FWD cars?
(doesnt mean that that cant be proportioned but it needs extra valves, either in the mbc or the abs actuator, and I think proportioning is always done with a valve, not with mbc piston size, but I'm not 100% sure)