Porsche brakes

[exdos]

I can recommend fitting brass bushes to replace the OEM rubber bushes (part 8 in the diagram) for the sliding calipers as in the parts diagram. A very easy install. They ensure that the calipers can't twist so that the pads wear evenly across their full face.




I've just found a cheap source for these on ebay. Put a bid in for less than £18 equivalent to avoid paying import taxes and handling charges.

http://www.ebay.co.uk/sch/i.html?_sacat=0&_from=R40&_nkw=bmw+brass+bush&LH_PrefLoc=2

 

[Lower]

No it wont but it must be an improvement on the single piston calipers on our cars.

Why do you say that?

I agree with Exdos and everything he's about to write once provoked enough :wink:

There's nothing wrong with the way the Z4M stops. In fact, I can readily lock all four wheels in the dry at anything over about 30mph (until the ABS kicks in) so the limit to deceleration is at the interface between tyre and road, not in the brakes themselves. The problem is one of heat dissipation when you ask it to decelerate hard from warp speed again and again on the track. I run a near stock set up with dot 4.1 fluid and race spec pads with no fade or over heating. I frequently use mine on track including airfield tracks with short (120-130mph) straights interspersed with tight hairpins. Such track eat brakes, tyres and fuel as they're all about changing acceleration not V max. The stock system will tolerate this with a few mods that will cost much less than a BBK.

The fact you can lock the wheels does not mean the braking system is good.

I could lock the wheel on a pushbike by sticking a twig through the spokes, but that doesn't make it an effective braking system.

What makes brakes effective is how well you can modulate the braking pressure just before the point the wheels lock as well as how well they resist heat.

Pad compound makes a huge difference to both of those. Brake cooling will also help with the heat problem but not how well the brakes can be modulated.

 

[carl]

Do Mov'it still use Porsche calipers?

 

[BMWZ4MC]

The fact you can lock the wheels does not mean the braking system is good.

I could lock the wheel on a pushbike by sticking a twig through the spokes, but that doesn't make it an effective braking system.

What makes brakes effective is how well you can modulate the braking pressure just before the point the wheels lock as well as how well they resist heat.

Pad compound makes a huge difference to both of those. Brake cooling will also help with the heat problem but not how well the brakes can be modulated.

I realise that Although a stick through the spokes is most certainly the fastest way of stopping a bicycle...
I was simply alluding to the fact that the stock set up does not lack stopping power. I find no difficulty with modulating the applied braking force with my set up and I didn't with stock pads and fluid on road or track (except when the brake assist interferes as I said before).

 

[Lower]

I realise that Although a stick through the spokes is most certainly the fastest way of stopping a bicycle...
I was simply alluding to the fact that the stock set up does not lack stopping power. I find no difficulty with modulating the applied braking force with my set up and I didn't with stock pads and fluid on road or track (except when the brake assist interferes as I said before).

You might be able to modulate the applied braking force easily within in the capabilities of the OEM set up. However, the OEM setup will be limited by its original design criteria, specifically the the pad compound and the piston size ratios.

Pulling numbers out of the air to try to illustrate my point, with the standard setup you may be able to modulate the brakes effectively up to 90% of the limit of the tyre's adhesion and before the wheel locks. A big brake kit with better piston size ratios and/or improved pad compound will let you modulate the brake pressure up to 98% of the limit if the tyre's adhesion. That's why there is a benefit to the BBK kit etc.

However, 90% of the limit of the tyre's adhesion on the Z4M is still pretty good braking performance.

 

[exdos]

A big brake kit with better piston size ratios will let you modulate the brake pressure up to 98% of the limit if the tyre's adhesion. That's why there is a benefit to the BBK kit etc.

Please can you explain what you mean by "better piston size ratios"?

 

[Lower]

I mean a better size ratio between the piston/pistons in the callipers and the master cylinder so that brake pad clamping pressure can be controlled more easily.

Our callipers are single piston callipers, so the calliper piston size is limited by the diameter of the brake disc as the piston is round. BBK callipers are usually much longer and contain multiple pistons which means you can have more piston area and also apply that clamping force more evenly over the pad. More piston area in the caliper obviously means more clamping pressure for the same pedal pressure which in turn means less pedal pressure for the equivalent braking performance. Less pedal pressure means you have more control and therefore better modulation of the brakes closer to the lock up point giving an ultimate improvement in retardation prior to lock up.

 

[exdos]

I mean a better size ratio between the piston/pistons in the callipers and the master cylinder so that brake pad clamping pressure can be controlled more easily.

Our callipers are single piston callipers, so the calliper piston size is limited by the diameter of the brake disc as the piston is round. BBK callipers are usually much longer and contain multiple pistons which means you can have more piston area and also apply that clamping force more evenly over the pad. More piston area in the caliper obviously means more clamping pressure for the same pedal pressure which in turn means less pedal pressure for the equivalent braking performance. Less pedal pressure means you have more control and therefore better modulation of the brakes closer to the lock up point giving an ultimate improvement in retardation prior to lock up.

Thanks, I thought that's what you meant.

The single piston used in the Z4M brakes, Part number: 34 11 1 157 037, is also used by BMW for several different models including the heavy 5, 7, 8 and X3 series cars as well as its M cars. The diameter of the single piston is 60mm, which is an area of 2827.4 sq.mm, which gives 5654.8 sq.mm total piston area. In comparison of a BBK, the Stoptech 6 pot brakes for the Z4M have piston sizes of 2x 30mm (706.9 sq.mm each) and 4x 36mm (1017.9 each) = 5485.4sq.mm total piston area. The Stoptech 4 pot brakes for the Z4M have piston sizes of 2x 38mm (1134 sq.mm each) and 2x 44mm (1520.5 each) = 5309 sq.mm total piston area.

So, the Z4M's OEM calipers have a larger piston area than either of the Stoptech BBKs for the Z4M. Also, all the hydraulic pressure of the brake lines goes to just one piston of 2827.4 sq.mm in the OEM caliper, whereas in the Stoptech BBKs the same hydraulic pressure has to be split either 6 or 4 ways. I'm wondering how all this translates into better stopping power and brake pedal modulation of the BBKs?

 

[Lower]

Well, I clearly wouldn't recommend either of those BBKs on piston ratio then! My comments were not specific to the Z4M.

There are other benefits to BBKs such as increased pad contact area and more even pressure on the pad due to the increased number of pistons but in my opinion the main benefit to a BBK is increased cooling and the improved braking performance often comes from the pad compound and larger brake discs.

I personally wouldn't bother with a BBK. I've personally had substantial improvements in braking performance and fade resistance just going for discs with better cooling but most importantly better pad compounds with higher coefficients of friction.

 

[ga41]

Well i have the Stoptechs and i am extremely happy with them! Modulation is great, pedal stiffness is the same as stock and the larger discs and callipers can take up heat much, much better than before.

 

[Wondermike]

Yeah I always thought the benefit in getting aftermarket brakes was predominantly to go with a bigger diameter on the discs, with a few other benefits after that.

 

[exdos]

What I'm trying to discover is a genuine objective comparison of braking capability of BMW OEM vs aftermarket BBKs. I've read lots of subjective assessments of OEM brakes (= bad) and BBK (= good) but when I compare the specifications of the different BBKs to OEM, to the "physics of braking", I can't see how so many owners seem to think that a BBK will be so much better than OEM, in stopping power and brake pedal modulation, all else remaining equal (i.e same brake pads, tyres etc.).

I can easily understand the improvement in duty cycle of an aluminium BBK vs cast iron OEM, because the specific heat of aluminium is twice that of cast iron. Therefore, aluminium calipers (BBKs) can absorb more heat before they would overheat, but the provision of cheap auxiliary brake cooling ducts can easily sort that problem out for both OEM and BBKs.

 

[Beedub]

from my BBk i get....

Better braking performance.... not speculation, track time proven.
Better pedal FEEL.
Much much longer fade resistance.
Swap brake pads in 5 mins once the cars in the air and wheel off.
Much more confidence in my braking.
My AP BBK can take every last drop of my cars monumental power and they do it ALL day.
The are noisy as f**k.
They are s**t when cold.
The parts to replace things are painfully expensive. :-(

right up their with my fave mods....

 

[exdos]

Hi Byron,

from my BBk i get....

Better braking performance.... not speculation, track time proven.

It's the proof that I want to see. How much quicker does a BBK bring a car to stop in comparison to OEM brakes? What is the reduction in stopping distance?

right up their with my fave mods....

I can certainly see the attraction of a BBK and I have experience of them, but as for stopping power, I'm pretty impressed with the Z4M OEM brakes and I'd rather spend the same money on suspension first.

 

[Beedub]

Hi Byron,

from my BBk i get....

Better braking performance.... not speculation, track time proven.

It's the proof that I want to see. How much quicker does a BBK bring a car to stop in comparison to OEM brakes? What is the reduction in stopping distance?

right up their with my fave mods....

I can certainly see the attraction of a BBK and I have experience of them, but as for stopping power, I'm pretty impressed with the Z4M OEM brakes and I'd rather spend the same money on suspension first.

cant accurately give those figures without measuring distance, but im picking up better laptimes, i can brake later with much more confidence, their is not one, not one area in terms of braking where their isnt an improvement over stock.... the pedal feel is bordering perfect for me.... drop AP an email to ask how they develop these kits and for specific details.... also let not mention my kit was LIGHTER on each corner ;-) not bad at all......

 

[goon]

A lot of the conversion kits have 4 or 6 piston s too not the single one of the oem brake.

 

[exdos]

I was just reading a thread about the need for servicing of AP Racing brakes on the cslregister.com forum and someone had posted the information below. To me, that reads that AP Racing recommend fitting auxiliary brake cooling.

 

[Beedub]

I was just reading a thread about the need for servicing of AP Racing brakes on the cslregister.com forum and someone had posted the information below. To me, that reads that AP Racing recommend fitting auxiliary brake cooling.

nahhhh im happy as is.... i'll get mine serviced every 4 years job done.
that said if someone devised a proper setup for maybe connecting up some proper ductings with a specialist rotor back plate, id be in..... im just not up for knocking up anything with cable ties ect ect... it just goes against my modding rules.

BTW my calipers have temp strips on them...... im pleased to report ive never got them near those type of temps despite my best efforts.!!!

 

[tombar]

What kind of herresy is this.

 

[olivierZ]

for details about BBK from Brembo/ Porsche 996.....

i have 2 calipers 4pots and the rallyroad kit for the z4m....

just know about the CSL's replica front and spacer,

for the brembo kit, it necessary to have 10mm spacers per wheel on front

before offset 38 >>> after offset 28

is not too bigger for a z4m??

(have Bilstein/eibach kit also)