Anyone know or have a source for the area of the piston on the stock front brakes and/or a braking system that’s designed for these cars (like stoptech, etc).
Wondering how much something like ECS’s Porsche 19z conversion with 365/380mm rotors would change brake bias. From what I’ve found the Porsche calipers have piston areas of :
17Z - 3060mm2
18Z - 2750mm2
19Z - 2956mm2
unfortunately I don’t have the data but I installed the RS5 brakes and for track duty i needed to swap the rs5 master cylinder and booster. The S4 and S5 use the master booster that the A4 and A5 does. Bias wasn’t/isn’t to bad I think the bigger issue is pedal pressure and how much you need with the much larger calipers. I think there is more rear bias built into this platform for comfort. Rear pads seem go more quickly than any other gen S4 i’ve owned.
You probably know this, but to correctly calculate the bias shift from one brake caliper to another, you should also factor in the rotor diameter variation. So the units should be m^3. As an example, there are different piston sizing on ST60 calipers for 355mm vs 380mm, as seen by the separate rebuild part numbers.
Right - brake torque is going to be a function of piston area, brake line pressure, rotor size, and pads (plus caliper flex, and other stuff that I’m inclined to ignore for this). To just get a simple 1-1 comparision I was going to assume the pads and brake line pressure don’t change - I’d only factor in piston and rotor sizes.
From OldSport// 's BBK install thread - he has some pictures of his 380mm stoptech trophy pistons (Thanks Ryan) - they are labeled 28mm, 30mm, 34mm - which would be an area of 2230mm if you just calculate the areas of those 3 numbers. Mike - do you happen to have pics of your stoptech 355 kit or info that shows the piston sizes?
2230mm^2 (stoptech) vs. 2956mm^2 (19z) both on 380mm rotors seems like it would be a big shift in bias toward the front - it’s still “stable” but may give up a fair amount on total braking power.
From some other reading ( http://www.zeckhausen.com/Testing_Brakes.htm ), I think stoptech BBK’s may move the bias slightly rearward vs. the stock balance to improve overall stopping power, but the 19z’s are nearly 33% more piston area than the Stoptechs if this info is correct - doubt they moved it that much… ???
Rear bias, if I understand correctly (freely admit I might be wrong here) - won’t be comfortable… This shows rear bias as being unstable - http://www.zeckhausen.com/Testing_Brakes.htm, and kind of makes sense to me - if the rears lock up first it’s like doing a handbrake turn 
Most cars have some understeer built into them for safety and driving “comfort” for the masses…
Maybe with the advance of stability control systems and the better weight distribution of the B8/8.5 chassis the rears are being used more to improve the stopping power, but it’s still front biased? The S4 data shown in that article was from 2004, so it was likely a B6 (or maybe a B5 depending on how old the data was) - so the “factory” bias may not be representative.
I’ve reached out to Stoptech, but have not heard back yet.
Where did you see the labels on the trophy kit? I’d be happy to take a look at some future date. Otherwise, guess I’d have to measure as below. I could email them and inquire about the kit for my car, they seem to make it difficult, maybe they don’t publish to make it harder to reverse engineer? But in theory, I hope my 355mm kit has the same bias as ryan, but would be interesting to verify.
http://www.stoptech.com/technical-support/piston-size-matrix
That’s a useful link – thanks. Although now I’m seeing that the area the brake pressure acts on may be design specific even for the same “piston size”, so calculating super accurate clamping force differences may not be easy. Then again - the 19z calipers are Brembo I think - maybe they have something similar to the stoptech matrix…
As for the piston size lables – they were so much labels, as just etched into the pistons : http://www.audizine.com/forum/showthread.php/637201-Stoptech-Trophy-Sport-380mm-BBK-Realtime-Install?p=10493698&viewfull=1#post10493698
Either way - it would be interesting to know if your 355 kit has different sized pistons than the 380 kit – ECS confirmed to me on AZ that the calipers between their 355/380 kits are identical. I also found a post of another stoptech 380 kit (non trophy) that had the same pistons as Ryan’s.
I wouldn’t take what ECS says as gospel. Especially for a product they don’t make. Unless they can show you pictures (which they can’t because ST packages the calipers in a shrink wrapped mini-box)…
Honestly, I wouldn’t get too worried in the nuances, as long as you are ballpark, you should be fine IMO. Since you are asking about bias, I assume this is for track driving? If so, many people don’t match the front/rear pads, and even with the same pads, I bet the temps on the front are quite different than the rear. Probably a reason race teams have proportioning valves ;D
I may have given the wrong impression – agree that with non-matched pads and/or suspension differences (coil overs vs. stock, etc) you can’t dial things in exactly. I’m just trying to get ballpark values and understand things better.
Yes - this would be for track - I had some house damage, which put car stuff on hold - but starting to looking into what I’d like to do again to make the car something I can trust on track while pushing it (well… myself more than the car)…
Believe me … I’ve learned not to take the vendor’s words as gospel ( the hard way in a few instances ) - thus the questions, research. I think these Porsche 19z big brake kits ECS is selling are something they are putting together themselves from components. I was just wondering if there would be a significant bias change fitting these to a B8/B8.5. Based on the info I have it looks as if it might be as much as 33% more front clamping force than the Stoptech 380 kit, and both have 380mm rotors.
I’m still very much learning about braking systems - but it seems like 33% more clamping force on the fronts would be detrimental.
You run XP12/10 for track days still? From their description of the XP12 : “initial bite, torque and fade resistance over and above the XP10” it seems that this combination would tend to bias braking toward the front over an otherwise identical XP10/XP10 or XP12/XP12 setup?
In the end - I’m unlikely to go experimenting with different pad compounds (given the cost per axle of these things), or install a proportioning valve to perfect the brake bias - I’d just like to understand the variables a little better so that I don’t get something that is overly front biased and thus down on overall performance without realizing it. I’m sure I wouldn’t know if my car was too front biased - at some point when I wanted more performance I’d probably just think I needed more grip and try to change tire width or compound…
Is there any good/easy way to know if the rears are doing their fair share of braking? Unless there is a good rotor temperature difference front/back that you can get with a pyro after being on the track (is there?) – without using different combinations of brake pads to move the bias rearward until things get “squirrely”, I’m not sure how you would tell. Even then it might vary track to track, right?
Thanks everyone for the help/education, and sorry for the diatribe.
Trust me Jim, this is a great thread. I’ve debated this kind of stuff many times on AZ, but never seen actual numbers. So in one sense, I completely put my trust in stoptech :-\ I learned most about BBK’s from arguing after my purchase lol. The porsche kit ECS is selling is definitely a parts bin kit. But this isn’t the first time to the rodeo for those calipers, I’ve seen some B6/B7 S4’s at the track with that BBK. The truth is, people are going to benefit most from the additional rigidity of the pedal, and the vast improvements to heat rejection at the piston surface (which is ultimately the cause of boiling your fluid). The minute you throw hoosiers on, and push the car at the advanced level with stg 2 power, I bet you overwhelm the system no matter what. Hence west going with Brembo GT-R’s.
Moving the bias forward should not make things too dicey, just as you said, reduce the overall straight line performance. I’d also be concerned about the other physical parameters in the system, like MC, pedal geometry, line diameter etc, since that caliper was made for a giant SUV (eg your calculation might need to be more comprehensive to truly compare the bias for this 19z kit).
As far as what I run, castrol SRF with .5mm Ti shims, and carbotech xp20 front xp8 rear. I have since got a set of xp24’s for front, but missed my last event with the broken test pipes. So I’ll have to wait till next spring most likely.
As far as knowing about brake bias, I’m pretty sure I got some strange tail wiggle at the back straight of mid-ohio, which is slightly declined. I’d assume going downhill makes the rear light, so honestly I’m not sure why that happened. Could have been due to the different pad compounds. Or could be due to the suspension geometry. I run with ESC fully off, but I bet it would still stabilize yaw during straight line threshold braking.
Anyways, I’ll keep adding to this thread where I can. I will mention that the ST kits can be had for much cheaper than the MSRP. I got mine for 2450 and achtuning was selling a trophy kit for the same price a few months back.
Any regrets with your kit? If you were starting again, would you go trophy, or 380mm, something else? Were the Ti shims just insurance, or did you find you needed them?
I’ve seen the 355 regular kit for as little as 2250, and the trophy 355 at 2725. The 380’s trophys’ are around 2950, but I’ve not contacted any vendors yet…
Didnt read this entire thread but I’ve gone down this road.
First of all, the P car calipers will be totally incorrect for this application assuming 355 rotors. It’s going to require a lot more fluid displacement to move those pistons, and your pedal travel will be low. At worst, you will lose stopping force at the limit–esp as fluid heads up.
The stoptech 355 ST60 caliper is entirely different from the ST60 caliper found in the 380 kit. The pistons are different in each. Long ago, I reached out to Stoptech to inquire about simply getting a bracket for my caliper to make it work with a 380mm rotor (my st60 is 355mm). They gave me the pn for the bracket, but advised my bias would be totally off and very front biased if i did this due to the piston sizes in this kit. I’ve since reached out to ask if I can use the 380mm rebuild kit on my st60 (I am assuming they just use spacers in the piston bores to allow for different piston areas to make it a “universal” application).
If you want some light improvements–especially in brake feel, get the tyrol sport rigid brake caliper slider kit. If you want one, pm me. I have one BNIP that I never installed. Otherwise, you can step up to the Q5 4 pistol fixed calipers. If you are serious about braking, skip the 355 and go straight to 380. I regret doing 355…
No regrets on my kit, very pleased so far. The only downside is how fast I go through pads (1 set = 4 events), but I’m hoping the xp24’s help longevity.
It’s always hard to re-evaluate a purchase like this, since it seems like you could get a lot more for slightly more money. With the 355mm kit going for 2250 new, I think that’s pretty much the sweet spot on the value spectrum. I needed 355mm so I can fit my winter wheels over. Would not have been financially viable to get a new winter setup. And I don’t want to be swapping brakes like boro I’m not sure what the trophy kit would get me other than a finish that doesn’t maroon itself away. Maybe slightly lighter? Would a revo alcon kit or brembo GT be better…probably…but does it warrant the additional cost…I don’t think so personally. You are getting fully floating rotors with a very available pad profile (which helps a lot finding pads). I got the shims for insurance.
And last time, I needed it…didn’t realize how bad these were till I started driving home. I can’t understand how this didn’t manifest itself on the track, maybe I got lucky with the cat breaking during my last session.
https://c2.staticflickr.com/6/5639/20688050169_19f70f9786_k.jpg
Ti shims are needed on those 355’s.
They dont have THAT much heat capacity that they can deal with a 4000lb fast sedan all that well.
Having them in there to reduce the temps dealt with by the fluid is great insurance at maintaining a firm pedal.
Though a proper (read: more racey) BBK would have stainless or even Ti caliper pistons which would do the same thing without needing to take up space in the pad. Worth it.
So stoptech got back to me and more or less confirmed my calculations for piston area - they are very slightly off due to the 32mm piston not being exactly 32mm, but it’s close. They agreed that if the piston areas are what I quoted for the 19z that they are “absurdly oversized (~20-30%)”. 19z pistons are 32/36/38mm while the stoptech 380 ST60 are 28,30,34mm - and with the brembo and stoptech pistons apparently being nearly identical (posts on planet-9 : http://www.planet-9.com/archive/index.php?t-106042.html) the calculations seem to match.
Erik also confirmed this (which is what I thought) - the area for the 380mm ST-60 is 2229mm^2, the 355 ST-60 is 2430mm^2 to make up for the smaller diameter. Both give in the ball park of 421,000mm^3 in piston-area*rotor radius, while the 19z for 380mm would be 561,000. 33% higher. On their 365mm rotor it’s only 28% higher. Stoptech’s 2 options are within 2% of the stock value.
[quote]If you are serious about braking, skip the 355 and go straight to 380. I regret doing 355…
[/quote]
Problem is I’m not sure how serious I am. 380’s only fit my 19" wheels, not the 18’s, and consumables are going to be more.
Mike/Boro - do you think Mikes pads are overheating with the smaller rotor leading to the very high pad wear rate, or is a set of pads every 4 events sort of par?
Thanks again…
Take this for what’s it’s worth as it’s definitely not 100% accurate… I measured my stock (B8 S4) front caliper piston with a digital caliper. It’s not super easy to get the digital caliper inside the brake caliper to get a really good measurement, and you have to deal with the piston boot and deposit buildup, so…
The front piston outer diameter was right around 58mm. I measured 3-4 times and got between 58mm and 58.2mm, so… That would make the area around 2642 mm2 (29x29x3.14). Or around 2660mm2 if you go with the 58.2mm reading.
I had measured the pistons of the 6 piston AP kit I have on my car when I installed them, but I can’t find where I wrote it down. If I find the measurements, I’ll post those too.
So the stoptech 18" would be moving the brake bias rearward by a scaling factor of 2430/2660 * 355/345. For a single piston floating caliper, the brake force is supposed to be 2x the single piston force, but I don’t really think that is accurate due to friction in the slider assembly. It’s entirely possible that 24xx * 355 * 2 ~= 26xx * 345 * 1.9 (whatever the fudge factor is for the floating setup).
http://stoptech.com/docs/media-center-documents/the-physics-of-braking-systems
Regarding my pad wear, I think this is more of a compound issue. But I could be wrong. Certainly larger rotors would do better in terms of heat dissipation. So maybe the wear is due to excessive heat? Not sure if wear changes at the pad/rotor interface because the same amount of energy is being converted from kinetic to heat. While the clamping torque on the larger rotor BBK is greater, the velocity differential at the pad/rotor interface would be greater. Not sure how that affects pad wear.
That’s a little larger than what inferred from the Stoptech numbers Erik sent me - I backed out around 2500mm^2, but hard to argue with actual measurements vs. my spreadsheet 
Then again - Mike makes a good point that the single piston stock setup will have some loss due to friction in the slider, and can’t really apply the same kind of optimized pressure that a 6-pot caliper can… maybe Erik was quoting 1-2% of resulting torque and not simply (piston-area * rotor radius) as a quick and dirty way to calculate torque.
The 19z’s 2950mm^2 on larger rotors still sounds to me like it would probably adversely move the bias forward vs. the optimum, along with needing more pedal travel as Boro pointed out.
I just went out there and hit the piston with some brake cleaner so I could get a bare metal surface to measure against. It is actually a little less. All 4 measurements were around 57.25mm, or about 2574.2mm2… Short of getting the piston out of the caliper, I don’t know if I can get a better reading.
I have no doubt that you have it right - Thanks for looking. I made a lot of simplifying assumptions to get to the 2500mm^2 number for the stockers - and there seem to be enough difference between the designs of 1 piston to 6 piston to account for the variance… Still – we should be able to compare the 19z 6-piston 380mm rotor directly to the Stoptech 6-piston 380mm rotor areas - 32.6% larger area for the 19z…
If you do find your AP numbers please post - I’m curious now …
Thanks everyone - learned quite a bit…