B8/B8.5 Front brake piston areas?

[drob23]

The front piston outer diameter was right around 58mm… That would make the area around 2642 mm2 (29x29x3.14). Or around 2660mm2 if you go with the 58.2mm reading.

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 wonder if you are measuring the boot diameter, which according to this, is slightly larger than the diameter of the actual piston http://www.stoptech.com/technical-support/piston-size-matrix

But the stocker is much different by design and way larger, so maybe this ambiguity is unimportant. I wonder if there are caliper rebuild kits out there that specify the dimensions.

[jran76]

The front piston outer diameter was right around 58mm… That would make the area around 2642 mm2 (29x29x3.14). Or around 2660mm2 if you go with the 58.2mm reading.

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 wonder if you are measuring the boot diameter, which according to this, is slightly larger than the diameter of the actual piston http://www.stoptech.com/technical-support/piston-size-matrix

But the stocker is much different by design and way larger, so maybe this ambiguity is unimportant. I wonder if there are caliper rebuild kits out there that specify the dimensions.

The first time I might have been. There was enough pad deposit material built up on the piston, it was hard to tell where the piston stopped and the boot started. The second time after cleaning I could see what looked like bare metal, and then where the rubber boot started. I’m pretty sure the second time I was on the bare metal on the outside of the actual piston. Again, hard to see everything with the piston still in the caliper…

[jran76]

OK, found Stasis’ whitepaper on their brake kits. I’m off by 0.1mm according to their measurement. They state the stock piston diameter is 57.15mm, or about 2565mm2.

Reference: http://stasisengineering.com/sites/default/files/whitepapers/B8%20S4%20370-390%20monobloc%20V.1.pdf

They also list their piston sizes as another reference point.

[jran76]

My AP kit uses caliper part CP8522 according to the box, and 27mm, 31.8mm, and 38.1mm pistons according to the AP site. This is on a 370mmx36mm rotors. That’s a hair over 2500mm2. AP calls it 50.1cm2 total…

http://www.apracing.com/product_details/performance_upgrades/road-aftermarket_brake_calipers/world_radi-cal_calipers/6_piston_caliper_variants/cp8522_family-50.1cm²_piston_area.aspx

[drob23]

Interesting, nice finds Josh 8) Sweet to have a solid informative thread to reference during these AZ debates.

[b5b4b8]

OK, found Stasis’ whitepaper on their brake kits. I’m off by 0.1mm according to their measurement. They state the stock piston diameter is 57.15mm, or about 2565mm2.

Thanks for the link…

P.S. – and wow… www.stasisengineering.com is still alive…

[b5b4b8]

Just to wrap this up, and consolidate the info :

[tr][td]Caliper[/td][td]piston sizes[/td][td]Area[/td][/tr]
[tr][td]--------------[/td][td]--------------------------[/td][td]-------------[/td][/tr]
[tr][td]Stock[/td][td]57.15mm[/td][td]2565 mm^2[/td][/tr]
[tr][td]ST-60/355[/td][td]28mm, 30mm, 34mm[/td][td]2430 mm^2[/td][/tr]
[tr][td]ST-60/380[/td][td]28mm, 30mm, 34mm[/td][td]2229 mm^2[/td][/tr]
[tr][td]19z[/td][td]32mm, 36mm, 38mm[/td][td]2950 mm^2[/td][/tr]

Stoptech told me that their 355/380 ST-60 kits are +2 and -1% respectively of stock output to maintain proper ABS and chassis control system function.

ECS posted on AZ that the stock, RS5, and 19Z kits they sell have the following brake balance :

[tr][td]Caliper[/td][td]Rotor[/td][td]Front %[/td][td]change[/td][/tr]
[tr][td]--------[/td][td]--------[/td][td]-----------[/td][td]---------[/td][/tr]
[tr][td]Stock[/td][td]345[/td][td]64.1%[/td][td]–%[/td][/tr]
[tr][td]19z[/td][td]365[/td][td]67.7%[/td][td]+5.6%[/td][/tr]
[tr][td]19z[/td][td]380[/td][td]68.8%[/td][td]+7.3%[/td][/tr]
[tr][td]RS5[/td][td]365[/td][td]68.4%[/td][td]+6.7%[/td][/tr]
[tr][td]RS5[/td][td]380[/td][td]69.4%[/td][td]+8.3%[/td][/tr]

Stoptech’s kits would be :

[tr][td]--------[/td][td]--------[/td][td]-----------[/td][td]---------[/td][/tr]
[tr][td]ST60[/td][td]355[/td][td]65.4%[/td][td]+2%[/td][/tr]
[tr][td]ST60[/td][td]380[/td][td]63.5%[/td][td]-1%[/td][/tr]