[M3SQRD]

Originally Posted by vasillalov

I believe your math is incorrect.

A single 51mm piston has 2,041.785mm area.
A single 30mm piston has 706.5mm area
Four 30mm pistons have 2,826mm area

The difference is 28% change, which can be very significant.

If the rear pad compund was the same between the two calipers, you would most definitely feel a significant change in brake bias. In medium to hard braking, you would feel the car squatting first before the front brakes bite. This can be good on the track in the hands of a good driver but I caution anyone who is not ready for something like this.

Of course the increased rear bias can be mitigated with a softer rear pad or a pad with lower initial friction coefficient.

Your total piston area calculation for the single-piston floating caliper is incorrect. For a floating caliper to be in equilibrium (equal and opposite forces on both sides of the caliper), the floating caliper must first compress the inner pad on the piston side of the caliper into the inner brake rotor surface. At this point, the piston continues to extend out of the caliper and begins to start pulling the outer half of the caliper inward against the outer pad, compressing it against the outer brake rotor surface. Now, the single piston starts to develop equal and opposite compressive forces between the inner and outer brake pads with additional piston displacement. The magnitude of the compressive forces continue to increase until they are equal to the brake line pressure times the piston area. This means the total piston area of a floating caliper is actually 2x the area of the single piston to maintain equilibrium between the inner and outer pads.

So for a 51 mm-dia single-piston floating caliper, its total piston area (one caliper) is

Afloat = 2*pi/4*51^2 = 2*2042.8 = 4085.6 mm^2

For the 4p-fixed caliper with four pistons having the same diameter of 31.4 mm, its total piston area (one caliper) is

Afixed = 4*pi/4*31.4^2 = 4*774.4 = 3097.6 mm^2

% change in clamping force is

%Clamp = 100 * (Afixed - Afloat) / Afloat =
100 * (3097.6 - 4085.6) / 4085.6 = -24.2%

The 4p-fixed caliper produces 24.2% lower clamping force than the oem 1p-floating caliper. In the past this would’ve created a firm brake pedal with poor modulation but I’m not sure how this change would affect brake feel in the g8x brake-by-wire system.

The change in F-R brake bias can be calculated if anyone knows the piston sizes of the front 6p-fixed caliper.

Edit: Below are links to two online brake caliper piston area calculators. Both discuss the factor of 2.0 on calculating the total piston area of a floating/sliding single-piston caliper.

https://brakepower.com/piston-area-calculator.htm

https://www.tceperformanceproducts.com/bias-calculator/

[dentprotony@gmail.com]

Originally Posted by M3SQRD

Your total piston area calculation for the single-piston floating caliper is incorrect. For a floating caliper to be in equilibrium (equal and opposite forces on both sides of the caliper), the floating caliper must first compress the inner pad on the piston side of the caliper into the inner brake rotor surface. At this point, the piston continues to extend out of the caliper and begins to start pulling the outer half of the caliper inward against the outer pad, compressing it against the outer brake rotor surface. Now, the single piston starts to develop equal and opposite compressive forces between the inner and outer brake pads with additional piston displacement. The magnitude of the compressive forces continue to increase until they are equal to the brake line pressure times the piston area. This means the total piston area of a floating caliper is actually 2x the area of the single piston to maintain equilibrium between the inner and outer pads.

So for a 51 mm-dia single-piston floating caliper, its total piston area (one caliper) is

Afloat = 2*pi/4*51^2 = 2*2042.8 = 4085.6 mm^2

For the 4p-fixed caliper with four pistons having the same diameter of 31.4 mm, its total piston area (one caliper) is

Afixed = 4*pi/4*31.4^2 = 4*774.4 = 3097.6 mm^2

% change in clamping force is

%Clamp = 100 * (Afixed - Afloat) / Afloat =
100 * (3097.6 - 4085.6) / 4085.6 = -24.2%

The 4p-fixed caliper produces 24.2% lower clamping force than the oem 1p-floating caliper. In the past this would’ve created a firm brake pedal with poor modulation but I’m not sure how this change would affect brake feel in the g8x brake-by-wire system.

The change in F-R brake bias can be calculated if anyone knows the piston sizes of the front 6p-fixed caliper.

I have front calipers in stock, ill try to measure front piston dia when i get to the shop

[M3SQRD]

Originally Posted by dentprotony@gmail.com

I have front calipers in stock, ill try to measure front piston dia when i get to the shop

Do you also have stock pads? If you do, please measure the radial depth of the front and rear pads. Also, please provide the radial depth of the pad material used in the 4p caliper. Thanks.

[vasillalov]

Not following then formula. Why are you using diameter for circle of area calculation. You should be using radius.

[M3SQRD]

Originally Posted by vasillalov

Not following then formula. Why are you using diameter for circle of area calculation. You should be using radius.

Area of a circle is pi*R^2 where R is the radius. The diameter, D, is equal to D = 2*R or R = D/2 so the area of a circle expressed in D, instead of R, is

Acircle = pi*R^2 = pi*(D/2)^2 = (pi/4)*D^2. You should get the same area whether the radius or diameter is used.

[ryanb06]

Would be interesting to see what the piston calcs are for the F80/F82 rear caliper. I think that would be a nicer option overall.

[dentprotony@gmail.com]

Originally Posted by ryanb06

Would be interesting to see what the piston calcs are for the F80/F82 rear caliper. I think that would be a nicer option overall.

I am currently finishing front kit for an f80, rear wont take long because its a normal parking brake set up vs epb challenges of the newer platforms

[M3SQRD]

Originally Posted by dentprotony@gmail.com

I have front calipers in stock, ill try to measure front piston dia when i get to the shop

Where did the 51 mm-dia rear floating single-piston diameter come from? I have an extremely reliable source that shows it’s a 44 mm-dia, not 51 mm-dia. This makes a huge difference and, when this diameter is used, the stock and aftermarket F-R brake biases make sense.

[TopJimmy]

Guys, there is wayyyy more to brake bias calculations than expected. Take a look at what’s involved in the true calculations:
https://brakepower.com

Now to do a piston area calculation…that’s less involved, but there’s still a lot to it…especially if going from a fixed caliper to a floating caliper. Read:
https://brakepower.com/piston-area-calculator.htm

Do we have a “brake Yoda” here who wants to play with the calculators?

[M3SQRD]

Originally Posted by TopJimmy

Guys, there is wayyyy more to brake bias calculations than expected. Take a look at what’s involved in the true calculations:
https://brakepower.com

Now to do a piston area calculation…that’s less involved, but there’s still a lot to it…especially if going from a fixed caliper to a floating caliper. Read:
https://brakepower.com/piston-area-calculator.htm

Do we have a “brake Yoda” here who wants to play with the calculators?

I know everything needed to perform a true brake bias calculation. It’s not difficult. However, you need the correct information before you can start calculating the brake bias.

[dentprotony@gmail.com]

Originally Posted by M3SQRD

I know everything needed to perform a true brake bias calculation. It’s not difficult. However, you need the correct information before you can start calculating the brake bias.

I apologize for misinformation, i asked ebay seller to unwind the piston and measure it for me, hence the wrong diameter. I see how this threw everything off in the brake bias calculations. I have m3 calipers here that show 30/34/36 mm piston dia stamped on the caliper (se pictures) i believe these are actual diameters because outer dust seals are smaller than these dimensions. As for the m3 pad i dont have one in stock, i carry f90/f95 pads that are much wider, but I attached an image off rockauto to at least get the ballpark idea on outer measurements. Also attaching pictures of the 19z actual pad , hope this helps to move further with this kit potential

[M3SQRD]

Originally Posted by dentprotony@gmail.com

I apologize for misinformation, i asked ebay seller to unwind the piston and measure it for me, hence the wrong diameter. I see how this threw everything off in the brake bias calculations. I have m3 calipers here that show 30/34/36 mm piston dia stamped on the caliper (se pictures) i believe these are actual diameters because outer dust seals are smaller than these dimensions. As for the m3 pad i dont have one in stock, i carry f90/f95 pads that are much wider, but I attached an image off rockauto to at least get the ballpark idea on outer measurements. Also attaching pictures of the 19z actual pad , hope this helps to move further with this kit potential

No problem. Thanks for the measurements. I found the rear floating single-piston caliper is 44 mm in diameter, not 51 mm. I also found the stock front and rear pad radial depths as well as the front 6p- fixed caliper piston sizes which match your measured values. I’ll have summaries of the brake bias calculations for stock F & R, stock F & Porsche R, and Essex/AP Racing F CP9668/9660 & R CP9449.

[M3SQRD]

I found on Racing Brakes’ website that the G8x front 6p-fixed caliper has 30 mm (1.18”), 34 mm (1.338”) and 36 mm (1.417”) diameter pistons. That means the total front caliper piston area (one caliper only) is 5265 mm^2. I also found out the rear floating single-piston caliper has a 44 mm, not 51 mm, piston diameter. Floating rear caliper has a total area of 3041.1 mm^2.

Total stock piston area is 5265.3 + 3041.1 = 8306.4 mm^2

front brake area ratio = 5265.3 / 8306.4 = 0.634
rear brake area ratio = 3041.1 / 8306.4 = 0.366

If you factor in the brake rotor diameters (380 mm F, 370 mm R), pad radial depths (77 mm F, 64 mm R) and pad COF (assumed same value for front and rear pads), you get the following brake biases:

Front brake bias = 0.629
Rear brake bias = 0.371

For the stock front & Porsche 4p-fixed rear caliper

Total piston area = 5265.3 + 3097.6 = 8362.9 mm^2

Front brake area ratio = 0.630
Rear brake area ratio = 0.370

Again, factoring in brake rotor diameters, pad radial depths and the same COFs, the brake biases are:

Front brake bias = 0.625
Rear brake bias = 0.375

For comparison, I also looked at the Essex/AP Racing CP9668 or CP9660 front calipers (same piston sizes but different pad thickness, 25 mm and 18 mm, respectively) with 372 mm-dia rotors + 54 mm radial pad depth, and the CP9449 rear caliper with 365 mm-dia rotors + 42 mm radial pad depth.

Front brake bias = 0.611
Rear brake bias = 0.389

So the two aftermarket kits: (1) stock F & Porsche R and (2) Essex/AP CP9668/9660 F & CP9449 R shift the brake bias rearward by 0.6% and 2.9%, respectively.

[M3SQRD]

Originally Posted by ryanb06

Would be interesting to see what the piston calcs are for the F80/F82 rear caliper. I think that would be a nicer option overall.

Putting the f8x blue calipers on the g8x? I can understand using the f8x blue calipers and rotors on an e9x but not a g8x. The f8x front 4-p fixed caliper has ~4.5% lower total piston area than the g8x stock front 6p-fixed caliper. The f8x rear 2-p fixed caliper has the same total area as the g8x rear floating caliper. Using f8x blue brakes on the g8x will shift the brake bias a little rearward. However, the f8x front brake pad is very small compared to the g8x front brake pad so it’ll make controlling brake pad temperatures harder.

[TopJimmy]

M3SQRD Thank you for the math workout. It’s good to know that Essex shifts the bias rearward more than the BMW+Porsche setup. I worried that bias would shift rearward too much so I considered choosing a rear pad with slightly less coefficient of friction (rear ISweep 1500 with my front ISweep 2000 pads). Thanks to your math I’ll take ISweep 2000 in the rear too.

[M3SQRD]

Originally Posted by TopJimmy

M3SQRD Thank you for the math workout. It’s good to know that Essex shifts the bias rearward more than the BMW+Porsche setup. I worried that bias would shift rearward too much so I considered choosing a rear pad with slightly less coefficient of friction (rear ISweep 1500 with my front ISweep 2000 pads). Thanks to your math I’ll take ISweep 2000 in the rear too.

No problem

I will actually run a more aggressive pad on the rear on track sometimes to shift the bias even farther to the rear. With high front spring rates, you can get away with more rear bias to help the front out a bit. You can go measurably deeper into a brake zone with the right springs and damping to make use of the rear brakes and reduce lap times. I have the Essex/AP Racing CP9668/CP9449 setup.

[dentprotony@gmail.com]

Originally Posted by ryanb06

Would be interesting to see what the piston calcs are for the F80/F82 rear caliper. I think that would be a nicer option overall.

Sorry I misunderstood your question, i dont think f80 pad is wide enough to cover the whole rotor sweep section, another thing to keep in mind is that the caliper is made to work with a 30mm rotor vs 22/24mm g80 rear , caliper piston travel will not be enough and could cause catastrophic failure once pads and rotor are wore

[M3SQRD]

Originally Posted by dentprotony@gmail.com

Sorry I misunderstood your question, i dont think f80 pad is wide enough to cover the whole rotor sweep section, another thing to keep in mind is that the caliper is made to work with a 30mm rotor vs 22/24mm g80 rear , caliper piston travel will not be enough and could cause catastrophic failure once pads and rotor are wore

The f8x rear is a 370 mm-dia, 24 mm-thick rotor with a 63 mm radial depth pad.

The g8x rear is a 370 mm-dia, 24 mm-thick rotor with a 64 mm radial depth pad.

F8x 2p-rear caliper is a great fit, except for the 1 mm radial depth difference, unless I’m missing something? The BMW 2p-rear caliper probably costs more than the Porsche 4p-rear caliper.

[dentprotony@gmail.com]

Originally Posted by M3SQRD

The f8x rear is a 370 mm-dia, 24 mm-thick rotor with a 63 mm radial depth pad.

The g8x rear is a 370 mm-dia, 24 mm-thick rotor with a 64 mm radial depth pad.

F8x 2p-rear caliper is a great fit, except for the 1 mm radial depth difference, unless I’m missing something? The BMW 2p-rear caliper probably costs more than the Porsche 4p-rear caliper.

I thought he meant using f80 front 4 piston calipers for the g80 rear set up. I didn’t think about 2 piston rears off f80. Availability is very limited, and the mounting is very odd , would require an extension bracket probably

[M3SQRD]

Originally Posted by dentprotony@gmail.com

I thought he meant using f80 front 4 piston calipers for the g80 rear set up. I didn’t think about 2 piston rears off f80. Availability is very limited, and the mounting is very odd , would require an extension bracket probably

I’m confused with you! The f8x blue brake setup is perfect for the e9x although you have to buy f8x uprights but still a solid upgrade. The f8x blue front caliper definitely would not work on the rear of the g8x. The rotor thickness difference front to rear, as you pointed out, is 6 mm and the pad radial depth is 70 mm so 6 mm too deep for the g8x 64 mm rear. No way you could practically mount the f8x front rotor and caliper on the g8x rear.

[TopJimmy]

Final product arrived!

Thanks dentprotony@gmail.com, excellent work!

Unboxing:
Calipers came professionally shrink wrapped, bubble wrapped, and in a fully foam-padded box for brake sets. Brackets shipped separately for extra protection. Nothing rattled or jostled, and all parts arrived 100% perfect.

As soon as weather warms up enough for my race tires I’ll get these mounted. I’ve already checked with Ben r33_RGSport and ISweep pads are available. My order for a set of IS2000 will go in soon.

[dentprotony@gmail.com]

Glad we were able to make this happen, can’t wait to see these installed ��������