Brake bias calculations for BBK

[OrbitalEllipses]

Originally Posted by Fizz

Based on the spreadsheet I linked earlier, the rear FHI 2-pots has the same piston size as well as rotor diameter, so overall rear bias remains unchanged compared to stock BRZ/FRS. But you do gain a lighter stiffer caliper with 2 opposed pistons....

Hm. The 2 pots have a larger effective disc diameter, compared to yours anyway. For the 290mm rear folks (US/Cad/high-spec) it's aesthetic - which is why I haven't done it yet.

[Fizz]

Originally Posted by OrbitalEllipses

The FHI 2 pot pistons appear to be the same diameter piston as the stock 1 pot slider, however the 2 pot has double the pistons versus stock - meaning increased piston area.

Paging @jamal

Hmmm somehow I think the extra piston does not mean double the piston area as such....something to do with total clamping force being similar even though your got more piston)....because if that was correct then we wouldn't be having the issue of brembo rear having less piston area versus stock (coz brembo rear caliper has 2 smaller pistons instead of 1 larger in the stock caliper). Anyway I might be totally wrong here

[gramicci101]

Originally Posted by Fizz

Hmmm somehow I think the extra piston does not mean double the piston area as such....something to do with total clamping force being similar even though your got more piston)

I was told the below when I asked a similar question:

Originally Posted by gramicci101

I would have thought a 4 opposed piston caliper would have a fair amount more clamping force than a 2 piston sliding caliper.

Originally Posted by Dave-ROR

Piston area and line pressure determine that. The OEM calipers actually produce more torque per given line pressure than the WRX 4 pot fixed calipers, unless they changed the piston sizes. I honestly never measure the OEM ones but that was true with the WRX a few years ago.

Originally Posted by wheelhaus

That's a common misconception, but you have to measure the surface area of pistons on only one side. It has to do with how pressure is applied and measured. Look at it this way... With a four piston caliper, each piston is matched by it's opposing piston, so the fluid pressure of the system is the same on each side, it is matched and equalized, not added. With a one-sided caliper, the piston(s) push against a bracket, which has to push back with equal force.

From this thread.

[Racecomp Engineering]

Yes, you only need piston area on one side with opposed piston calipers. And also yes, there is a very slight bias shift rearward with the subaru wrx 4 pot front.


EDIT: I'm wrong on the bias change, the rear FHI 2 pots shift bias forward. Front FHI 4 pots have no effect on bias.

- Andy

[gramicci101]

Originally Posted by Racecomp Engineering

Yes, you only need piston area on one side with opposed piston calipers. And also yes, there is a very slight bias shift rearward with the subaru wrx 4 pot front.

- Andy

So is there really a benefit to going to a 4 pot front caliper vs. staying with the OEM 2 pot sliding caliper?

Aside from looking cool?

[Racecomp Engineering]

Originally Posted by gramicci101

So is there really a benefit to going to a 4 pot front caliper vs. staying with the OEM 2 pot sliding caliper?

Aside from looking cool?

For the subaru 4 pot front it's just the coolness, a slight change in weight, and a slight change in feel.

The brembos get you a much bigger heat sink, but more of a shift in bias.

- Andy

[OrbitalEllipses]

The FHI 4 pots have smaller piston area than the stock 2 pot sliders, using the same effective diameter - hence they move bias rearward.

Originally Posted by Fizz

Hmmm somehow I think the extra piston does not mean double the piston area as such....something to do with total clamping force being similar even though your got more piston)....because if that was correct then we wouldn't be having the issue of brembo rear having less piston area versus stock (coz brembo rear caliper has 2 smaller pistons instead of 1 larger in the stock caliper). Anyway I might be totally wrong here

Yeah I was editing my post when you quoted that.

[gramicci101]

Originally Posted by Racecomp Engineering

The brembos get you a much bigger heat sink, but more of a shift in bias.

- Andy

Switching to brembos is going from a 295mm disc to a 326mm disc, plus adding 6mm more width to the rotor and god knows what to the piston area. I wasn't able to access Fizz's spreadsheet.

Just switching to the 326mm disc should multiply the applied torque by 1.1 because of the extra distance from the point of rotation to the point of applied pressure. I wonder if our calipers would fit over a 30mm wide rotor?

[OrbitalEllipses]

Originally Posted by gramicci101

I wonder if our calipers would fit over a 30mm wide rotor?

No. Legacy GT fronts however, will. Those are paired with a 316x30mm OEM rotor.

[gramicci101]

Originally Posted by OrbitalEllipses

No. Legacy GT fronts however, will. Those are paired with a 316x30mm OEM rotor.

Curses, there goes that idea. Thanks for the tip about the LGT calipers.

The OEM rear calipers should work. Ours is 290x18, a 2004 STi's is 316x20. Just need to make a bracket for the caliper.

[OrbitalEllipses]

Originally Posted by gramicci101

Curses, there goes that idea. Thanks for the tip about the LGT calipers.

The OEM rear calipers should work. Ours is 290x18, a 2004 STi's is 316x20. Just need to make a bracket for the caliper.

You will need the full assembly from the front of the LGT, including dogbones. I don't recall where the bleeder is, but you might have to swap the calipers right to left to get the bleeders in the right place. Both pistons should be the same size in the caliper, so no concern with flipping them side to side.

[King Tut]

Originally Posted by OrbitalEllipses

What you can do and I know plenty of WRX/STI folks who do this as a solution: run a more aggressive rear compound.

This.

[OrbitalEllipses]

Also @gramicci101 you could do this: http://www.knsbrakes.com/c/caliper-items/2279

Nissan Z32 adapter bracket, Nissan Z32 caliper, LGT rotor.

[gramicci101]

Interesting, thank you.