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Originally Posted by diamondnik  Ok, long time ago I installed an e36 M3 master cylinder because I hated the squishy feel of the stock brake pedal, and I knew that I would upgrade my brakes at some point. Except that instead of M3 fronts (60mm piston) I installed e46 330 front brakes (57mm piston). I've been driving around like this for some time now and have mixed feelings about how well this arrangement works. I have larger rear calipers on the way (38mm piston) because I noticed that the rear just wasn't working well at all; pads and rotor would never really warm up and it was just causing galling on rotors (new rotors, brake lines and rebuilt calipers). Also, I don't think its normal to get over 100 000km out of same set of rotors and brakes...
So here is what I found regarding this. The M3 master cylinder is stepped; it's ID is 25.40 Front and 20.64 Rear, while on the TI it is 23.81 F/R. I looked at the 330 master and it is also stepped (23.81/22.20). So I looked into this reason, and figured that it must have something to do with ATC or DSC (on e46+). For this reason rear lines have different pressure, presumably to soften/dull line pressure when traction control kicks in. And since my Ti doesn't have traction control, I don't need a stepped master cylinder? And was the reason I got so much life out of my rotors and pads because rear was 20.64 ID?
So to recap:
e36 M3 60/40 caliper pistons and 25.40/20.64 MC
e36 Ti 54/34 caliper pistons and 23.81 MC
e46 330 57/44 caliper pistons and 23/81/22.20 MC
I can see how the M3 needs a larger diameter MC for front pistons, given their size, but if the 330 is fine with 23.81 front ID, then maybe I need to switch back to my original Ti master cylinder. Another thing concerning 'feel' generated by MC sizing. Originally I wanted the larger MC to stiffen the pedal and give better feel, like what you'd get in a competition car without power assist. But I think that the large MC just gives, what we'd call in motorcycle circles 'wooden lever' feel, in other words, you need lots of force to push fluid and get same result that you'd get from smaller MC and less needed force (it also dulls feel of line pressure). The Ti MC would give a softer pedal and still same braking power. The thing is, I can't even remember what the original Ti brake pedal felt like. On my one motorcycle I found that original MC made braking a chore and I'd need 4 fingers to modulate brakes, while two fingers with smaller diameter MC. I wonder how things would feel with the original MC.... Any thoughts on this issue? |
Are you sure the e36 m3 has different bore front and rear? According to ATE's web site the master cylinder for the e36 m3 (s50b30 and s52b32 (us-model)) has 25.4mm both front and rear.
Your findings regarding the handling of the car with your setup with e46 330 up front and stock ti-rear fits within my expectations. The stock ti has a brake bias of 72.62%. With your current setup this is changed to 77.05% (assuming equal MC-bore front and rear), which in my opinion is too much front bias.
When replacing brake parts, doing the bias calculations would indicate how the balance in the car changes. How to do the calculations could be found in the following link:
http://www.318ti.org/forum/showthread.php?t=12852 My philosophy is to choose brake parts to keep the bias close to stock or maybe a tiny bit less front biased.
Then assume:
Fb = The force applied the brake pedal by the foot (typical 170 N).
FbAmp = The amplification of the brake leveler and the vacuum system (typical 4*6.9)
uPad = Coefficient of friction (typical 0.5)
dcal = diameter caliper piston (mm)
dmc = diameter master cylinder (mm)
dbd = brake disk diameter (mm)
The front brake torque (TF) can then be calculated as
TF= Fb*FbAmp*2*F.uPad*(F.dcal/F.dmc)^2*((F.dbd-F.dcal)/4)/1000 (F. means front)
The equation can be used for the rear torque as well (TR), using the appropriate data for the rear brake system.
What is interesting to analyze is how the changes in the brake master cylinder affects TF, i.e. the term (F.dcal/F.dmc)^2. You have the data to do the exercise yourselves.
Hence comparing to systems where one only changes the master cylinder, one should expect to use more force on the pedal to achieve the same brake torque when increasing the diameter on the master cylinder.
However, since increased diameter on the master cylinder means higher liquid volume pr. mm-master-cylinder-piston-travel, the pedal travel should decrease since the amount of brake fluid 'requested' by the calipers are the same in both cases (assuming a stiff/incompressible system).