How to choose the proper caliper piston diameter.

[Paul318iS]

What have you guys found when swapping to different calipers? When given the choice, should I try to come close to the stock piston area? Or should I increase a bit and then install the 25mm MC?

For example if the 25mm MC is 18% larger then the stock e30 one, then I assume I should increase the piston area by 18%. Although, to make the pedal a bit more stiffer, how much larger can I go? It looks like I will be running a 12.2" rotor, or 310mm.

[IS]

No, it's over 40% larger since area is Radius^2 * pi.

The point of going to a larger Master cylinder is to avoid having too much brake pedal travel when using calipers with larger piston surface area.

To maintain a relatively stock brake bias, stick with stock size piston calipers, go with larger ones front AND rear, or just increase the size of the fronts and install a custom brake bias valve. The latter option is easier said than done.

The pedal will not be "stiffer" no matter what mc or caliper you use, unless you are using so much pressure as to actually bend the calipers under braking. This will not happen on street tires, though. If you want a stiffer pedal, change all those old brake lines.

[BeirBrennerE30]

Paul... i am assuming you are doing this because you want to increase your braking ability....

step one: 25mm mc, SS lines, Nice set of street brake pads... you WILL get a stiffer pedal because of the MC and lines but with a stiffer pedal you will be better able to modulate the brakes.

see how you like thsi for while and unless you are running an s50 or turbo or something this should be fine for any e30

if you want more braking after step one you need to start looking into a BBK with your calipers with larger pistons... the larger pistons in yoru calipers are not to balance out the larger MC diameter as i think you were getting at... the point of larger or more pistons in a caliper is do better apply evenly the pressure in the brake line onto the pad to ensure maximum contact area with the rotor

example: whats easier... to stop a spinning bike wheel by grabbing it with just one index finger in either side of the wheel or by mashing your hands on either side.... by mashing yoru hands on either side, even though you are using the same force as with your fingers you are applying more area... pressure=force x surfacearea... more area of pistons more force on rotors under the same pressure... and force x coefficient of friction=friction force that slows you down

[IS]

Larger pistons apply more force, more pistons help spread the force throughout the brake pad.

A master cylinder has no affect on stiffness. How can it? It is just a means of getting hydraulic advantage. force/area = force/area

[BeirBrennerE30]

Larger pistons apply more force, more pistons help spread the force throughout the brake pad.

Thats what i said...

A master cylinder has no affect on stiffness. How can it? It is just a means of getting hydraulic advantage. force/area = force/area

umm... you are exactily right except your stiffness does increase... but if you INCREASE the AREA in the MC you have to add an larger amount of force to move the same amount of fluid as before

lets say you apply 1lb of force to an MC area of 2in^2.... thats 1/2psi pressure in the line.... if you double the area of our theoretical MC to 4in^2.... you now need to press with 2lb of force to get that same 1/2psi since 1/2=2/4

[PeaveyBassist]

Wanna stop faster? Get wider, stickier tires.

Will

[rwh11385]

25mm is waste unless you want a change to go with a BBK. Doesn't add much to an otherwise stock system. Actually will length your stopping distances.

SS lines are a PITA. And not significantly better performing than NEW oem lines, besides the new rubber lines won't have to be replaced every 2 years...

Will has got it. Sticky tires like the ones people use in STS will help braking performance, as will better pads.

Hell, some kid locally got a stoptech BBK and then still used crap Axxiss Metal Mushers pads.

[PeaveyBassist]

Will has got it. Sticky tires like the ones people use in STS will help braking performance, as will better pads.

Also help cornering and acceleration. ;) Most people dont understand that EVERYTHING your car does is limited by it's tires. Upgrade to better tires and you can do everything faster/better than before.

Will

[rwh11385]

What have you guys found when swapping to different calipers? When given the choice, should I try to come close to the stock piston area? Or should I increase a bit and then install the 25mm MC?

For example if the 25mm MC is 18% larger then the stock e30 one, then I assume I should increase the piston area by 18%. Although, to make the pedal a bit more stiffer, how much larger can I go? It looks like I will be running a 12.2" rotor, or 310mm.

if you have already purchased the big brake kit, you may decide you want to install the larger MC

http://www.outlawdiscbrakes.com/downloads/info.doc

[IS]

Ok, now you are just playing with words.

Pedal 'stiffness' or 'firmness' means as force increases, after you already have enough pressure to have the pads firmly against the rotors, the pedal movement doesn't increase. That is the ideal brake system - completely incompressible. However this isn't the case because lines do expand afterall - even steel has some ductility. Brake fluid soaked, old brake lines have a lot more expansion, therefore an even further decrease in firmness or stiffness.

And as mentioned above, tires do have the greatest impact on braking, only assuming that your brake system/leg is powerful enough to break the limit of adhesion of your current tread. This is why I run hankook rs2s even when I drive 30k miles per year.

And another thing to consider - for all of those running lots of camber, that hurts braking too.

[BeirBrennerE30]

i think i am taking crazy pills today... when you intall a 25mm master cylinder.. your pedal is firmer(it is harder to push and you have to push it less distance) thats all there is to it... thats how physics works... having a firmer pedal is always desireable... it will NOT lengthen your stopping distance(most retarded statement of the day)

having stickier tires will decrease braking distance(and overall performance)... having wider tires will not... this thred is not about tires however

ALL BRAKE LINES REGUARDLESS OF TYPE SHOULD BE REPLACED AS NEEDED... SSlines are an upgrade from stock... they are no more or less a PITA than rubber lines to install or remove... the only difference between a ss line and a rubber one is that ss lines have metal wrapped around the rubber line toprevent the line from bulging a bit