Originally Posted by JAJ
Stock piston sizes are cast into the calipers.
As you say, the Stoptech ones are available from ST. Brembo doesn't say anything. AP Racing diameters are on the UK site in the assembly drawings.
Thanks for the pointers. I updated the 1st post with new numbers. Given the stock piston sizes (60mm front 46mm rear) and the data found on stoptech's web site
, the piston sizes should be based on my calculations as follows:
st40 355 front = p1: 44mm p2: 42mm (44mm is max size for st40)
st40 355 back = p1: 34mm p2: 32mm
st60 380 front = p1: 36mm p2: 34mm p3:32mm (or 40 34 29 who knows?)
These sizes would allow mixing and matching stoptech and stock rotors with negligible change to brake balance.
Stock balance is: F/R = 1.74
Stoptech ST40 all around is: F/R = 1.69
Stoptech ST60/ST40 is: F/R = 1.70
Stoptech ST60/stock is: F/R = 1.78
Stoptech ST40/stock is: F/R = 1.77
(All the above given same friction material front and back). Hence, stoptech can rightfully claim they provide a balanced upgrade no matter what you go with. All the balances above are within at least 2.5% of stock. Nice!
Would be nice to figure out piston sizes for the other brands. I ran out of time while looking for piston sizes on APs website... Some other time maybe.
Keep in mind that when you change pads, all your careful calculations go out the window because the mu of the new pads could be anywhere from half to double the mu of the pads you took off.
All the calculations I want to perform do not yield absolute values. So friction material does not factor as long as you use the same pads on both front and rear.