Need to replace my rear disks and pads so now seems like the perfect time to decide what to do to add some more rear bias.
First some general info as I’ve found it difficult to find in one place.
Standard Front Calipers. (2 pot), 2x44mm
Standard Rear Calipers. (1 pot), 1x36mm
Lotus S2 BBK AP Calipers. (4 pot), 2x36mm + 2x31.8mm
I’m sure I’m being overly simplistic here, so if you’ve got any better insight on how the size of the pads or the diameter of the disks alter the bias then delighted to be educated. But for my simplistic view if you just take the combined piston area front and rear to estimate the bias then.
Standard: 553mm front, 226mm rear. 71% front bias.
BBK: 852mm front, 226mm rear. 79% front bias.
Putting standard 2 pot calipers on the back with BBK on the front
std to rear: 852mm front, 553mm rear, 61% front bias.
Is that too much of a switch in the opposite direction? Would it really be necessary to run with a bias value, and is that even possible with the ABS?
Alternative is to get AP two pots with smaller pistons. EP do some with 41.3 mm pistons. That would give a 62% front bias.
AP also do the same calipers with 39mm pistons, which would be 63.5%.
Other thing I want to avoid is a long pedal. Given the increased piston area on the rear then that will consume more fluid. I know some of you have previously told me that isn’t an issue with the std 2 pots on the rear, but is that really the consensus? Is it twice the fluid volume going to the rear, or is that offset by the fact that with a single piston caliper the piston would need to move further?
Will
I have 4 pots and 315 up front and 308 and STD 2 pots on the rear.
Don’t waste your money on a master cylinder!! Mine is fine with the STD one.
Not sure your calcs are correct as you also need to include pad area, disc size and weight distribution
Cheers
Dave
The calculation for brake torque is the total piston area on ONE side of the disc; times the effective radius of the caliper; times the hydraulic pressure in your brake line; times the friction coefficient of the brake pad on cast iron disc. Most of that information can be gotten from the AP Racing customer installation drawing for each caliper…Lotus M/Sport caliper can be found here… http://www.lotustalk.com/forums/attachments/f91/185830-308mm-lotus-sport-ap-racing-rebuild-kit-cp7635-1cd-sheet1.pdf
Area from that drawing is 18.1-sq-cm and the effective radius is 13.34-cm… go from there… assume line pressure to be equal and pad coefficient can also be the same when comparing brake torques…
Piston clamping pressure is just one thing, leverage based on caliper distance, friction of the pad (also dependant on current temps when mixing compounds), is another.
Will, I have had the same dilemma, and now after many changes and messing around, I think Dave’s setup is about the best you can get, I found now I have 315mm all round with 4 pots up front and original 2 pots on the rear there is just a little too much rear bias, to be fair it only seems to show up in the wet where rear locking has now become an issue. I am now going to use 308mm on the rear and 315mm up front. All this is with the standard master cylinder.
I’m no expert but really you’d want more a rearward bias in the wet, as you’re getting less weight transfer.
Any which way, a way to manually adjust bias is useful and considering they should only be used on the rear axle, more bias that you can bleed away is better
I have the same set up as Dave. I use a Viper separate spot handbrake. As supplied by JSR, also used by Ade.
I have no complaints about balance nor pedal travel on standard MC
