Toymods Car Club Inc: Tech & Conversions => FC Caliper downgrade

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improvedae86
Forums Junkie

Location:
Melbourne
Registered:
January 2003

FC Caliper downgrade	Sat, 30 October 2004 00:19

Found this really good write up by Gabriel Tyler on a small US forum . I have also seen the reverse of this theory on a S13 sr 180sx which had GTR front calipers fitted . It had no rear brakes , due to the smaller area of the Four piston compaired to the Big single piston used normally on that car . And thats not as easy to fix as more brake power to the rear . Basically don't just bolt on shit thinking its better , it might not be safe . Well Heres the Write up By GT . Did some research, some of you may find this interesting...others may not. It was brought up in my thread about our big brake kit, but I figured this particular info was more pertinent to the Tech Talk Forum. MidTenn Drifter brought up the issue of the FC front caliper upgrade causing the rear calipers to lock up first. So I figured I'd look into it. I did some research on the FC calipers today and figured out exactly why they make the rears lock up first sometimes. Suprisingly enough...shock shock horror...the FC calipers are NOT an upgrade for the AE86... I know I know...but they are 4 piston etc.... But lets to the math...Remember, you have to use Pie R Squared and all that to get actual surface area of the piston. 2x1" pistons does NOT equal a single 2" piston for surface area. FC calipers use 2 pistons pn each side that are 1.375" in diameter Making a TOTAL Piston surface area of 2.97" per side. Remember that with a Fixed Caliper, you can't add the force together from both sides, since the SAME master cylinder is pushing both sides. All that having 2 pistons per side does is split the force in half that goes to the pad. For example, a 2 piston sliding caliper puts EXACTLY as much force on the pads as a 4 piston fixed caliper. Every time you double the number of pistons, ALL OF THEM MOVE HALF AS MUCH. Stock AE86 Calipers use 1 piston 2" in diameter Making a Total Piston surface area of 3.14" The stock AE86 caliper is a sliding caliper and puts out an equal amount of force that a 2 piston fixed caliper with two 2" pistons would. Again, all that multiple pistons do is cut the force aplied to each piston in half. What all this means is that the FC calipers have LESS clamping force than the the stock AE86 calipers!!!!This could easily cause the rears to start locking up before the fronts. Now the FC kit does still offer the larger rotor, which equals more leverage and more braking, as well as more heat handling capabilities, but all in all, the caliper is not something to get up and shout about. Anyone that want's to check my math or just mess around with piston calculations etc... there is an excellent calculator here: http://sth2.com/Z-car/Brakemath.xls A reply to the post : neat info. But I'd argue that the bigger rotors would still make this mod worth while. Better braking generally comes from better heat dissapation which the bigger rotors will do. But yes, this mod should be done with a an adjustable brake bias valve to get the front locking up first again. A more suitable master cylinder might help restore the pedal pressure lock up point back to stock too. Another Reply : well it may be an upgrade but for the price of the adaptor kit and all the parts needed you could probably get alot more out of your oem brakes with that money Another Reply : Gabriel, I like your approach in trying to analyze this braking problem. However, I think you have misinterpreted a couple of points. My comments are included below, encased in { }. ********** But lets to the math...Remember, you have to use Pie R Squared and all that to get actual surface area of the piston. 2x1" pistons does NOT equal a single 2" piston for surface area. FC calipers use 2 pistons pn each side that are 1.375" in diameter Making a TOTAL Piston surface area of 2.97" per side. Remember that with a Fixed Caliper, you can't add the force together from both sides, since the SAME master cylinder is pushing both sides. All that having 2 pistons per side does is split the force in half that goes to the pad. {Not quite true. The force is equal to the hydraulic pressure divided by the area, so if the area is the same the force is equal.} For example, a 2 piston sliding caliper puts EXACTLY as much force on the pads as a 4 piston fixed caliper. {but only if the area on each side is equal.} Every time you double the number of pistons, ALL OF THEM MOVE HALF AS MUCH. {The distance traveled is a function of how much the piston(s) retract, and perhaps the rotor run-out. In an opposing piston setup, each piston only needs to move half as far. But, this doesnt affect the braking force.} Stock AE86 Calipers use 1 piston 2" in diameter Making a Total Piston surface area of 3.14" The stock AE86 caliper is a sliding caliper and puts out an equal amount of force that a 2 piston fixed caliper with two 2" pistons would. {That is correct.} Again, all that multiple pistons do is cut the force aplied to each piston in half. {This is not correct. As you stated above, the SAME master cylinder is pushing on both sides, therefore the pressure and force is identical provided the areas are the same.} What all this means is that the FC calipers have LESS clamping force than the the stock AE86 calipers!!!!This could easily cause the rears to start locking up before the fronts. {This is correct.} {Because the brakes have been modified the front/rear braking ratio has changed. Perhaps an easy fix is a brake proportioning valve, which allows the amount of braking to the front and rear to be adjusted. Hope this helps! [Updated on: Sat, 30 October 2004 03:47] [Report message to a moderator]

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