ABS technical question

[Mark911]

I've installed Brembo F40 calipers and 14" rotors front and Lotus calipers and 13" rotors rear. My calculations showed I have about 20% too much rear bias compared to stock and road testing has verified this. So, I have three choices, 1) using different pads front and rear, 2) yank my ABS, stock M/S and booster and add a bias bar pedal assembly, 3) add proportioning valves to the rears. Obvisously, the proportioning valve is the standard answer. However, since two valves are needed this required a set of "matched" valves. In talking with the various mfgs (AP, Willwood, etc), they do not recommend a tandem setup since output pressure can vary as much as 15% between valves at any given position (lever type). No matched valves. Also, I'm a bit concerned about any kind of valve being downstream of the ABS as well.
The best solution would be to put the valve in line to the rear brakes before the ABS system. But that darn diagonally split system makes this more challenging. It would be relatively easy to re-route the ABS brake output lines and repin the solenoid connectors so that the system would be split front and rear. However, I noticed that the caps retaining the modulators compression springs are all different heights. Hummmm....... The caps seem to vary in height in relation to brake line length (resistance?) with the shortest being the left front and tallest the right rear. This is where I start speculating. I'm assuming that the short cap (left front) provides the greatest spring rate (assuming the springs are all wound the same and vary only in length) which results in the least change in volume in the modulating chamber and the least delta pressure differential between caliper and ABS chamber given a specific solenoid "on" time. The others chambers would receive a greater change in volume and delta pressure based on their given spring rates (I'm assumping preload is equal on all springs). My conclusion is that the longer the brake line the greater the delta pressure must be to flow the same volume fluid given the same solenoid signal due to line resistance.
Assuming I'm correct, I should be able to swap caps (probably machine custom ones) to again match the correct spring with the correct line. Now the big question; not having actually seen the inside of one of these systems, I'm not sure if everything else is equal. Are all the solenoids the same? Are all the modulator assemblies the same? etc. Any differences between front and rear solenoids or modulators would change things to the point where it probably would be impossible to make the switch from diagonal to conventional split system.
Has anyone taken one of these buggers apart and studied it? Any help would be appreciated as I can't seem to find any indepth technical info on this ABS system. Thanks, mark

 

[patric s]

why not to reduce rear brake pad area? just cut or grind and you can achieve exact bias

 

[Briank]

Two things:

1) Your ABS is 4 channel so it will control the braking on all four wheels independently so you should have no problem there.

2) Reducing the brake pad area and or using different compound in the rear should be an easy fix.

 

[Mark911]

Two things:

1) Your ABS is 4 channel so it will control the braking on all four wheels independently so you should have no problem there.

Yes, this is true. However, the ABS is working way too hard in the rear now, even on relatively easy (racing wise) braking conditions. When the ABS functions, it's difficult to get a good "feel" in the pedal due to all the pulsing. Under good traction conditions, the ABS shouldn't be working unless it's a true panic stop. Maybe other people with oversized rear brakes don't mind when the ABS cuts in prematurely, I do.

2) Reducing the brake pad area and or using different compound in the rear should be an easy fix.

Reducing the pad area only makes the pads wear faster and run hotter, it shouldn't affect the bias as it's simply a pressure over area relationship and the coefficent stays the same (unless the pads are way over heated). Running different compounds is looking better to me so long as the different pads have the same coefficent curves over speed and temperature. My only problem is it might take several trys to get it right and at almost $300 a pop, it's could get costly (and still no ability to adjust for different track/weather conditions).

 

[Larry Bastanza]

Mark911,

If I can make a suggestion: Speak with Andie Lin at www.cobaltfriction.com . I am confident he can assist you in regard to compounds and choices to suit your needs.

HTH,
LarryB

 

[ncdogdoc]

Also talk to Larry at www.Carbotecheng.com, he does this for SCCA guys all the time. You can call him at 877-899-5042. He makes his own pads, plus carries Hawk, Axxis and will even give advice on Performance Friction. Plus, NSXCA members get a 10% discount.