Brake bias

[jimmystratos]

Since my car is SORN, I can't go and road test to check where the balance is set. I've read that you can check which end locks first by jacking up one side, and getting people to turn the wheels while gently applying the brake to see which locks first. Does this work? Since I've got an LSD on the rear, i suppose I'd have to jack the whole back end up. Any other ideas?

[GreenV8S]

jimmystratos said:

Does this work?

Total waste of time. There are a few simple tests you can do to check the static and dynamic brake balance, but they all involve driving the car or using expensive test equipment.

[HiRich]

You can get a passable starting point by jacking the car up and turning the wheels - I have used the technique on a couple of competition cars between sessions. It is far from perfect, but is better than being totally blind. It requires that you have a race-style set-up, no load compensator and probably no servo.

First, given that the car's off road, bleed the system. Next, I'd jack the car up completely.
"Driver" presses the pedal progressively while "Mechanic" turns a front wheel. Increase the pressure until the wheel is almost impossible to turn, then hold firm at that pressure. Now check the other front wheel, which should be similarly hard to turn.
If the front's OK, check both rear wheels. These should have noticeable drag, but be well away from locking (how much comes from experience). Holding the pressure constant on the pedal can be tricky for the time to go around the wheels, so if the driver has a problem, start again. And always double check when you think you are satisfied.

Most importantly, remember that it's only a starting point, so get a better set up asap (rolling road, test session, even a run around the paddock is better than nothing).

[JWB]

I agree with HiRich. Its a good starting point and one I use before venturing onto the road to do a better setup.

[jimmystratos]

Thanks HiRich, that worked well to give me a starting point. I was able to use a clamp on the balance bar to squeeze it towards the chassis, so even pressure was no prob. Just add another turn until the right point is reached.

[jonamacg83]

I dont really agree, granted it will tell you which brake 'bites' first, but when the car is in motion it is far more to do with the amount of clamping force that is applied. Any tiny amount of clamping force will be enough to lock the wheel doing the above test, and will not give you an indication at all.

[Crash Test Dummy]

Surely this theory only works if the pad/disks all brake evenly in a linear fashion.

[GreenV8S]

In my view feeling the brake force by turning the wheel by hand tells you the caliper is connected and working and that's about it. In use the brakes will be producing somewhere in the region of 500 lb ft of brake torque per corner and by hand you will only produce a few percent of that. At these low loads, the stiction in system becomes significant and you won't be getting any indication of the brake balance when they're used in anger. The person who designs the brake installation should already know what sort of line pressure and displacement they're expecting and for adjustable brakes roughly where the static balance is likely to be and what front/rear pressure curve they want. You shouldn't ever need to make this up from scratch at the track side.