Brake Bias

[FATGTS-R]

First up I will give the current brake setup.

HR31 coupe with R32 GTR rotors and calipers up front with RB74 pads.

Rear has standard rotors and single piston calipers with unknown pads.

The problem I am having is the braking seems to be biased towards the rear. In an emergency stopping or even a hard braking appplication the rears will lock up first every time swinging the arse end around. Which is nerve racking and obviously not right.

Now I dont know much about braking systems but I would of thought that having the significant brake upgrade on the front would of made it even less likely to cause this situation.

My questions are: what could of caused this situation? Can it be rectified by taking it in to a brake place?

Lastly, could it be simply the RB74s in the front not being up to temp and therefore not biting forcing the rears to do all the work? It most commonly happens in the wet when I am driving very slowly and the brakes are obviuosly a lot colder.

[slow_r31]

its because of your front brake upgrade

the total surface area of your front brake pistons has increased, thus for a given pedal displacement the front brake pistons are going to be pushed less distance.

when u push the pedal x distance, its pushing the front brake pistons xy distance, and the rear xz distance, where y < z.

simple solution is an adjustable brake bias valve, or possibly different master cylinder from a car with larger front brakes.

sorry for the unconcise explanation, hope it can help

[Sydneykid]

Five choices;

1. Upgrade the rear callipers and rotors to R32GTR

2. Change the master cylinder to R32GTR, this may still give problems with the R32 rear callipers.

3. Change the bore/piston size of the master cylinder to R32GTR

4. Cut down the rear pads, this will reduce their effectiveness. You can "tune" the reduction by cutting off a little pad material, then try it, if still locking rears, cut a little more, continue until braking effort is equalised. Draw a diagram of how much you have cut off so that you can duplicate when you next buy pads.

5. Get a lower CoF rear pad material.

Hope that helps

cheers

Edited September 4, 2005 by Sydneykid

[blind_elk]

Nissan often puts a sort of pressure limiting valve in their braking systems. You could look at changing that over to a more favourable bias setup.

[GTS20X]

Fit R32 GTR rear calipers to the standard HR31 discs, that's what i done with my upgade. Only sign of lock up is from the front under extreme braking although i haven't investigated it much.

Carlos

[blind_elk]

its because of your front brake upgrade

the total surface area of your front brake pistons has increased, thus for a given pedal displacement the front brake pistons are going to be pushed less distance.

when u push the pedal x distance, its pushing the front brake pistons xy distance, and the rear xz distance, where y < z.

<{POST_SNAPBACK}>

I don't agree. The hydraulic pressure is what moves the pistons and provides clamping force. Physics says that the hydraulic pressure in a closed system is equal at every point in that system. Therefore, the pressure clamping the front pads is the same as the pressure clamping the rear pads, regardless of how far the pistons had to move. The effect you are describing is the change of pedal travel that comes with this sort of upgrade.

Might I propose that the cause is that the front is now working much better, causing increased weight transfer onto the front ie off the rears, so the rears don't have the load to make the tyres turn against the road surface, against the braking force. Therefore the rears now lock up more easily.

Throw a bag of cement in the boot and see what happens.

[slow_r31]

blind elk, what you said is 100% true, however if you look at your master cylinder and braking hydralic system you will notice that it is actually a dual circuit, and split front to rear. thus the pressure in the front circuit is unrelated to the pressure in the rear circuit, this is incase of a leak in one part of the system to retain pressure (and thus braking) in the other part.

[blind_elk]

Point taken

[Cubes]

I had this issue with my R32 GTST running stock brakes with Ultimate brake pads.

I would flush the system, brake bias would come good.

A couple of weeks later it would go crappy again, locking the rears.

I began flushing the system every few weeks. Finally it came good.

The brake fluid prior was extremely dirty and would constantly turn the fluid blackish.