Yellow73SB

My front brakes seem to lock up early, so I'm going to put an in car bias adjuster for the front.

Do I want all the brakes to lock up at the same time or front slightly before...etc.?

C3 BTW

0Randy@DRM

Rule of thumb, don't restrict front brake fluid flow. Work on the rear brake setup.

I don't know anything about C3 cars. But in all track cars the fronts need to lock up sooner then the back.

Randy

Solofast

A number of things can result in it being necessary to change brake bias. Changes to CG height, brake system changes and obvioulsly differences in tire sizes from front to rear can all have an effect on bias needed. In addition going from the poorer tires of yesterday to today's sticker rubber will shift more weight forward and can tend to make the back end lighter and need less bias than GM originally intended.

Is this a problem that has just come up, or is it something that you noticed in conjunction with some other changes to the car?

Some C3's had a rear pressure limiting system, and you should first make sure that, if your car has it, it is working correctly and isn't preventing pressure from getting to the rear brakes.

If the system is working properly, then ask yourself if you have made mods that would make the front end lock up earlier, such as bigger back tires than the fronts, or something like that.

You need to tune the brakes to have more front bias than the rears. Too much rear bias will have you looking out the side (or worse in the mirrors) to see where you are going...

Yellow73SB

I have bigger rear tires 27 vs 26.5 and the rear is about 1.5 inches taller

Good year slicks BTW

I wasn't worried about restricting the front because I have very little travel till I lock them up.

This problem just came up after I changed to hawk blues and also braided stainless rear brake lines instead of rubber. The fronts were already braided.

It seems like it stopped better with the discount auto pads because I could modulate the pedal better and the didn't lock up so fast. Also I have a hydraboost. I think I need a smaller MC.

The front doesn't seem to have a chance to dive down as it locks up. The front springs have a 660# rate with bilsten shocks.

Solofast

Could be that you aren't getting the back brakes hot enough for the better pads to start working... Maybe wasn't an issue with the other pads, but now the backs arent getting them hot enough to work...

Yellow73SB

Hopefully that's the issue

I have my first autocross this Saturday so we'll see

Solofast

When we run more agressive track pads for autocross we drag the brakes to the starting line from grid before the first run to get some heat in the brakes so the pads will work as intended. Didn't realize you were using them for autocross. Higher temp pads aren't the best for autocross first runs since they are usually cold and you are asking the pads to do something that they weren't intended to do. We had bias problems with our BSP car and often found we were adjusting the bias one way or the other depending on the course, the amount of heat in the brakes and the phase of the moon...

Maybe a less agressive pad is in order that will give you consistent braking for the range of temps that you are seeing for autocross work.

AU N EGL

It is a tire issue.

When I ran the GY G19 slicks my fronts locked and I flet like I was doing a motocycle stoppie ( standing on the front tire) when I hit the brake hard.

The G19s have a lot of grip for traction and for stopping.

To releave this I went to a slighly softer front brake pad vs the rear. This helped changed the brake bias reward slightly with those tires.

Independent1

I think it has already been said but bears repeating: You want the fronts to lock up before the rears, but not too much so you getting maximum braking power.

What width tires are you using front versus rear?

Seems like you are transfering too much weight to the front when you brake hard causing front lockup. A wider tire in the front, if you can do that, might help. Anything to reduce weight transfer forward would help also.

Slalom4me

I agree with Solofast about reconsidering the suitability of the pads for
the application. I defer to AU N EGL's greater knowledge about the
tires.

Another consideration is whether the rear circuit may benefit from
further bleeding. It was opened to upgrade the hoses and perhaps
some air remains? This could contribute to the shift in balance to
the front.

Did you suggest a smaller master cylinder because of the increased
gain this would provide? With a Hydroboost such as those sold by
Hydratech, I would expect that you do NOT lack for clamping force.
My vote is a smaller m/c bore would give more travel and additional
modulation or 'feel' between application and lockup. It would be
important to check that there was still sufficient m/c piston travel to
ensure that the piston would not bottom under the worst case
scenario (worn pads/rotors, soft hoses & ect.)

James Walker's book is a current look at the subject, $19 @ Amazon.

High-Performance Brake Systems: Design, Selection, and Installation
S-A Design (February 1, 2007), 144 pgs
ISBN-10: 1932494324

Yellow73SB

I think I know what is wrong

I didn't turn the rear rotors. And they looked pretty glazed over. I'll pop them off and have them machined today. It's worth a try as I don't have to pay for machining

Yellow73SB

I don't think nose dive is the problem. I didn't notice any when breaking them in.

Same width at 11.75

I'll try a small bore then

I'll buy that book now thanks