I just reciently spun out doing 80 down a mountain road. Luckily everything was fine, i didn't hit anything. The only damage was my majorly flat spotted tires. But what I couldn't figure out was why I spun out other than my own irresponsibility. Well today in the rain, i was driving around, and I decided to try to have a little fun, tailed out, and tried to lock up all 4 wheels in hard breaking. Well, I found out that only my rear wheels would lock up if i pushed the brakes in hard enough. The fronts work, but the rears would lock. This is BAD. I have really no experience with breaks other than changing pads and bleeding them. Can anyone give me some insight on steps to take to fix this? I checked my front pads, and they have ALOT of life left, the rears look good for another couple thousand miles. Thanks.

 

If you are in a turn and squeeze the brakes hard enough, the rear tires will lose traction since you were already using a large portion of the traction for all tires to hang on in the turn. When you apply the brakes, weight is transferred to the front of the car, reducing traction in the rear and increasing it in the front. If you brake hard enough, the unloaded rear tires will lose traction (lock and slide) first. If this is what happened, it is most likely a driving technique issue rather than a problem with the car. It sounds like this is what happened, since you already had the tail out when you hit the brakes. Next time, counter-steer to straighten the car out and apply light throttle to shift some weight to the rear axle to regain traction.

If you were braking in a straight line, the fronts should lock first. Try it in a straight line and see what happens. If the rears are still locking first, you have an issue with the car to resolve. Stuck front caliper, front tires that grip better than the rears (in the rain, especially), lightened rear end (empty tank, spare tire removed, interior removed, etc.) with stock or heavier-than-stock front end, parking brake dragging, or higher friction rear brake pads could possibly do this. But most cars are heavily front-biased so something pretty significant would have to be in effect to make the rears lock first in a straight line.

-Max

 

Pay for some driving lessons...
What you're describing should not be done on public roads.


-Ted

 

Yeah I tested it in a number of conditions, straight line down hill and up hill. The rears lock up and the fronts do not. I even had a friend look while I breaked next to him. The rears locked up and the fronts wouldn't lock up. I looked at my pads again, and there is alot of break dust stuck on the pad and rotor. I'm going to change front pads and see how that helps. Oh, yeah I understand that I shouldn't be doing them on public streets. But I live i the middle of no where, after 10pm, no one is anywhere ever. So that leaves alot of free space to enjoy. I never do anything stupid when I see a pair of headlights on the road other than myown.

 

Something is seriously wrong if your rears are locking first if you brake in a straight line on dry pavement. About the only simple/easy thing I can think of that would do that would be if you had high friction coefficient (track) pads on the rear, and stock (or worse than stock) pads on the front, or grippy tires on the front and rock-hard ones on the rear. In general, I would suspect the fronts are not gripping well enough, rather than the rears gripping too well, but if the car is braking in a straight line (i.e. not pulling to one side under braking), it would have to be something that affected both front brakes equally. Are there any mods to the braking system? Is there a single line out from the master cylinder before it splits to the two front calipers that could be pinched? I don't know the FC, but if the brake plumbing is such that one chamber of the master cylinder operates the fronts and the other the rears (rather than a dual triangular setup), a leaking master cylinder seal could do this as well. Listen to max - you need a full diagnostic on the brake system.This isn't safe even for sedate street driving.

 

I've been doing some research on other forums, and other people have been having the same problem with older FC's. Tomorrow I'm going to bleed my breaks to see how that helps. My front pads look like they have alot of life left, and they are covered with break dust. So its obvious my front calipers aren't working as hard as they should. Someone mentioned they fixed their problem by installing a break proportioning valve, so I'm considering that too.

 

I have never seen it.

If you're talking about cars with original suspensions with dead shocks, then this is possible.


-Ted

 

Ive seen this happen on a vehicle with the wrong brake shoes, pads on the rear.

 

Don't do a prop valve unless you are replacing a bad stock prop valve (if you find that to be the cause of the rears locking first). That would be a bandaid on a gunshot wound.

-Max

 

If new pads and bled brakes don't fix it then I would say that points to the brake proportioning valve as the problem.