[Guest]

I have developed a warped rotor sensation. This can be felt in the brake pedal it's self. If stopping slowly at low speed, I can feel the pedal pulsate a bit. Also the stopping power feels like the pads are grabbing then not grabbing as well then grabbing then not grabbing so well in relation to the revolution of the wheels. At high speeds it is a very fast sensation and adds rumble into the cabin. Also some minor brake fluid had been pushed out the top of the reservoir.



A few weeks ago I ran the brakes hard to clean the surface off. Had (1 year old)Brembo rotors with Axxis Metal Master Pads at the front. Stock rotors and pads at the back. Soon after this I developed this problem.

Since then I took off the front pads and sanded them thinking they were glazed. No improvement.

I put on new front Brembo disks and Greenstuff pads. Slight improvement but sensation still very evident. Also brake fluid still got pushed out the fill cap on the reservoir. The brake fluid level is fine.



Could the rear disks be so warped???? They are stock (7 years old). Any ideas? Any test things I can check?



1998 SE Turbo 143k km's.

 

[PMI]

One thing you can do is look for uneven wear of the rear pads. The front calipers "float" and have only one piston. The rears are "fixed" and have two pistons, one on each side. If the rear calipers start to rust (like mine did), the pressure from the two pistons may start to vary, so the pad on one side pushes against the rotor with a much greater force than on the other side. This is bad for the rotor as well as the caliper and if it goes on long enough, the wheel hub.

The easy way to tell is to take the pads out and compare the wear on the two sides. A small difference (up to 1 mm) is ok, but anything larger is not. There is also a previous thread with a note about measuring the rotor to tell if it warped.

 

[rotate]

I think when people claim that their rotor is wrapped, that's a misnomer. You need lots and lots of heat before you can wrap metal that beefy. Unless you're racing, you're not likely to warp the rotor.

The feeling of pulsation is more likely to come from uneven wear of the rotor (bad quality casting, work hardening, seized caliper pin, etc). The only way to determine this is to measure the run out using dial indicator on BOTH sides of the rotor with the lug nuts (bolts) tightened to the proper torque. You should also measure the thickness of the pad so that it's even on all spots, since this too can cause pulsing sensation, especially during light braking.

The problem is that most mechanic won't go through the trouble of making these measurements and determining the root cause. They'll tell you that it's cheaper for you to replace the rotors/pad.

If the rear rotors are stock and 7 years old, then I think it's due for replacement.

 

[earthworm]

I'd be a bit suspicious of the brake fluid - even if it has nothing to do with the pulsations/rumbles,etc.

Evidently there is too much fluid in the reservoir - the Max mark is poorly defined- but it is there and the fluid should only be at that full mark when all the pads are new.
This must never be "topped off", but allowed to become lower due to pad wear.
When the Min level is reached, then it is time for new pads - or at least a mearsurement...

 

[Guest]

ok. I will check the rear pads tonight if I have time. The brake fluid level is normal and I have never toped it off.
What do I do to the calipers if they are "rusting"? What do I look for? Can I clean and lube?
I suspect the problem has to be in the rear since front is all new...again.:roll:
Would this uneven pad wear cause this pulsating sensation?

Thanks!

 

[ragtopcav]

Nearly all brake discs are cast vertically, if the thermal gradient within the mould at solidification is incorrect then you have rotors with differing graphite structure within the iron that'll wear at different rates, expand and contract with heat differently and have variable friction co-efficient, to make matter worse this can be on new rotors from the moment they're bedded in or can alsso manifest in old rotors through wear; scored discs can cause a wobble feel also as can patchy corrosion.

Just to clear up a myth, a warped rotor will not be felt through the pedal because it's thickness is not varying is it? so it cannot push back against the pads, a truely warped rotor [on the front] gives a steering wheel vibration when the brakes are applied; due to the metalurgical gumph above you can have discs that vary in thickness when hot but return to even thickness when cold so you can never verify where the fualt is unless you put the car on a brake dyno [UK MOT testing stations have these, an MOT station can spot which rotor is manky even at full temperature due to this]

 

[PMI]

Just to clear up a myth, a warped rotor will not be felt through the pedal because it's thickness is not varying is it? so it cannot push back against the pads

Is that correct? (we have fixed rear calipers) I am hardly a metalurgical expert, but won't an uneven or wobbly rear rotor present a variable resistance to the pad and pistons on the two sides, because the caliper is bolted to the back plate?

 

[rotate]

The rear calipers are fixed but the pads are floating with two pistons, so it should have a very similar dynamic characteristic as the front calipers.

I think a warped rotor can be felt, since the floating caliper is now forced to slide back and forth, and this imparts momentum to the piston.