Warped brake disc(?)
 

Just saved some more money re. the GV after getting an insurance quote from Admiral on renewal saving over £100 from last time. This time re. so-called "a warped disc" on front driver's side brake, I've saved even more money by not changing the disc. As mentioned in a recent posting I was getting a steady "thump-thump-thump" through the brake pedal during moderate braking.
I researched this, and many pros. were saying there was no such thing as a warped disc. Found this was right. But there is a similar prob. that's very like warping. After having the GV. MOTd., this unpleasant effect appeared, so I worked out from the pros. that the disc wasn't warped, but acquired "variable thickness disc" abbreviated to VTD.
The pros said this was something transferred from the brake pads to the brake disc during heavy braking from a very hot disc, ESP. when the brake is left on after a full stop. Ah-ha! I thought, that's happened from the MOT tester's rolling road, and whatever it is, I'm going to grind it off, with no need to dismantle the brake or replace the disc and pads.
So I jacked up the GV's front driver's side wheel off the ground to see what's what. Selecting neutral, I was able rotate the wheel by hand, detecting a regular slight grab in one spot. So off with the wheel and chalk-mark the slightly binding spot as it passed into the brake caliper and pads.
Careful observation of the disc at that spot revealed absolutely NOTHING!
Taking notice of the pros. advice to use only garnet powder discs on my drill to grind off this invisible contamination, I did as instructed and found the disc was EXTREMELY hard, esp. at that spot. Hence the advice about the garnet discs for my drill.
It took some time to get the contamination off, but it was gone eventually and as subsequent road testing showed -- so had the "thump-thump-thump" through the brake pedal.

It seems that the disc got slightly thicker at that crucial spot with a layer of 'cementite' produced by the brake pad pressure on a very hot disc after stopping fully (MOT). This cementite is iron carbonite (Fe3C) and is as hard as some ceramics, only being produced at high temps.

So just now I'm a very happy-chappy thank you -- I think I've saved a couple of hundred quid here.

Leedsman.

Interesting, you went out and solved something that others would say didn't exist. But you went ahead and proved it, for us to take note. We'll remember that one for future issue...
Goggs..

nice one

Regards
Richard

Great thread! And very informative indeed...

Addendum to posting:--
Apologies for calling the substance iron carbonite, it should be called carbonide. It's similar to ferrite, the material used for the magnets in your loudspeakers.

From researches, it seems two things have to be present to cause this stuff to be on the brake disc.
There has to be high temperatures -- and there can be very high temps. on a brake disc.
There has to be high pressure -- and there is, tons of it pressing the brakepad onto the very hot disc.

Can you stop this carbonide forming? A qualified yes here. If you are an "enthusiastic" driver using the brakes frequently, or you've been going down a long hill and got the discs very hot, RELEASE THE BRAKE PEDAL AS YOU COME TO A COMPLETE STOP. This action removes one of the necessary causes of the carbonide "thump-thump-thump" from the brake pedal, the tons of pressure on one section of the hot disc. Without both causes being present (heat and pressure) carbonide cannot be formed on the disc surface.
However, one would suppose very light braking after the full-stop for safety's sake would not produce carbonide.

Another thing about a carbonided surface-section on your brake disc -- as soon as you know something is wrong, do something about it immediate. If you just leave it to progress, it will reach a self-propagated point where it cannot be got rid of, getting worse and worse. The only solution then is to discard the disc and fit new. Expensive. A well worn and thin disc is more likely to get carbonided simply because the mass of the disc is less and therefore heats up faster and to a higher temperature, other things being equal. Disc thickness and mass is a problem because it's all unsprung mass. Make the disc a lot thicker and yes you'll get a longer life from it, but then the car's handling won't be as good. Excellent case for inboard brakes here.

If new discs must be fitted, they have to be bedded-in carefully, otherwise the same problem is going to rear it's ugly head again in the near future.

Leedsman.

Totally agree with Leedsman see my thread below from a couple of years ago.

https://www.chryslerforum.com/forum/...-judder-13482/

I have started to think that it affects the rear more than the front in that the rears might not get that hot to create an even surface for adhesive braking....

Rodger

No matter what caused a rotor to get hot in one spot and made the thickness change, its still classified as a warped rotor. Heat, bad guide pins/bolts, and many other factors can cause a feeling through the brakes pedal and if bad enough even through the steering wheel.