Rear brake rotor

74westy

74westy

Member
When replacing the rear suspension bushes on my 3500 P6 I noted that the brake rotors had a couple of slight cracks in the apex of the curved section between the pad area and where the rotor bolts to the diff. I noticed these due to a weeping of oil form the opposite side due to a slightly leaking diff side oil seal.

What is the safety factor of these cracks? They are only small about 25mm long in two places. Myth instinct tells me to replace them. Has anyone had similar experience of cracked rotors in a very bad way?.

Craig
 
Hi Craig,

I have never seen a brake disc (rotor) with a crack before, but given the consequences that would result should it shatter, I can only advise that you replace them, and soon.

Ron.
 
I've seen loads of rear discs that have cracked there. Every time I removed them I had a test procedure to pick up even the smallest of cracks before they got to the point of being a real problem.
 
Were the cracks a consequence of corrosion Harvey?

Aluminium alloy parts when subject to an appropriate level of cyclic loading can crack and fail through fatigue, but the brake discs are steel, so what is considered the primary reason for them cracking?

Ron.
 
SydneyRoverP6B said:
Were the cracks a consequence of corrosion Harvey?
Click to expand...

Brake discs crack due to thermal cycling. Motorbikes have the disc mounted 'loosely' on the hub to allow for controlled expansion.
 
That's interesting, thanks for that Pat.

I'll have to have a closer squizz next time.

Has anyone had a disc completely shatter and separate from the central mount?
 
Hi,

We have, now we do suspect that it was contact with the road that finally broken the disc but it possible that some cracking existed before the final failure.

Picture shows the "get me home" fix, I'm told the brakes felt OK like this but you wouldn't want to drive for too long like it.

Thanks, Tim
 

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^^^ You could have removed outer pad so it didn't fall out and get lost and then driven home like that. There would have no difference in the feel of the pedal, just massive unevenness in the rear brakes, which you had doing it your way anyway.

I've had cars come in with one inner pad missing after people have started a pad replacement and given up, and Vaultsman had an inner pad fall out completely IIRC.

Why is one of the links missing from the handbrake?
 
You could have removed outer pad so it didn't fall out and get lost and then driven home like that. There would have no difference in the feel of the pedal, just massive unevenness in the rear brakes, which you had doing it your way anyway.

Not sure I understand how having no disk between the pads could not effect the feel of the brakes? The small piston within the caliper would surely go full stroke with no resistance, leading to a long brake pedal, before any pressure built within any of the remaining calipers? The full stroke movement of the piston would also activate the self adjustment mechanism as well, would it not?

I've had cars come in with one inner pad missing after people have started a pad replacement and given up, and Vaultsman had an inner pad fall out completely IIRC.

There are some crazy people out there :shock:

Why is one of the links missing from the handbrake?

Not sure what bits Gav removed to make the running repair (if any), this was on one of the events he drove and I stayed at home :cry: This car had a super handbrake, certainly by P6 standards, it could lock the rear wheels on all but the most bone dry of roads/tracks.

Gav is on Holiday at the moment, he may add some more detail when he returns

Thanks, Tim
 
Kman1600 said:
Not sure I understand how having no disk between the pads could not effect the feel of the brakes? The small piston within the caliper would surely go full stroke with no resistance, leading to a long brake pedal, before any pressure built within any of the remaining calipers? The full stroke movement of the piston would also activate the self adjustment mechanism as well, would it not?
Click to expand...

You need to think about how the caliper is assembled. If you remove the pads and pump the pedal, the hydraulic piston will move until it hits the stop washer, at which point it stays there and it's just the same as if the piston was seized in the bore or if you clamped a hose. You'll have one maybe two pumps on the pedal to get it there, just the same as you would if you forgot to pump the pedal after fitting front pads. If you don't have a heart attack if that happens when driving, then you're good to go. (Obviously not adviseable, but if it's that or walk what do you think most people will do? Especially as they won't know what's happened and why the pedal went long and then came back again.) You just lose the handbrake, but the footbrake feels the same, it's just that it's only working the one caliper.
 
Hi,

I have to say that is not how I see the caliper working.

As far as I can see the internals of the caliper are sprung loaded and will return the hydraulic piston back to its home position each time the brake pedal is released, so the pedal will be long for every fresh application of the brakes. It’s been a few years since I rebuilt them so maybe I’m remembering things wrong?

With the inner pad in place the self-adjust will, as far as I see it, try and correct for the excessive movement each time and with no disc in the way it will keep doing this (a little bit at a time) until it can do it no more, ie falls out or bumps into something else. If there is no inner pad I can see how this may not happen as the tang on the screwed “piston” won’t have anything to stop it just turning around and therefore the “piston” could simply rotate with the adjuster.

I’m pretty sure all of the above is why Gavin went to the bother of the part disc cable tied in place solution to get him home.

Thanks, Tim
 
Well, I've seen it on several occasions, and although you describe the operation of the self adjusting mechanism correctly, what actually happens in practise is the piston ends up against the stop washer and stays there, probably because the mechanism over travels.
 
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