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MG MGB Technical - Brake disc concentricity problems


Guys,

I have a bit of a problem, I have recently fitted new discs and part of the instuctions was to ensure that once on the vehicle there is no more run out than 0.15mm. I measured my run out with a DTI and seem to be getting 25Thou (0.5mm), I guess that if I use the car like this im going to have horrible vibrations through the steering.

Having taken everything back to pieces and tried every orientation of the disc on the hub I have measured the hub up and seem to have 30Thou of non-conventricity. I have phoned around a few places to see if I can get the assembly ballanced but am having no joy.

What do people think to taking a skim off the OD of the disc (using the hub as a centre point) to align
the disc edge to the hub centre? Any other ideas massively appriciated.

Regards

Mark
Mark

Mark. You can either mount the hub between centers and make a facing cut on the rear surface, where the rotor contacts it, or you can use a brake lathe to turn the rotor after it has been attached to the hub.

The latter is the easier process to perform on a "one-off" basis. But, if the rotors are changed out in the future, or the relationship of the rotor to the hub changed, you have to skim cut the rotor again.

Making a facing cut on the hub corrects the root cause of the problem and, in the future all you would have to worry about would be any run-out on the rotor.

I have seen both processes used with success. If the car is for pleasure use only, and you do not expect to have to change the rotors again during your ownership, skimming the rotor is the least trouble and any larger mechanical shop should be able to accomplish it.

Les
Les Bengtson

Nopt sure how you are measuring this. The run out is to to the braking surface of the disc, not the rim from the rotational centre. I've never measured that and do not think 0.5 mm is going to have any effect. If it is the braking surface going in and out by 0.5 mm as it goes past a dial gauge measuring in the horizontal plane then its a big problem.
Stan Best

Stan, I was wondering if that's the way Mark was measuring, too. Clear it up for us, will you, Mark?
Tom


Cheers guys,

Was measuring the "wobble" on the rim of the rotor. However I also have run out on the face of the disc of 35 thou. I think im going to have to get the whole lot to a machine shop to have the rotors skimmed to suit the hubs.

I think the cause of my problems may be partly due to cheap secondhand parts off e-bay. An expensive lesson I think!

Mark
Mark

If it's the hub face that is out of true then the whole thing has has a nasty crack at some time. I assume that it's only the one side you have the trouble with and this being the case I would try to source a second hand hub from MGB Hive or the like and this would completely eliminate your problem without resorting to non standard machining. At the end of the day you should be able to achieve a disk face run out of better than 3 thou and this will giva a satisfactory braking condition.
Iain MacKintosh

This thread was discussed between 26/03/2008 and 27/03/2008

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