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MG Midget and Sprite Technical - runout disc

The right brake disc was crooked 0.40 mm and too thin 7.22 mm. I replaced the brake disc, the runout is now 0.20 mm. It must be < 0.1 mm. My conclusion is that the hub is swinging. Is 0.2mm acceptable for a Sprite? The left disk has a runout of 0.002 mm.

Flip
Flip Brühl

Two checks to do:

1. Are mounting faces clean?
2. Have you tried rotating it 90, 180 and 270 degrees? (Easy to try than 1, no need to remove the hub).

Cheers,
Malc
Malcolm

Flip,

2 micron runout? Has an extra zero slipped in there by accident?
David Billington

No extra zero, At first I could not believe the micrometer. The needle did hardley move. What I saw were the groves.
I already cleaned up both faces and rotated.
Do I have to buy a new hub? (Expensive!)
Flip Brühl

Flip. I agree with cleaning up the mounting surfaces and checking them out as a first step. The runout figure you state as being the maximum translates to .0039", about the thickness of a coarse human hair. And this is measured near the outer edge of the disc/rotor which would emphasize any dirt or rust at the mating surfaces of the hub and rotor joining surface.

I have only seen one instance of a warped, our made out of specification, hub where the hub was causing run-out even after the rotor was turned on a brake lathe. We turned the hub and rotor as an assembly and corrected problem of run-out for the assembly.

The other thing that I would check is your measuring equipment. You note a measurement of .002 mm. A measurement of .002 mm is about .000078 inches, something that only precision laboratory equipment would be able to properly measure. I wonder what your readings would be with another measuring instrument?

Les
Les Bengtson

Surely the test that matters, is the foot test.

Can you feel it?

https://youtu.be/jZpoDbFHwE4?feature=shared



anamnesis

David an Les you were right! one zero to much. I saw it today when I went back to the garage.

Flip
Flip Brühl

Flip,

I assumed that might be the case as I've seen it happen before. The indicators I use most of the time are 0.01mm/division and fine for the vast majority of what I do including checking disc runout but I do have a micron indicator used mainly for setting accurate angles like this afternoon setting the lathe for a number 5 Morse taper, tomorrow it's coming out again for a 3.5"/foot taper.
David Billington

Just curious - I wondered how many people actually check disc runout. I never have (on any car) as I don't have a dial indicator.

So I'm with Anam on this - if the pedal feels OK, that's good enough for me.
Bill Bretherton

Bill. The only times I have had disc/hub combinations that allowed you to feel the pedal pulsate, you had significantlfy more than the factory standard for run out on the outer edge of the disc. In that particular instance, the hub was the root cause of the problem and, since no other hubs were easily and quickly available, the rotor/hub assembly was turned on a brake lathe and the run out of the system brought into specification. I suspect that the factory spec is more of a guideline than a hard and fast rule.

Les
Les Bengtson

The brakes are ready. The runout is 0.2mm and I can't feel that in the pedal or when braking. I'm braking in slowly. I was used to doing that quite vigorously and I think that made the brakes too hot and warped. I was shocked at how hard I had to pedal the first few times, but after a few kilometers it is almost normal.
Flip
Flip Brühl

This thread was discussed between 11/12/2023 and 24/12/2023

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