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Rear brake disc uneven wear

 
Old 07-27-2011, 09:41 AM
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Leon Speed
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Default Rear brake disc uneven wear

I have refurbished all brake calipers with new pistons and seals two years ago. New brake lines. New rotors rear. I have foolishly ignored the old adagio "always change rotors and pads at the same time"

The right rear doesn't look to good; left rear looks ok. I have maybe driven 2000 kms since changing out the rotors.

So what is the best course of action? Do I leave it and eventually the pads will form to the rotors, or do I change the pads?

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Old 07-27-2011, 09:52 AM
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turn the rotors, new pads.
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Old 07-27-2011, 11:13 AM
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Originally Posted by Ducman82 View Post
turn the rotors, new pads.
+1 ... bare minimum off the rotors to true them and new pads.
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Old 07-29-2011, 08:06 AM
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Thanks guys.

Turning rotors is not common here, I'm not sure whether that is due to safety/regulations/insurance or just because it's more attractive to sell new rotors.

I am contemplating just installing new pads, or would I make the same mistake all over again?
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Old 07-29-2011, 10:11 AM
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Tom in Austin
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Our rotors tend to have 2 mm from new to minimum (like 32 to 30 mm), and most places take a minimum of 1 mm when they refinish, so it's rare to be able to reuse a rotor.
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Old 07-29-2011, 10:40 AM
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As Tom said, there's little tolerance for machining, but as the rotors are almost new they should clean up very easily with an amount less than 1mm ground off them.

Rotors are commonly turned, but some engine rebuilding shops and brake shops here can grind the rotor, which allows a very small amount to be taken off to get rid of ridges and inconsistencies only, without removing more material as is common with turning. If you try to take less material off with turning, the tool tends to ride up on the surface and not cut in, whereas grinding can take minute amounts off. Grinding is commonly carried out on flywheels and gives a nice fine finished pattern to bed friction material in .. as it does with rotors.

It would be better/preferable if you can get someone to true the discs up, because those rusted areas are going to glaze or chop into the pads, and you may be in a similar situation all over again.

I had a flywheel ground recently and they removed hardly any material, but trued the surface and restored a good friction surface.
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Old 08-02-2011, 06:05 PM
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I am starting a quest for a shop that will do this type of work, if there are any.

So I've been checking the specs in the tolerances and dimensions book. Seems that minimul thinkness for remachined rear rotors is 22.6 mm where 22 mm is the wear limit.

Thickness tolerance is 0.02 mm: is that the max. difference in thickness measures in different places?

Lateral runout is max. 0.05 mm uninstalled and max. 0.1 mm installed. This is the "side to side movement" of the disc.

Then there is mention of max. surface finish after machining: 0.006 mm. How the heck is this measured??
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Old 08-03-2011, 01:22 AM
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......
Originally Posted by Aryan View Post
I am starting a quest for a shop that will do this type of work, if there are any.

So I've been checking the specs in the tolerances and dimensions book. Seems that minimul thinkness for remachined rear rotors is 22.6 mm where 22 mm is the wear limit.

The reason they set a minimum is to ensure that you still have reasonable thermal mass of the rotor to store heat, that it isn't so lacking in bulk that it will crack and warp, and so the thickness is still within the working range of the caliper pistons ... in spite of all that I have seen some that are under the minimum, it's just that they won't perform to spec in some respects.

Thickness tolerance is 0.02 mm: is that the max. difference in thickness measures in different places?

Yes ... I would take that as being the variation in thickness tolerance as a result of out of parallel sides, or concave/convex sides. I'm assuming they're not referring to an absolute thickness tolerance of new and machined rotors relative to specified values.

Lateral runout is max. 0.05 mm uninstalled and max. 0.1 mm installed. This is the "side to side movement" of the disc.

This tolerance is of allowable 'wobble' of the rotor relative to the mounting face of the hat of the rotor uninstalled, and then the cumulative 'wobble' tolerance as a result of some runout of the hub on which the rotor is mounted. There are some refinishing machines which mount on the car and spin the rotor and hub, to eliminate runout from mounting discrepancies. Cleaning of mounting faces is really important to minimise runout ... also don't leave any paint on mounting faces, as it softens with heat and the rotor will move/loosen

Then there is mention of max. surface finish after machining: 0.006 mm. How the heck is this measured??

I assume this would be the 'roughness' of the surface as a result of grinding or turning ridges/marks. The only way you could measure that would be with some sort of pin adaptor for a micrometer? ... I'm sure there's some gadget for measuring that 'valleys' are no more than .0006mm. What you need is sufficient roughness/bite to get the rotors to bed in tandem with the pads, without glazing. A good experienced machinist will be able to achieve that by eye/appearance. .....
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Old 08-03-2011, 06:22 AM
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Install a higher rear bias valve, and give it a few good hard stops to bed the pads.

What you see is not uncommon or dangerous. But the rear brakes in the 928 do very little unless the bias valve is changed.
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