Checking Rotor Runout with Coated Rotors

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As most of you are aware, verifying the rotor runout is a key step in performing a proper brake job. Some aftermarket rotor manufacturers (and most Euro OEMs) have begun to coat their rotors on the friction and non-friction areas of the rotor (for rust/corrosion protection) and this is making it difficult to check the rotor runout. The coating on the friction areas feels rough and the needle on the dial indicator will skip, which makes it impossible to get an accurate reading of the rotor runout.

Here is a picture of the rotor in question from a recent brake job I did. This is a Genuine Mercedes rotor that came coated with the aluminum/zinc coating on the friction and non-friction areas.

Untitled by The Critic, on Flickr

There are only 2 solutions that I can think of:

1) Find a way to efficiently sand off some of the coating (to make the surface bare metal again)

2) finish the brake job, go for a test drive, then come back and check the runout.

#2 is a terribly inefficient method. I haven't tried #1 yet.

Does anyone have better ideas? Thanks.
 
For a genuine rotor not made in china I think I would not worry about it.

Otherwise 2 is your only way

Rod
 
Originally Posted By: ragtoplvr
For a genuine rotor not made in china I think I would not worry about it.

Otherwise 2 is your only way

Rod


The issue is with runout in the hub. The rotor may be perfect, but when if your hub has excessive runout, the final assembly will still be out of specification. Or, if both pieces have runout, the total runout will be out of specification. Many vehicles only allow a maximum of 0.0016", and this can be quite difficult to achieve without an on-car-lathe or correction shims.
 
You need a larger, beveled tip on your dial, and you need to go slow.

I've never had an issue on BMW or VW with their OEM coated rotors. Cants say I've done a Mercedes, but their coating looks similar, if not the same.
 
That's a problem I haven't encountered, yet. If brake clean isn't removing the coating then I guess sanding would be the only other option other than driving the car and possibly doing the job twice.
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Put it in the brake lathe and hit it with 320 grit in the pad area instead of cutting it. No brake lathe, I would wire wheel it with a drill mounted brush.
 
Use a "rocker arm" follower on the pickup instead of the plunger of the DI directly on the disc.

Smoky
 
That's only for rust protection?
I bought some rotors and pads recently and the coating was part of
the final honing process. I had to drive them to bed them in.


Can google for non-contact runout measurement rotor.
But, the results are production-line based, not suitable for the field.
MTI Instruments is one of the hits.

(I remember your post from a few years back about why you measure.
Very nice post.)

What happens if you do find excessive run-out in this situation?
Will you try to correct it now, then re-coat the rotor to allow
it and the pads to bed-in correctly?
Seems you can only try to reposition the rotor for it to be
less. Maybe let the customer bed in the pads for 100 miles then
return and measure (and possibly correct) at that time.
 
Roller tip on the gage like smoky14 said - if the coating is even and thin.
You can buy tapered correction shims to eliminate TIR.
 
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Your measurements may be skewed if the coating thickness isn't consistent all the way around the rotor.

Is there a way you can safely remove a thin ring of the coating down to the metal surface at the outer circumference of the rotor? You only need a thin ring removed so that your dial indicator feeler can ride over smooth metal.
 
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