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      12-15-2008, 11:21 AM   #53
Mark II
The Grim Reapers Love Child
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Drives: BMW e92 M-Sport Coupe
Join Date: May 2008
Location: A little bit left of Sane, near Tongue in Cheek.

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Originally Posted by dxb335d View Post
what does radial run out mean bud?
Sorry mate - been busy - as others have said, looking side-on at the wheel, it's how out-of-round (or 'egg shaped') the wheel is.

As So Yank said - if you were to mount the wheel on a calibrated axle shaft (so you know the shaft isn't out of kilter) and positioned a dial guage with the tip of the guage onto the radius (outer edge) of the rim, pre-loaded and zero'd the guage and then rotated the rim one revolution, the difference between the maximum and minimum dial guage readings would be your radial run-out.

I believe BMW are saying that should be max 0.3mm and seem to be suggesting that any figure above 0.3mm indicates that the wheel has received impact damage which caused the crack in the first place.

I don't doubt that some form of impact caused the crack but my point is why the alloy cracked: I'm not a metallurgist but as an engineer I believe that if you do a radial run-out test on these damaged wheels you may well find that the lowest reading is 180 degrees opposite to the crack and the highest reading it at the point of the crack - which at the very least suggests that instead of the crack being the point of impact it was actually 180 degrees opposite to the point of impact and therefore perhaps if the alloy material had not been flawed in the first place (by a weakness such as a casting inclusion or suchlike) then a crack would not have formed.

In my theory the impact force transmitted into the edge of the rim deformed the rim at the point of impact but the rim burst (for want of a better word) on the opposite side.

To me, the fact that the wheel isn't round is not an adequate explanation for the existence of cracks - poor quality control on the alloy is, so ergo it was a manufacturing fault and should therefore be covered under warranty.

As far as alloy casting is concerned this article is very interesting (for the techie):

Just my tuppence
Mark II

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