[s-cars] RETorquing wheels

Bernie Benz b.m.benz at prodigy.net
Tue Oct 15 19:01:05 EDT 2002

Comment interlaced below.

> From: "Trevor Frank" <tfrank at symyx.com>
> Date: Tue, 15 Oct 2002 15:45:18 -0700
> To: "Theodore Chen" <tedebearp at yahoo.com>, "Bernie Benz"
> <b.m.benz at prodigy.net>, "s-cars" <s-car-list at audifans.com>
> Cc: "200q20V mailing list" <200q20v at audifans.com>
> Subject: RE: [s-cars] RETorquing wheels
> You might be right on getting movement, but lets hope not.  A properly
> designed bolted union is one that has the tention in the bolts way in
> excess of the forces that would ever act on it.  This means that once
> you have bolted it together you have gone up on the stress strain curve
> to a point that will not be exceeded.  If it is exceeded and often then
> you get into a situation where you can have a fatigue falure, bad
> design.  If this was the case then you have one of two options, increase
> the torque "tention" and or at the same time go to a stronger bolt and
> increase the tention.
I buy your explaination of the design requirements of a good bolted union in
the first paragraph above, but then apparently you wrongly imply that Audi
has violated the principle in the second paragraph, below.
> I do agree that you might be seeing localized yeilding of the aluminum,
> spacers or more likely the rims.
Like the annular contact ring directly under the tapered head of the bolt.
> Expecially under heavy breaking and increased temps.
Especially not so!  Your heavy braking forces are trivial relative to the
hub to wheel clamping friction.
> Look at the strenght of aluminum at 400 deg F.
Your 400F is right out of the blue, irrelevant!  Have you ever felt an
aluminum wheel after heavy heat soak braking?  Maybe warm to the touch,

> I think Ted is right on with tighten, eventially you should reach a point
> where the density of the aluminum is to a point where you shouldn't have
> this problem.
You imply that the wheel material suffers porosity problems, to be
compressible.  A condition to be avoided in wheel selection, but way off
point.  We are discussing the yielding, creep, deflection over time of the
wheel material under the bolt applied clamping force.  This clamping force
reduction, as measured by the loss of bolt torque, ocours because of wheel
material creeps at the point of maximum compressive stress, the annular
contact ring directly under the tapered head of the bolt.  As the contact
area spreads out the compressive stress is reduced until the creep rate
becomes insugnificant.

No need to retorque as often now, Scott!  That is, until you change wheels
> Moral of the story, buy rims that have been forged.
Maybe a slight improvement, but maybe not cost effective.


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