[s-cars] RETorquing wheels

[Theodore Chen]

--- Trevor Frank <tfrank at symyx.com> wrote:
> 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.

trevor,

i'm not necessarily claiming that the loads are going high enough to
exceed the tension in the bolts, but they could be high enough to
reduce the clamping force and friction force enough for the wheel to
move under heavy loads.

and as you know, you can torque the bolts only so high before the
bolt fails.  you could go to a stronger bolt, but then what's the
next weak link in the chain?  are your rotors going to warp?

> I do agree that you might be seeing localized yeilding of the aluminum,
> spacers or more likely the rims.  Expecially under heavy breaking and
> increased temps.  Look at the strenght of aluminum at 400 deg F.  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.
>
> Moral of the story, buy rims that have been forged.

i suspect you're talking about aluminum, not aluminum alloy.  you're
right that pure aluminum is relatively soft and weak, but the alloys
are a lot stronger and endure high temperatures better than pure
aluminum.  brake calipers are made from aluminum alloys and regularly
go over 400 degrees.  still, you raise a good point - who knows what
kind of aluminum alloy was used by the wheel manufacturer?

the wheels in question were CSA 16" wheels.  they are cast, not forged.
they're also quite light, at 16 lbs. for a 16x8 rim.  once i got rid
of the spacers, i never had any problems with losing lug nut torque
after running them down to 85-90 lb-ft.

forged wheels are better, but there are plenty of good cast alloy wheels
and they're a lot cheaper.  you just have to make sure to buy the good
ones.

-teddy


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