Wheel Torques.

Jun 11, 2012
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Bertieboy I think its something to with two different metals and its ally wheels that wheel nut tend to ease of f a little when they get warm .
Sir Roger
 

JTQ

May 7, 2005
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Hi,
I know that different torques are required for steel and
alloy wheels.The question is - why?

Based on designing bolting systems for some similarly challenging applications, IMO.

The decision in any bolting system is based on what it is asked to do, and ensuring the arrangement stores enough energy locked up to do it.

Here our wheels are clamped to the hub and the friction developed between the two takes nearly all the shear forces.
The bolts or any spigots can't, as designing to do that would require so tight a fit to resist the forces encountered, that these fits would be impossibly "tight" to allow a user to change a wheel.

Thus the requirement is for a high clamping force, but one that still retains a high percentage of that should the contact surfaces settle at all. That means things being "elastic", ie springy.
The length of the bolt that is elastically stretched contributes part of this, but so does the wheel surrounding the bolt that gets compressed.

Now to answering the key bit of the question. The solid metal of the alloy wheel is far from elastic so can't contribute much. By contrast, the pressed steel wheel and the pressed in bolt seatings can as these are effectively "disk springs".
So in like for like situations the alloy wheel bolts need to be done up tighter, to store similar amounts of energy to live with any settling without too greater loss of the clamping force. Additionally, the aluminium being softer, is more prone to allow settlement, so could need the energy, to stay tight enough.

The steel wheel has other features, one it can flex to enable each bolt to seat properly and so pick up some shear carrying, thus it is that bit more accommodating, in living with achievable manufacturing tolerances.
 
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