[SystemSafety] Wheel detachment from a moving car

[Mike Ellims]

I've been looking for an answer to this since the original email came out
but getting a good; but perhaps not definitive answer has been a bit of a
slog.

First we need to consider that older wheel lugs (e.g. 1940's) are different
to the ones currently in use in. Current wheel lugs are a tapered design and
so there is no precession, thus having left and right handed nuts on either
side of the vehicle body is of no use as there is no self tightening action.

Talking to various people established the view that on light vehicles at
least (Class M1, N1 for instance) if all the lugs are correctly torqued down
a wheel won't come off. Indeed the only example I know of where this
happened is from 1974 on an Original VW Beetle where a back wheel came off;
my guess is that had un-tapered lugs.

As to why this is the case is more difficult to answer and I resorted to
reading the SAE wheel test standards in the end (not recommended). It all
depends on some details of the design involving the interaction between the
lug and the wheel which is generally a beveled surface at about 60 degrees
which is much easier (i.e. less costly) to machine accurately than say a
flat surface.

More importantly it seems that the fact that the hole in the wheel into
which the lug fits (the nut seat) isn't flat but is rather raised in a
truncated cone forming a type of spring washer called a Belleville washer or
a coned-disc spring. Applying the correct torque to the lug compresses the
spring and locks the lug in position. Conversely if you apply too much
torque you can warp the dish and end up with "sub-optimal" brake
performance.

There are other considerations for wheels on large vehicles and the design
differs somewhat and I haven't even tried to dig down into that one in any
detail. One important difference however is that on light duty vehicles the
lugs are also used to centre the wheel which is why it's important to
tighten the bolts across the wheel to prevent the nuts seats from being
distorted.

Probably not the complete story but I hope this helps.

Cheers.





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