Re: [SystemSafety] Wheel detachment from a moving car

From: Mike Ellims < >
Date: Tue, 26 May 2015 20:00:28 +0100


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.

---
This email has been checked for viruses by Avast antivirus software.
http://www.avast.com

_______________________________________________
The System Safety Mailing List
systemsafety_at_xxxxxx
Received on Tue May 26 2015 - 21:00:55 CEST

This archive was generated by hypermail 2.3.0 : Wed Feb 20 2019 - 20:17:07 CET