Re: [SystemSafety] Wheel detachment from a moving car

From: Mike Ellims < >
Date: Tue, 26 May 2015 22:13:10 +0100

There are two questions here,

First, if fitted correctly will a car wheel wheel come off? The answer seems to be if it's fitted correctly no, it won't come off, the designers thought of that one.

Second, how often are wheels fitted incorrectly?

I should be able to at least partly answer the second question as all accidents attended by the police in the UK and in particular all fatal accidents have to have a STATS19 sheet filled out, part of which attributes causes and code 999 can be used for a write-in e.g. "wheel came off". However that's the part of the data set that isn't publically available and at the moment I can't get access to the data as;

  1. for the data sets I have the surge protector has gone phut (tech term) on the set of computers that hold the data so I can't turn them on.
  2. the UK data archive seems to have decided my login details are no longer valid so it won't let me download anything so I have to brace myself to try and get readmitted :-(

I may of course have the wrong user ID/password so I could look it up in my email - except that's on an old email system that's mounted on the set of computers detailed in point a) above...

I should have the old data on a backup... somewhere though I'm not sure I have the will to go searching just yet. A new surge protector is possibly simpler.


-----Original Message-----
Sent: 26 May 2015 20:19
To: Mike Ellims
Cc: David Crocker; The System Safety List Subject: Re: [SystemSafety] Wheel detachment from a moving car

I don't think you'll ever get a definitive answer. There's no requirement for data collection or reporting. Even when it does happen, nobody has to ever tell anyone about it. So no way to capture any objective data about it. All you will ever get will be anecdotes--unless some law change ends up requiring a reporting mechanism.

ҵ iPad


> 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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Received on Tue May 26 2015 - 23:13:32 CEST

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