I thought a further word would help clarify the difference in the
situations, beyond Kelvin's impressive explanation. It boils down to
this: the center lock nut on the wire wheel is at the center of rotation
of the wheel; the five (or four) individual lug nuts are not. Therefore
the lug nuts do not experience any actual rotational friction between
them and the wheel (that is, rotation on the center of the threads) --
they experience (if anything) linear friction (across the center of the
threads, therefore not contributing to loosening or tightening), if the
wheel shifts on the studs under braking/acceleration. Presumably what the
Chrysler (and other) engineers were worried about was angular momentum
loosening lug nuts that were *already* loose (less than finger tight), to
the point they would fall off.
In other words, the rotation of the wheel itself acts directly on the
knock-off, rotating it in the direction of either tightening (forward) or
loosening (reverse). The rotation of the wheel does not act directly on
the lug nuts, since no lug nut is at the center of rotation. The lug nuts
experience angular momentum and centrifugal force, but not rotation on
the center of the threads. This is why modern motor manufacturers are
satisfied with non-handed lug nut threads, and no one warns about towing
steel wheel vehicles backwards.
BTW I have no special knowledge of this and am just reasoning from first
principles, so if you have a better explanation, fire away...
Dodd, Kelvin had this to say:
>Rick:
>
[snip]
> A knock off nut is never tight. The surface area that it seats upon
>is conical and of very large area. The wheel itself seats upon a conical
>surface and does move slightly between the knock off nut and the hub
>mounting face. Even with perfect, new splines on hub and wheel there is
>always some movement, which is one reason why grease is important.
>
> The knock offs are handed so that the momentum of forward movement
>will tend to tighten them. This system works well unless you decide to
>drive in reverse as one list member pointed out. I hope the list member
>does not attempt this with a traditional American car, as the self adjusting
>brakes on such a vehicle may also cause problems. Not to mention the
>possible chiropractic bills.
>
> Lug nuts have a much smaller fastening surface and are under far
>higher clamping force. The frictional force between wheel and nut is far
>higher than that of a wire wheel. Check out how much metal is damaged on a
>typical steel wheel nut surface. Compare that to the lack of damage to the
>much softer wire wheel nut and wheel taper. The momentum effect on the much
>smaller wheel nuts is also much less, so the non-handed lug nut became
>industry norm.
>
[snip]
>> Kelvin et al,
>> OK, I believe you guys about towing a wire wheel car backward.
>> However there is something that has always bugged me about
>> this. Back in
>> the old days Chrysler used right handed thread lugs on one
>> side of their cars
>> and left haned thread lugs on the other side. The theroy
>> being, I suppose,
>> the same as the knock off that they would not come loose when
>> driving. OK
>> fine, I'll buy that. Every other car maker in the world only
>> uses right hand
>> thread studs on both sides of the car. Now since we don't
>> see the road
>> littered with wheels off just one side of all the cars made
>> EXCEPT Chryslers,
>> and LBC's with wires, it must mean that the Left handed
>> threads aren't as
>> necessary on cars as Chrysler thought.
>> Agreed?
>> So why do knock offs come off when towed backward? On a car
>> with all right
>> handed thread lugs, at any one time, half of them are going
>> in the "wrong"
>> direction and should be comming loose. I can tell you from personal
>> experience that if lug nuts get a little loose the studs (all
>> of them) will
>> break. Ask me how I know this<G>. The only answer I can
>> think of is that a
>> properly torqued fastner won't come loose. Am I missing
>> something here?
>> Not trying to start a flame war, just want to understand.
>> Rick Ewald
>> 67 MGB
>> In a message dated 4/20/99 4:30:24 PM SA Eastern Standard Time,
>> doddk@mossmotors.com writes:
>>
>> > > DO NOT DO THIS WITH A WIRE WHEEL CAR! Towing a wire wheel car
>> > > backwards will
>> > > cause the knockoffs to loosen and spin off.
--
Max Heim
'66 MGB GHN3L76149
Runs great,
looks particularly bad since some SUV clown backed into it.
If you're near Mountain View, CA,
it's the red one with the silver bootlid.
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