John Bertsche wrote:-
>I'm afraid your theory is wrong. The "tightening" direction is OPPOSITE the
>forward movement of the wheel, not the same. The "undo" arrow is
always rotating
>in the same direction as the forward rotation of the wheel.
>Were your inertia-based theory correct, hard braking would quickly loosen the
>nut.
>I've also heard several first hand accounts that support my point of view,
>including these-
(snip)
>I've heard a lot of ideas about this issue but no one really seems to have
>gotten to the heart of the matter.
> jb
My favourite theory is to do with the geometry of cones (tapers).
The inside of the spinner and the face of the wheel centre hub that it
contacts are tapered.
This has two benefits, firstly it centers the wheel but it also
provides the self
tightening (or self loosening!) property of the wheel.
How?
Imagine that you fit a wheel one day and forget to knock it tight, and
then go for a drive.
There's going to be a bit of play between the spinner and the hub.
Because of the taper
there's also play vertically between the tapers on the spinner and the
hub. Effectively
the diameter of the inside of the spinner is greater than that of the
hub. Because the spinner
is loose it will only be in full contact with the hub at one point, the
top, because this is where
the weight of the car is acting. This sets up a rolling action between
the hub and spinner
which causes spinner to turn very slightly slower than the wheel.
If you think about it this rolling action is inevitable if you have two
tapers in contact with
each other unless they are in full contact all the way round..
To take the example further suppose this play is only about 2 thou. The
diameter of the
spinner is about 2 inches so the difference is .1%. If you are cruising
at 60-70 with the engine
doing about 4000rpm and with a diff ratio of about 4:1 the wheels could
be turning at
say 1000rpm. If your spinners are turning at 999rpm you've got a big
problem not far down
the road unless the threads are arranged to tighten against the
rotation of the wheel!
Of course as the wheel tightens the play reduces and so does the
relative speed
until the there is no play at all possible and the surfaces are locked
firmly together.
If the thread is in the wrong direction though, any play at all will
cause this rolling
action to start, the play will increase and so will the relative speed
until the taper on the
spinner is no longer in contact with that on the wheel. Then and only
then the forces of
inertia will take over, but by then it's far to late!!
An interesting side note is that on some cars of 50s & 60s with
ordinary steel wheels,
handed wheel nuts were used. Here the geometry is reversed as there is
an inside taper
on the wheel and an outside taper on the nut. This means that this
rolling action will make
loose nuts turn faster than the wheel. The theory says that, in this
case, wheels on left hand side
of the car would have a left hand thread, and on the right hand side
would have a right hand thread,
which is the opposite of the situation with wire wheels. Does anyone
out there have such a car,
and can they confirm this??
Keith Bennett
|