At 01:41 PM 12/28/2000 -0600, Jeff Fayne wrote:
>Perhaps an answer lies here, at least the logic appears correct:
>http://www.vtr.org/maintain/wire-wheels-tightening.html
Most of the _guesses_ are so far off the wall that you couldn't play
handball with them, but this particular explanation is very close but still
not quite complete. In fact the cause of self tightening has nothing to do
with the construction of the wheel being a wire wheel, as the center lock
alloy wheels will do the same thing. And the self tightening action in
fact _depends_ on a small amount of clearance in the splines so that the
parts can move slightly if the nut is a little loose.
This man's description of the hub construction is correct in that the wheel
hub only touches on the conical surfaces at each end of the hub, and the
splines come into contact with the driving torque. This part of his
statement is also correct, but not doesn't fully explain what that
"relative movement" actually is.
Quote:
"As the car moves forward, a different portion of the wheel rim takes the
weight and relative movement occurs between wheel centre, locking cap and
hub. The effect of this is to tighten the locking cap .... The clearances
involved are of course, minute but the locking action is completely
positive and entirely automatic."
The "relative movement" he speaks of here is orbital motion like the action
of a Hulla Hoop (Damn! just how old am I anyway?). As a Hulla Hoop orbits
around a person's waist it also rotates, with this rotational motion being
caused by the difference in the diameter of the two "round" parts.
Another example may be more intuitive. Stand a roll of racers tape on
edge, place a very large wrench socket inside of the tape roll, and proceed
to to roll this assembly along a table top. With gravity the socket
remains in the bottom of the tape roll, and as both parts roll along
together the smaller part inside will rotate faster because of the smaller
diameter.
Now the corresponding parts on the car are the wheel hub and the large
threaded nut, and the point of relative rotation is between the tapered
contact surfaces. This is easier to understand if you start with the nut
loose to begin with so there is a little intentional clearance between the
parts. The male taper on the hub is then smaller diameter than the female
taper on the nut at the point of contact. As a result, as the car rolls
forward both parts are rotating in the forward direction, but the hub will
turn slightly faster than the nut, so relatively speaking, the nut on the
left side of the car turns clockwise in relation to the wheel as you drive
forward, and the nut on the right side turns anti-clockwise, in relation to
the wheel. To make these parts self tightening the threads must be right
handed threads on the left side of the car and left handed threads on the
right side of the car.
And of course the opposite action will also result when the rotation goes
the other way. If you tow the car backwards with the front wheels on the
ground the large nuts will eventually unscrew (although it may require
quite a long travel distance, but less if the nuts is loose to begin with).
And if you assemble the hubs to the wrong side of the car the nuts will
unscrew while driving forward, and even if they start out very tight you
would be lucky to get 100 miles before you lose a wheel.
When the hubs are rotating in the wrong direction, the nut will come
totally unscrewed very quickly as soon as it becomes the slightest bit
loose. With just 0.001 inch difference in diameter the relative motion of
the nut to the wheel will be 0.003 inch with each rotation of the wheel.
You can work out the math, but with this much rotationthe nut will unscrew
one complete turn in about 3 miles of road travel. As the clearance
inscreases the relative rotation increases, and you are surely doomed.
The fellow with the race car with center lock alloy wheels and the hubs on
wrong may be okay as long as the knockoffs are very tight at the beginning
or each race, the races are not too many miles, and the knockoffs get
manually retightened periodically. For my own use, I wouldn't take two
laps with those hubs on the wrong side of the car.
As to Chrysler's use of left hand threaded nuts on the left side wheels
(with steel disk wheels), the principal is the same. But in this case the
tapered contact surface on the nut is the smaller diameter part, so the nut
rotates in the opposite direction. Theory says you need left hand threaded
lug nuts on the left side of the car, but we know that most newer cars have
right handed lug nuts all the way around, and so do our MGs. The reason
these right handed lug nuts on the left side do not come unscrewed is
because the combination of fine threads and large assembly torque applies
so much force against the tapered surfaces that the friction will hold the
part in place with no relative motion, so the nut stays put.
However, if you should happen to forget to fully tighten the lug nuts after
changing a wheel, the left side leg nuts will indeed work themselves off
the studs in a short distance of road travel. I suspect some of you must
remember having this occur to you at least once, where after perhaps a
somewhat hurried wheel installation you find the wheels getting very loose
within the first 50 miles of driving, and maybe even sooner. A somewhat
alert driver should notice the sligntly vague road feel as the wheel
becomes noticeably loose, and stopping to tighten the lug nuts is in order,
but larger cars with power steering would surpress the feedback considerably.
Please do pay attention to properly torquing the lug nuts. This especially
applies to us autocrossing fanatics who have a habbit of changing the
wheels twice a day on the weekends.
Barney Gaylord
1958 MGA with an attitude
http://www.ntsource.com/~barneymg
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