> Many years ago I was reading the driver's manual for an XK140
> and laughed at Jaguar's way of defining left and right. I
> wish that I could quote it verbatim but, in effect, it said,
> " as viewed by the driver, in the driver's seat, facing
> forward". Kind of hard to imagine sitting there while facing
> backwards.
>
> My OF memory bank seems to recall that the omniscient Barney
> Gaylord did a dissertation on the physics of wheel rotation
> versus knockoff threads a couple of years ago.
The hubs are indeed marked L and R. L for Left (quite simple really).
Left is the left side of the car, and as Bud has indicated, that is the
same as drives left.
As for Barney's message, it follows:
Larry Hoy
==============================================================
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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