So after 35 years I now must repent and retract at least one wrong
assumption about those 'dumb Brit engineers'.
When I used to run around in a beater XK150, the right front wheel
hub splines eventually wore right off - smooth as a baby's bottom.
The student budget solution was to whack on the knock off as tight
as possible with a sledge hammer. But the damn wheel would always
undo and depart under hard braking, followed by long searches for
the lost knock off, usually in tall grass pastures. And so it was
that I came to form the belief that all Brit machines, at all
times, simply did things the Lucas Way.
Forever since, all threads on this topic have been relating to
rotational spline clearances. Now thanks to Barney's One CPS
Vertical Relative Motion Theorem, the Universal Order has been
finally restored.
Mike L. (P. Eng.)
60A,67E,59Bug
I see the light - its the ----- Original Message -----
From: Barney Gaylord <barneymg@ntsource.com>
To: <mgs@autox.team.net>
Cc: Jeff Fayne <jeff.fayne@tellabs.com>
Sent: December 28, 2000 10:07 PM
Subject: Re: wheel hubs on the "wrong" side - the real answer
> 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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