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Tire balancing (was: free tire-purchasing adivce)

Subject: Tire balancing (was: free tire-purchasing adivce)
From: (Russ Wilson)
Date: Thu, 20 Jun 1996 09:33:02 -0800
Denise is correct: tire-balancing machines with cones usually don't get it
right because they are not *the* cones with *the* correct angle.  But
simply balancing a wheel until it has no tendency to rotate on its own is a
static balance that puts the center of mass of the wheel (and hub, rotor,
etc.) at the center of the axle.   It is comparable to the old
bubble-balance technique.
A statically-balanced wheel can, and generally will, still wobble when
rotated, since its mass will not necessarily be distributed symmetrically
*along* the axle.

Years ago, I bought a front MGB hub from a wrecker with the intention of
modifying it to allow mounting, with a wheel and knock-off attached, on a
balance machine.  This seemed the best way to assure the right cones were
used.  However, before I ever used it, I found a shop that still did
dynamic, on-the-car balancing.  This used to be a much more common practice
than it is now, but fell out of favor because off-the-car dynamic balance
is, in principle, better (that is, if you don't have unusual rims - but
they don't tell you that).

The last person who did this for me was a bit of a perfectionist who wasn't
satisfied when the machine said it was done.  After, typically, two
iterations of adding/moving weights, the instrument was happy, but the
technician wasn't.  He fully extended the radio attenna and watched the
vibration of its tip.  I don't know what he was using for a guide (probably
intuition), but he got the balance so good that, with a wheel singing at 60
mph, there wasn't ANY discernible movement of the antenna.  (When he
started, each wheel caused the fender to thump loudly.)

I don't worry about remounting the wheels in the exact position in which
they were balanced.  It's unlikely there will be a great enough inbalance
in anything else that rotates, because of the small masses and diameters
involved, to make any difference; if there is, it ought to be fixed
independently, not buried in a wheel-balance job.  You can't really sweat
the small stuff - the first rock picked up in the tread will make a
difference that is detectable on these balance machines.

(Denise - when your guy retires, try the tire shop on Convoy Ave just south
of Balboa for a dynamic on-the-car balance.)

Russ Wilson

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