From: Dave Baker
Newsgroups: rec.autos.tech
Date: 26 Apr 2001 18:54:55 GMT
Subject: Re: Wheel Balance

>From: Robert MacNab w4xg@nova.org
>
>I got a Snap-On manual wheel balancer on Ebay last year.  It just
>clamps the wheel hub on an axle supported on roller bearings and
>then the heavy point on he wheel drifts to the bottom.  I like
>it.
>
>They tell you to put weights on the wheel on radials spaced 120
>degrees apart.  That is, if the heavy point of the wheel is at 6
>o'clock, you put weights on the rim at 10 and 2 o'clock.  When
>you do this, you have to use twice the weight of lead which would
>balance the wheel if one weight were put at the 12 o'clock
>position.

That sounds completely pointless to me unless someone can explain an advantage
that I'm missing. It would never be standard practice when balancing a crank or
flywheel and I've never seen anyone doing it when balancing a wheel.

> To get the best dynamic balance, you further split the
>total weight between the inboard and outboard edge of the rim.

That's a separate issue and on your machine you can't tell whether there is any
"rocking couple" caused by separate imbalances across the width of the wheel.
Imagine the wheel was out of balance by 20 grams at 12 o'clock on the outer rim
and 20 grams at 6 o'clock on the inner rim. On your machine the wheel would
indicate as balanced. But when the wheel is rotating it will generate an
oscillating force about the mid point of the width of the rim. This is called a
"rocking couple" and is something that has to be balanced out when balancing a
crankshaft. In the absence of actually knowing whether any rocking couple
exists it is probably pointless to split the weights between the inner and
outer rim but it would do no harm.

Professional wheel balancing machines measure the rocking couple and show the
amount and position of weights required for the inner and outer rim separately.
The wider the wheel the more important this is. In general though the forces
generated on a road wheel are small enough that you don't have to worry about
it much. The last wheels I had balanced, the guy put all the weights on the
inner rim so as not to spoil the looks of the wheel on the car and it worked
fine - no vibration at all.

You can make a perfectly good balancer of the type you have with a couple of
ordinary roller bearings and a shaft turned accurately on a lathe. That's how I
balance flywheels and it cost about £5 for the bearings and a hour or so of my
time to make. It indicates 1 gram without any trouble. Hope you didn't pay too
much for the one on Ebay. Maybe one day I'll make a bigger one for road wheels.


Start every day with a smile :) You might as well get it over with before
everything goes to hell.
http://www.pumaracing.co.uk/