[Shop-talk] tire balancing

[Donald H Locker]

Static vs. Dynamic - consider a yardstick (or metre-stick) with equal 
weights on either end. With one weight mounted an inch below the stick, the 
other three inches below, with both equidistant from the center of the 
stick. This is statically balanced similar to a balance - neither side will 
tend to rise or fall. BUT if you put an axle through it and rotate it about 
that axis, the weights will try to rotate about a point two inches below 
the yardstick and will alternately pull it up and down a bit with each 
revolution.

Putting both weights equidistant radially and axially (which is what 
dynamic balancing does) removes that wobbling force. Here is some ASCII art 
(best viewed in a fixed-width font.)

Static balance only            Static and Dynamic balance
---------------+--------------- ---------------+---------------
Wt             |                               |
                |                Wt             |     Wt
                |             Wt

Donald.

On 05-Nov-2019 13:40, Steven Trovato via Shop-talk wrote:
> That's interesting.  I always thought of static balancing as being done 
> with a bubble balancer sitting still on the floor.  And any balancing 
> with the wheel actually spinning I would call dynamic. They obviously use 
> the terms differently.  I imagine it wouldn't hurt any if you did both 
> for the rear as well, even if it isn't necessary.  The whole premise of 
> on-the-car balancing is that you don't want to move the tires around.  
> Even if you remove and replace a wheel, you want to mark it so you put 
> the same lug studs through the same wheel holes.  I would think in modern 
> off-the-car balancing they should just balance all of the wheels the best 
> they can regardless of position.  I suppose they could use a lower 
> standard for the rears, but that seems pretty lame to me.
>
> -Steve
>
> At 01:21 PM 11/5/2019, Randall via Shop-talk wrote:
>> I don't know that this is relevant, but the manual for my on-the-car 
>> balancer (an antique so old it actually uses vacuum tubes) talks about 
>> "static" and "dynamic" imbalance as two different tire/wheel conditions. 
>> Its sensor only works in one direction, so first you check for the tire 
>> hopping up and down (which it call static imbalance), then for wobble 
>> from side to side (dynamic). IIRC, the dynamic check is only done on the 
>> front axle.
>> -- Randall
>
>
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