On Sun, 11 Jul 2004 19:17:52 -0600,
[email protected] wrote:
>
>Dear Bob,
>
>Your plea raises an interesting question.
>
>It's often been stated here that wheel-balancing is
>pointless on bicycles because the wheel mass is so small
>compared to the bike and rider.
>Carl Fogel
Hi Carl:
I do wish that I did know more about the theory involved here. My
statements here are based more on experience (which may not carry much
weight here.. G).
I know that when you spin a bike or cycle wheel at speed, such as we
used to do with a stroboscopic automotive wheel balancer machine, that
the forces on the free wheel are not all in the plane of the wheel and
that some side to side motion is generated. You can easily
demonstrate this by spinning a freely suspended bike wheel with a shop
compressed air nozzle and you will see a "shimmy" component as well as
the vertical out of balance forces.
I am told, ( as were you, it seems - G), that this can not effect a
bicycle at speed. But, we used to balance the motorcycle wheels by
wrapping heavy gauge solder on the spokes at the "light" point as
indicated by the stroboscopic balancer (or simply by letting the
wheel seek its own heavy spot if the bearing and brake drag would
permit. This would usually eliminate the deadly "speed wobble" which
surely was a form of what we are refering to as "shimmy".
I agree, it happens at harmonics and can be very exagerated if
occuring at a resonant frequency of the structure of the machine
involved.
>
>How heavy would an unbalanced weight on a wheel have to be
>before wheel-balancing became worthwhile in the eyes of the
>naysayers?
>
An out of balance condition of 1/2oz. on a car at the wheel rim can
easily be felt at road speed as a vibration (probably mostly in the
vertical plane), with a much heavier wheel and tire than the bike, so
I feel that the reflector weight on a bike tire is sure to cause some
effect even if mounted opposite the valve stem, which is supposedly
the "heavy spot".
In my case, with one of my first pedal powered bikes, a Trek 750
"Cross", I recognized what was happening just before it shimmied me
off of the road into the woods. After spinning the wheels with shop
air to see what the wheel balance was like at speed, and removing the
reflectors, which made a big improvment in the amount of vertical
movement (and the side to side as well), I never had the shimmy event
occur again in many thousands of miles of riding this bike. I realize
that this is purely emperical and therefore of little value here.
So while I am admitting that I cannot argue this point in theory, I
ask only that doubters put a reflector ( or an equivalent weight) on a
front bike tire, suspend the bike so that the fork can turn freely,
spin at road speed by whatever means (shop compressed air nozzle is
great for this) and observe the effect and then surely someone here
can (and most likely will), theorize. BG.
Bob..
[email protected]
P.S. Something to do with gyroscopic preccesion as the out of balance
mass revolves around center creating a force at right angles to the
primary out of balance force... rambling now..sorry ??????
Bob Flumere
[email protected]
