Low speed shimmy?

Bob Flumere

On Sun, 11 Jul 2004 19:17:52 -0600, carlfogel@comcast.net 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.. rflumere@comcast.net

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 rflumere@comcast.net