High-speed shimmy, Speed wobble

In article iQlfb.678016$uu5.110598@sccrnsc04,
Shayne Wissler wrote:

Tim McNamara wrote:

Hmmm. Neither my front nor my back tires are perfectly round.
The front rim seems mostly round and true, but the tire itself
is off by 1-2mm at certain points (Bontrager 700x23 Race X Lite
Silica). This is the recently installed tire.

All bicycle tires are slightly out of round in my experience and
don't cause dramatic shimmy. In the case of my fiend's bike, the
culrit was that the tire was able to brush against the underside
of the crown.

That's what I suspected. Oh well...

Can the rim itself wobble without wrecking it or knocking it
permanently out of true? Or is its role in this confined to
modifying the overall tension of the system? Because it seems to me
that either it was caster motion or rim instability as the primary
component of the shimmy (the rim instability idea is just a
hypothesis). But rims are quite stiff...

Do you mean "can the wheel go temporarily out of true" under some kind
of stress, causing the bike to shimmy violently? Caster motion seems
far more likely, if for no other reason than a small arc of rotation
in the headset being the line of least resistance. As the frame whips
back and forth, the steerer rotates in the headset through a small arc
and return. The tire proceeds in a straight line on the ground while
the headest can be whipping back and forth a few inches.

Of course, if you looked down you'd likely have some optical illusion
of the fork, frame or wheel bending, just like you can produce by
holding a pencil lightly between the finger and thumb and shaking it
up and down. It appears to curve, even though it is not doing so.

On Fri, 03 Oct 2003 21:07:51 GMT,
wrote:

I think you'll find that the course the front wheel traces while
shimmying is nearly a straight line. Looking over the fork at the
tire contact point, only the top of the head tube (and bars) seem to
be moving from side to side as the wheel steers gyroscopically from
left to right, the trail of the wheel limiting its stroke.

So, if we increase a bike's trail, can we assume there will be a
reduction in the severity of the shimmy? Will the converse be true?

I would
like to see a slow motion movie of this event because it is just fast
enough to be seen but not scrutinized while riding.
-------------------------------
John Dacey
Business Cycles, Miami, Florida
Now in our twenty-first year.
Our catalog of track equipment: eighth year online.
http://www.businesscycles.com

John Dacey writes:

I think you'll find that the course the front wheel traces while
shimmying is nearly a straight line. Looking over the fork at the
tire contact point, only the top of the head tube (and bars) seem
to be moving from side to side as the wheel steers gyroscopically
from left to right, the trail of the wheel limiting its stroke.

So, if we increase a bike's trail, can we assume there will be a
reduction in the severity of the shimmy? Will the converse be true?

I don't know. I've noticed that shopping carts, with all their trail,
can shimmy with the best of them while cars with almost no trail do it
as well.

I would like to see a slow motion movie of this event because it is
just fast enough to be seen but not scrutinized while riding.

As I descended Tioga Pass last Saturday (25 Oct 03), a cool breeze,
for which I chose not to put on more clothes, made me hold the bars
loosely to avoid the "death grip shimmy" we so often hear about here.
Just gripping the bars was enough to make my reliably steady bicycle
begin to wobble. Just the same, we got some high speeds there and the
next day in warm air on Sonora Pass.

Jobst Brandt

On Sat, 01 Nov 2003 04:36:48 GMT,
wrote:

I think you'll find that the course the front wheel traces while
shimmying is nearly a straight line. Looking over the fork at the
tire contact point, only the top of the head tube (and bars) seem
to be moving from side to side as the wheel steers gyroscopically
from left to right, the trail of the wheel limiting its stroke.

So, if we increase a bike's trail, can we assume there will be a
reduction in the severity of the shimmy? Will the converse be true?

I don't know. I've noticed that shopping carts, with all their trail,
can shimmy with the best of them while cars with almost no trail do it
as well.

Thanks. I wasn't sure if you were suggesting that the range of the
excursion a shimmying wheel would make was a function of its trail.
-------------------------------
John Dacey
Business Cycles, Miami, Florida
Now in our twenty-first year.
Our catalog of track equipment: eighth year online.
http://www.businesscycles.com