Developed a shimmy

Apparently I've developed a no-hands shimmy -- you know, when you descend down a hill at 20+ mph or
so, and the front end starts to wobble. This has never happened before. Now I've read the FAQ and
some old threads on this topic, and I think these are the two possible relevant factors:

a. I recently replaced the back tire with a tire 4 mm wider. It's otherwise the same tire as I had
before and the same as the front tire. I need to make sure the inflation is high enough (right?)
and even, in both tires, to reduce the likelihood of shimmy.

b. I fell a couple times on ice recently -- is it possible that the tires got a little unseated to
induce the shimmy? Or otherwise knocked something out of alignment that could be a factor?

The wheels do not appear to be out of true and the spokes all seem fine. Other ideas that might
be a factor?

--Claire ([email protected]) No sig this afternoon

 

> b. I fell a couple times on ice recently -- is it possible that the tires got a little unseated to
> induce the shimmy? Or otherwise knocked something out of alignment that could be a factor?
>
> The wheels do not appear to be out of true and the spokes all seem fine. Other ideas that might be
> a factor?

Yes. Could be that you're not as relaxed while riding, after having taken those falls. You might
have even subtly changed your position on the bike. I doubt, though, that you "unseated" your
tires when you fell (but why not take them off and reinstall them anyway, if you have any question
about it?).

I've found that, once somebody's confidence is shaken (unintentional pun there), it's considerably
more difficult to exorcise a shimmy.

--Mike-- Chain Reaction Bicycles http://www.ChainReactionBicycles.com

"Claire Petersky" <[email protected]> wrote in message
news:[email protected]...
> Apparently I've developed a no-hands shimmy -- you know, when you descend down a hill at 20+ mph
> or so, and the front end starts to wobble. This has never happened before. Now I've read the FAQ
> and some old threads on this topic, and I think these are the two possible relevant factors:
>
> a. I recently replaced the back tire with a tire 4 mm wider. It's otherwise the same tire as I had
> before and the same as the front tire. I need to make sure the inflation is high enough
> (right?) and even, in both tires, to reduce the likelihood of shimmy.
>
> b. I fell a couple times on ice recently -- is it possible that the tires got a little unseated to
> induce the shimmy? Or otherwise knocked something out of alignment that could be a factor?
>
> The wheels do not appear to be out of true and the spokes all seem fine. Other ideas that might be
> a factor?
>
> --Claire ([email protected]) No sig this afternoon

 

On Thu, 16 Jan 2003 20:03:43 -0500, Claire Petersky wrote:

> Apparently I've developed a no-hands shimmy

> a. I recently replaced the back tire with a tire 4 mm wider. It's otherwise the same tire as I had
> before and the same as the front tire. I need to make sure the inflation is high enough
> (right?) and even, in both tires, to reduce the likelihood of shimmy.

Almost any change can start, or stop, shimmy. It's the whole mechanical system response to the
back-and forth motions we all make to stay upright, which are, at certain speeds, reinforced rather
than damped. It's not one specifiic thing, it's everything.
>
> b. I fell a couple times on ice recently -- is it possible that the tires got a little unseated to
> induce the shimmy?

Not really, except as above.

> Or otherwise knocked something out of alignment that could be a factor?

See above. I know people who claim that replacing the seatpost fixed a shimmy, or replacing a wheel
started it.

If you are worried about a shimmy when going downhill, unload the saddle and it will go away. If
you want to change things enough to stop it, change whatever you wanted to replace anyway. It might
just work.

--

David L. Johnson

__o | Accept risk. Accept responsibility. Put a lawyer out of _`\(,_ | business. (_)/ (_) |

 

I developed a front-end shimmy once from bungee-ing something onto my rear rack. Have you added any
weight to your rear rack? (Do you have a sister named Kate?)

Claire Petersky wrote:

> Apparently I've developed a no-hands shimmy -- you know, when you descend down a hill at 20+ mph
> or so, and the front end starts to wobble. This has never happened before. Now I've read the FAQ
> and some old threads on this topic, and I think these are the two possible relevant factors:
>
> a. I recently replaced the back tire with a tire 4 mm wider. It's otherwise the same tire as I had
> before and the same as the front tire. I need to make sure the inflation is high enough
> (right?) and even, in both tires, to reduce the likelihood of shimmy.
>
> b. I fell a couple times on ice recently -- is it possible that the tires got a little unseated to
> induce the shimmy? Or otherwise knocked something out of alignment that could be a factor?
>
> The wheels do not appear to be out of true and the spokes all seem fine. Other ideas that might be
> a factor?
>
> --Claire ([email protected]) No sig this afternoon

 

In article <[email protected]>, Claire Petersky
<[email protected]> wrote:
>Apparently I've developed a no-hands shimmy -- you know, when you descend down a hill at 20+ mph or
>so, and the front end starts to wobble. This has never happened before. Now I've read the FAQ and
>some old threads on this topic, and I think these are the two possible relevant factors:
>
>a. I recently replaced the back tire with a tire 4 mm wider. It's otherwise the same tire as I had
> before and the same as the front tire.

The 4mm wider tire may very well also be taller than the old one (ie, you changed the distance from
rim to road and jacked up the rear of the bike a hair). I wouldn't think it enough to matter, but
when you've introduced a shimmy you need to undo any changes and revert to the last known good state
if possible.

Regarding your recent falls, any change caused by that event can't be undone since you don't know
the state of the frame/fork alignment prior to the fall. You can have the alignment checked, but
even if it's perfect you still don't know how it was before the fall. I think it's still worth
checking and I would probably correct any significant problem, but you can't assume that proper
alignment will make the shimmy go away - on the contrary it may get worse, you just don't know til
you ride it after the change.

--Paul

 

> > Apparently I've developed a no-hands shimmy

it's from an involuntary input from your arms, believe me I've been through it. The way I solved
it was I got more narrow handlebars (44 to
42), and then I had to concentrate on relaxing those arms.