I was also experimenting with descending shimmy this past weekend, not by
choice at first.
I just built a bicycle with upright steel handlebars (older schwinn type)
using an older Specialized Expedition touring bike (long wheelbase). The
wheels are Mavic MA-2 with Campagnolo 36h hubs, 126mm spacing in the rear
13-24 6sp. The tire are the older Avocet Duro Plus 32's, tire pressure 85
psi.
On a descent grade of 3-6 % for about 1/3 mile I was able to induce shimmy
with my hands on the handlebars.
My speed was 26-32mph.
I've also noticed that with upright bars the steering is very responsive
from what I was used to, riding drop bars.
I'm not sure if this is because of the handlebars being further behind the
stem. In any-case it was easy to induce shimmy.
I thought this was interesting, so I rode back up the hill and repeated it
again. As I got the bike to shimmy I wanted to see if I could correct this.
I took weight off the handlebars by every so slightly removing both hands
about an inch away from the bar. My speed was about 27 mph, and before I
knew it, the shimmy corrected itself and was completely eliminated.
I repeated this a 3rd time, to make sure it wasn't a fluke and actually
timed how long it took for the shimmy to correct itself. The time was about
5 to 7 seconds for the shimmy to completely vanish.
I'm going to try this again since it's one of my training routes. This time
I changed tires, with Avocet Carbon 12 Road 700 x 25c.100psi.tire pressure.
These are the tires I normally ride with my roadbike. Strange, but my
roadbike doesn't shimmy at all on this stretch of my training ride.
-tom

wrote in message
...
I suppose we need another FAQ item on this. What is missing in most
of these testimonials is how and at what speed these shimmy episodes
occurred. In prior airing of this sort of thing, it was claimed that
balancing the front wheel would prevent shimmy or that a different
weight wheel would do it.

I put a large imbalance on a front wheel and made test runs with and
without imbalance (I balanced the wheel as well as greatly imbalanced
it) with no discernable difference in shimmy. I use a 5% grade,
smooth half mile section of road, coasting at between 25 and 30 mph
no-hands and can consistently develop shimmy.

The next test was to completely fill the front tire with water. This
changed the shimmy frequency slightly but did not stop it from
occurring.

That said, I descend mountain roads often at speeds up to 50mph with
no problem while placing my hands on the stem, knees together. Shimmy
is a characteristic of most larger bicycle frames and should not be
allowed to occur by the rider. SInce most people cannot ride
no-hands, they are not aware of the tendency of their bicycle to
shimmy. Then when it occurs, it is a disaster about to happen.

http://draco.acs.uci.edu/rbfaq/FAQ/8h.5.html

Jobst Brandt