I went for a 16 mile ride yesterday evening with a couple of guys who like to push the pace. We got up to about 25 mph on a slight downhill and I got tired so I decided to coast a little and took my hands off the bars. the bike immediately went into a sever shimmy. Now I have coasted hands free at higher speeds but as best I can tell, this was the fastest Ive been hands free on fairly level road. I'm guessing that going downhill keeps more weight on the front tire so thats why I have not had this happen before. My knee is hurting too much to tackle the mountain anytime soon so it may be a few days before I can test this theory.
This is the same bike (XL Trek 520) I suspected a thump in when I first got it. That has went away. The bike rides as smooth as silk and coast as smooth as silk. I have even got to where I will coast hands free around a fairly sharp curve at 15 mph. I checked all the bolts on the stem and handlebars and all feel tight. Just wondering if there is something going on I need to get fixed. Money is tight but I don't want to spend more later or have a wreck sooner...
PS this all happened unloaded except for a couple of water bottles. the racks etc are all tight.
Interesting...I have a similar problem with my 2006 Trek 520. When I first got the bike I could ride it no handed with no problem. Recently, I've put the Surly Nice Rack on the front fork and now I get a pretty severe front end shimmy when I ride hands free. Otherwise it rides nice and steady. The rack looks like it is properly centered. The Surly rack is pretty heavy, so maybe its the extra weight? BTW, I haven't tested it fully loaded with no hands yet.
"Shimmy is not related to frame alignment or loose bearings, as is often claimed. Shimmy results from dynamics of front wheel rotation, mass of the handlebars, elasticity of the frame, and where the rider contacts the bicycle. Both perfectly aligned bicycles and ones with wheels out of plane to one another shimmy nearly equally well. It is as likely with properly adjusted bearings as loose ones. The idea that shimmy is caused by loose head bearings or frame misalignment seems to have established currency by repetition, although there is no evidence to link these defects with shimmy. "
Interesting...I have a similar problem with my 2006 Trek 520. When I first got the bike I could ride it no handed with no problem. Recently, I've put the Surly Nice Rack on the front fork and now I get a pretty severe front end shimmy when I ride hands free. The Surly rack is pretty heavy, so maybe its the extra weight?
Try attaching a pair of visegrips to the forks/bars to see what effect you get from moving the centre of gravity of the steering assembly backwards or forwards.
I had a shimmy problem with an old Centurion touring bike. However, the shimmy would always develop with my hands on the handlebars on fast downhills. After a couple of accidents, I became quite nervous on very fast downhills. The shimmy was far worse if I was nervous about the speed. I'm not sure what that says about the origins of the shimmy. I recall one occasion where the entire bike was shaking unbelievably violently. It felt like the frame might possibly fall apart while I was on it, and I had great difficulty in braking and stopping the bike on that occasion. When I got a new bike, the problem went away, even though I would still get nervous when I built up great speed on steep downhills.
The shimmy was getting worse, and would occasionally disappear if the rack was really weighted down. While replacing my front brake pads, I noticed that the fork would move a little when the front wheel was braked. Turns out the headset was a bit loose. Tightened it up and the shimmy is gone.