budopo

I'm trying to figure out the cause of some front-end shimmy/vibration on a descent.

There's a short, moderately steep hill at the end of my usual ride. I usually hit about 30-33 mph, and at the bottom is a 90-degree turn onto another road (I'm usually at about 25 mph at that point). Usually, I just have my hands on the hoods and tuck down. Since there's no shoulder for that section, I have to take the lane. Near the bottom, I signal to cars that I'm turning right by placing my left hand on the bar top, next to the stem and pointing to the right with my right hand. Often, that's when the front end starts to shimmy (doesn't shimmy every ride).

I first noticed this after I moved my saddle back about 1cm, which is really where it's best for me. It could be coincidence. Also, they've been doing road construction on that hill for a while, so I may be hitting some uneven asphalt, but it does seem smooth, so I don't think it's that.

I checked the headset and hubs for play, and they were fine. I did my annual tear-down/rebuild, making sure there's not play, and I still get the shimmy.

The bike's a 1990 Davidson Impulse (58cm), Shimano RS-80 wheels, Nitto stem (90mm quill), Nitto bar (115mm reach), Cane Creek headset (cartridge bearings).

Thanks in advance for any ideas.

Bmach

Could your hand near the stem be the cause?

Andrew R Stewart 

Classic speedman's wobble (shimmy) is commonly thought to be a harmonic oscillation with the rider's body, the CG of the combined system, the rotating masses, the frame and the fork's stiffness, the steering geometry being primary factors. That it will happen in the "same" place and speed sometimes but not all times suggest that small and overlooked differences in the above (and likely other actors) are just different enough to not trigger or to damp the initial shimmy. Even the rider's clothing comes into play. I had situations where a shivering body has triggered shimmy. Claims of dented headset bearings, out of balance wheels and loose bearings abound although I place less value on these factors.


In your case the move from the bar's outer portions by both hands, one returning to the bars near the stem (which BTW is a good technique to use when riding one handed) and the other extending out changed the forces acting on the bars as well as the steering balance. Other factors likely also changed.


When shimmy happened it can be addressed by your changing the bike body relationship. Classic techniques include pressing one's kenns against the top tube. Same for heels against the stays. Sitting up or lowering one's upper body. Loosening one's grip or stiffening one's arms. Changing pedaling, cadence or direction. And (once again) there are likely other methods. Andy.

budopo

Harmonics as the cause actually sounds reasonable, since it started when I moved the saddle back 1cm, thus moving the cg back very slightly. I'll try sliding forward a bit on the saddle and seeing if that helps. I never got the hang of knees on the top tube, but I never really had the need. I'll give that a try, too.

AlexCyclistRoch

FWIW, sometimes just lifting you @$$ off the saddle will change the harmonic structure.....or, if the vibration is due to insufficient trail on your fork, it might make it worse. Regardless, I'm more concerned that you are riding 1 handed and signalling at 30-33 mph. I'd slow it down.