Speed wobble on a 2-wheeled bent?

DIscuss anything relating to non-standard cycles and their equipment.
Post Reply
UpWrong
Posts: 2409
Joined: 31 May 2009, 12:16pm
Location: Portsmouth, Hampshire

Speed wobble on a 2-wheeled bent?

Post by UpWrong »

Anybody ever had this happen to them? It happened to me yesterday whilst descending at around 25 mph on a road with many patched repairs. The handlebars were oscillating at more than 5 times a second. I had plenty of time to ponder what to do. I stopped pedaling and relaxed my grip (a mini cooper hovering within a few feet of my rear wasn't helping). Eventually it faded out. I had travelled something approach 100m I estimate.

So today I've taken the lock out of the single rear pannier and have stowed it behind the seat. My guess is that there was too much weight behind the rear axle and the bumpy road started a harmonic oscillation.
User avatar
[XAP]Bob
Posts: 19793
Joined: 26 Sep 2008, 4:12pm

Re: Speed wobble on a 2-wheeled bent?

Post by [XAP]Bob »

Not with hands on the bars...
A shortcut has to be a challenge, otherwise it would just be the way. No situation is so dire that panic cannot make it worse.
There are two kinds of people in this world: those can extrapolate from incomplete data.
Geoff.D
Posts: 1982
Joined: 12 Mar 2010, 9:20pm

Re: Speed wobble on a 2-wheeled bent?

Post by Geoff.D »

I'd be checking the obvious possibilities first, as I'm sure you have. Tyre pressures. Tyre wall condition. Broken spokes, Stiff/loose headset. Wheels running true.

However, it does sound as if the problem is connected to that particular stretch of patchy road. If you descend at 25 mph (with the same stuff on board) on another stretch of road without the wobble, then I'd put it down to the patches. It's true that your arms will dampen the effect of any oscillations, but not necessarily eradicate it unless they're (your arms/muscles/ligaments/etc) "tuned" exactly to do so.
UpWrong
Posts: 2409
Joined: 31 May 2009, 12:16pm
Location: Portsmouth, Hampshire

Re: Speed wobble on a 2-wheeled bent?

Post by UpWrong »

I think it was a combination of the loading, the bumpy road, and being too tense with a car hovering at my rear. The shimmy dissipated when I eased my grip on the bars. It's not happened since and I've been at higher speeds.

On an upright the advice is to come forward of the saddle and/or grip the top tube with your knees. Not really an option on a bent. I wonder if gentle braking of the rear wheel would have helped or not. As I said, I had plenty of time to consider the options but my priority was not to aggravate it. It was alarming but I was still going in a straight line at the time.
Geoff.D
Posts: 1982
Joined: 12 Mar 2010, 9:20pm

Re: Speed wobble on a 2-wheeled bent?

Post by Geoff.D »

I know what you mean about being tense. When I hit a rough patch at speed I consciously try to relax. I've never had oscillations, but I have had a lot of bouncing around. I don't worry about it because the frame is stiff, the geometry good and the bouncing is much a result of lack of suspension. I start the relaxation in my shoulders and right down to my hands. Sometimes I've even made "circles" with my index finger and thumb and then let the bars ever so slightly "shimmy" within the circle, thus almost disconnecting the steering from my body. It's a matter of trust that the bike will cope with the bumps, and only needs minimum input from me to cope.

I'd actually quite like to try a wider tyre up front, to gain some suspension and avoid the harshness, but the maximum I can fit is 1.35. Pity.
Post Reply