Peculiar Instability Going Downhill
 

I did a 35 mile jaunt yesterday on my Marin. I've got the factory "dirt/street" Kenda, Krossplus 26"x 1.95" tires on it. I was a little heavier laden with gear than usual, and I noticed that coming down long hills once I got beyond about 25 MPH I started feeling an intense wobble, almost like the whole rear wheel was coming off. I stopped, checked it and everything was kosher. Nothing loose at all. Then I'd start again and get this death wobble. I did some experiments and noticed it only happens when I'm coming down the side of the road on light road sand and gravel. On good solid pavement I feel no wobble even at high speeds.

Given that roadside sand and gravel is ubiquitous in Alaska, I'm wondering if I should be eyeing a different tire. Speed is not important, but I don't want to wipe out! I've never been a fan of the dirt/street concept and actually don't mind light knobbies for what I do.

It's called a speed wobble or death wobble. Touring cyclist (at least those who carry their own gear) are quite familiar with it. It's a harmonic oscillation that occurs on vehicles with a single pivot point for steering. Some people think that it's due to a weak top tube or other frame member. Often, you can stop it by touching your leg to the top tube. The fact that you had more gear then usual also contributed...especially if you had most of the gear piled on a rack. Lots of racks aren't as rigid as they could be and contribute to the oscillation.

The tire, by the way, has little to do with it.

Interesting. Can the problem be reduced by keeping the gear lower to the center of gravity or something?

I've had that problem when riding fully loaded during tour. Like Cyccommute said, pressing one leg to the top tube seems to help, particularly when cornering going downhill. This is where low-center of gravity front rack and panniers can help, it seems.

All I can say is maybe. I've had a steel touring bike that developed a death wobble around 45 mph with a load (never without) and my daughter's steel bike does it around 40 with a load. My T800 won't do it for love or money even up to around 50 with a load. These are all bikes that are loaded 60/40% front/rear with the front load on low riders. Loads carried high might exacerbate the problem but putting the load low doesn't completely cure it. Time to experiment:D