Advertise on Bikeforums.net



User Tag List

Results 1 to 5 of 5
  1. #1
    Biscuit Boy Cosmoline's Avatar
    Join Date
    Apr 2006
    Location
    Speeenard 'laska
    Posts
    1,358
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)

    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.
    ''On a bicycle you're not insulated. You're in contact with the landscape and all manner of people you'd never meet if you were in a car. A fat man on a bicycle is nobody's enemy.''

    Tom Vernon.

  2. #2
    Mad bike riding scientist cyccommute's Avatar
    Join Date
    Nov 2004
    Location
    Denver, CO
    My Bikes
    Some silver ones, a black one, a red one, an orange one and a couple of titanium ones
    Posts
    15,965
    Mentioned
    24 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by Cosmoline View Post
    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.
    Stuart Black
    Solo Without Pie. The search for pie in the Midwest.
    Picking the Scablands. Washington and Oregon, 2005. Pie and spiders on the Columbia River!
    Days of Wineless Roads. Bed and Breakfasting along the KATY
    Twisting Down the Alley. Misadventures in tornado alley.
    An Good Ol' Fashion Appalachian Butt Whoopin'.

  3. #3
    Biscuit Boy Cosmoline's Avatar
    Join Date
    Apr 2006
    Location
    Speeenard 'laska
    Posts
    1,358
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Interesting. Can the problem be reduced by keeping the gear lower to the center of gravity or something?
    ''On a bicycle you're not insulated. You're in contact with the landscape and all manner of people you'd never meet if you were in a car. A fat man on a bicycle is nobody's enemy.''

    Tom Vernon.

  4. #4
    M_S
    M_S is offline
    All Mod Cons M_S's Avatar
    Join Date
    Dec 2006
    Location
    Face down in a snowbank
    My Bikes
    K2 Enemey Cyclocross franken build; Redline D660 29er, Volpe SS Cross
    Posts
    3,694
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    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.

  5. #5
    Mad bike riding scientist cyccommute's Avatar
    Join Date
    Nov 2004
    Location
    Denver, CO
    My Bikes
    Some silver ones, a black one, a red one, an orange one and a couple of titanium ones
    Posts
    15,965
    Mentioned
    24 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by Cosmoline View Post
    Interesting. Can the problem be reduced by keeping the gear lower to the center of gravity or something?
    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
    Stuart Black
    Solo Without Pie. The search for pie in the Midwest.
    Picking the Scablands. Washington and Oregon, 2005. Pie and spiders on the Columbia River!
    Days of Wineless Roads. Bed and Breakfasting along the KATY
    Twisting Down the Alley. Misadventures in tornado alley.
    An Good Ol' Fashion Appalachian Butt Whoopin'.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •