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Thread: Front end shimmy, trail and weight distribution

  1. #21
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    Default Re: Front end shimmy, trail and weight distribution

    Quote Originally Posted by Andrew R Stewart View Post
    had hoped for some general guidelines that I didn't already know.
    Don't make bikes out of steel?

  2. #22
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    Default Re: Front end shimmy, trail and weight distribution

    I discovered this thread and velocipede (yes, I'm new here) a couple weeks ago while on a multi-day bike tour with my daughter--our first such tour.

    I picked up a ~1990s vintage Giant Sedona hardtail mountain bike that was in very good working condition, and came w/ a rear rack ready to load. I swapped out the mtb tires with Schwalbe Marathon 2"-wide tires, better for road touring. We loaded the rear rack with 27 pounds of gear--two big panniers & a bikepacking tent strapped to the top of the rack.

    All went fine until the first significant descent when we got going ~20 mph, give or take, her bike shimmied and wobbled violently, almost causing loss of control. Fortunately, she reacted quickly with both rear & front brakes to slow down, and lose the shimmy. Very scary moment, as we were on a ~3-foot shoulder on a busy highway. Loss of control could have been deadly.

    The bike is a chromoly frame, ~1" tubes I believe, 18", pretty standard issue hardtail. 26" tires.

    She did some descents unloaded, and no shimmy problems. One tip she tried was press knee firmly against top tube So, after googling and reading up on this problem, I have some thoughts and/or questions. Any insights on this would be greatly appreciated.

    1. I attribute the problem we experienced to a grossly unbalanced weight distribution; all of gear loaded onto rear rack, and virtually none (except a water bottle) on the frame or front. We're thinking next time: (a) bring less gear and (b) better distribute the gear, front, mid, and rear.

    2. She was riding kind of high in the saddle; less than optimal saddle height because we were lazy, and she had a tent packed under her seat making adjustments more difficult. I'm guessing dropping the saddle down ~ 1/2" to 1" might have made a big difference.

    3. Tires: The Marathons we got had an inflation range of 30-70 psi. I ran them at 60 psi, which is pretty stiff. I'm thinking running them low, or trying an even fatter, lower pressure tire might help dampen some vibration. And had we left the knobby mtb tires on, the play in those tires may have dampened vibration, although she would have had much worse rolling resistance on the highway. (other reason to focus on tires: all the bumps, crevices, and canyons we encountered on the road were kind of punishing).

    4. Is this bike usable as a touring bike? Or is fiddling around with the above parameters likely to lead only to more frustration and potentially dangerous situations? Daughter weighs about 120 pounds; if she carried load of ~25 pounds, the total load seems quite manageable for any adult-sized bike to carry. But if we're likely to have shimmy problems no matter how we distribute the load, maybe our time & money would be better spent on a different bike rather than trying to make this one work for us.

    Again, useful insights appreciated!

  3. #23
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    Default Re: Front end shimmy, trail and weight distribution

    I would say it is #1 - weight distribution. You have a decent load that is all on the rear and sitting fairly high.

    If you google "low rider bike panniers" you will likely find examples of the traditional way to distribute full-on loaded touring gear. I am sure there are more modern ways that people are doing this for shorter trips and someone will chime in on those set-ups. The low riders can take a decent amount of weight and keeps it distributed around the front axle (not up high).

    My experience with touring tended to be 1 to 6 week self-supported adventures which meant I carried a fair amount of stuff. I never experience front end shimmy when fully loaded with the weight on the front and rear. Trying to get equal weight side to side is important and the old rule of thumb was to put ~40% of the weight in the low rider panniers.

    You can also google "great divide race bike" and see how people carry loads for a multi-day off road race. Also self-supported. These folks only bring the bare minimum to survive a very intense event.
    Brian McLaughlin

  4. #24
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    Default Re: Front end shimmy, trail and weight distribution

    I agree with Bewheels; it's weight distribution.

    It could be the speed factor alone that caused it, or the shift in weight distribution while going downhill (mass shifts toward the front of the bike.)

    It's all caused by the amount of weight toward the rear of the bike, and the higher it is the more potential for shimmy.

    I have a sport touring bike with which I commuted to work for decades. Actually, I've been through three of them.

    All three had a rear rack. With loaded panniers, shimmy was not an issue.

    If I commuted to my part time job at a bike shop, and strapped my change of clothes and my mechanic's tool bag to the top of the rack, shimmy was clearly an issue if I took my hands off the bars.

    Some panniers mount lower than others; some racks are designed for lower mounting of panniers. Those are two options.

    You can also shift the load to a pair of front panniers. I had a friend who rode front panniers exclusively for their ability to damp shimmy. Front panniers don't usually have as much capacity as rear panniers but they are a third option.

    The popularity of bags which fit inside the main triangle would be another solution.

    If the bike rides fine unloaded, then the bike is not the issue.

  5. #25
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    Default Re: Front end shimmy, trail and weight distribution

    Peter & Bewheels, thanks for your thoughts. Supports what my gut feeling is about how to solve this issue.

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