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Deciding if this is a speed wobble/shimmy

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Deciding if this is a speed wobble/shimmy

Old 10-14-17, 01:20 PM
  #1  
ktk90w
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Deciding if this is a speed wobble/shimmy

I recently bought a used Surly Ogre (29" steel frame.) I'm experiencing a weird instability starting at around 25-30 km/hr and I am trying to figure out if this is what people call a speed wobble or shimmy.

The feeling I'm getting reminds me a lot of when my physics professor had us hold a bicycle wheel while it was spinning, and try to turn it from side to side. The spinning wheel causes a force that resists the lateral motion. It's not violent, but it feels like the front wheel has a mind of its own. I can't tell that it's getting worse with higher speed; it just starts being unstable and stays that way until I slow down.

I read what Jobst Brandt had to say on speed wobbles or shimmies, and watched a video or two of people experiencing a speed wobble. But what I feel is a slower motion and just makes me feel like I really have to control the bike carefully when I'm coasting downhill.

The bike's got some pretty worn-down Specialized MTB tires on it right now. I'm going to try putting road tires on it, but I wanted to see if anyone had experienced this and had some idea of what else I could check and/or change on the bike. Am I just describing a speed wobble?

Thanks!
ktk
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Old 10-14-17, 01:37 PM
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Sounds like a type of speed wobbles. This is a broad category that includes all sorts of harmonic steering issues.

Notvall are the same, and there are various causes and solutions.
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Old 10-14-17, 03:05 PM
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I'd go ahead and change the tires. Problems like this are so inter-related, the tire change is likely to make it go away.
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Old 10-14-17, 03:14 PM
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I would check the headset bearings for play just in case. Also you could look at the wheel alignment and centering in the frame/forks.
Sometimes a speed wobble can be damped by pressing one of your knees against the top tube, but it's better when it doesn't happen.
Steve
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Old 10-14-17, 05:14 PM
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Too Tight or rusted head set.

bike wanders from notch to notch, back and forth...

"Slow Wobble"... first time i've heard it called that... extra points for creative description!

lift the front wheel... see if the steering feels notchy or "slow"... bet it does.

ideally, the weight of the bars/controls should make the steering flop to one side or the other without help... if it doesn't, it's too darn tight or rusted.... some cable housing resistance is normal...

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Old 10-15-17, 05:33 AM
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Thanks for the help. That gives me several places to look. I was having trouble getting the right adjustment on this headset, so I might start there.

I'd not been able to calm down this problem by putting a knee against the top tube but I did discover yesterday that pushing my weight backward over the saddle eliminates the problem.

I'll keep working on it. Thanks again!

ktk
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Old 10-17-17, 04:31 AM
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+1 to Maddog34. At that low speed, its probably a tight headset that is not allowing the front wheel to self-center and the bike feels like it is wandering from side to side with each pedal stroke.
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Old 10-25-17, 06:35 PM
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Yep, you all were right. The bottom bearings on the headset were damaged. I hadn't noticed that yet and that's why it was so hard to get the bearing adjustment correct. Some combination of the bad bearings, bent retainer, and slightly mis-adjusted headset caused the wobble I was seeing. I'm getting it put back together now, but it looks like the problem is solved.

Thanks for the help!
ktk
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Old 10-25-17, 07:30 PM
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Old 10-25-17, 08:16 PM
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Originally Posted by ktk90w View Post
Yep, you all were right. The bottom bearings on the headset were damaged. I hadn't noticed that yet and that's why it was so hard to get the bearing adjustment correct. Some combination of the bad bearings, bent retainer, and slightly mis-adjusted headset caused the wobble I was seeing. I'm getting it put back together now, but it looks like the problem is solved.

Thanks for the help!
ktk

Not sure I agree with this yet. I don't think you've ridden the bike since this take apart if I read correctly. Damaged ball retainer is very unlikely to be any cause. Worn balls maybe, rusted ones not usually. "brinelled" bearing track, sure but not an if/then that's an always true. There are so many bikes with worn out headsets that don't shimmy so saying that you've found the problem before actually confirming it with an after service test ride is likely premature.


Now I'm not saying that the headset issues are not a problem unto themselves. They do need correction. BTW there's a strong chance every other bearing on this bike could use servicing. Andy.
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Old 10-28-17, 05:00 PM
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Your frame is out of alignment. Search here how to measure and straighten the frame.
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Old 10-28-17, 08:57 PM
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Originally Posted by San Rensho View Post
Your frame is out of alignment. Search here how to measure and straighten the frame.

Agreed that frame/fork/wheel alignment can contribute to shimmy. But if this cause was absolute then many more bikes would shimmy. Again any miss alignments are bad and corrections of these are only good, but not a guaranty of no shimmy. Andy
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Old 10-29-17, 07:01 AM
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I have always found that most of my cycling problems, especially chain lubrication, can be solved with "Up Dog".
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