Bicycle Mechanics - What is behind my low-speed wobble?
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10-20-10, 09:55 AM
So about a week ago, on my way home from work, I was rounding a turn at a residential intersection. Going maybe 10mph, my inside pedal connected with the pavement and nearly threw me off the bike. The only apparent damage was a pretty good scrape on the corner of the pedal.
So on my next bike commute, I noticed an extremely annoying wobble at low speeds (10-15mph). It resonates all the way from the handlebars to the seatpost. I thought, oh no, I bent my frame! But after reading a little online it sounds like pedal strikes are fairly common and no one seems to have bent a frame doing it. (BTW the bike is an '08 Volpe, steel frame)
So here are the other possible culprits that I can think of:
(1) I did tighten up my...headset, I think? This is the allen bolt at the top of the stem; I cranked it down a little because I just got the bike and found some loose bolts. I also tightened up the bracket that holds the front brake cable housing to the stem.
(2) I just noticed that the rear wheel is pretty out of true. Might be my riding weight (~210 w/backpack) is too much for the stock wheels.
Would either one of these or both result in what I am describing? Should I be looking at something else?
Thanks for any help you can offer.
10-20-10, 10:23 AM
Shimmy is a function of you and your bike resonating at a certain frequency. There can be a number of causes, but a couple of them come to mind:
1) Tightening the headset makes it stickier, which requires more steering input to keep on track. At a certain speed, the speed of steering correction reaches the frequency that the correction takes effect, which makes things wobble. To help correct this, adjust the headset correctly. If you can't ride no-hands, something is wrong- the steering should be really free-moving without any lash. The more work you have to do to keep the bike going straight, the more likely it's going to shimmy.
2) The accident could have thrown off the steering geometry, making the resonance occur at a different frequency than it ordinarily would have.
3) The rear wheel could be more flexible after having a few spokes loosen up, which would change the resonance. Being out-of-true doesn't help; the wheel wobble could be adding to the shimmy. Get your wheel trued and tensioned- it's safer and stronger.
4) Wearing a backpack changes the resonance of you and your bike. You should get some panniers and a rack.
To test for shimmy, try resting your knee against the top tube- if it goes away, then it's classic shimmy. Anyway, it's best to eliminate all of the potential causes. Shimmy is scary and dangerous.
At the speed you're feeling it, I suspect it's simply because of the out of true rear wheel. However make sure that the headset is free enough that the steering is totally frictionless while tight enough that you can't cause any fork movement if you either bounce the front end, or push the bike forward and back with the front brake on.
BTW- you say you tightened the headset by tightening the top cap. That won't do the job, You have to loosen the stem clamp screws so the stem is free and the top cap screw can push it down against the headset, then after adjusting tighten the stem to keep it there.
10-20-10, 10:48 AM
If it is a threadless headset you didn't tighten the headset you just tightened the cap. The stem needs to be loosened then tighten the cap bolt, then re-tighten the stem. But to check if the headset is loose, pull the front brake lever tight and rock the bike back and forth, if the headset is loose you will feel movement in the headset otherwise it is fine.
Also if it is a threaded headset, all you did was tighten the stem.
an untrue rear wheel could be the culprit.
10-20-10, 12:29 PM
Low spoke tension in the wheel can cause a wobble. How many spokes in the back wheel?
10-20-10, 12:45 PM
Far more likely that when the bike re-landed after it skipped sideways from the pedal strike that you deformed the rear wheel and now that is what you're feeling. Good hand built wheels can often survive such things with little or no damage. But if this is a more basic bike with machine built wheels that have not been tensioned and trued anytime recently then you could easily pull the wheel out of true. If you have the skills then re-true the wheel and it should be fine. If you don't then have a shop do it. For a few bucks more they could also go around it and balance the tensions up a bit and add a little more tension to the spokes if they (very likely) need it and fix the wobble all at the same time. If you get this extra done then the wheel will be far more durable for years to come. And you are not too heavy for the wheel. Just that the regular stock machine build does not properly bed the spokes in so the wheel needs some touching up over the first year. Or a good builder can stress relieve it and balance the tensions and true it up all on one go and it will be stable for a long time.
10-20-10, 12:56 PM
Thanks for all of the great input! I am relieved that it sounds like, in my ineptitude, I actually did no harm whatsoever with the headset. And it is even more relieving to know that the wheel is so likely to be the culprit. BCRider, your theoretical explanation for what could have happened is entirely believable. It seemed so strange that I wouldn't have noticed before that the wheel was so out of true.
I think I will pick up a spoke wrench and read up on how to tighten things up. If I can't do it, I imagine my local co-op can. Oh, and davidad, to answer your question, Bikepedia says the rims are 32-hole so I presume that means there are 32 spokes.
10-20-10, 02:34 PM
Low spoke tension in the wheel can cause a wobble. How many spokes in the back wheel?How low of tension do you need for wobble?
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