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Old 01-14-06, 04:22 PM   #1
Lobalobo
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Shimmy in True Rear Wheel

On an old hybrid Trek bike I use to transport my 3-year-old (in a seat attached to a rear rack) the back wheel begins to shimmy at about 10 miles an hour, gets worse if I go faster than that, and is worse when there is weight on the wheel (that is when my 45 pound son is in the seat). Here's what I've checked: (1) the wheel is true, or is best I can tell by judging it on a bike rack, spinning and watching the rim against a fixed point on the frame; (2) there are no broken or obviously loose spokes, so I see no reason that the wheel would lose its shape under pressure (though this could be it anyway I suppose); (3) the wheel is set completely and clamped tightly into the drops. The only thing that I see even remotely off is that the tire might be slightly irregular; not sure this is the case, as it could be my imagination, and I've put it on and off the wheel twice. Anyway, I doubt this could cause of what I observe, as the shimmy is too pronounced at high speed and not evident at all at low speed. So I'm stumped. Any suggestions are welcome.

Thanks.
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Old 01-14-06, 04:54 PM   #2
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It could be a number of things. Spin the back wheel and look at the tire very closely. It could be the tire. It happens once in a while. Even though it seems like the back tire it is more likely the front tire as you have made the bike very rear heavy and front light, with the kid in the child seat. Check the front wheel and tire just as carefully as the back. This is more common. Also try putting a lot more weight on the front to see if the shimmy goes away. This is common too. It may be hard to fix if it's just the weight on the rear.
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Old 01-14-06, 05:05 PM   #3
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Do you have the "slime" filled puncuture proof tubes on it? If so, and the bike has sat in one position for an extened period of time, all the slime can run to one spot inside the tube and kind of take a "set", causing the wheel to be imbalanced and shake at speed.
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Old 01-14-06, 05:22 PM   #4
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Check for loose hub bearings.
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Old 01-14-06, 08:14 PM   #5
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Thanks to all for the responses. I'll check everything and let you know what I find (on the chance any of you are interested to know out of curiosity or if you keep track of problems and solutions for future reference). Although my repair attempt will wait until morning, the most intriguing suggestion is the slime theory. The fact is that the shimmy did not start until after the bike had sat in one spot for quite a while, which makes it seem that this is the likely explanation except that I'm almost sure it is not a slime-filled tube. I hope I'm wrong as this is the easiest and cheapest fix. In any case, I've learned something;I'm never going to use a slime-filled tube on bikes I use only on weekends, particularly because I sometimes skip a weekend. Thanks again.
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Old 01-15-06, 05:45 PM   #6
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What you are seeing is something present in all bikes at some particular speed.

The front wheel, being a spinning rigid body, has a property called nutation which is a tendency to oscillate around the spin axis. The frequency of oscillation is proportional to rotational speed.

Normally, you won't see anything because this oscillation is suppressed by the rest of the bike being attached to the front wheel. However, if the bike has the possibility to flex, especially like with a rear load, then this flexing will have its own natural frequency; at a particular speed the nutation frequency of the front wheel will match the wobbling frequency of the bike+load and shimmy is the result. The entire bike will shimmy and gets worse if you increase speed.

What you can do to check if this is your problem, is to take the bike by the saddle and handlebar and shake it from side to side. Is the child seat + rack wobbling from side to side? If so, try to stiffen the setup. This will not eliminate it (that is impossible) but it will make the speed at which it happens higher.
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Old 01-15-06, 08:26 PM   #7
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Trueness involves not only lateral runout on the wheel but also radial or out-of=roundness. Be sure you check that.
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Old 01-16-06, 08:41 AM   #8
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Thanks so much; I was snowed out of attempting a repair this weekend. But I'll post what I find.
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Old 01-16-06, 10:29 PM   #9
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Although I still haven't had a chance to ride the bike (too much ice on the road, particularly for a trip with my son on it), my bet is now with nutation. When I examined the bike, as suggested by jur, I discovered that the Rhode Gear child seat had been twisted from use to the point of loosening some of the connections between parts of the seat--the parts did not disconnect, just slid from where they should have been fixed. There was not, I think any chance of the seat failing altogether--I check it before every ride to see that everything is connected--but there is now significant flex and I'm now confident this was causing the shimmy. The seat is off and in the trash--I should have paid more attention to the 40 pound weight limit and to the age of the seat--and I'll bet the bike won't shimmy anymore. (By the way, to get another spring and maybe part of a summer with my kid on the back of the bike--he's not yet four but tall and about 45 pounds and doesn't like being enclosed in a trailer--I may buy a Kettler seat with a 50 pound weight limit; this may not be the right forum, but I'd be welcome to know of a reliable seat with an even higher weight limit.) Thanks again.
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