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Discussion Starter · #1 ·
Yesterday when riding my '86 Trek without hands going downhill at about 15 mph the front wheel/handlebars began to rapidly shaking back & forth, so much so that it scared me and forced me to grab back on. It happen again on another descent so I ruled out the road surface as a cause. With hands-on though I don't notice the wobble even at speeds in the mid 30's. Question is should I be worried? Could this indicate a serious safety problem with the bike?

Note that prior to this I had my LBS do a $69 tune-up on the bike to make sure it was still fit to ride after being in the garage unused for several years. Also the bike has never been crashed (short of one or two tip-overs at traffic lights 10's of years ago when clipless was new.) Tires are brand new Conti Gatorskins that I'm running at 90 PSI.
 

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Discussion Starter · #2 ·
I should mention that if I lift the front of the bike off the ground and spin the wheel by hand it looks perfectly balanced and true.
 

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ragweed said:
Yesterday when riding my '86 Trek without hands going downhill at about 15 mph the front wheel/handlebars began to rapidly shaking back & forth, so much so that it scared me and forced me to grab back on. It happen again on another descent so I ruled out the road surface as a cause. With hands-on though I don't notice the wobble even at speeds in the mid 30's. Question is should I be worried? Could this indicate a serious safety problem with the bike?

Note that prior to this I had my LBS do a $69 tune-up on the bike to make sure it was still fit to ride after being in the garage unused for several years. Also the bike has never been crashed (short of one or two tip-overs at traffic lights 10's of years ago when clipless was new.) Tires are brand new Conti Gatorskins that I'm running at 90 PSI.

Loose headset? (is must a threaded headset right?)
 

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Resonance

ragweed said:
Yesterday when riding my '86 Trek without hands going downhill at about 15 mph the front wheel/handlebars began to rapidly shaking back & forth, so much so that it scared me and forced me to grab back on. It happen again on another descent so I ruled out the road surface as a cause. With hands-on though I don't notice the wobble even at speeds in the mid 30's. Question is should I be worried? Could this indicate a serious safety problem with the bike?
Agree with 02tones02 to check the headset. Any kind of shimmy, whether hands on or hands off, is all about exciting resonant frequencies. When you take your hands off the bars, you change the mechanically coupled system and so can have a bike that resonates easily when riding no handed. There's not a lot you can do about it (check that headset adjustment) and since you don't ride no-handed much, it's not really an issue.
 

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Could be a number of different things, front wheel though probably not it. May be the fork, could be headset.

FWIW my DeRosa has a shimmy up around 20mph+ when I take my hands off bar. Simply resting my knee against the top tube stops the shimmy.
 

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It doesn't hurt to check the headset/front wheel bearings for looseness.

The way I check the headset: while standing next to the bike grab the front brake and try rocking the bike back and forth. It should be fairly solid with maybe some flexing coming from the forks but no clicking. Clicking indicates a loose headset.

The way I check the wheels: while standing next to the bike grab the wheel and try to rock it sideways relative to the bike. Again, you are looking for clicking.

Both tests check for looseness, not overtightened components.
 

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One more thing to check; make sure the wheel axles are sitting properly in the dropouts. It's possible that when the LBS pulled your wheel, they put it back in slightly cockeyed, which might cause a "new" shimmy on an old bike. Pop the wheels, and put 'em back in, and see if that doesn't fix things. Certainly easy enough to try.

You could also get a string, to see if the frame is out of alignment. You mentioned that the bike had never been dumped, but maybe it got "virtually dumped" by, e.g., spouse's/roommate's car, or kids in the garage, or even at that bike shop. This one is doubtful, but again, it's easy enough to check.

You may know this, but grabbing the top tube between your knees (not code) will often arrest the shimmy.
 

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Discussion Starter · #12 ·
Thanks for the excellent list of things to check for! I'll print this thread out and give the bike a look-over this weekend.
 

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I have this identical thing going on. No handed, relaxing, the frontend starts to wobble.

I have another model of the same bike, same size geometry, different wheels, different hubs-cartridge bearings-no wobble. One is 631 Reynolds and the wobbler is the 520 Reynolds with ball & cone hubs.

Also the 520 wobbler has a very long steerer tube/wider handlebar. Which I'm going to cut down and get a narrower bar. Could the long steerer/wide bar make it prone to wobble?


I'm rebuilding the 520 one right now and am toying with maybe a new headset. The one on it is a cheapo headset. I'm thinking maybe go to a sealed bearing model.

Also the BB shell was not faced at intitial building in Taiwan, what are the odds the head tube is not faced?

I hate to dump the coin on a new headset/face job and the thing still wobbles...
 

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I had the same symptoms on my old Lemond. It was my winter bike, so I didn't really care. Eventually bought new wheels, problem solved.
 

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Are Treks good bikes?

It could be a misaligned frame or a headset issue. It could also be a center of gravity/weight distribution issue. Too much or too little weight over the front wheel will give you the same shimmy. You mentioned that you hardly noticed it with your hands on the bars at higher speeds, but that sounds to me as though it's still there but your hands are keeping it "under control."

After checking the headset and front wheel bearings, try descending again but shifting your weight slightly fore or aft as you descend. This should tell you if you're weighting or unweighting (I know it isn't a word) the front end. Do this on a mild descent for safety reasons, you want to keep the pucker factor to a minimum.
 

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Discussion Starter · #16 ·
The main contributor to my original shimmy problem turned out to be the new 25mm Conti Gatorskin tires I had installed. I say that because after putting a few hundred miles on them the shimmy became significantly reduced. It also only occurred on certain patches of road so there must be some kind of interaction between the road roughness and tire tread.
 

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My old commuter did that. No hands at whatever speed the front end would shake. Everything checked out ok. I eventually replaced the tires (Conti Town & Country) and it never happened again. The tires were the last thing I would have thought of faulting.
 

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I had same problem, tried everything, was out riding and hear some strange noises, LSS the spokes on my rear wheel were coming loose. Bike shop that did build re-trued and tightened and problem solved
 

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That's funny you should say that. I just dropped off the wheels to get trued up by my wrench. The front wheel looked fine while the rear one had 2 abnormally loose spokes and there was a very slight clicking which sounded like it was coming from the rear wheel which I was atributing to the loose spokes rubbing and then finally shifting on each other.
 

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It's good practice to squeeze the spokes regularly to check for loose ones so you can tweek them back to tension.
 
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