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Old 06-08-09, 01:33 PM   #1
yangmusa
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Good, good, good vibrations... (or, wheel weight imbalance at high speeds)

This weekend I took BART to Walnut Creek and rode up Mount Diablo, then back via Danville and the Iron Horse Trail. About 45 miles or so.

There are some fairly steep sections, and I had went pretty fast on the way down. I find the Birdy's handling very confidence inspiring, I was the fastest descender in the group (which also included a Moulton and a 3Rensho).

However... Above a certain speed (30 mph?) the whole rear of the bike started vibrating. It's disconcerting, but didn't seem to affect handling. I've noticed this before when I had the bike suspended and cranked the rear wheel up to high speed. I still have the wheel reflector on since I also commute on the bike after dark. I wonder if it could be enough to cause this unbalance at high speeds? I read elsewhere on BikeForums that it might - and can only imagine that the problem is worse on small wheels since they rotate more times for a given velocity.
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Old 06-08-09, 02:36 PM   #2
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A reflector? Seems like a pretty light thing that is centered from a lateral perspective. If it was something about its position radially, why wouldn't the valve affect steering?

Well ... this should be an easy test. Take off the reflector and ride downhill. See what happens.

On full size bikes, there is a recommendation of placing a knee against the top tube to reduce a shimmy.
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Old 06-08-09, 02:50 PM   #3
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Originally Posted by invisiblehand View Post
A reflector? Seems like a pretty light thing that is centered from a lateral perspective. If it was something about its position radially, why wouldn't the valve affect steering?
I guess you're right - the easiest way to find out is to take it off and see...

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On full size bikes, there is a recommendation of placing a knee against the top tube to reduce a shimmy.
I don't get shimmy, front-end or otherwise. And the "top tube" on the Birdy is too low for me to do that on. At least without some acrobatics that I'm not going to engage in while bombing down hill
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Old 06-08-09, 03:45 PM   #4
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I don't get shimmy, front-end or otherwise. And the "top tube" on the Birdy is too low for me to do that on. At least without some acrobatics that I'm not going to engage in while bombing down hill
I thought about that. It crossed my mind that with a low monotube we could put a foot on top of it.

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Old 06-08-09, 11:22 PM   #5
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Check the rear suspension pivot for some side-ways play. Wild guess here.
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Old 06-10-09, 02:05 PM   #6
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I checked the rear pivot. Thankfully no play there!

I hung my bike up and spun up the rear wheel. The back end shook all over the place! Then I took off the rear wheel reflector, and it was quite an improvement though still quite noticeable. Finally, I bit the bullet and changed out the original inner tube with a smaller one fitting the Stelvios better. The original was so big, it was bunched and folded in one particular place! With the new innertube fitted and the wheel back on, the vibration at high speed is almost gone - though now I noticed that the back wheel is slightly out of round - I'll have to borrow a friend's wheel truing stand to fix that.

I think all these small things are probably accentuated by the small wheel size, since it rotates so much faster for a given speed. But it seems like I've pretty much solved the problem now, so all it well
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Old 06-10-09, 06:52 PM   #7
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Just a thought for what it's worth, probably not much - but you never know:

Lift the rear of your bike and see if your rear wheel will rotate freely on it's own. If it's balanced it won't. If it does, it should finally come to a stop with the heaviest part of the wheel at the bottom after a bit of "see-sawing." With welded rims the inflation valve would usually end up at the bottom. With "tongue-pressed" rims the valve usually ends up top (the metal tongue is heavier than the valve). Clamp your reflector opposite of the heavy side (or the magnet sensor trigger for the cyclocomputer if you've got one).

It might get better still.
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