front end wobble on bike

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2008-09-19 9:00 PM

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Gilbert, Arizona
Subject: front end wobble on bike
I was riding down a hill the other day on pavement that was a little rough and my front end really started wobbling.  I have carbon forks and I was going over 30 miles an hour before I had to start breaking.  I am new to road biking and I have never experienced this before so I was a little freaked out.  I have been close to 30 mph on smoother pavement and not had a problem.  After the ride I checked things out and nothing seems obviously loose, and it didn't do it on any other part of the ride.  Does anyone have an idea what might be causing this?

2008-09-19 9:14 PM
in reply to: #1685470

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Subject: RE: front end wobble on bike
Maybe it was the road if the smooth pavement presented no problems, try going down the hill again, or just spin the tire with your hand and check for a wobble or bend in the rim.
2008-09-20 10:30 AM
in reply to: #1685470

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Champion
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Tacoma, Washington
Subject: RE: front end wobble on bike

Front-end shimmy, death-wobbles... What's happened is that the harmonics of your rider/bike system (the interaction of the stiffness of the frame and fork, where your weight is on the bike, the amount and frequency of bumps in the road) have found a resonance frequency, so instead of damping each other off, they build on each other.

First, make sure that (1) your headset isn't loose, then (2) your front quick release is tight enough.

When wobbles happen, though, there are many things you can do, but the basics of the situation is that you need to change something to change the harmonics of the system. You can shfft your weight forward, which often helps. You can start pedaling hard to get past the frequency that the pavement is inputting. But one of the most effective methods, though, is to clamp your thighs on the bike top tube, which radically changes the natural frequency of the frame.

2008-09-20 10:38 AM
in reply to: #1685758

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Elite
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Armpit of Ontario
Subject: RE: front end wobble on bike
briderdt - 2008-09-20 11:30 AM

Front-end shimmy, death-wobbles... What's happened is that the harmonics of your rider/bike system (the interaction of the stiffness of the frame and fork, where your weight is on the bike, the amount and frequency of bumps in the road) have found a resonance frequency, so instead of damping each other off, they build on each other.

First, make sure that (1) your headset isn't loose, then (2) your front quick release is tight enough.

When wobbles happen, though, there are many things you can do, but the basics of the situation is that you need to change something to change the harmonics of the system. You can shfft your weight forward, which often helps. You can start pedaling hard to get past the frequency that the pavement is inputting. But one of the most effective methods, though, is to clamp your thighs on the bike top tube, which radically changes the natural frequency of the frame.

X2. stopping death-wobbles with a death-grip using your legs works like a charm everytime. Buttpuckering is optional, but often unavoidable.

 

 

2008-09-21 10:28 PM
in reply to: #1685470

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Gilbert, Arizona
Subject: RE: front end wobble on bike
Thanks for the ideas.  I will be hitting that hill again this week, so I'll give it a try.
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