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High speed jitterbug
Last Post 06/18/2013 09:44 AM by Le Professeur. 10 Replies.
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abrickinthewall

Posts:13

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06/17/2013 04:11 PM
Descending on a curvy, no-shoulder downhill section somewhere above 40 mph (this bike has no computer) this weekend. Luckily there were no other riders or cars close by. My bike went into a speed wobble beyond anything I've ever experienced. This particular bike, an old OCR has a lot of miles. It has sometimes had a gentle shake at high speeds before, but pretty controllable. This time it was violently shaking right up until I got it stopped. After checking my bibs for leakage I continued on, still a good ride. I'm not sure I will ever take that bike downhill fast again. Am I nuts to even use it as a rain-bike?
Orange Crush

Posts:1161

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06/17/2013 04:26 PM
If it's indeed the bike, then that is pretty omnious. I know how scary that shaking can get, had it twice this year. Except it was not because of the bikes, it was because of me, shaking so hard from the cold descending that I transferred that vibration onto the bike. Normally they are dependable descenders.

That's hillclimb training in PNW spring for you.
vtguy

Posts:234

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06/17/2013 04:51 PM
I'd say you're nuts to ever ride it again except on a trainer. I had it happen once with a misadjusted head set -- dreadful sensation!
abrickinthewall

Posts:13

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06/17/2013 05:00 PM
OC you mentioned shaking from the cold. I was using the same bike on a hill climb (Schweitzer ski hill) on a cold, foggy day. Got all sweaty from the climb. Went into a coffee shop for a bit then jumped on the bike to descend. Within a minute or two I was so cold my shaking was uncontrollable. Had to squeeze the brakes and go very slow (still wobbling) until we got into a bit warmer elevation and then was fine. Quite shortsighted of me, but I'd never had it happen to that extent before. Too bad I've gotten attached to that bike.

79pmooney

Posts:1081

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06/17/2013 05:33 PM
Speed wobbles are (almost) never just the bike. They happen when 3 different things add up just right (or wrong). Those things are 1) a cyclical driving force (on bicycles there are lots: wheels of even slight imbalance - how many of us do dynamic balancing with weights and how many wheels hang stem up or down?, pedaling forces, road vibrations, etc. 2) Mass and inertia. Wheel weights, bike weights and our weight all play into this. 3) Stiffness and damping - frame, wheels, bar/stem set-up, how tight or loose we keep our arms and grip - we almost invariably go in the wrong direction when we get scared. How many of us relax our grip when the bike starts to shake?

Both of my newish customs can get the wobbles if I am nervous or cold, especially on less than smooth roads. Both are flawless handlers when I am loose and trusting.

Ben
winterale

Posts:43

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06/17/2013 06:06 PM
Got a couple of bikes that have really flexy frames that I'm fond of so not getting rid of em, that sometimes under right conditions will shimmy. I've found that a knee placed against the top tube will always stop it. Wish I could remember who told me this.
79pmooney

Posts:1081

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06/17/2013 08:44 PM
winterale, that is an old trick, probably learned simultaneously on several continents a century ago. One place it doesn't work is on half my bikes. Pulling that trick is introducing a LARGE cyclical force! (Fix gears!)

Ben
Tortue Volante

Posts:22

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06/17/2013 09:54 PM
Brick,

Did you check to see if the headset was loose? I've experienced some pretty scary moments due to my not paying attention to maitenance. FWIW, this seemed to be more of an issue with the old quill stems than the new stuff.

Ditto on the knee / top tube thing. In fact, I've pretty much made a habit of defaulting to that position when descending. The only time one of my knees are out is when I'm carving a turn.
abrickinthewall

Posts:13

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06/17/2013 11:14 PM
Thank for the input. I did check the headset and there was no play. I forgot the knee against the top tube. I have had mixed results with that.

As I thought it over, just as I was really picking up speed I passed a MTB rider to the inside (he was almost stopped and turning left) and I turned my body a little to keep an eye on him and that's when the fun began.
THE SKINNY

Posts:378

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06/18/2013 08:47 AM
here in the flat lands i've never had that happen. is it the frame that starts oscillating?
How we spend our days is, of course, how we spend our lives.
professeur

Posts:12

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06/18/2013 09:44 AM
There's no more helpless feeling than barreling down a slope, picking up a worsening vibration, and having the vibration go nuts when you touch the brakes. Glad to see you recovered from this one.

As posted above, the vibration can come from several sources. Loose headset, loose skewer, out-of-balance wheel, even a pinched tube. Hard to track down, the bike just happened to have react harmonically at just that frequency. I remember when my Mavic Ksyriums picked up a harmonic flutter when descending in a cross-wind. The spokes passing behind the fork at a certain frequency was all it took.

I wouldn't give up on the bike because of this, I would be really careful at speeds above 25mph though.
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