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Old 07-11-09, 10:21 PM   #1
Widsith
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My first high-speed wobble

I had my first experience tonight with the dreaded "high-speed wobble." It happened on a fast descent, in the most dangerous part (a long curve with lots of road hazards to avoid -- gravel patches, manhole covers sticking up a couple of inches above street level, etc.). Without warning the front wheel began to shimmy violently. My first instinct was to panic, as the bike was becoming very hard to control and I had visions of planting my face on the asphalt. However, I suddenly remembered advice I'd seen here on BF several times to remedy just this situation: "Grip the top tube between your knees." I did so, and instantly the bike became steady as a rock. I finished the rest of the descent smoothly without even having to slow down.

There's been lots of useful information that I've gleaned from BF in the past year, but this time, it saved me from what could have been a serious injury. This place is a great resource.
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Old 07-11-09, 11:43 PM   #2
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I read "knees to top tube" here, too. Saved me at 43 mph.
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Old 07-12-09, 12:46 AM   #3
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I was going to post it- but on Friday I followed a rider on our coast road. First met him on a stiff climb and he was going like a train. Traffic and he overtook me just at the top of a 16% descent. The rider looked good and was obviously good at the rate he climbed the hill-but on the descent- he was slow and riding the brakes. He only did about 20mph on a hill where I can get around 50. I had plenty of time to look at his lines and I noticed his front wheel had a wobble. I put it down to an out of true wheel and did not think anymore about it.

Then yesterday I went into the LBS and his bike was there. It was in to be checked over as the bike had a speed wobble. They had checked the whole bike over and taken it for a test ride and no problem. The only thing they could put it down to was the combination of frame- wheels and rider weight distribution. Then while I was there- they found a Fault. One of the head set Bearings was not fully located in the frame and may have been the problem. It will be interesting to find out if they have cured the problem.

I had a few problems on the TCR-C when I first got it and the main one was a harsh ride over rough roads that made it bounce a bit and I did not like high speed descents on it. Change of wheels cured the harsh ride and on fast descents I get my weight as far to the rear as possible. This has improved the handling but I am still a bit wary when Gravity taking over.

But wheel wobble and "Known" cures have been gripping the Top tube with the knees- Change of wheels- getting the weight further back in the saddle or a fault on the bike and normally steering set up with loose headset or damaged fork.
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Old 07-12-09, 04:45 AM   #4
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Quote:
Originally Posted by stapfam View Post
getting the weight further back in the saddle .
I believe weight more forward helps - whether it is moving weight over the bars or moving a little forward on the seat. At least that has been my experience and consistant with what I have read.
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Old 07-12-09, 06:34 AM   #5
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I would get the bike checked out. My only bike ever that had a high speed wobble was a 1970's Schwinn Varsity. The headset was loose.
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Old 07-12-09, 07:20 AM   #6
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I've made much faster descents, some on curves, without having this happen before. In this case I believe it was a combination of the speed, the sharp curve (I was leaning the bike way over), and the rough road surface. This road goes through a relatively new housing development that is built on the side of a valley. I had climbed to the upper end of the road, beyond where the last houses were built, so there is little traffic up there to sweep the road clean (hence the large amount of gravel present). Also, though the road is several years old, it's clear that it isn't finished yet; it's due for at least one more layer of asphalt, as evidenced by the protruding manhole covers.

On the way home there is a series of descents on a better road, where I like to shift into my highest gear and crank it hard until I'm going too fast to keep up with the pedaling any longer and I'm really flying. That one was as smooth and safe as ever when I returned home, so I don't think there's a problem with the bike. But I'll check the headset anyway, just to be sure.
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Old 07-12-09, 01:50 PM   #7
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Sometimes just oversteering a little will cause the wobble. Tried to steer (read aim) at about 35mph last year going down a long hill into a tight curve. I really tightened up as I turned, wobble began, but ended as soon as I loosened up. Nothing wrong with the bike in that case, just the rider's style.
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Old 07-12-09, 02:49 PM   #8
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A particular combination of frame and rider can resonate at certain wheel speeds. Gripping the top tube with the knees is a good solution, but often one needs only to press one knee against the top tube to break the resonance. Since no wheel set is perfectly dynamically balanced, I think almost any road bike can wobble at some speed, although the problem is more pronounced on some bikes than others, and each bike-rider combination will have its own vulnerable speed.
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Old 07-12-09, 07:31 PM   #9
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In my 300,000 miles of cycling have experienced at least half dozen hi speed wobbles (harmonic vibration).
However did have one at only 12 mph on a new tandem. Had fork replaced with one that had a bit more rake and that solved the issue.
On the hi speed wobbles, clamping legs to top tube and lifting butt off saddle always worked, except about 3 years ago.
Descending at 38 mph down a newly paved canyon road in northern Utah, got harmonic vibration. Put usual knees to tt, lifted butt . . . no luck. Then lifted butt and leaned forward . . . still no joy. Harmonics were so bad that bike was slaloming across the yellow line uncontrolably and around the mountain curves. Pictured myself as a hood ornament on the next pickup truck coming at me (not a pretty thought!) . . . so decided to tap rear brake and expected to ummersault over the bars; which I did. All this happened in seconds, not minutes. As I did my paratrooper roll looked under my right arm and saw another bike rider coming down full tilt! He ran over me as it hit pavement and flipped over the bars and cracked his helmet.
My injuries were a double fracture in the left shoulder (not a collarbone).
Bike was fine so pedaled home for the next ten miles before heading to hospital.
Checking headset, increasing rake on fork or even putting on a wider diameter tire can prevent issues like that from recurring.
Just my experience.
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Old 07-12-09, 08:51 PM   #10
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Scary stuff, zonatandem! Yes, everything from headset to fork rake to rear wheel dish to tire/rim imbalance can contribute to wobble, but yours is by far the worst case I have ever read about. However, 38mph is very fast by my timid standards.
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Old 07-12-09, 10:27 PM   #11
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Sometimes it's due to bad design of the frame and fork with the wrong combination of steering angle and fork rake, resulting in inherently unstable steering. I experienced this about 30 years ago when I took a camp-touring trip on an R.E.W. Reynolds steel framed touring rig with panniers on the back only. It was light on the front end, and would wobble severly on descents unless I leaned as far forward as I could like the TDF racers. I also noticed that the front end would rise and fall as I turned the bars back and forth. Afterwards, I replaced the fork with one that had about 1/2" less rake, which increased the trail and the problem never recurred. I should also point out that it had 27" wheels, not 700c, which also affects steering. So, increasing rake does not necessarily make steering more stable, rather, it's the proper combination of steering head angle and fork rake for a particular wheel diameter.
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