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#1
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High-speed shimmy, Speed wobble
This weekend I was descending a canyon, and at around 42MPH my LeMond Buenos
Aires started to get a bad speed wobble. This is the first time it's ever happened (and I've been at that speed many times over the summer). I've read about speed wobbles but have never experienced one. At first I thought my front tire had gone completely flat and was going out of control, or as if the front tire had become extremely out of true--I'd guess that the wobble amplitude was about 2-3", and a frequency of about 5Hz. But the braking was responsive. As I slowed down it continued to wobble for a while though lower speeds (I'll guess through 30MPH--but I don't know). I finally stopped, checked that my front tire was in tact and not out of true. Everything appeared OK, so I continued down the hill, this time not going over 30 or so. My owners manual says that if I experience this, to take it into the shop. I called the shop and they want to see the bike. But based on my reading of the shimmy FAQ, I'm half expecting that they won't find anything. I have made two changes to my bike recently. The most recent was that I lowered my handlebars by about 2". They used to be almost parallel to my seat. I did tighten the headset well. The other is that I replaced my front tire with a lighter, skinnier one. I have descended with the tire before, but I can't say whether I reached 42MPH or not. I can say that ever since I put that tire on, I have noticed there being less stability while riding no-hands--it does start to shimmy a little, but at lower speeds and not so violently. Is it probable that this low-speed (~20MPH?) no-hands shimmy is from the same source as the violent shimmy? Any suggestions would be appreciated. Shayne Wissler |
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#2
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High-speed shimmy, Speed wobble
On Mon, 29 Sep 2003 16:23:31 GMT, "Shayne Wissler"
may have said: Is it probable that this low-speed (~20MPH?) no-hands shimmy is from the same source as the violent shimmy? Any suggestions would be appreciated. Observing that the problem appeared after the tire change, the connection to the reduced diameter is possible but not proven. An observation: Reducing the diameter of the front tire will move the steerer axis more toward the vertical, which moves the point of intersection of the steerer axis with the ground toward the tire's contact patch. The distance between the axis intersection and the patch is the "trail" of the fork. Insufficient trail will cause exactly the problem that you describe...but if the amount of trail before the tire swap was small enough that the change produced this problem, I would be a bit surprised. If the bike has a suspension fork, try cranking the preload up to the max, as that will tend to minimize such a problem. Somehow, though, I think the lbs is likely to find that there is something else wrong. What I describe is, in my opinion, not terribly likely to be the problem. -- My email address is antispammed; pull WEEDS if replying via e-mail. Yes, I have a killfile. If I don't respond to something, it's also possible that I'm busy. |
#3
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High-speed shimmy, Speed wobble
I've read about speed wobbles but have never experienced one. At first I
thought my front tire had gone completely flat and was going out of control, or as if the front tire had become extremely out of true--I'd guess that the wobble amplitude was about 2-3", and a frequency of about 5Hz. But the braking was responsive. As I slowed down it continued to wobble for a while though lower speeds (I'll guess through 30MPH--but I don't know). I finally stopped, checked that my front tire was in tact and not out of true. Everything appeared OK, so I continued down the hill, this time not going over 30 or so. Once you get a shimmy, my advice is not to slow down, but stop, take a breather, and then get going again. Lack of confidence doesn't help the situation any. I have made two changes to my bike recently. The most recent was that I lowered my handlebars by about 2". They used to be almost parallel to my seat. I did tighten the headset well. The other is that I replaced my front tire with a lighter, skinnier one. Simplest experiment would be to raise the bars back up again and see if the shimmy goes away. As you've read in the FAQ, shimmy isn't a function of the bicycle alone, but the entire system, which includes you on it. The combination of your position being altered and the skinnier tire (which speeds up response on the front end) sounds like a good place to start, almost a no-brainer. --Mike-- Chain Reaction Bicycles http://www.ChainReactionBicycles.com "Shayne Wissler" wrote in message news:70Zdb.634208$o%2.291570@sccrnsc02... This weekend I was descending a canyon, and at around 42MPH my LeMond Buenos Aires started to get a bad speed wobble. This is the first time it's ever happened (and I've been at that speed many times over the summer). I've read about speed wobbles but have never experienced one. At first I thought my front tire had gone completely flat and was going out of control, or as if the front tire had become extremely out of true--I'd guess that the wobble amplitude was about 2-3", and a frequency of about 5Hz. But the braking was responsive. As I slowed down it continued to wobble for a while though lower speeds (I'll guess through 30MPH--but I don't know). I finally stopped, checked that my front tire was in tact and not out of true. Everything appeared OK, so I continued down the hill, this time not going over 30 or so. My owners manual says that if I experience this, to take it into the shop. I called the shop and they want to see the bike. But based on my reading of the shimmy FAQ, I'm half expecting that they won't find anything. I have made two changes to my bike recently. The most recent was that I lowered my handlebars by about 2". They used to be almost parallel to my seat. I did tighten the headset well. The other is that I replaced my front tire with a lighter, skinnier one. I have descended with the tire before, but I can't say whether I reached 42MPH or not. I can say that ever since I put that tire on, I have noticed there being less stability while riding no-hands--it does start to shimmy a little, but at lower speeds and not so violently. Is it probable that this low-speed (~20MPH?) no-hands shimmy is from the same source as the violent shimmy? Any suggestions would be appreciated. Shayne Wissler |
#4
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High-speed shimmy, Speed wobble
shayne
i used to have similar experiences on an older road frame and i managed to "tune" it out. as you read in the faq, there are many reasons for the shimmy, but basically, you're looking at a dynamic system which has a resonance, just like that famous footage of the tacoma narrows bridge. the initiation of the resonance aside, and tires are one, the frame and the wheels facilitate the resonance. just like the bridge, the wind initiated the oscillation, but the resonant frequency of the bridge was disasterously "in tune". so, if you are to address this problem with your bike, you have to address these two main areas - frame & wheels. i was lucky enough to have several rear wheels and noticed that different wheels had different effects on the resonance. in my case, the lighter spoked wheels were much more prone then the heavier spoked ones. following this pattern, i ended up building a "stiff" rear wheel with straight gauge 2.0mm spokes on the drive side and butted 2.0/1.8/2.0mm on the non-drive. it's very rigid and quite a harsh ride, but it killed the shimmy completely. i have since replaced that frame and can report that while i can induce some degree of shimmy in any of the three others i now have if i really try, none of them are susceptible in the same way. ymmv, but in my opinion, start with wheel variants and if that doesn't work, move on to a different frame. my aluminum frame is the most stable. jb |
#5
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High-speed shimmy, Speed wobble
On Mon, 29 Sep 2003 16:23:31 +0000, Shayne Wissler wrote:
My owners manual says that if I experience this, to take it into the shop. I called the shop and they want to see the bike. But based on my reading of the shimmy FAQ, I'm half expecting that they won't find anything. Quite lkely. I have made two changes to my bike recently. The most recent was that I lowered my handlebars by about 2". They used to be almost parallel to my seat. I did tighten the headset well. The other is that I replaced my front tire with a lighter, skinnier one. The maddening thing about shimmy is that it can be just about anything that will cause, or cure, it. It's as likely the bar height (and the change in your position that results) as the tire. I have descended with the tire before, but I can't say whether I reached 42MPH or not. I can say that ever since I put that tire on, I have noticed there being less stability while riding no-hands--it does start to shimmy a little, but at lower speeds and not so violently. Is it probable that this low-speed (~20MPH?) no-hands shimmy is from the same source as the violent shimmy? I'd say it's possible. Any suggestions would be appreciated. Try it with the bars a bit higher, or with another front tire. If it _does_ shimmy, press one knee against the top tube. That should stop it. Also, unloading your weight from the saddle will stop the shimmy. -- David L. Johnson __o | "Business!" cried the Ghost. "Mankind was my business. The _`\(,_ | common welfare was my business; charity, mercy, forbearance, (_)/ (_) | and benevolence, were, all, my business. The dealings of my trade were but a drop of water in the comprehensive ocean of my business!" --Dickens, "A Christmas Carol" |
#6
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High-speed shimmy, Speed wobble
"Shayne Wissler" wrote in message news:70Zdb.634208$o%2.291570@sccrnsc02...
The other is that I replaced my front tire with a lighter, skinnier one. I bet this is it. When I developed a "high-speed" no-hands shimmy earlier this year, I read everything on rb* on shimmies. It seemed like the fact I had replaced the rear tire with a slightly fatter one was the likely culprit. I returned to a skinnier rear tire, and the high speed shimmy disappeared. Warm Regards, Claire Petersky ) Home of the meditative cyclist: http://home.earthlink.net/~cpetersky/Welcome.htm Books just wanna be FREE! See what I mean at: http://bookcrossing.com/friend/Cpetersky |
#7
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High-speed shimmy, Speed wobble
Is it probable that this
low-speed (~20MPH?) no-hands shimmy is from the same source as the violent shimmy? Any suggestions would be appreciated. .. I'll tell you this just once, and I assume you don't want the sh*t scared out of you again. The skinny tire did it. Put a 25 width tire on the front and enjoy life. |
#8
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High-speed shimmy, Speed wobble
It seemed
like the fact I had replaced the rear tire with a slightly fatter one was the likely culprit. I returned to a skinnier rear tire, and the high speed shimmy disappeared. It doesn't matter what you have on the back wheel, it's the front wheel that counts. |
#9
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High-speed shimmy, Speed wobble
I suppose we need another FAQ item on this. What is missing in most
of these testimonials is how and at what speed these shimmy episodes occurred. In prior airing of this sort of thing, it was claimed that balancing the front wheel would prevent shimmy or that a different weight wheel would do it. I put a large imbalance on a front wheel and made test runs with and without imbalance (I balanced the wheel as well as greatly imbalanced it) with no discernable difference in shimmy. I use a 5% grade, smooth half mile section of road, coasting at between 25 and 30 mph no-hands and can consistently develop shimmy. The next test was to completely fill the front tire with water. This changed the shimmy frequency slightly but did not stop it from occurring. That said, I descend mountain roads often at speeds up to 50mph with no problem while placing my hands on the stem, knees together. Shimmy is a characteristic of most larger bicycle frames and should not be allowed to occur by the rider. SInce most people cannot ride no-hands, they are not aware of the tendency of their bicycle to shimmy. Then when it occurs, it is a disaster about to happen. http://draco.acs.uci.edu/rbfaq/FAQ/8h.5.html Jobst Brandt |
#10
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High-speed shimmy, Speed wobble
I was also experimenting with descending shimmy this past weekend, not by
choice at first. I just built a bicycle with upright steel handlebars (older schwinn type) using an older Specialized Expedition touring bike (long wheelbase). The wheels are Mavic MA-2 with Campagnolo 36h hubs, 126mm spacing in the rear 13-24 6sp. The tire are the older Avocet Duro Plus 32's, tire pressure 85 psi. On a descent grade of 3-6 % for about 1/3 mile I was able to induce shimmy with my hands on the handlebars. My speed was 26-32mph. I've also noticed that with upright bars the steering is very responsive from what I was used to, riding drop bars. I'm not sure if this is because of the handlebars being further behind the stem. In any-case it was easy to induce shimmy. I thought this was interesting, so I rode back up the hill and repeated it again. As I got the bike to shimmy I wanted to see if I could correct this. I took weight off the handlebars by every so slightly removing both hands about an inch away from the bar. My speed was about 27 mph, and before I knew it, the shimmy corrected itself and was completely eliminated. I repeated this a 3rd time, to make sure it wasn't a fluke and actually timed how long it took for the shimmy to correct itself. The time was about 5 to 7 seconds for the shimmy to completely vanish. I'm going to try this again since it's one of my training routes. This time I changed tires, with Avocet Carbon 12 Road 700 x 25c.100psi.tire pressure. These are the tires I normally ride with my roadbike. Strange, but my roadbike doesn't shimmy at all on this stretch of my training ride. -tom wrote in message ... I suppose we need another FAQ item on this. What is missing in most of these testimonials is how and at what speed these shimmy episodes occurred. In prior airing of this sort of thing, it was claimed that balancing the front wheel would prevent shimmy or that a different weight wheel would do it. I put a large imbalance on a front wheel and made test runs with and without imbalance (I balanced the wheel as well as greatly imbalanced it) with no discernable difference in shimmy. I use a 5% grade, smooth half mile section of road, coasting at between 25 and 30 mph no-hands and can consistently develop shimmy. The next test was to completely fill the front tire with water. This changed the shimmy frequency slightly but did not stop it from occurring. That said, I descend mountain roads often at speeds up to 50mph with no problem while placing my hands on the stem, knees together. Shimmy is a characteristic of most larger bicycle frames and should not be allowed to occur by the rider. SInce most people cannot ride no-hands, they are not aware of the tendency of their bicycle to shimmy. Then when it occurs, it is a disaster about to happen. http://draco.acs.uci.edu/rbfaq/FAQ/8h.5.html Jobst Brandt |
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