Front wheel oscillation

Discussion in 'Cycling Equipment' started by ad5at, Apr 9, 2006.

  1. ad5at

    ad5at New Member

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    I might be posting this in the wrong forum, but I am not sure if it is an equipment problem or just poor control from myself as a rider. Yesterday, while coming back down a popular winding mountain climb, I got a bad shake in the front end at about 35-40 mph. Bad enough I thought I was going to lay it down. Since I was already down in the drops, I tapped lightly on the rear brake little by little to scrub enough speed to stop the shake. Needless to say, I remained below those speeds until back in the flats. It was very unnerving to say the least. The funny thing is about 6 months ago, a friend of mine layed his down up there at around the same speeds. They are both Trek bikes with carbon forks. His, a 2500, and mine 1500. So tell me, I know this happens to others. What is the cause, and what can I do to prevent it in the future? Is it the wheels ? I thought that my Bontrager race x-lites were a decent wheel.:confused:

    Thanks for the help!
     
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  2. John M

    John M New Member

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    Lennard Zinn discussed this topic of front end shimmy in a velonews a few years ago. The discussion regarded geometry of Lemond frames, but might also apply to Trek bikes since they have been from the same company for several years.

    http://www.velonews.com/tech/report/articles/2226.0.html
     
  3. jur

    jur New Member

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    This phenomenon is commonly called shimmy. It happens on all bikes at various specific speeds depending on geometry and rider mass among other things. Pressing your knee against the top tube helps.
     
  4. rickt

    rickt New Member

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    I don't know about it happening on all bikes. I have travelled at every speed from 0 to 110km/h (about 70mph) on my custom built steel bike without ever having had a hint of the problem (the 110km/h was down a hill, slipstreaming behind a station wagon). While it may happen at speeds greater than 110km/h, it is very unlikely that I will ever reach that again.
     
  5. jur

    jur New Member

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    :D

    It's one of those things that do not necessarily happen on al bikes riding normally; but if you ride no-hands it usually comes out. It happens when the front wheel reaches a certain rotational speed, such that the associated wheel nutation frequency matches the rider-frame wobblyness. Any small amount of disturbance will set it off. Putting hands on handlebars is usually good enough to stop it, or putting knee against top tube if riding no hands is continued. It is especially bad if you have a loaded rack onm the back - the speed is quite low when it sets in.
     
  6. Deanster04

    Deanster04 New Member

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    Doesn't happen on all bikes. It only happened once to me on a early bike with a British type touring fork popular in the 60's. Never since. I ride a lot of miles and my test of a bikes stability and balance is to take my hands off the bars going down hill and slap the head set. If the oscillations dampen immediately I will buy the bike.
    I don't pretend to understand the problem because I have had friends who have seen the problem. My 3 classic steel bikes are Italian geometry with Cinelli type fork crowns and Italian forks. My moderne bikes are Ti and Carbon compact frames. Never an inkling of a problem. Weigh 220lbs and love to descend fast on my own built 32 H conventionally spoked wheels. Your method of stopping the problem is exactly what I did on my bike 45 years ago to stop the problem...and it is scarey when it happens.
     
  7. OscarC

    OscarC New Member

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    I'm up to about 45 mph every weekend on my Trek Madone 5.9 (all carbon) and have never exeprieced this. Bontrager X-lites for me turned out to be great tires. see my review.

    http://www.cyclingforums.com/t318835.html


     
  8. unicos

    unicos New Member

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    I didn’t notice any limitation to Lemond frames. If you read the article there is a question from a person with a Lemond frame getting shimmy at a particular speed. Most shimmy at speed is usually caused by the rider being very tense. If you were to do a survey on where shimmy occurs among riders you would probably find that it is beyond a reasonably comfortable speed for the rider. I will agree that there are physics involved for shimmy involving but not limited to frame geometry, frame size, CG, rider weight, wheels, etc. Geometry may be the case here but; I would be curious to see if ad 5at were to ride these speeds more regularly that the shimmy would go away at 35-40 and move to a higher speed.

    First suggestion is to check possible mechanical causes:
    Have your LBS look over the bike.
    Headset properly tightened.
    Wheels properly true (front and rear).
    Axle/Quick release properly tight and seated (front and rear).
    Stem bolts tight (all of them).

    Then work on rider:
    Try and ride the 30-35 speed more regularly and see if you get more comfortable. Then move up to the 35-40 and see if the shimmy goes away. You may see it rise with comfort level. Even the pros have a speed comfort level.
     
  9. dhk

    dhk New Member

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    I'd like to add lack of adequate frame and fork stiffness to your list of possible causes. Since the front triangle of our frames is flat, with very little thickness, there isn't much resistance to twisting between the axis of the head tube and seat tube. As a result, oscillation started by any steering correction can be amplified by the rider's weight on top of the seatpost. As the rider moves to one side, the rider needs to pull on the bars on that side to keep the bike under him and upright. This new input causes the weight to shift past centerline to the other side. Depending on speed and frame/fork stiffness, a resonant condition can occur.

    Clamping the frame with the knees works because it more tightly connects the rider to the frame while greatly reducing the feedback through the handlebars. If the frame and fork are stiff enough, any oscillation will occur at a higher frequency which is easily damped by the riders weight.

    Long-winded explanation, but that's my theory anyway. If I'm correct, bikes with stiff enough frames and forks for rider weight and size shouldn't have this problem.
     
  10. unicos

    unicos New Member

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    I agree that lack of adequate frame and fork stiffness could be a contributing factor. This particular fork is relatively stiff and therefore could be shifting most of the energy to the frame. The two Trek frames, mentioned here, are relatively close in geometry and I think are stiff enough unless there is a very heavy rider on them. My guess would be in the 250lbs+ weight range to be a contributing factor in this case. I am still very curious about my speed comfort level theory.
     
  11. jur

    jur New Member

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    I have a background in gyroscopes and am therefore attuned to the properties of rigid spinning bodies. (This by way of explanation. Apologies for rather lengthy discussion:)
    One of these properties of spinning bodies is called nutation; you can easily see it in any wheel by taking say the front wheel out of the fork, hold it lightly by the axle with one hand so it dangles with axle vertical; now spin the wheel up with the other hand. When it is spinning, give the tyre a sharp bump parallel to the axle; the wobble you see is the nutation. It is an underdamped harmonic oscillation whose frequency depends on the wheel's angular inertia and angular speed. Nutation is only one component in wheel shimmy in a bicycle, and is a property of all wheels.

    The second component is less clear; some hold it is a function of frame stiffness and rider weight; I suspect frame stiffness while contributing is not the major part; rather it is the compliance of the rider to the saddle. But whatever the exact factors, what you have is a second underdamped system comprised by frame and rider and steering mechanism. You can imagine that if the bars are suddelny turned one way, the inertia of the rider and frame will shift momentarily and then re-center; but if poorly damped, may overshoot to the other side, and so on until fully damped. A loaded rack on the back is almost guaranteed to produce shimmy, because it can quite easily sway from side to side, reducing the damping in the system.

    Now on to the third part: At some speed, the wheel nutation frequency matches the frame+rider frequency. If there is insufficient damping in these 2 systems, they will reinforce each other and become a noticable shimmy. In severe cases the oscliation amplitude will be so bad that the rider loses control.

    If you ride no-hands and give the bars a light bump, the front wheel will immediately exhibit the nutation, but usually it damps out because there is no positive feedback from the frame+rider. So you will be under the impression that this bike is fine from a shimmy point of view.

    Only thing is, this thing is entirely speed dependent, so there is likely another speed where the oscillation would not damp out as quick, or not at all until you put your knee agains the top tube, or perhaps lift yourself slightly off the saddle, or take the bars in hand. Any of these actions should normally be sufficient to damp out the shimmy.

    Again, apologies for this long-winded explanation, but it is complex and I suspect even gurus like Brandt poorly understands the phenomenon, because in all his writings I have read he nor anybody else ever mentions nutation as a necessary part.
     
  12. rickt

    rickt New Member

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    Jur, you have certainly explained the phenomenon well and I thank you for that. I have a reasonably sound knowledge of physics in general and understand what you explained, even if I hadn't heard the terminology before. Perhaps what you could have said in your original post on this thread is that all bike will exhibit signs of shimmy albeit at extremely high speeds in some cases. This would have shut up people like me straight away. I believe that the bike I mentioned earlier could only exhibit shimmy at speeds WAY beyond the average persons capability to reach and possibly beyond elite athletes capabilities.
     
  13. dhk

    dhk New Member

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    Your explanation seems sound to me as well. It certainly explains why speed is a critical factor. Would it follow that for a given bike/rider, going to lighter wheels (lower rotational inertia) would raise the speed at which the wobble occurs?

    Agree that the "connection" of the rider mass to the saddle and handlebars would likely be a bigger contributing factor than frame stiffness. Also, wouldn't a stiff CF fork with high degree of damping, eg, Reynolds Ouzo Pro, act to dampen the wheel wobble before it got to the frame/rider?

    When I was discussing tubeset choices for my custom frame with the builder, he recommended Zonal in the Megatube shape for both downtube and toptube on my 58cm frame. He said that in this larger size, he wanted to make sure the frame was stiff enough for speeds up to 55 mph. The bike, with Ouzo Pro fork, has in fact been wobble-free so far up to that speed.

    Also good to hear that someone else feels that not every explanation from the "guru" Brandt rings true. I've had problems with some of his writings.
     
  14. ad5at

    ad5at New Member

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    Well, I will tell you one thing, you guys are much smarter than me on all this technical jargon. But, I really do appreciate the time taken and consideration in the replies. I should have said in my post that I am about 155lbs, 6ft 1in tall. I was fitted properly to the bike by my LBS.

    Still don`t understand much about this higher speed "shimmy", but I am going back up that hill this weekend with some friends and will certainly try and not "tense up". If I can get through this year with the bike, a new one is definately in my plans next spring.

    Thanks a bunch to all for the replies.
     
  15. jur

    jur New Member

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    The very first time I ever experienced wheel shimmy was when I was a boy of about 12; I was riding my friend's Raleigh Chopper, the one with sissy bars and a stick shift on the horizontal. We were motoring down a very long downhill; I decided that was a good moment to mess with the shifting and took one hand off the bars to shift. Now we were going at a nice clip and all was perfect at that point, no shimmy; however, taking one hand off the bars disturbed the system and a horribly violent shimmy set in. I grabbed the handlebars but was completely unable to bring it back under control and rode like that with violently shaking Chopper for another minute or so before we reached the bottom of the hill. The speed bled off and the shimmy damped out. It was touch and go or I lost it on that downhill.

    A steel frame Chopper is not a flexy thing but it shimmied most horribly. I got off the bike and was cured of my desire to own one.

    That chopper had a typical padded seat allowing the rider to shake from side to side due to the padding; a racing saddle being usually quite hard and supporting a bum with not much excess padding would be quite a stiff system.

    More later; my daughter is kicking me off the PC :D
     
  16. jur

    jur New Member

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    OK I'm back.

    I just did an experiment; I thought about the rider-saddle interface and how stiff that might be, so I just sat myself on a hard chair and shook myself from side to side. It was easy and quite clear that the movement of your skin and tissue between chair and sitbones allows quite a lot of movement; what's more, it seems like it is a bit under damped. So the rider shaking from side to side despite the skin being firmly positioned on the saddle would furnish the necessary side-to-side underdamped system. The rider's weight would play a large role in the speed where the wheel nutation frequency matched the rider+bike frequency; likewise a lighter wheel would increase the speed where matching occurs.

    I am not sure the damping of a CF fork would have much impact on nutation damping due to the different frequencies. A fork damps high frequency vibrations, like several tens of cycles per second, while shimmy is typically a few cycles per second. This is also the reason why I think frame stiffness is not a major player in this phenomenon - the frequencies are too far mismatched.
     
  17. dhk

    dhk New Member

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    Agree on the CF fork damping; it's for higher-frequency road buzz. Was really thinking of fork stiffness. But a perfectly stiff fork may not be the answer either, as it would feed the wheel nutation directly to the rider via the handlebars. You mention natural frequencies; the answer's likely in there somewhere.

    Thinking a bit about your experiment, seems a rider down on the drops in a good aero tuck would present a different (higher) natural frequency than one sitting upright with hands only loosely on the bars. What do you think about weighting the pedals by easing the butt off the saddle just a bit? Seems that would raise the wobble speed by lowering the CG more closely to the level of the front wheel axle.

    My bike doesn't wobble, at least not up to 55 mph, so I can't really experiment here...not that I'm complaining. The only time I encountered speed wobble was on a motocycle with a bar-mounted windscreen at 75 mph, and it's memorable to this day. Needless to say, the screen came off when I got home, and the dropped "clubman" bars went back on.

    jur, your location in lat/long UTM coordinates inspired me to pull out the old Rand McNally atlas. Appears you are near the Silvan Reservoir, about 35 km east of Melbourne....fair guess?
     
  18. jur

    jur New Member

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    Spot on.
     
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