Downhill Shimmying

Discussion in 'The Bike Cafe' started by millark, Dec 17, 2003.

  1. millark

    millark New Member

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    I'm just looking for some general advice in this area. I'm having a hard time getting my downhill speed up and I'd love to get some tips. One of my biggest issues is wheel "shimmy" I keep experiencing at high speed, generally about 30 MPH. This isn't an equipment issue; my wheels are tru and my front fork and handlebars are adjusted properly. I think it's because my upper body is too tense. I've tried repositioning myself on the bike and relaxing, but I still occasionally get that "shimmy" and feel really unstable. Sometimes it gets so bad the bike feels totally out of control and I have to slow down completely to to regain control. I want to race in the Spring and I know I've got to be able to really fly on the big downhills without riding the brakes. Can anyone suggest some things I might try?
     
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  2. davidbod

    davidbod New Member

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    If it happens all the time you could get another experienced rider to ride the bike and see if it realy is the bike or not. If you have another front wheel or someone willing to trade with you for a test you could do that too. Also check your headset for play.

    When it does happen bring your knees together and clamp the frame top tube between them.
     
  3. djg21

    djg21 New Member

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    It does sound like you may have headset problems, or perhaps a frame alignment issue.

    Like the previous poser said, when descending, press one or both of your legs against the toptube whenever you aren't pedaling. That will kill the shimmy.
     
  4. Ratface

    Ratface New Member

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    Heh! I get the impression that you meant "poster" there huh? ;-)


    One thing I wondered about the original poster's question is whether the bike has a particularly short wheelbase (distance between where front and back wheels touch the ground). That can lead to instability in steering at times.
     
  5. djg21

    djg21 New Member

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    My typing skills are clearly lacking! Excuse me!:p
     
  6. AvgTdFsizeguy

    AvgTdFsizeguy New Member

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    I'd say clamp your knees together on the top tube if you'd said it happens above 40-45mph, but in this case it sounds like it might be a frame problem because 30mph isn't exactly fast. You haven't crashed it lately, have you? How stable does it feel when you ride hands off? At least have someone else ride and compare or have the alignment checked.

    How much do you weigh and what is your frame material and geometry? If you are relatively heavy and your frame material is flexy, your frame might not have enough lateral stiffness. That might be the problem even if you're not heavy.

    If you are convinced it's not the bike, you might try putting your weight on the pedals (feet level) and not on the seat, stay on the drops but with as little weight as possible on them and drop your shoulders (push your elbows down), and steer only by leaning and, on sharper curves, putting your outside pedal down. Back and stomach muscles are really important in this position because you've got to support your upper body and transfer that weight to the legs (not lean on your arms) but stay relaxed enough to breathe deeply. This will get your weight as low and centered on the bike as possible. Also, look as far ahead as practical, not at your front wheel, especially if it starts to shimmy, which will keep you from overreacting to visual cues (seeing the wheels or handlebars move and wanting to correct the motion) and get you responding to your sense of balance by seeing a horizon and the line you are traveling (the big picture) so you can stay ahead of the bike.
     
  7. Alnamvet

    Alnamvet New Member

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    It's either a bent/misalinged frame, or the wheels are not true or balanced.
     
  8. millark

    millark New Member

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    The frame is brand new (I've only got 1000 miles on it) and I haven't crashed on it, so I really doubt that it's a bent frame or an equipment issue. I think it's more a product of my inexperience with high speeds downhill. I'm 6'1" and 165 lbs. The frame itself is a 60cm Cannondale R800 with a Gippimme wheelset. I think I just need to relax more and work on positioning myself on the bike. I'll also try to borrow another person's bike, or have them ride mine to see if it's really an equipment issue. But any other input on positioning or tips on how to relax when riding downhill fast would be welcomed.
     
  9. Feanor

    Feanor New Member

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    That has to have been the GREATEST typo I have seen in a long while :) I'm still chuckling :)
     
  10. Feanor

    Feanor New Member

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    I don't think it is unheard of for frames to be out of alignment from the factory... maybe rare, but still worth checking... Yet again, you never know what might have happened to the bike while it was still in the showroom... I was in a bike shop the other day and one of the employees was cruising around the floor on one of the road bikes... He made several attempts at a trackstand, the final attempt of which he dumped it and ended up in a clattering heap... the bike had flipped up and struck one of the steel bike stands that was in the vicinity of the wreck and he quickly got up looked at the bike for a moment and put it back in the rack... Probably an ultra rare occurence, but still...

    Good luck in trying to sort out the shimmy!
     
  11. zaskar

    zaskar New Member

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    On OLN they were road testing rd bikes and on the down hill test some bikes did shimmy. they did'nt say what caused it just said it was point deduction on the test. i notice my bike when going fast on flats or down hill 30mph plus it feels like i have a flat, i can let go of the bars withy no shimmy but the rear just feels flexy, a few times i have stoped to ck my tire. i want a new frame, i feel safer and can decend the same mountain faster on my mtb.
     
  12. rv

    rv New Member

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    could be a fork rake problem. if I remember correctly, less rake generally means more stability. lennard zinn has written about this on velonews.com tech talk; do a search. changing the rake changes the frequency at which the bike will oscillate (shimmy). as someone suggested earlier, clamp the toptube tightly w/your knees while going fast downhill. clamping the toptube changes the wavelength and hence the natural frequency. go back to the dealer where you purchased the bike and have a fork with different rake installed for a test. no reputable shop wants you riding a dangerous bike.
     
  13. yamaha_mike

    yamaha_mike New Member

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    i experienced this for the first time today and i can relate to it being spooky: mine happened on a 50mph descent, on a cold and rainy day here in norcal (i thought i was just taking it easy, in lieu of the conditions but slowed way, way down for the rest of the ride)

    at first i was thinking it was headshake, like i have heard about with motorcycles, and besides the strong emotion of "am i going to crash?" i wondered, do i have a flat? is my wheel or headset loose? wtfo?!?!?!?!

    in that sort of quick nano-second scene assessment i saw no way out, such as a soft dirt place off the asphalt to crash into, so i just yelled out loud "come on baby!" and eased into the banking right hander and it went away. it was too wet to really get much out of the breaks, so i just let the bike do its thing in gravity and slowed as best i could.

    looking over the bike, everything seems fine. hmmm????

    during the ride today, i was able to reproduce it by sort of tightening my pecs and lats, a tensing or shivering like movement with my upper body got the whole bike shaking like a dog coming in from the rain

    dunno if it's part of the wheelbase or frame material??? (giant tcr composite) but i am glad to have the great advice given in the thread of using the knees against the top tube to abate the oscillations

    i think if your bike is OK then it's your tense upper body (as you sense already) that is causing the trouble. easing off of that just takes practice
     
  14. dhk

    dhk New Member

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    I agree with the "posters" that say something is out of alignment, bent or otherwise wrong with the bike. I'd take it back to the LBS and complain. This is definately a warranty issue. A CAAD 7 frame shouldn't shimmy down hill at 30 mph, IMO, and I'm sure Cannondale wouldn't want you riding it in this unsafe condition.

    Dan
     
  15. mike frye

    mike frye New Member

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    The question may not be frame alignment but basic Frame geometry. The aforementioned "shimmy" seems to happen when a natural "wave" stets up between the front and rear wheels. I have had this happen with the first "good" bike I bought .

    I was riding down one of the steeper hills in Portland Or. and I made it around the first corner ok, then the next . Finally, rounding turn number three the handle bars start to shimmy
    and get progressively worse untill I flew off the road. The result of that experience was a good set of road rash and a bent frame.

    Later when I started building frames I had the same thing happen with the first road bike I brazed together.(although to a much smaller degree.) I found that If your head tube is too long or not "stout" then it seems to me that your frame losses the very stable " triangle" shape and becomes more of a parallelogram. This will help set up a oscillation between the front and rear wheels, and if that gets out of hand then it will be time to visit the emergency room.

    Steps have been taken to keep this wave from "starting up" You might try moving your handle bar a little lower or maybe getting bigger tires in order to "damp out" that wave form at the start of the cycle. If you have reflectors on your wheels you might try moving them(or removing them) or even go with a different number of spokes in the front than the rear. At this point its just pure conjecture in my part. Chances are that you don't have to get a new frame. I would start by using another wheel on the front and see if that works.

    Mike Frye A.K.A. Frye Bikes
    by the way HAPPY NEW YEAR!
    :)
     
  16. stevek

    stevek New Member

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    I have not had the shimmy but I was on a concrete bike path going about 35 and the cracks between the slabs rattled me so much my eyes were vibrating (G) I could have gone faster but it was too hard to see (G)
     
  17. mike frye

    mike frye New Member

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    I think that maybe you need some "big tires". With the type of frames being manufactured today(IE: Really short wheelbase coupled with very steep head and seat angles and super small tires.) The idea of a road bike that can be ridden on anything but a super smooth surface is pretty uncommon. If you can squeeze it in, I would recommend going with 28c tires and if they don't fit maybe go with 25s or like I did on my fixed gear commuter 23s.

    One last thought if you go with those "bigger" tires you might get by with only 100psi (I don't know the metric equivalent ) The last road bike I made for myself I set it up so you could put a 28c tire and a fender on the back. I could not do the same for the front because I don't "do" forks.

    Mike-it looks like I'm missing out on the New Years Day ride because of all the snow-Frye :p
     
  18. stevek

    stevek New Member

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    I have 25's on it. but I also weigh about 240 right now (G) so I feel every bump. but hey I got the racing bike for a fantastic deal and can't afford another bike.
     
  19. mike frye

    mike frye New Member

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    Well I guess that you will have to deal with what you have.( sigh.)
    :rolleyes:
     
  20. stevek

    stevek New Member

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    the trials of life (G)
     
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