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Messy Shorts...high speed shimmy !(9 posts)

Messy Shorts...high speed shimmy !yellowspox
Jun 16, 2003 12:46 AM
I did a club ride over the weekend that got into some serious Wi. hills for an Illinois flat-lander. On Saturday I hit the low 40 mark coming downhill with a hard left turn. As I dodged a large rock and hit some rough pavement, the front end of the bike started to shimmy and I ended up off the road and on the gravel/grass. I was able to scrubbed off most of the speed before locking up the brakes about 5' in front of a mailbox. At that point, my bike and I swapped positions. The vibrations switched from the bike to my legs but no injury. I started braking on the downhills the rest of the day. By Sunday afternoon I was gaining courage and started to let it go on the downhills I could see all the way to the bottom. Then it happened again. I was hitting 42, in a tuck, and coasting when the front end started the shimmy. I was all over the road. I put a death grip on the top tube with my knees and shifted my weight off the seat moving out over the bars to regain control. I got lucky twice and don't want to push it further. My fellow riders suggested things from loose headset to the Spinergy Spox rims that I ride. I'm 6' @ 210 (at the start of the ride) riding a Cannondale R1000. I've only been riding about 18 mo and never on these type of downhills or speeds over 38. What is the proper position to coast down a hill and how do you prevent the shimmy? I love the speed but at 58, I want to live awhile longer !
re: Messy Shorts...high speed shimmy !Mariowannabe
Jun 16, 2003 4:00 AM
Welcome to life on a stiff bike with a short wheelbase. Same thing used to happen to me on my Cannondale - one of the old ones with the crit geometry. I switched bikes.... But, I know what you mean, its VERY disconcerting to have happen. I bet you won't find anything wrong in the headset, hubs, etc. It used to happen to me when too much weight was over the handlebars but you should check the bike mechanicals out. Try descending in a different position, maybe shift your weight back a bit? Good luck.

If all else fails, its a good excuse to buy a new bike;-)
take it to a shop! if nothings wrong, time for a new bikeandy02
Jun 16, 2003 4:50 AM
I can't belive that even Cannondale would sell a bike that would be that unstable at only 38mph! If your headset is lose you should be able to apply the front brake and move the hnadlebars front to back if it move or acts funny its the headset. I would take it in anyway.
Great questionfiltersweep
Jun 16, 2003 5:12 AM
I had a similar issue (minus any wipeout) hitting the 40s on a descent- it felt like the fork was made of rubber (I lost all road feedback). The road surface was poor (very rough). One thing that concerned me is that the spoke magnet started hitting the pick up, telling me the wheel itself was noodling (possibly resonating from the road). You might want to take another look at what the spox are doing- starting with the spokes, rather than the rims.

Or if anyone else has any better ideas...
Used to get that occasionally on my Vitus.dzrider
Jun 16, 2003 6:07 AM
It was flexible aluminum frame and not a particularly short wheel base. After 3 incidents, spread over 3 years, all on windy days, I learned to let go of the handlebars whenever I felt anything like the start of the shimmy. Letting go was scarey, but the bike sort of found its equilibrium without any invountary screaming from me. Grabbing on tighter and braking to a stop left me gasping for air and waiting to calm down before riding again.
happened to meFrith
Jun 16, 2003 6:35 AM
And I agree that it is a chamois soiling experience. Like you I'm a pretty new rider. I managed to hang on and have been working my speed and confidence up ever since. One thing I try to concentrate on now is to loosen my grip on the bars letting them do a bit of natural compensation for the road noise. My understanding is that input from the rider is actually usually over-compensatory causing a increasingly large feedback waveform.
Loosening the death grip has thus far worked for me and like you I'm on a bike with a short wheelbase (giant TCR). My weight distribution now goes something like this...
Concetrated mostly over pedals.
Ass just lightly touching seat.
Hands on grips just tight enough to steady my balance.
Fore-aft position should be so that you have to do very little with your hands to balance your weight.
I may be causing the problem & bike is OKyellowspox
Jun 16, 2003 7:38 AM
Both time this happened, I was sitting back on the seat but still as low as possible over the bike with the thought that should I need to brake hard, I'd be in a good position. It may be awhile until I get up enough nerve to try it again. I did leave my prints on the bar trying to hold on. Once it started, I wouldn't have dared to loosen my grip. I've already been warned that my fellow riders are presenting me with some type of ride "award" tonight on our recovery ride. I can only guess !!!!!
I may be causing the problem & bike is OKBirddog
Jun 16, 2003 8:59 AM
I'd almost bet that the MAJOR culprit is your own "death grip" on the bars. I've had this happen to me on a couple of different bikes, and when I learned the trick of loosening the grip, I have not had the problem. Try it in a controlled area. In other words, try to get the bike to shimmy by doing your normal thing. When you just start to feel it, loosen your grip or take your hands off altogehter. Yes it takes a leap of faith, but it works. Doing this will give you a feel for what is right/wrong about your grip. What others have said about supporting most of your weight on your pedals is good advice. I just returned from some hellacious hills where I reached 52 mph, and at one split second thought I might be getting a shimmy started, I immediately loosened my grip a little, and it went away just as quickly. BTW, on this particular ride, going past "Deadman Vista" at 45 mph was a little disconcerting.
Birddog
Let go?!wolfereeno
Jun 17, 2003 3:20 PM
Ok, I'll try!

This has happened to me several times. Only on really rough road that caught me by surprise - causing me to brake > 30mph. But since the bike's skipping over the rough spots theres not much stopping power. At that point the oscillating would start.

My bike's pretty stiff - a Kestrel 200sci with a Kestrel fork.

I've also been advised to clamp top bar with knees. And stand out of saddle (this didn't help the last time).