Stopping the shimmy -- a theory

16-03.-2005, 01:14 AM	#1
analogkid333 Registered User Join Date: Mar 2005 Posts: 12	Stopping the shimmy -- a theory I'm having trouble escaping this shimmy thing; every bike I've ever toured on has wobbled horribly. I used to tour on "sport" bikes but a few years ago actually managed to find a NOS Miyata touring frame (618GT), cantilever mounts and all. I rode this one cross-country, and it shimmied like a washer on spin cycle the whole way. One day, on an 18-mile descent in the Cascades, it very nearly dumped me on the tarmac at 40mph. So I've been brainstorming on cheap ways to stop the problem. I already use lowriders and have trimmed my touring weight to maybe 30 pounds. This has barely helped. I have a new idea, and I'd like to hear opinions: Since shimmy is transmitted greatly through a bike's top tube, I'm thinking of finding some 1" steel pipe in a hardware store, having a machinist cut it in half lengthwise, and then securing one half to the underside of the top tube using hose clamps. No doubt this would look UGLY, but would it arrest the shimmy? I think I'm going to try it. The other question this raises, assuming this trick works, is how would buttressing the top tube affect tube stresses in other parts of the frame? Would an inflexible top tube mean greater load stress at the joints or in the seat stays? I want to stop the shimmy, but not if it means making the frame unsafe. I'm getting technical here, but if there's anyone who has any ideas on this, feel free to respond. Thanks. Last edited by analogkid333 : 16-03.-2005 at 01:23 AM.

16-03.-2005, 06:23 AM	#2
EmmCeeBee Registered User Join Date: Jul 2004 Location: SW Washington, USA Posts: 52	Re: Stopping the shimmy -- a theory What you're thinking of is a drastic bandaid approach. I doubt that any flex in the top tube is the sole cause of the shimmy, and I'm even more skeptical that a 1" steel half-pipe and hose clamps will do anything except add a couple pounds. I'm not a bike designer, and not a professional bike mechanic. But when my wife had a bad shimmy with her bike, we solved it (about 90% solved, actually) by balancing the load, back to front. If most of your gear is on the back, this can greatly affect the dynamic balance. Don't go overboard by weighting the front too much, but try to move some of the pounds onto your lowriders. Another thing I'd check: Have you changed the fork, or put larger/smaller tires on (from what came with the bike)? The geometry of the steering angle and the contact point of the front tire (the "trail") are finely tuned for handling. The contact point is designed to be "self-correcting", with positive caster it trails the angle of the steering tube so that the wheel will tend to straighten out when you turn (the effect is hardly noticeable, unless you ride with no hands). If you've gone to a larger tire, you increase the caster and could cause shimmy. It's probably impossible to mount a tire small enough to reverse the caster, but this would make steering horribly unstable. Just a thought.... A good bike shop should be able to help, especially to check for tube flex. -- Mark

16-03.-2005, 11:29 PM	#3
analogkid333 Registered User Join Date: Mar 2005 Posts: 12	Re: Stopping the shimmy -- a theory I think the weight is pretty well balanced already, especially since I got rid of my handlebar bag, which had a notorious effect on the shimmy. It could very well be a trail/geometry problem; my bike is a 63cm with a 56cm top tube, shallow head angle, and a long fork rake. No matter where I put the weight, this stubborn mule just doesn't want to carry it. I thought of stiffening the top tube because I remember looking down many times and seeing the top tube vibrating back and forth uncontrollably. Wherever the shimmy actually begins, I thought that making the top tube rigid could somehow prevent the shimmy from spreading throughout the entire bike. Perhaps that's naive thinking, but I'm prone to seeking out handyman solutions to things. (Once, in North Dakota, my riding partner's trailer tire completely fell apart. I wrapped the whole thing in about five layers of duct tape, which got us to the next town.) Anyway, thanks for the suggestion. I'll figure it out eventually.

17-03.-2005, 02:20 PM

daveornee

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Join Date: Sep 2003

Location: Suburban Chicago

Posts: 2,778

Re: Stopping the shimmy -- a theory

Quote:

Originally Posted by analogkid333

I'm having trouble escaping this shimmy thing; every bike I've ever toured on has wobbled horribly. I used to tour on "sport" bikes but a few years ago actually managed to find a NOS Miyata touring frame (618GT), cantilever mounts and all. I rode this one cross-country, and it shimmied like a washer on spin cycle the whole way. One day, on an 18-mile descent in the Cascades, it very nearly dumped me on the tarmac at 40mph.

So I've been brainstorming on cheap ways to stop the problem. I already use lowriders and have trimmed my touring weight to maybe 30 pounds. This has barely helped. I have a new idea, and I'd like to hear opinions: Since shimmy is transmitted greatly through a bike's top tube, I'm thinking of finding some 1" steel pipe in a hardware store, having a machinist cut it in half lengthwise, and then securing one half to the underside of the top tube using hose clamps. No doubt this would look UGLY, but would it arrest the shimmy? I think I'm going to try it.

The other question this raises, assuming this trick works, is how would buttressing the top tube affect tube stresses in other parts of the frame? Would an inflexible top tube mean greater load stress at the joints or in the seat stays? I want to stop the shimmy, but not if it means making the frame unsafe. I'm getting technical here, but if there's anyone who has any ideas on this, feel free to respond.

Thanks.

What will you do with your rear brake cable?
Maybe you could experiment with half of a thick wall PVC pipe on the bottom of the top tube.
Most bicycles have some resonant frequency.
I have seen steering dampers that attach to the stem in triathalons.
Make sure that your headset is properly adjusted and that your wheels are true/centered/stress relieved/spoke aligned/tension balanced/and bearings adjusted. Check your frame and fork alignment. See if temporaily swapping to a different wheelset makes a significant improvement. If it does, you are on to something.
On my old Miyata 1000T changing to Phil Wood hubs and my own hand built wheels made a huge difference. I used 36 DT Alpine III spokes all around with Mavic T217 rims. I descended stretches at 40 - 45 MPH without any shimmy.
I saved those wheels when I sold that bicycle, with the idea of using them on my new touring steed. So far I haven't had the need.
Your load/racks/panniers will make some differences too.
Have you tried tubular steel racks from Beckman, Bruce Gordon, or Tubus? How secure are your panniers tied to your racks? Have you had a nearby rider look to see if they can identify any potential shimmy sources?
Have you lifted your weight from the saddle to see what happens to the shimmy?
Have you squeezed the top tube between your knees?
See if you can find some touring riders in your area that will let you test ride their wheels on a descent with your load.
The stock Miyata fork is comfortable, but I wouldn't put more than 20 pounds of dead weight on it... including rack/pannier weight.
David Ornee, Western Springs, IL

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David Ornee, Western Springs, IL USA

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