No-Hands Shimmy (2019 Version)
 

Okay, so I've done my homework and have discovered a great many things that can cause the front end to shimmy when taking your hands off the bars. But there hasn't been a thread on this in a while so hey, let's try a new angle on it. Rather than "what causes this?" we'll play "can this cause it?"

The bike: '79 Raleigh Competition GS, set up for commuter duty: rear rack with a trunk on top, no panniers, rarely is there more than 5-6 lbs of stuff back there. Headset and all the other maintenance-type things I've seen mentioned in previous threads are in good order.

The recent change and primary suspect: Switched from a 90mm SR Royal stem to an 80mm Technomic. Bars are now 5cm higher than they were, and are set back probably 2cm(?) from previous.

Could this really be enough to tip things into general instability? The bike seems to think so, but I'm curious if anyone has run into a similar situation after going to a Technomic.

Pic, because it's what we do here:
https://cimg2.ibsrv.net/gimg/bikefor...c93443f084.jpg

I'm far from an expert on these things, but my Bianchi Sport SX also gets wiggly when I take my hands off the bars. It was a bit unstable for the previous owner, @balindamood, as well. Interestingly, the only hardware in common for our riding is the frame (excepting the fork), and I've used the string test to make sure it's pretty well aligned. What does seem to be in common, is having weight too far back, and not enough of it on the front wheel. Brian used a rear rack and trunk bag, I had it set up in sort of a French/Eddy fit with knees behind KOPS and an 80mm stem. The new fork has more rake for less trail -- I'm not sure if that helped or hurt the stability, but it makes the bike easier to control with a light touch, so I'm content. I've scooched my saddle forward a bit and am pretty much at KOPS now, considering a 90mm stem since I can now reach forward a bit more. Moving my weight forward seems to have helped. The bike still gets shaky if I take my hands off and pedal at 20+ MPH, but now I can ride no-hands smoothly at 15-18 MPH.

I think the ultimate solution might be to find a frame that's 10mm shorter in the top tube, and make up for it at the stem so that a little more of the front wheel is under me. (My Bianchi Eros, one size smaller, doesn't wiggle.) Perhaps longer chainstays would help, too. Might be looking at a custom frame if too many of these odd demands add up. ;)

That's a lot of blathering to say moving your bars up and back may have indeed hurt your cause. One thing to try is to pull your rear wheel back in the dropouts -- I like to line up the rear axle with the seat stay, and can see that your wheel is ahead of that.

Contrary to what many have reported, I found that moving the saddle forward tends to make my bikes less stable when taking my hands off the bars. I discovered this after moving the saddle forward to compensate for too long of a reach. When I tested the bike, I found I could not ride it no-handed, the wheel would quickly turn to either the left or right.. Previously, I could easily ride no-handed. Moving the saddle toward the rear 1/4 to 1/2 inch corrected the problem and the bike was stable no-handed again.

Shimmy is a resonance problem, and if you are now sitting further back in the saddle or not leaning as far forward, you have changed the mass distribution of the rider-bicycle system. Try bending your elbows and sitting and/or leaning slightly farther forward. The lean may be the key here, because it alters the height distribution of your weight over the frame.

I have a lightweight bike that shimmies too. I only noticed it after swapping the wheels. But my guess is that it shimmied with the previous wheels too — I just hadn’t tried riding no hands at speed. I do think the shimmy is more pronounced the further back I am on the saddle. My guesses: frameset resonance, unbalanced wheel weight, tires and/or wheels not mounted perfectly straight; replacement fork/geo. Also maybe a bent frame, but IIRC mine passed the string test too. Shimmy is quite the mystery.

I have one set of wheels that has induced identical shimmies on two different steel frames, fairly stiff ones. A 531 Raleigh Competition and a Lemond Tourmalet. These both occurred while coasting, with one foot down and no hands. I'm wondering if maybe it could be the radially-laced front wheel lacking spoke tension? Or something else? Headsets are both in good shape, so i think I can rule that out. Is it strictly a "steel frame thing"?

I agree, it's quite the mystery. I have noticed that European pros ride no-handed all the time without problems (Brad Wiggins notwithstanding) so I'm suspecting there may be a mechanical fix that the professional mechanics know about.

Genius! :) This will be tried immediately. The looong Campy dropouts mean I might be able to make a real difference this way -- I've got more than 2cm (maybe even 3) available behind the current position.

Thanks to all who replied. You gave me at least a couple of things to consider now that I may not have otherwise.

Solution to shimmy, explained.

My motorcycles get shimmy if I sit too far back on the seat and take weight off the pegs. Reweighting the pegs immediately straightens things out. My Centurion Pro-tour gets some shimmy with out tension on my front bag, strings to the handlebar or a decaleur stop it. If riding no hand I tend to put my weight on my seat, putting weight on the pedals stops the tendency to shimmy but that is hard riding no hands.

Not sure how a stem change could induce a no-hands shimmy. I suspect something else is the cause.

FWIW, I just changed a 95mm SR Royal stem to an 80mm Technomic with no shimmy problems.

Dang, I thought you were posting the Russo shimmy scene (or waggle.... or whatever..)

I'm convinced that strong no-hands low-speed shimmy is related to some kind of wheel tracking problem. I have cured that kind of shimmy by aligning the rear end of a bike. OTOH, I have seen forks that weren't aligned on bikes that didn't shimmy at all. People report that fork alignment problems can cause shimmy. No-hands shimmy seems to be more prevalent the less trail a bike has. So front weighting those is a good idea.

The pros are on bikes with a lot of trail. Probably has a lot to do with their lack of problems with shimmy. I am a little surprised they don't have high speed shimmy/speed wobble though.

Race bikes having more trail might be part of it. Jobst Brandt claimed that experienced riders just knew how to deal with it, so they didn't talk about it. ;)

Last summer my 1982 Competition GS was returned to me with all the original parts I had on it. Took it out for a ride and entered a tank slapper at 35mph down hill. Controlled it and made it home. Put it in the stand and found spoke tension to be exceptionally low. Brought it up to tension and have been riding without a hitch. Hit 40mph the other day without trouble.

One trick I learned with motorcycles was to tighten down the tension on the headset. helps to stabilize shimmy's.

I built up a 1987 Centurion Ironman with brand new Vuelta XRP racing wheels, Microshift brifters, cables, brakes, chain, cleaned and polished. It weighs 21-1/2 lbs. The very first time I took my hands off the handlebars cruising downhill about 20mph it started to shimmy. Scared the #@%^& out of me. Never had that happen before. Went away as soon as I put my weight on the handlebars again. If anybody can figure it out let me know. I love that bike. It's a fantastic ride.

Jan Heine has suggested that a roller bearing headset like the Stronglight A9 can dampen shimmy.

Well thanks to all. You've given me a lot to think about and explore as I track down a solution.

FWIW, moving the axle rearward in the dropouts -- and I moved it all the way back -- didn't resolve things. I was hoping it would, as the weight was effectively moved slightly forward. No dice.

In the meantime I suppose I'll keep my hands on the bars. :D

Can concur. Put an IRD roller bearing headset on mine and that fixed virtually all shimmy. And when it does happen it is a whole lot more manageable.

Lighter wheels (or tyres) helps too.

I think wheel weight/balance and tire/tire mounting issues may have something to do with shimmy. If a wheel is unbalanced for some reason, or the tire tread(s) aren't in a straight line, couldn't this contribute to "resonance"? I also think that lighter and more compliant frames may be more prone to resonating.

My Viner would shimmy at higher speeds if I moved my butt back on the saddle further than normal, I have had a couple of scary incidents on that bike. I could ride it no hands without any problems though, the speeds where I would want to do that are much lower than where it would shimmy. My understanding is that all bikes will shimmy at some speed, it's just that the good ones only shimmy at speeds higher than anyone rides.

My mind keeps drawing parallels to electronics, particularly tube amplifiers. Lots of opportunities for those to oscillate, and oftentimes luck plays a role. A draggier headset could be likened to putting a low-pass filter on the steering input. :geek:

Have you checked either/or rear hub cone adjustment and rear wheel tensioning and dishing? I have found that too loose rear cones, too loose rear spoke tension, and a rear wheel out of dish can all contribute to some shimmy.

Random update: Front wheel has been ruled out after swapping in the front from my Paramount. Same size/brand tire, Continental GP Classic, but on H+Son rim and Campy high-flange hub. Just about as perfect a wheel as I could ask for.

...the shimmy remains.

Might just tear the whole thing down and take the frame/fork for a proper alignment. Was hoping to avoid that.

I salute your tenacity, and welcome any new data points on the mysterious issue of shimmy. Good luck!

There might indeed be a frame alignment problem, but you might be able to get around it by replacing the rear rack with a front rack.