I'm sure there's a dozen threads on this subject, since the words "fork shimmy" are, according to the search function, too common to search for. If anyone knows of such a thread, a link would be appreciated.

Anyway.

My question is, does anyone have first-hand experience figuring out what caused fork shimmy, and fixing it?

- Slightly "unsquare" fork.
- Headset too loose.

Two that have affected me personally.

I suffer the same problem. My Schwinn Shimmies pretty bad around 18-20. havent really taken it faster than that. Assuming its the dish on my front wheels.

My Centurion has the same problem up around 20, but its not as pronounced as the Schwinn. Did it with the stock wheelset, the wheelset that is now on my schwinn, and my 105/cxp22's. I think it might be me.

How did you diagnose (and fix) the fork issue?

Keith, what do you mean by "unsquare" fork? I'm slow today.

Edit: I removed my question as it wasn't pertinent to fork shimmy.

Hi RHM -

I encountered this once before in a major way, on my Trek 760.
As I purchased it, it has grossly mismatched wheels and tires (Tubular Campy front, and a clincher rear - with a warped rim!)

I got a lot of pointers and possibilities from the Mechanics forum with much about the headsets - but truing the rear wheel helped, and replacing the wheel set obviated it completely.

So - if you can't find the cause of a wobble or shimmy up front, look to the rear for the possible cause.

Recommended reading: https://sheldonbrown.com/gloss_sa-o.html#shimmy

In addition, try "high speed bicycle instability" or "... vibration" and similar searches. Many causes have been suggested, and it is likely that there is truth in each, in some cases, some of the time. Some wheel/frame/rider combinations seem more prone than others to this problem, which I used to encounter w/ my first Capo, which had a double-butted 531 frame and a Pletscher rack over the rear wheel.

Nishiki reportedly changed from a butted to a straight-gauge seat tube on its 25" Competition frames because of customer complaints about shimmy. The heavier gauge would alter the frame's modulus of elasticity and move its resonance frequency.

Other culprits have been mistracking wheels, even a mis-dished rear wheel.

From what I've seen, there are lots of theories, and precious few actual solutions. Some claim too little trail is the cause, others say too much trail; too much frame flex is the culprit, or is it too little?. Heavy wheels seem to cause it, but light ones can make it worse. FWIW, I've had stiff-as-a-board frames that wobbled, and willowy ones that didn't; low-trail bikes that were rock-steady, and high-trail ones that shimmied if you looked at 'em funny. Even weight distribution isn't a consistent issue. My 'Traut Limited tourer wobbled with a rear load, but was rock steady with the same load up front. My Gunnar Crosshairs was a regular hula dancer with a bar bag.

Word is that headset friction may be part of it: more friction seems to damp things out pretty well. So tighten up your hs maybe 1/8 turn (or less), or install a needle-bearing unit. Cartridge-bearing headsets have the least friction, so may aggravate the problem.

There's been a lot of attention to the issue in BQ lately, and one theory to come out of that is that the distribution of rigidity within the frame matters. A slightly heavier downtube and lighter top tube seems to work better, which my experience bears out. Most of my favorite bikes have been made from the old Reynolds 531 db set with 1.125", .9/.6/.9mm downtube and 1" .8/.5/.8 top tube, and in retrospect those bikes also had the fewest wobble issues. Of course, that doesn't help much for an existing bike...

SP
Bend, OR

Loose/worn headset bearings can also cause a shimmy at the front end.

Chombi

My theory, based upon experience, is as follows: Shimmy starts in the wheels (usually the front). Tires, untrue rims, whatever, it starts there. Forks, tubesets, headsets only serve to amplify the medium-high speed shimmy problem that the wheel is out of balance. Low speed shimmy is almost always a problem with tires. 99.9% do not balance the wheels on our bikes, but we ALL balance them on our cars. Balance your wheels (very hard to do), and the shimmy will go away. Sometimes just shifting weight (different tires, or even a stylish spoke reflector) will do the trick. Fooling around with headsets, forks etc is always a goood idea to keep a bike in working order, and may reduce the noticability of shimmy, but it will not eliminate it.

Jan Heine and Mark Vande Kamp wrote an article, Curing Shimmy on a Bike in Bicycle Quarterly, Volume 6, Number 3 (Spring, 2008).

They believe that frequently the problem is caused by the head tube upper and lower edges not being perfectly parallel. They contend that on a standard ball bearing headset the balls on one side tend to run looser than those on the other, causing shimmy. On Mark's Ti bike that had a very strong shimmy under certain conditions with a Chris King headset installed, they swapped out the Chris King headset with a Stronglight needle-bearing headset. Stronglight headsets are different from all others in that the bearings align automatically, and thus compensate for imperfections in the facing of the head tube. After they swapped the headset, Mark tried to induce shimmy as he had before, but the bike no longer shimmied. After numerous attempts, he finally got the bike to shimmy briefly, but instead of continuing until Mark put his knee on the top tube, the shimmy now attenuated on its own within a few oscillations.

There's more to the article, but that's the jist of it. When I read the article in 2008, I realized I had never had a shimmy problem under any condition while riding my '87 Paramount with the OEM Stronglight A-9 needle-bearing headset.

YMMV.

Bingo! I introduced a shimmy to my Rockhopper when I added spoke reflectors. It only shows up when I have a load on the rack, but I removed the front reflector and it vanished.

I put the reflector back on ultimately - the shimmy isn't always present, and it's pretty minor in my case.

Shimmy can be due to poor geometry, especially if it is a tall frame,
since most mfgrs use crappy geometry in their tall frames.
I cured a tall Litespeed of shimmy by replacing a fork with 45mm rake,
with a custom fork with 38mm rake.
See the following links for more info:


https://davesbikeblog.squarespace.com...f-history.html

https://davesbikeblog.squarespace.com...-steering.html

https://velonews.competitor.com/2010/...-poorly_121162

I recently had a bike powdercoated and did not have the head tube faced. I can't quite get the headset adjusted properly. I'd install a needle bearing headset, but I'm not aware of a JIS model.

Apply the front brake and "feel for little vibration" at the headset when you push the bike back and forth. A little vibration is OK with an older headset. Now raise the bike off the floor to maintain a downward tilt of about 30 degree. If the front wheel can swing from side to side with ease when you tilt the bike, then you're good to go. You can also drop the front wheel to the ground from a height of 6-8". If you hear a rattling sound around the headset, then the bearing is too lose.

The saddle and saddle height can also affect high speed vibration. A sprung saddle like Brooks Flyer may induce vibration at high speed. Changing the saddle height or getting off the saddle may alleviate the problem. My Bridgestone 400 would go into oscillation mode around 32-33 mph with a Flyer. Problem goes away when I put in the Imperial narrow.

I've always suspected weight distribution, at least in loaded touring and/or fast downhills.

This happened to me with my Raleigh comp. It was so bad that when the fork was turned one way, it was tight, and when it was turned the other way, it was loose. I believe this only happens when you have a compound problem: one or more faces of the head tube not being parallel, AND the crown race not being parallel with the head tube. I ended up pseudo-facing the head tube myself, using sandpaper to remove the powder coating from the upper and lower faces, being careful not to sand away a significant amount of metal. I also removed the crown race and used a soft carbon-steel brush wheel to remove the powder coat that had oversprayed onto the crown race seat. This solved the problem entirely, and now the headset (a Campy HS) is pretty smooth and solid.

But that was then, before I really understood how important proper facing is. I think I was lucky that my improvised facing method even worked. Now I just make sure that I have the head tube (and BB, for that matter) professionally faced after powder coating. Most LBSs will do it inexpensively. My new powder coater is also a frame builder, and he includes facing of the head tube and BB in the price of his powder coating.

Thanks, Anton.

My LBS doesn't have facing tools and I doubt the "mechanics" at any of the other LBSs would have a clue what I'm talking about.

The BB obviously needed facing, but I didn't have it done. If I posted a pic I'd likely be ran out of this place!

Speaking of needle-bearing HSs, doesn't VO sell one? The concept does seem superior, and makes you wonder why this type of headset isn't more common.

Thanks for all the replies, guys, this is very interesting.

I have never notice shimmy when riding frames that are "too small", but have noticed it on frames one would consider my size (60 - 62 cm); so frame size is probably a factor.

I have never noticed any shimmy on a bike with small wheels, and I've ridden a lot of bikes with 16" to 20" wheels -- folding bikes, Moultons, and a recumbent.

The thing that intrigues me, however, is that I sometimes my Trek 720 has been prone to shimmy, while at other times it has not. On one occasion I solved the shimmy by rebuilding the rear wheel with new spokes; evidently I hadn't tensioned the spokes right, and they started popping. After rebuilding that, the shimmy went away.

I will spend some more time truing my wheels, and maybe balancing them. Though I fear the dynohub may interfere with the latter process.

A needle bearing headset might be nice, but I don't think I can get one that will fit my frame, which has an old-fashioned "integrated" headset.

Yes. I was torn between using their Grand Cru cartridge bearing unit or the needle bearing unit on my Voyageur. I bought the Grand Cru. It didn't matter anyhow as, when I went to install it, I quickly learned the fork was JIS. Damnit!

Laid the fork on a nice flat table, bag of litty litter on the steer tube to hold it down, squared up the fork legs to the table just below the crown, measured from the table to the dropouts. One drop out was SLIGHTLY higher (about 2mm) off the table than the other. I then muscled it a lot closer than 2mm, shimmy all but eliminated. Tweaked the headset a little tighter. Pretty much gone.

I also think it's exacerbated by tall frames, tall riders, tall seatposts, HIGH centers of gravity. And I have some "high" gravity...

Not a fix, but a survival tip - If your steed starts to shimmy at speed, clamp your knees onto the top tube for all you're worth. That should change the harmonics enought to stop or at least contain the shimmy. I had a speed wobble start at 35-40 mph once, and I thought I was going to die - until I put a death grip on the top tube with my knees. Problem solved, and my heart rate eventually came back down.

The knee-clamp trick is is no substitute for trying the various solutions offered above, but it's a damn good thing to know about, because in my experience, speed wobbles at speed (a) are no fun at all and (b) tend not to announce themselves ahead of time.

+1 to dampening shimmy by pressing your knees into the top tube.

+2 and don't forget (c) they don't go away by themselves. Living in Colorado this is something I tell my kids when we ride in the hills. I've never had this happen to me but if it does I'll know what to do.

I can't believe we've gotten to post #25 without anyone mentioning FEA (finite element analysis). Shimmy can have lots of causes but it is really just an instability in the mechanical structure that is the bike and rider. Add a disturbance (unbalanced wheels, hitting a bump, normal road vibration, etc) and the system can begin to oscillate. Like a tuning fork when struck or a crystal glass when you rub your finger around it you can excite the natural resonant frequencies of the mechanical system and cause shimmy.

I've sometimes wobbled my bike intentionally (when loaded with panniers, etc) to see how prone it was to shimmy. Do that at different wobble frequencies and magnitudes and you should have a good idea on how stable your bike is. As rules of thumb, stiffer structures will shimmy at higher frequencies and adding dampening (by putting your knee against the top tube) is the best way to stop a shimmy.