Andrew R Stewart 

The understanding of speed shimmy that I have come to believe in is it's being a harmonic vibration/oscillation and that by changing the frequency the parts can vibrate at can disrupt/change the nature of the shimmy. Stiffer frame "tubes" are one way to do that, IMO.

Rarely mentioned when frame stiffness and shimmy are discussed is the contribution that the fork has, more specifically the steerer as this is the one tube on the whole bike that is only solidly connected at one end, so the aspect of triangulation (which the rear end of the bike is an example of) adding to stiffness doesn't exist.

I have made 4 different touring frame/forks for myself over the years and as I became unhappy with the current version's shimmy I would tweak the design and make another. Initially I changed the steering geometry (and tried a second fork even) for #2. #3 saw heavier walled tubes but still traditional diameters. It was #4 that saw the shimmy essentially end. It had a 1.125" steerer and a 1.125 top tube but of a medium wall thickness. It is from this that I came to feel that the fork is more involved than most will talk about. Especially these days when a fork is just an off the shelf component that the frame builder really doesn't control much with. Andy (whose #4 touring bike doesn't have anywhere nearly as much "inch worming" when riding over bumps, the fork doesn't flex fore and aft as much)

unterhausen

That's not how structural vibration works, it's really pretty deterministic in a structure like a bicycle. With the right equipment, you can measure the frequencies where it will vibrate. There is such a thing in nonlinear systems of frequency splitting, but I have never seen any examples where that leads to vibration modes at lower frequencies than the linear elastic frequencies. Again, I'm not seeing an argument against the fact that there is a hinge in the middle of the frame that can't support bending moments in the direction a bike fame moves in shimmy. So in shimmy, the frame moves like a rigid body. Physics doesn't play games. In a really large frame, there might be a bending mode that would see the head tube rotate in relation to the seat tube. But that would show up in a modal analysis. And it's at a higher frequency than shimmy for all the frames that we measured.

It's my understanding that all bicycles have a critical frequency where they shimmy. I have seen no evidence that shows that less-stiff tubes lower that frequency. Builders like Rob English would have lots of problems, but they don't. I am pretty sure shimmy is just a function of how straight the frame is. I'm trying not to argue from authority here, and I think I have avoided it pretty well so far. Maybe that last sentence is an argument from authority, I don't know.

guy153

Thanks. This makes a lot of sense. And reading a bit about motorcycle shimmy it sounds plausible that trail is important. The two bicycles being compared by HW had very different trails (the shimmying bike's was in the 30s, the other's 50s or 60s), besides one having an oversized TT.

(about 10m in)

unterhausen

Bicycle steering geometry is self-righting, the front wheel automatically goes back to center. There is also the matter of flop, which drives the wheels away from center. It's a situation that's ready-made for shimmy. Shimmy is a fundamental feature of steering systems similar to the one used on bikes.
Here are some thought experiments:
--Why don't swing bikes have horrible shimmy problems? They have virtually no stiffness in the direction of shimmy.
-- Did slingshot bikes have shimmy problems? The name of the bikes with the seat attached to a cantilever are escaping me, but it seems like they should also have shimmy problems, since the riders weight is allowed to move sideways.
-- when you get a bad cart that shimmies at the grocery store, do you think they should have used stiffer tubing?

My main theory about why bigger bikes are more likely to shimmy is that they are also more likely to have alignment problems. I really need to strip down a couple of bikes I have that are known to shimmy and see if I can find alignment problems with them.

bulgie 

My working assumption has always been that frame alignment was not a factor. I can't explain how I came to that conclusion exactly, other than knowing some frames that shimmied were near perfectly aligned.

I'll leave "perfectly aligned" undefined here since FBs don't totally agree on what that means, but the bikes I'm talking about were fastidiously aligned on a top-quality pro framebuilder alignment fixture (Marchetti) by a competent experienced operator (me). So at least by some definition, the frame was near perfect.

Paul Brodie, in his video on frame alignment, complains about the fixtures like the Marchetti that use the BB faces as the datum. He says something like "nobody ever flips them over to see if it measures the same on the other side". Well I'm here to tell ya, some people do flip them over. Repeatedly, habitually, a lot. I'm intimately aware that the BB faces aren't even parallel to each other, (yes, even after facing with a Campy tool), but that doesn't mean the BB face can't be a useful datum, it just means you have to be aware of its limitations. But I digress. I assert that different ways of measuring a frame all result in the same thing (a straight-riding bike) if they are all done correctly — I hope that's non-controversial.

One frame that comes to mind, we sponsored a top Swedish amateur team, just to the extent that we sold them a bunch of frames below wholesale, not a gift. They were in the US and got to ride the bikes while still here. Their biggest guy rode a 66 cm, which was not a custom, just the largest size our stock lightweight road frame came in. We had made literally thousands of those frames, nearly identical, but this one shimmied. We tried many things, like heavier wheels, forks with more and less rake. Nothing worked. We triple-checked the alignment, it was spot on (by our definition).

That was back in the era of 1" toptubes on everything. Larger frames got thicker wall tubes but the diameters stayed the same, due mostly to lug availability. We had just gotten our OS lugs and tubeset (designed by us in collaboration with Takahashi and Tange), which had only a 30 mm DT (slightly OS) and 1-1/8" TT, so I quickly threw together a "team issue" custom for him. (For us that meant as close to stock as possible, and with a lower level of aesthetic polish, i.e. left a little rough here and there). Angles, lengths, wheels and other parts, and his position on the bike were identical to the previous bike, and the frame weight was about identical too, but the new one never shimmied.

All frames from 62 cm and up got OS TTs from then on. The DT stayed at 1-1/8" though, for that light road race model, even on the 66 cm. (The full OS tubeset and lugs was for a more crit-oriented model.)

I know for sure I can't analyze shimmy in my head, but I think I have noticed a positive correlation between frame flex and shimmy. Maybe with low trail being a secondary factor. But the one thing I am pretty sure about is that it's the bike + rider system that shimmies. The same bike that shimmies with one rider aboard often will not when someone else rides it. For example I wasn't able to get that Swedish racer's bike to shimmy for me.

Anyway I'm not nearly sure enough about any of it to rule out unterhausen 's theory that it's caused by poor alignment, as long as he'll allow for exceptions, frames that still shimmy even though properly aligned. Which would mean there has to be more than one cause.

unterhausen

Like I said, shimmy is a fundamental aspect of the steering geometry. I'll admit this doesn't prove it's not stiffness, but I have presented extremely good evidence that it's not on normal bikes. I'm not convinced that it's alignment either, although I know alignment will cause shimmy. Different riders will definitely change this behavior. Although I think it's mostly weight distribution. Someday I'm going to instrument a bike with strain gauges and show that there's no bending in shimmy. I need a tall person with a bike that shimmies and doesn't mind some weird decorations glued to their bike.

Speaking of alignment, I have been wondering about flipping the bike over and checking it using the other face of the bb shell. But since I almost exclusively use silver and lugs, and I think it's less important because the faces of the bb shell don't suffer as much distortion as with TIG or fillet brazing. I was just looking at my alignment table today. I'm thinking of using the head tube as the basis of my alignment system. I'm not sure how the people that do that check bb shell alignment. Maybe use the bb shell and the HT in different operations? I have a bench center just collecting dust in the basement, and there is a place to put it on my alignment table.
There is an Ellis Briggs youtube where he answered the video from Paul Brodie. The thing that impressed me was how many different ways he checked alignment. I think some aspects of my system are better, but he does a lot of checks. Especially on forks.

guy153

The shimmying bike in the video is likely to be perfectly aligned. Of course we don't know this but it's a custom build by someone who looks like he knows what he's doing. However it still sounds like a good theory in general.

unterhausen

Maybe I shouldn't have brought alignment into it, but I feel like it's a common reason for shimmy. Bob Dopolina (BDop bike parts founder) used to post in every thread about shimmy that he knew that in most cases it was fork alignment related. And I have fixed shimmy by aligning a rear triangle. Basically the steering geometry of a bicycle leads it to wander a little under normal conditions, but if it gets thrown into the shimmy limit cycle, it will violently oscillate. This is a pretty common dynamic system. Unstable equilibrium point surrounded by a stable limit cycle. There are various effects that will throw it close enough to the stable (shimmy) limit cycle that it gets captured. As I posted in my first comment about it, a bike that is much less stiff than any bicycle that has ever been made will shimmy because of stiffness. It's not really a proper experiment to just replace a bike with one with bigger tubing and say that shows it's stiffness. Even the worst framebuilders make more bikes that don't shimmy than do. A low trail bike has lower self-centering forces, which means that any other factor is more likely to drive it into shimmy. But most people that have low trail bikes have a front bag that totally changes that. I think my travel bike (ridiculously low trail) is one of the most stable bikes I have ever ridden. But I never ride it without at least a few pounds of stuff in my front bag, wouldn't be prudent.