corwin1968

I've got a 20 year-old frameset that has a very slight wobble when riding it, very much like an incompletely seated tire or untrue wheel. I've taken my custom built wheels (that run 100% smooth) and put them on this bike and the wobble is still there. Is there some frame misalignment that could account for this? I know for certain the headset is properly adjusted and I don't know about the bottom bracket and if that would even make a difference.

Andrew R Stewart 

Explain more in depth when and how this wobble happens. All the time and any speed with any load on the bike? Or, say, only when coasting and riding no hands?

The "classic speed man's wobble"/shimmy has a long history to it, if that's what you're experiencing. Andy.

unterhausen

if it "wobbles" when you have weight back, it's probably an alignment problem. My guess is that this is the case. Google, "bike frame string test" for something that will probably show you right off the bat. Although that doesn't show misaligned forks. Does the wheel sit straight in the fork?

corwin1968

I can feel the wobble at any given speed with just me riding the bike. The wheel seems straight in the fork but the clearances are a bit different in the back so maybe the rear triangle is out of alignment. I'll look up that string test and check that out.

Andrew R Stewart 

Most frame/fork miss alignment is felt as a "pull" or a need to lean to one side while riding no handed. Most shimmy is due to the flexing harmonics of the various parts of the system we call a bike. So a badly out of line frame/fork won't cause a shimmy by default. And a straight frame/fork that is other wise straight but has the wrong load/flex/geometry/rotating inertia can shimmy.

Having said that it's prudent to start with frame alignment (and component condition) as that's something one can measure/service and change/replace. Andy.

unterhausen

compliance in the frame is not a primary cause of shimmy. https://en.wikipedia.org/wiki/Speed_wobble Maybe if the frame is broken

Andrew R Stewart 

If shimmy was completely understood and easy to avoid then no bikes would. But it's not that simple. I can ride the same bike and suffer shimmy at some point (speed, loading, weight position) and another can ride the same bike and never see shimmy. I have done this very comparison many times. I have spent hours trying different wheels, load placements even different geometries (but still with the same intentions of ride use) and the results are that each effect the possibility of shimmy. Included on this list is making a handful of frames with increasingly stiffer tubing. This last variable also effected my results. Stiffer frames made the other variables less of a factor.

So I won't say that frame stiffness is the sole factor or the most important one, just the one that I find has a strong influence for my situation. Andy.

unterhausen

shimmy is actually pretty well understood if you follow my wiki link. It's just that there is so much disinformation in the bike world that the literature is simply ignored. I've gotten used to it for the most part. People have studied 2 wheel vehicle dynamics for decades now, and the simulations are pretty accurate. Not that you can do much about it when a bike shimmies in most cases. If we had better metrology, I'm pretty sure it would be possible to figure out why a bike shimmies.

I know how to make a bike that will shimmy, and I think a very straight bike will almost never shimmy at the kind of speeds that we ride. The common thread on most bikes that shimmy is shifting the weight rearward at higher speeds in combination with a misalignment to get things going more easily. As I understand it, there is a set of eigenvalues in the system equations that becomes complex over a certain speed, and thus there is an oscillation waiting to happen for most bikes given an input at the right frequency. Maybe a well aligned bike will shimmy at high enough speeds that frame stiffness plays a part, but that has to be 60mph or more from what I can tell.

I'm pretty sure that the OP's bike has something obvious wrong with it and either a fork alignment check or the string test will reveal all. Or maybe it's broken and he didn't notice, it could happen


I know what you are talking about regarding a "pull," but that may just be a shimmy that is unstable at zero speed. Which is a weird concept, I know. I suspect that most bikes that pull would shimmy if you could get them to speed. Or else the pull is so hard to overcome that they never shimmy, I just don't know. The bike I made (30 years ago) that shimmies badly had a misaligned rear triangle. I fixed the alignment and it stopped shimmying. You couldn't tell anything was wrong with the bike except the shimmy.

corwin1968

I just picked up a 1992 partial Rockhopper that will take the place of the bike in the OP as my single-speed project. The first bike will probably move to the back burner for now but I'll eventually come back to it and spend some more time experimenting with it to figure out what's going on.

triathloner

Are the wheels tight, look for loose skewer or nut. Make sure the wheels are aligned. Also check to make sure there isn't play in the axels, loose can cause a shimmy if they are too loose. good luck and be safe, get it figured out.

Andrew R Stewart 

"The oscillation frequency can be changed by changing the forward speed, making the bike stiffer or lighter, or increasing the stiffness of the steering, of which the rider is a main component"

This from Eric's Wiki link. I feel that it confirms two of my points. First that a stiffer bike does make a difference. Second that the rider makes a difference. I feel we are more agreed then not but perhaps are looking at this issue from differing starting points, or something like that.

BTW I had to look up a couple of definitions before replying. Wish I had a engineer/math guy to talk about these aspects with. Andy.