Shimmy help

[emleyman]

The tyre fits well on the new rear. It's a Ryde Sputnik rim that I built onto the new Aikema hub motor, fitted with a Schwalbe Silento tyre that was transferred from the previous wheel. The wheel runs true with no dragging of the brakes.

I do wonder if the no-hands shimmy was there previously. It's not something I do normally so I might not have noticed it. The additional weight at the back has obviously made it worse to the extent that I noticed it one handed.

[pwa]

So far on the Wayfarer I have experimented a little with varying the weight in my saddlebag and my speed, but neither seems to make a difference to whether or not shimmy happens. I suppose rider weight might make a difference, but that is not something which I can vary (I weigh around 70kg). Tyres are 46mm semi-slicks, and the frame is the longer version (size 54cm).

I had a speed wobble on a bike and decided my Carradice SQR bag behind the saddle was the source of the problem. See if you can get a wobble if you have no saddle bag at all.

[531colin]

We have 2 threads running now about shimmy; recent additions to this one [link removed by moderator] are about shimmy.

Is there is a sensible way to combine?

I think a proper shimmy is a "resonant frequency" flexing of the frame in time with a snaking of the front wheel; so it has to occur at/above a particular speed when the wheel revolutions drive the resonant frequency frame flex.

Is there a "start speed" for your shimmy?

I think the irritating thing about shimmy is the unpredictability.....which is, I guess, another way of saying I hate that I don't understand it.

I guess a shimmy which only happens riding no hands isn't a problem if you never ride no hands. A shimmy riding one handed is a bit more of a worry, but I think the real worry is whether or not this mildly irritating little waggle is just waiting for an opportunity to turn into a real head-shaker and throw you off.....particularly as I don't know exactly what form that "opportunity" might take.

Long ago we had a discussion where Brucey advocated riding no hands and giving the handlebars a bang with the flat of your hand to try to initiate a shimmy. I did make a start, but as I tried higher and higher speeds, it occurred to me that this pursuit of knowledge might result in a very nasty accident!

[emleyman]

I first noticed the shimmy when coasting at about 25mph downhill. After that, I tried to regenerate it to check if it was a one off due to some unique set of conditions, but it was happening at speeds above about 16 or 17 mph.

I'll try to do a bit of experimenting and report back, but it might not be for a week or so as we have visitors all weekend.

[cycle tramp]

How rigid is the hinge in the middle of the frame?

[AndyA]

I bet if you relocated the battery somewhere low and central the shimmy would move above 25mph. Another bet that I'd be willing to take is that the wheels don't line up perfectly

[the snail]

I think a proper shimmy is a "resonant frequency" flexing of the frame in time with a snaking of the front wheel; so it has to occur at/above a particular speed when the wheel revolutions drive the resonant frequency frame flex.

I think it's more complicated than that, pretty much any part of the system - wheels, frame, bearings, the rider and any combination of factors can cause it. I had a few motorbikes that developed fairly severe tank-slappers. Taking weight off the front wheel is a common cause though, ime. It might be a good idea to try removing weight from the back, and adding some at the front and seeing if that helps.

[Norman H]

I still suspect that this has something to do with weight distribution associated with the recent conversion to electric assistance. I see that it's a rear wheel motor. Could you fit a bar bag and try running with a little more weight on the front wheel?

[emleyman]

Thanks for all the comments. I have a rack battery as the geometry of the bike and the fold doesn't allow for fitting anywhere else. I think that the change to weight distribution has highlighted the shimmy rather than caused it, since it is still there but much less when the battery is removed.
As I said yesterday, I'll try to investigate and report back. The hinge is solid so I don't think that is the issue.
I'll check the alignment of the wheels when I get a chance.

[Jdsk]

Sheldon Brown and Jobst Brandt on shimmy... and weight distribution:`
https://www.sheldonbrown.com/shimmy.html
https://www.sheldonbrown.com/brandt/shimmy.html

Jonathan

[emleyman]

It's been a while since I posted about this issue, but I thought I'd report back on what I've found in case it helps anyone else.

I've checked the geometry/alignment and everything seems correct. The hinge is solid. I've stripped the headset, regreased and rebuilt.

The bike still exhibits shimmy when ridden no handed or with very light touch on the handlebars. I suspect that the length of seatpost I have showing (I'm 6'2") probably makes the shimmy worse for me than someone shorter.

I've decided that the bike probably always did this, but I never noticed for a number of reasons:
1. I don't ride no-handed.
2. The extra weight of the rack battery definitely makes the effect worse.
3. When doing the conversion, I swapped the flat pedals for SPD. I didn't consider this originally, but I've realised that with flats you need to keep some weight on the pedals when coasting to stop the feet from slipping off. With SPDs I tend to have the pedals fairly unloaded when coasting.

I've decided that I can live with the shimmy as I know about it, and more importantly know how to stop it.

1. additional weight on the handlebars stops the shimmy (not gripping the bars, but just leaning on them a bit). To assist with this I have moved the saddle forward slightly, increasing the weight on my arms.
2. putting some weight on the pedals when coasting.

It's not a perfect solution, but I can live with it. Hopefully this might help someone else with a similar issue.