Shimmy help

[slowster]

[moderator note: some posts were moved from Spa Cycles' Disc Braked Tourer and Audax]

The current thread discussing fork rake [link removed by moderator] and its effect on steering has prompted me to revive this thread to post about my experience of the difference in handling between the Spa Tourer and the Wayfarer, because I would be interested in the insights of others.

I have found the Wayfarer prone to shimmy when I ride it no handed. It only happens on tarmac, and it's easy for me to control and stop by pressing my calf against the top tube, so much so that I have deliberately been experimenting to see under what conditions the bike will start to shimmy. In contrast my Spa Tourer has never exhibited shimmy under any circumstances (it is probably the most predictable and assured handling bike I have ever owned).

Despite my deliberately trying to provoke the same shimmy off-road by riding no handed, it does not happen. I presume that the rapid, constant tiny steering inputs caused by the tyres running over uneven and stony surfaces must be preventing the shimmy from occurring, probably by disrupting the oscillation. My supposition is that such oscillation is something which occurs in the background on every bike, usually at a very low level which the rider does not notice and which does not noticeably affect the steering, but that under some particular combinations of circumstances - depending upon the bike and other factors such as the rider/luggage weight and its distribution - the oscillation starts to create a feedback loop, with each oscillation progressively exacerbating the next one until the effect becomes noticeable and manifests itself as shimmy.

The shimmy does not greatly bother me because I mostly ride my Wayfarer off-road, and obviously I can prevent it by simply not riding the bike no handed on tarmac, but natural curiousity has made me want to understand better why it happens. The only other bike on which I have experienced shimmy was a 1980s Raleigh tourer with front and rear panniers fitted, and I was able to prevent it by reducing the weight carried in the front panniers.

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).

[Jdsk]

Sheldon Brown on shimmy:
https://www.sheldonbrown.com/shimmy.html

Jonathan

[slowster]

Sheldon Brown on shimmy:
https://www.sheldonbrown.com/shimmy.html

To clarify, Sheldon talks in particular about violent shimmy and speeds above 25 mph. I've not tried riding my Wayfarer at that sort of speed no handed on tarmac, and with my hands on the bars at high speed I do not detect any oscillation occurring. Conversely shimmy will occur at low speeds when I ride no handed on tarmac, e.g. 10 mph or less.

[Paulatic]

it's easy for me to control and stop by pressing my calf against the top tube,

Really?
Do you have to take your feet from the pedals to do it?

[531colin]

Wikipedia is worth a look if you want to understand the physics of shimmy. You may have to search Wiki using a term which the software of this forum always used to object to "tankslapper". Separate the 2 words on here, and it all falls apart.
I have seen a really good animation of speed wobble, but of course I can't find it now!

What most people don't realise is that any bike (frame) built light enough to ride nicely un-loaded will exhibit shimmy under some circumstance. Often, this circumstance will include something the rider never does, so the rider never gets shimmy.

Shimmy is the frame flexing at a "resonant" frequency. Now if you twang a ruler on a desk, the ruler has a fixed point (the desk) to flex about. The nearest thing to a fixed point which your bike frame has is your backside....your bodyweight provides an "anchor" about which the frame can flex. So, as John Allen says, shifting your weight a bit onto the pedals may work. The resonant shimmy is driven by forward motion and the front wheel snaking.....hence it happens at a particular speed.

I only ever had one bike shimmy....my first decent bike, built mid-sixties for me in 531 butted with cast lugs, and it shimmied "hands-on" at a particular speed on a smooth road. I was devastated. I found out that if I sat back a bit on the saddle, the shimmy was switched off; so I put the saddle back a shade and rode that bike for about 40 years with no more trouble. I don't recall that it shimmied no hands, but its a long time ago.

[slowster]

it's easy for me to control and stop by pressing my calf against the top tube,

Really?
Do you have to take your feet from the pedals to do it?

No. It is a sloping top tube, and riding sat up and no handed, I instinctively raise my left pedal/foot close to the top of the pedal stroke and press the top of the inside of my left calf against the top tube.

Doing it instead with pedals horizontal at 3 and 9 o'clock while riding no handed, and pressing the inside of my thigh against the top tube, might well be possible, but it would not come as naturally to me.

[531colin]

Further thoughts....
I don't think any particular type of steering geometry (twitchy, slow) is a good predictor of speed wobble....anybody?

Something which costs nothing and might just be useful....
If you spin the front wheel either in your hand or with the bike on the stand, is the wheel far out of balance?
If it is, I would try balancing it up either by adding a weight (an inch or two of solder around the spoke nipple always used to be recommended for motor bikes) or by swapping tyres front to back, or just moving the tyre round the rim.

[Jdsk]

I don't think any particular type of steering geometry (twitchy, slow) is a good predictor of speed wobble....anybody?

I've never seen a convincing connection to high speed shimmy.

Jonathan

[emleyman]

I have a Dahon Matrix 26" folder which has developed a nasty shimmy. I can't feel it until I take a hand off the bars, then it starts to shimmy whenever coasting. It's not noticeable when pedalling.

The bike has 26" wheels with cable discs. The wheel spins freely in the fork with no dragging of the brake or lateral movement.

The fork is a suspension fork which is locked out. There is a very small amount of movement noticeable if rocking the bike with front brake on.

It has an ahead headset which turns smoothly and has no movement when rocking the bike. The top cap was removed and refitted recently to swap the stem and spacers for an NVO adjustable one.

The bike has recently had a rear hub motor and rack battery fitted. This has moved the centre of gravity backwards.

I need some suggestions on what to check to cure the shimmy. I suspect it may be headset or suspension related but I'm not sure what to look for /adjust/replace.

[531colin]

Can you remove the battery and see if you still get shimmy?

[Norman H]

I'd be suspicious of the recently fitted rear rack and battery pack. My experience of touring loads on rear racks is that they can have just such an effect if the rear rack isn't rigid enough. Have you tried running without the battery fitted?

Edit: Colin beat me to it.

[emleyman]

The rack is very rigid, but I haven't tried without the battery. I'll give it a go and report back.

[emleyman]

I just tried a quick ride with the battery removed. The shimmy was less noticeable but was still there when riding no-hands.

[531colin]

It isn't quite Occam's razor, but continuing on the lines "a fault is often associated with the last bit of the bike that was worked on", does the tyre run nice and straight on the new/rebuilt back wheel?

[pwa]

Further thoughts....
I don't think any particular type of steering geometry (twitchy, slow) is a good predictor of speed wobble....anybody?

Something which costs nothing and might just be useful....
If you spin the front wheel either in your hand or with the bike on the stand, is the wheel far out of balance?
If it is, I would try balancing it up either by adding a weight (an inch or two of solder around the spoke nipple always used to be recommended for motor bikes) or by swapping tyres front to back, or just moving the tyre round the rim.

Many people have a computer magnet on the front wheel. When I used one I placed it opposite the the valve, which was the heaviest side of the tyre/wheel combo. If more weight were needed for balancing a bit of blu tac pressed into a spoke head recess (beneath the rim liner) would be an easy solution.