bit of a shimmy

[foxyrider]

Yesterday was a nice day so I headed out into the hills despite the cold After climbing Holme Moss and dropping to Woodhead I used the A628 to return to the right side of the Pennines. I've done the climb and descent towards Penistone quite a few times over the years without incident. GR SE 177 011

But yesterday was different, I suffered a bit, no a very big, nasty 'speed' wobble. In short as I entered the last sweeping bend before it levels out the front of the bike started to shimmy. From a little wobble it quickly escalated into something which I was no longer in control of and so vicious that I was certain i'd be eating tarmac in short order. Luckily the road was fairly clear and the HGV's behind stayed there.

Not quite sure how but I stayed upright and managed to get it back under control, shaken and definitely stirred! Now i've ridden the same road faster, on a wide variety of bikes and just last week on the same bike loaded exactly the same without any issue.

My question is this, what may have caused the shimmy yesterday. (nothing loose on the bike, wheels straight, bearings well adjusted, no flappy clothing, its my steel framed audax machine) I was just hanging on for grim death, but anyone got any hints for controlling a major wobble?

And finally a warning to anyone descending the eastbound Woodhead pass - be careful and take the lane, its all too easy to get carried away with even a slight tail wind.

[meic]

I was just hanging on for grim death, but anyone got any hints for controlling a major wobble?

The first thing to try is touching your leg on the crossbar, this works wonders for the right sort of wobble (frame resonance), not all wobbles though.

[RickH]

There is a nice, fast hill that I used to go down quite frequently when I was on my way to do the afternoon school run by bike. Just occasionally I would get a bit a shimmy. I reckoned it was because of the variable contents of the rackpack, particularly if I had something heavier than normal in there, that would set up a resonant wobble. Knees against the top tube to damp any oscillation seemed to work in those instances to kill it off before it did the same to me.

[drossall]

It seems to be a resonant frequency thing. That's affected by various factors. Musical instruments use resonance - guitar strings of different lengths, for example. So, I suppose, in theory anything that altered key dimensions and other factors, by even a slight amount, might affect resonance. Maybe even holding the bars in a slightly different place? Or the weight in a water bottle which was providing damping? Exact wind direction, stimulating a bit of wobble that acted as an input? Not sure about road vibrations, with tyre pressures having an effect?

I'm speculating aloud here more about a cumulation of factors, with several needing to be just so to set up resonance - so this isn't saying that holding the bars slightly differently would fix it, but saying that you were really unlucky to have several things coincide. And it's more guesswork than science, hoping that others will chip in and comment. I've even heard spoke reflectors blamed, for example (not that I think you fitted those specially for this particular ride!)

But yes, the trick of damping by putting a knee on the top tube is widely used.

[pwa]

+1 for the knee pressing on the top tube. It works.

Try balancing the wheels, as you would with a car wheel.

[meic]

It isnt working on my present bike. I am pretty sure this particular wobble isnt resonance (because damping the frame doesnt work) and it is something entirely to do with the front end and as it occurs (in extreme circumstances only) with different wheels, tyres etc, I suspect it is a fork or headset problem.

[foxyrider]

It went from 'oo a bit of a wobble' to 'holy s*#t!' so quickly that I had no real chance to damp the frame with a knee as the bike was thrashing about so much, it was like being inside a washing machine!

The bike was loaded exactly the same as the previous week so i'm guessing it was set off by some outside force - there wasn't much wind and it's quite open at that point so no sudden gust was evident.

Resonance in the front wheel? Possibly, its a radial spoked Khamsin CX with 20 round spokes. Could the mudguard be contributory, causing a vortex at the rim?

[meic]

it was like being inside a washing machine!

I have experienced both types and can tell them apart but i appreciate that when it happens first time you are not doing a careful analysis of the symptoms.

The front end problem that I am getting now is noticeably from the front end, feeling coming from the handlebars and the rest of the bike remaining solid even if being thrown around a bit.
When you get frame resonance it feels like the whole bike has turned to rubber and is uncontrollable because of that. A touch of the leg on the cross bar and it is instantly steel again.

[Cyril Haearn]

Maybe a bit of discarded chewing gum or a slug or something was picked up by one of the tyres and unbalanced the wheel

[hamster]

Generally shimmy seems to happen with light wheels and tyres - otherwise the natural frequency is too far away from that of the frame. As you have a pretty exotic front wheel on the bike it fits the risk factors. Try it with (say) an Open Pro and Gatorskin and it's probably fine.

[mig]

So yesterday was a nice day so I headed out into the hills despite the cold After climbing Holme Moss and dropping to Woodhead I used the A628 to return to the right side of the Pennines. I've done the climb and descent towards Penistone quite a few times over the years without incident. GR SE 177 011

But yesterday was different, I suffered a bit, no a very big, nasty 'speed' wobble. In short as I entered the last sweeping bend before it levels out the front of the bike started to shimmy. From a little wobble it quickly escalated into something which I was no longer in control of and so vicious that I was certain i'd be eating tarmac in short order. Luckily the road was fairly clear and the HGV's behind stayed there.

Not quite sure how but I stayed upright and managed to get it back under control, shaken and definitely stirred! Now i've ridden the same road faster, on a wide variety of bikes and just last week on the same bike loaded exactly the same without any issue.

So my question is this, what may have caused the shimmy yesterday. (nothing loose on the bike, wheels straight, bearings well adjusted, no flappy clothing, its my steel framed audax machine) I was just hanging on for grim death, but anyone got any hints for controlling a major wobble?

And finally a warning to anyone descending the eastbound Woodhead pass - be careful and take the lane, its all too easy to get carried away with even a slight tail wind.

is this the long descent that sweeps to the right as it passes a pub on the right hand side of the road? ends up a a roundabout at the bottom.

[Audax67]

In general, I've found that gripping the bars tightly and trying to control the shimmy makes it worse, because your rigid body and arms then act as a feedback coupling from saddle to steering. It's hard to relax with the bike doing the twist, but it's often all that is required. I have a Ti bike that is a devil for that. I discovered the shimmy on the way down Mt. Ventoux, too - kinda hairy.

To counter it, you can change the dynamics of the system or fit a steering damper. In my case I added a handlebar bag, which changed the resonance frequency of the bars/fork system.

[foxyrider]

. . . And finally a warning to anyone descending the eastbound Woodhead pass - be careful and take the lane, its all too easy to get carried away with even a slight tail wind.

is this the long descent that sweeps to the right as it passes a pub on the right hand side of the road? ends up a a roundabout at the bottom.

that's the one

[mig]

blimey i bet you can get some speed up down there! i've never cycled down there but have driven it many times. i can see that taking the lane on the descent would be the only option given the speed and volume of traffic.

i've always wondered if the wind can bring this on. the weather wind that is!

[foxyrider]

blimey i bet you can get some speed up down there! i've never cycled down there but have driven it many times. i can see that taking the lane on the descent would be the only option given the speed and volume of traffic.

i've always wondered if the wind can bring this on. the weather wind that is!

I've certainly had wind buffeting on there before and yes, I always take the lane - just as well this last time.

I checked my speed out on Strava - 70kph at the start of shimmy, about 65kph by the time I had control again. Pretty certain i've had more on previous descents when i've actually been 'trying'