bit of a shimmy

[mig]

does the surface change on the way down?

[foxyrider]

does the surface change on the way down?

Not really, there are some rough patches in the centre of the carriageway about where things got hairy but I wasn't actually on that - you can take a line well in advance to avoid them.

[Gattonero]

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.

Not really, speed wobble happens because of several factors, alone or n combination:
-unbalanced weight distribution on the bike, i.e. upright riding position leaving lots of weight on the rear wheel but very little on the front
-tyres not seated correctly
-the wheel or rim are not balanced, i.e. if the rim is "sleeved" only the weight of the sleeve opposite the valve will make the wheel to wobble at speed
-lightweight forks not suitable for heavy loads and speed
-resonance with rough tarmac

[Pneumant]

Very very scary when this happens, as others have mentioned look carefully at your set-up. If this were my bike I would strip/check/adjust the headset and a thorough check of the forks /frame. I would then ride the route again (carefully!) to prove that the bike is good. This may sound a bit zen but sometimes it is best to be faster uphill rather than downhill. I tend to descend on the drops as this puts more weight on the front and permits more effective braking. Good luck!

[meic]

I tend to descend on the drops as this puts more weight on the front and permits more effective braking.

That rather depends on the slope, accepted practice is to lean back so get some weight on the rear allowing some traction there and avoiding going over the bars.
Having weight on the front may avoid the wobble setting off in the first place but shifting my weight onto the front while a wobble is in progress is something that I would need a lot of convincing about before trying.

I do see this as quite probably part of the bad combination on my bike as it happens with the lightweight forks and wheels fitted but not with the touring forks fitted.

[anniesboy]

Yes check your forks.
Some years ago on my tandem I got into a real shimmy on a hill I had descended many times.
I stopped checked everything as I thought, a few days later I realised the fork crown was broken could have been a real nasty.

[pwa]

It is also worth balancing the front wheel, not least because it is an easy and probably cost free thing to do. Spin the wheel in a truing stand or with the bike upside down and work out which bit of the wheel settles at the bottom. Most wheel/tyre combos have a heavier side. Then put the computer magnet (if you have one) on the opposite side. That should reduce the imbalance. Play around with other spoke mounted add-ons if necessary, until you have a wheel that seems to settle more or less in any position. Obviously, if you change tyre this will need re-doing.

I wonder if in the incident described the tyre had been changed or re-positioned after a puncture, resulting in a bit more weight on one side of the wheel. Just a thought.

[foxyrider]

It is also worth balancing the front wheel, not least because it is an easy and probably cost free thing to do. Spin the wheel in a truing stand or with the bike upside down and work out which bit of the wheel settles at the bottom. Most wheel/tyre combos have a heavier side. Then put the computer magnet (if you have one) on the opposite side. That should reduce the imbalance. Play around with other spoke mounted add-ons if necessary, until you have a wheel that seems to settle more or less in any position. Obviously, if you change tyre this will need re-doing.

I wonder if in the incident described the tyre had been changed or re-positioned after a puncture, resulting in a bit more weight on one side of the wheel. Just a thought.

What is this 'puncture' thing you mention?

Not had one of those on this bike ever and only one this year on any bike! I always replace the tube so you won't find me riding patched tubes unless i've had a really bad day!

I am however wondering about tyre pressure, it was a few pounds lower this week than the previous decent and today I noticed a distinct squidge once or twice. I'm talking 80psi to previous 100psi supposedly to put more rubber on damp, muddy roads.

Not expecting to go that way over that road again this side of chrimbo, its been behaving on the other fast descents i've done since, but maybe i've been a little more restrained as the roads have been damp.

[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.

Not really, speed wobble happens because of several factors, alone or n combination:
-unbalanced weight distribution on the bike, i.e. upright riding position leaving lots of weight on the rear wheel but very little on the front
-tyres not seated correctly
-the wheel or rim are not balanced, i.e. if the rim is "sleeved" only the weight of the sleeve opposite the valve will make the wheel to wobble at speed
-lightweight forks not suitable for heavy loads and speed
-resonance with rough tarmac

Read here, a pretty definitive article by CJ:
https://www.cyclinguk.org/sites/default ... 911052.pdf
Note you have to scroll down another page (54) to get the finish of the article.

[Chat Noir]

Thanks for the CJ article - seems pretty comprehensive to me.

Not had shimmy very often but, as people say, once bitten pretty shy. Going up and down the Dales fast accelerations and high speeds are standard. Some years ago I instinctively gripped the top tube between my knees when a bit of a wobble and now seem to do this not infrequently, helps feel more stable, and rarely experience shimmy - but always alert to it.

One other observation: I've never had a wobble on the Dolan I ride. Has long wheel base, strange oversize tubeset (Dedacciai steel), the most stable bike I ride. Use as my main tourer, 11.2 kg, little heavier compared to the rest but not really heavy. I've come down some really long and fast descents on this and the bike hasn't wobbled once (I was a bit more nervous), including off the Col de la Bonnette with full panniers. Coming down the gentle side of Ventoux (Sault) was as close to cycling heaven as I've been on a bike.

[meic]

The one sure-fire, 100%
guaranteed way to come out of a
shimmy is to apply more damping.

If that is true then clearly the term shimmy is being more tightly applied by CJ than by others.
I have noticed that my present problem appears different to frame resonance and is much more like an unbalanced wheel feels in a car (though it doesnt care much which wheels and tyres are fitted).
Possibly it is fork resonance rather than frame resonance, not so easy to touch your forks to dampen that.

I also notice that he says it needs a smooth road but the way my bike goes unstable is a combination of downhill, rough surface, slightly banked and a tiny bit of braking.

Knowing this isnt actually a shimmy does nothing to solve the problem.

[pwa]

The one sure-fire, 100%
guaranteed way to come out of a
shimmy is to apply more damping.

If that is true then clearly the term shimmy is being more tightly applied by CJ than by others.
I have noticed that my present problem appears different to frame resonance and is much more like an unbalanced wheel feels in a car (though it doesnt care much which wheels and tyres are fitted).
Possibly it is fork resonance rather than frame resonance, not so easy to touch your forks to dampen that.

I also notice that he says it needs a smooth road but the way my bike goes unstable is a combination of downhill, rough surface, slightly banked and a tiny bit of braking.

Knowing this isnt actually a shimmy does nothing to solve the problem.

You haven't changed anything recently that might account for the wobble? New inner tubes with longer, heavier valves perhaps.

[BigFoz]

There's a fork and wheel difference mentioned.

My take would be the fork swap either the lighter fork is damaged or the rake /trail are a bad match for the frame geometry.

I use Khamsins without issue at higher downhill speeds. Very solid wheels, not massively light either. I often come of the top of the moor here at well over 70kph, loaded and I weigh 100Kg nearly

[georgew]

The only time I've experienced this was on a custom-made Mercian tourer and with panniers on the rear.
I found that it disappeared when I moved the panniers about half an inch forward on the rack nearer the seat post.

[the snail]

Apart from bike /road surface, the rider is part of the system, perhaps this was rider-induced? The only time I've had a speed wobble was descending a hill I've ridden many times, in windy conditions. After I'd checked the tyres, frame and forks, I put it down to a combination of gusty wind, and me being freezing cold and tensed up.