bit of a shimmy

Commuting, Day rides, Audax, Incidents, etc.
mig
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Re: bit of a shimmy

Postby mig » 15 Nov 2017, 9:40pm

does the surface change on the way down?

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foxyrider
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Re: bit of a shimmy

Postby foxyrider » 16 Nov 2017, 8:52am

mig wrote: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.
Convention? what's that then?

Airnimal Chameleon touring, Orbit Pro hack, Orbit Photon audax, Focus Mares AX tour, Peugeot Carbon sportive, Owen Blower vintage race - all running Tulio's finest!

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Gattonero
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Re: bit of a shimmy

Postby Gattonero » 18 Nov 2017, 9:27am

hamster wrote: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
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since you have to sweat up the hills and coast down them.
Thus you remember them as they actually are...

Pneumant
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Re: bit of a shimmy

Postby Pneumant » 18 Nov 2017, 10:24am

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!

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meic
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Re: bit of a shimmy

Postby meic » 18 Nov 2017, 10:33am

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.
Yma o Hyd

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anniesboy
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Re: bit of a shimmy

Postby anniesboy » 18 Nov 2017, 11:11am

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
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Re: bit of a shimmy

Postby pwa » 18 Nov 2017, 11:25am

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.

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foxyrider
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Re: bit of a shimmy

Postby foxyrider » 18 Nov 2017, 8:24pm

pwa wrote: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.
Convention? what's that then?

Airnimal Chameleon touring, Orbit Pro hack, Orbit Photon audax, Focus Mares AX tour, Peugeot Carbon sportive, Owen Blower vintage race - all running Tulio's finest!

hamster
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Re: bit of a shimmy

Postby hamster » 20 Nov 2017, 12:15pm

Gattonero wrote:
hamster wrote: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
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Re: bit of a shimmy

Postby Chat Noir » 20 Nov 2017, 1:32pm

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.
Dawes Galaxy 1979; Mercian 531 1982; Peugeot 753 1987; Peugeot 531 Pro 1988; Peugeot 653 1990; Bob Jackson 731 OS 1992; Gazelle 731 OS Exception 1996; Dolan Dedacciai 2004; Trek 8000 MTB 2011; Focus Izalco Pro 2012

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meic
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Re: bit of a shimmy

Postby meic » 20 Nov 2017, 1:53pm

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.
Yma o Hyd

pwa
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Re: bit of a shimmy

Postby pwa » 20 Nov 2017, 2:40pm

meic wrote:
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
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Re: bit of a shimmy

Postby BigFoz » 24 Nov 2017, 2:13pm

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

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georgew
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Re: bit of a shimmy

Postby georgew » 25 Nov 2017, 11:35am

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
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Re: bit of a shimmy

Postby the snail » 26 Nov 2017, 1:51am

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.