High speed wobble today - advice?

[Jon in Sweden]

Hello all,

I had a nasty experience with high speed wobble today on my 531 framed Mercian.

I was doing close to 60kph, with a 60t lorry coming past me in the drizzle, throwing a tonne of spray at me and it was half dark. Not that there is ever a good time for your bike to start wobbling frantically, but this is probably the worst.

I got it back under control, somehow, with gentle braking, but it didn't stop until I was doing about 40kph.

The bike is an outstanding condition 61cm Mercian in great condition. It's got a fresh respray on it (so assuming that any cracks would have been identified at that point), running 3x10 Ultegra.

It's not a stable bike at the front end though, and has a tendency to wobble a bit generally. It's on an old style stem - there seems to be no play in the headset, but is there anything else I'm missing?

I've not had wobble on any of my other bikes, and it's a hill I go down all the time.

Any help much appreciated.

[Jdsk]

I recommend starting with "Shimmy, or Speed Wobble":
https://www.sheldonbrown.com/shimmy.html

Jonathan

[Jon in Sweden]

I recommend starting with "Shimmy, or Speed Wobble":
https://www.sheldonbrown.com/shimmy.html

Jonathan

Thanks for that Jonathan. Interesting reading for sure.

It's not a very stiff frame to be honest. Beautifully made, but I hadn't realised how outdated it was (in terms of performance) until I recently got a Canyon Endurace.

[rjb]

Yes as above. Search for shimmy on the forum and it returns several threads.
Unfortunately once you experience it you can become nervous and tense which can exacerbate the problem. Next time try damping the oscillation by gripping the top tube between your knees. Easier said than done.
Here's one of those recent threads viewtopic.php?t=147525

[Mick F]

I have a Mercian that I've had and used since 1986.
Mercian Vincitore 531c.
Close clearances, sporty geometry, 23.5" seat tube, 73deg parallel.

It's not easy to ride "no hands" but there's never a wobble and never an issue with it. I think I've done in excess of 50,000miles on the bike.

[Jon in Sweden]

I have a Mercian that I've had and used since 1986.
Mercian Vincitore 531c.
Close clearances, sporty geometry, 23.5" seat tube, 73deg parallel.

It's not easy to ride "no hands" but there's never a wobble and never an issue with it. I think I've done in excess of 50,000miles on the bike.

It's a beautiful bike. I think it's a 'me' issue, not a bike issue. Maybe being 2.03m tall, 105kg and on a 61cm frame I possibly need a stiffer frame though? I'm open to any ideas!

For reference, it's an audax frame, so a more relaxed geometry I assume.

[Jdsk]

...
Next time try damping the oscillation by gripping the top tube between your knees. Easier said than done.
...

Yes. Pressing one knee against the top tube is easier and might help.

But it's a difficult phenomenon to study both with a single bike and between bikes. Solutions might not be transferable!

Jonathan

[Jdsk]

...
It's a beautiful bike. I think it's a 'me' issue, not a bike issue.
...

It's an interaction, not caused by one or the other alone.

Jonathan

[Barrowman]

I have come across steering damping by way of wrapping the fork column in secondhand inner tube to create a light interference fit in the head tube. Was on a secondhand Trike, contacted the previous owner who explained he had done it to stop steering shimmy .
The forks weren't the originals, they were Tandem Spec ( I know the bloke who made them, on that spec after previous owner compromised the original forks with a hub brake) and I fear they may have been a bit too stiff.
I have replaced the forks with some Bob Jackson forks . Never had shimmy myself with either fork set up.

[Mick F]

I have a Mercian that I've had and used since 1986.
Mercian Vincitore 531c.
Close clearances, sporty geometry, 23.5" seat tube, 73deg parallel.

It's not easy to ride "no hands" but there's never a wobble and never an issue with it. I think I've done in excess of 50,000miles on the bike.

It's a beautiful bike. I think it's a 'me' issue, not a bike issue. Maybe being 2.03m tall, 105kg and on a 61cm frame I possibly need a stiffer frame though? I'm open to any ideas!

For reference, it's an audax frame, so a more relaxed geometry I assume.

61cm is only half an inch taller than my frame.
However, you are 28kg heavier than me as well as being nearly a foot taller ............ if you don't mind me mixing my metric/imperial units!

Sounds like you have the wrong bike frame for you.

[rjb]

Tensing up and gripping the bars tightly is often cited as an initiating scenario for a shimmy experience. Most would do the same if experiencing the event you described. The advice often given is to relax and loosen that death vice grip on the bars. It takes some willpower to achieve this.
Good luck.

[slowster]

Dave Moulton has some interesting blog posts on shimmy. It seems that the origin of the oscillation is in the (rear of the) frame, and it may be that it is difficult to design out completely. What matters is whether the riding parameters at whch a frame will shimmy will either never/very rarely occur in normal riding conditions, or will only occur under conditions which are benign (low speed) and/or easily controlled, e.g. by adjusting the weight distribution on a loaded bike between front and rear panniers.

http://davesbikeblog.squarespace.com/bl ... himmy.html

http://davesbikeblog.squarespace.com/bl ... himmy.html

http://davesbikeblog.squarespace.com/bl ... sited.html

http://davesbikeblog.squarespace.com/bl ... himmy.html

Next time try damping the oscillation by gripping the top tube between your knees. Easier said than done.

You need to press a *fleshy* part of even just one leg against the top tube, NOT a bony part like the kness. That acts as a damper and is quite easy to do. Jon in Sweden, I suggest you practise doing this a few times so you know what to do/where to position the leg in the event that it happens again. With a horizontal top tube, you may find that the thigh is the part of the leg you need to press, but with a sloping top tube I find that it is the calf.

As for frame stiffness, I am a bit sceptical. Greg Lemond and co. were winning the Tour and riding down mountains on TVT carbon frames which were floppy even by the standard of a traditional 531/753/Columbus frame. The rider in the video below was on a modern carbon frame, which was probably very stiff.

[Paulkentuk]

Motorcyclists would call your symptom a 'tank slapper.' A truck passing you, when you are both at speed, in the rain, with the predictable vortex is indeed a scary thing. That you walked away from it. Isn't all down to luck, so we'll done you.

On motorcycles a front fork damper usually clamped between the fork yoke and frame will significantly reduce 'uncontrolled' oscillating, but then other factors are likely to come into play, like tyre depth / pressure.

I've found on the rare occasion its happened to me on a motorcycle, the only sure thing is to get the speed down ASAP, & that is by applying the rear brake ONLY. I'm not aware that playing about with the front end during a tank slapper, has worked for mates of mine. One mate swears blind you 'power out' of a tank slapper, not sure its a solution on a bicycle

[axel_knutt]

You could try a stem with a longer reach if you can stand it, I've only ever had one shimmy in my life and that was just after I put a shorter stem on. I was on a familiar hill that I've been down countless times, very short, but 1 in 6 and the steepest point is on the apex of a bend, which makes it good for a play: laying the bike into the bend if the slowcoach cars keep out of the way and you can stay off the brakes. That was the point it shimmied, and it was a bit of a surprise having never done it before.

(I'm 196cm, 70kg, and ride a 64cm frame. A bit bigger would do, but 64 was the biggest size off the peg.)

[simonineaston]

Gosh sounds like a nightmare - glad you're OK