The Dreaded Speed Wobble

[Night Train]

I'd wonder if it was down to frame geometry and wheel imbalance.
Some bikes are more twitchy then others and wheels could be out of balance and a combination of the two could set off a resonance wobble at certain speeds.

I've only come across it on the Brompton without a bag on the front. With a weighty bag on the block it is fine and stable.

 

[fossyant]

All sorts play a part in this.

My old Raleigh 531c used to do it at about 40-45 mph, but only when mudguards were fitted. My Ribble 653 has done it once at 55 mph - hit by a side gust and just started the wobble, clamped top tube with knees, pulled rear brake, steadied, and then went for it again - was fine. My best bike (columbus SLX) has never done it.

My fixed hasn't done it either, but I can't pedal it fast enough.

 

[MacB]

60 cm frame here with a 6ft4in passenger weighing 16 stone so it could have a bearing although I must add that it isnt a regular occurence but when it does happen it shakes you up a little especially when you still havent found your own cure

Hmmm, well I'm not as tall as you, 6'1", but I've just realised another common factor, it's never happened when I've been lighter, both times I was upwards of 16.5 stone. I no longer have the bikes in question though that was nothing to do with the speed wobbles.

Thinking on it I'm sure the blurb I read, can't find it right now, was critical of some frame design for larger riders. It was along the lines of, a larger frame would be more prone to speed wobble, but a larger frame trying to be a smaller frame was far more likely to suffer, especially if the tubing wasn't upped to accomodate either. By that it meant where the bigger rider was accomodated by basically making the bike taller rather than bigger all round. For example I think the claim would be that a 60cm touring frame would be less likely to suffer than a 60cm race frame made as small as possible. It also mentioned tubing and increased flex due to heavier riders loading the frame more. Custom frames will actually have tubing selected to match the rider weight.

If you or I ordered a custom frame online and decided to put our target/desired weight in, rather than our actual weight, I bet we'd get a different frame to what would be built if the framebuilder saw us

Would be interesting to see if you could lose 2 stone and if that made the speed wobbles vanish.

 

[Fab Foodie]

Chapter and verse on the subject, one on which quite a lot of bollox is spoken:

http://www.sheldonbrown.com/brandt/shimmy.html

 

[MacB]

Chapter and verse on the subject, one on which quite a lot of bollox is spoken:

http://www.sheldonbrown.com/brandt/shimmy.html

Seems about right, though I note he indicates a longer, rather than taller, frame as being more prone. The bit that really makes sense is the idea of the shimmy occuring around a pivot point and that being the rider seated on the saddle. He's saying that weight distribution isn't the issue but where the weight contacts the bike is. I know he's right but most of us would class that as weight distribution as well. As in shimmy can be reduced by unweighting the saddle as that is the main pivot point. It also explains why a bike will shimmy for one rider and not another if they weigh significantly different amounts or one rides heavily seated while the other floats his bum above the saddle.

This ties in with another article I read, interviewing a team mechanic, he mentioned that the pros tend to minimise weight on the saddle when descending.

It also explains my personal experience - the first time was on the IOW ride, I wasn't as fit as I had been, was heavier and also rather ill. The IOW is quite up and down and we'd just climbed a short nasty one and my speed wobble was coming down the other side. I would have been knackered, sitting up and slumped in the saddle. This also matches the second time as well, so the combination of being personally heavier and sitting heavily in the saddle could well be the culprits.

Though I guess that only applies to bikes that don't normally shimmy on you, if a bike always has from day one then it may be more frame related.

 

[slowmotion]

I am fairly sure that I will never suffer from this...

 

[Keith Oates]

I had it on one of my bikes and could not find the problem, however one day I bought a new pump that had a pressure guage and when I pumped up the tyres to the maximum recommended pressure the wobble never materialised again!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

 

[MacB]

I had it on one of my bikes and could not find the problem, however one day I bought a new pump that had a pressure guage and when I pumped up the tyres to the maximum recommended pressure the wobble never materialised again!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

That does make sense, indications are that speed wobble is more likely on smooth roads, nothing interfering with the frequency oscillation. So pumping the tyres rock hard will turn all but the smoothest roads into bump fests that jar at every imperfection. No more speed wobble and no more comfort, two birds with one stone.

 

[Keith Oates]

All roadies are hard men and so we don't need this comfort thing you're talking about!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

 

[MacB]

All roadies are hard men and so we don't need this comfort thing you're talking about!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Sorry but I don't recognise that answer as it has less than 50 exclamation marks after it !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

By the way I looked up exclamation marks and apparently anything more than three is code for t***er

 

[Keith Oates]

That shows the quality of the items you use for referrence!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

 

[MacB]

That shows the quality of the items you use for referrence!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

that's an even lower exclamation count there Keith, button wearing out is it?

 

[StuartG]

Are we just talking about the resonant frequency of your bike?

It has one, hopefully outside one's riding envelope. If it is a regular feature then its case of thinking how you can change it. Stuff like tyre pressures and load/stress on the frame. I guess if you hit it then it is best to lift off if you are seated or the opposite if not.

Luckily I'm too slow and my bikes are too stable to have encountered so my advice may be flawed.

 

[gary in derby]

oh, to be able to go that fast, relax and enjoy it!

 

[pepecat]

Hmmm. Can't say it's happened to me but reading the thread two things strike me...... i don't weigh that much (which may help)...... I also appear to be a suicidal manic cos when I'm going down a hill and the wobble is most likely to happen, I never grip the handlebars tightly. In fact, I don't really grip the handlebars at all. What I do is have my hands in the 'holding the handlebars' position, but slightly more open, so the handlbars can move within my hands, if that makes sense. I have some grip, but not much. Don't know why i do that, but I have always done it. I guess it's more comfortable than having every judder of the road going through your wrists.