Interesting video on bicycle steering

[Tenkaykev]

Don't knock it, in a race the pelvic thrust can make all the difference between coming first and second.

A gentleman should never come first...

 

[All uphill]

I've often found a childish pleasure in looking at bicycle tyre tracks on a dry road after going through a puddle. It's clear to see that riding in a straight line is more accurately described as slightly weaving left and right.

Kind of falling slowly one way, correcting, falling the other way etc etc.

Our brains are amazing.

 

[Dogtrousers]

when you say "shimmy", do you mean "speed wobble"?

Are they different things? I don't think I've experienced either so I don't know.

 

[T4tomo]

Are they different things? I don't think I've experienced either so I don't know.

no idea, i don't know what was meant by "shimmy" in a cycling context that what i do when walking through a narrow gap

 

[matticus]

Are they different things? I don't think I've experienced either so I don't know.

That's cleared it up, thanks.

No, I don't think they are different things. (although "shimmy" can also be used to described how I enter a room on foot.) I tend to call it "shimmy" because I know it can happen at any speed.
"speed wobble" is perhaps more common; for most people/bikes it only happens at a certain speed i.e. higher than normal flat speeds.

And because it happens at high speeds - usually descending - it can be Somewhat Alarming.

The knee-on-top-tube trick is worthwhile, and has sometimes stopped my shimmys. But if it was that easy, I wouldn't be so worried about it
I also believe that keeping relaxed arms helps - of course this can be difficult when you think your bike is about to throw you down a cliff at 40mph...

(there are many threads about this on The Cycling Internet, but I'm pretty sure no-one truly understands it. A few have a better understanding than the many! Oh, and I'm happy to discount the "loose headset" theory that often pops up :P )
I'm certain it's about resonance. And that means it's a combo of frame material, geometry, weight distribution and probably wind + road surface.
(Anecdotally headwinds make my bike worse. I almost couldn't get out of the Alps into Italy!)

 

[Dogtrousers]

no idea, i don't know what was meant by "shimmy" in a cycling context that what i do when walking through a narrow gap

Oh good. We both don't know what it means then?

 

[matticus]

I have to say, I would LOVE to see my tyre tracks after a good shimmy descent moment! But I'm bu99ered if I'm going to retrace that kind of road.

 

[Dogtrousers]

And because it happens at high speeds - usually descending

Ah ha. That's why I don't know anything about it. If by "high" you mean greater than 55km/h / 35mph which is roughly as fast as I ever find myself going.

 

[matticus]

Ah ha. That's why I don't know anything about it. If by "high" you mean greater than 55km/h / 35mph which is roughly as fast as I ever find myself going.

I've probably had it at 30mph - but that's a guess, as I'm never in a position to check my speedo during these "incidents". Usually much faster though - that's why it's rare and hard to reproduce. It's rare for me to get upto 40mph, but often enough to be an issue, and when I DO reach those speeds, I really really really want a controllable vehicle!

The Simplon Pass Incident involved riding miles downhill with the brakes on, just to keep my speed below the resonance. It's not even particularly steep - hugely frustrating.

Some bikes will do it when riding no-handed at low speed. If you're brave enough to just keep pedalling, the oscillations don't build - it's just a small wobble that is harmless but looks very disconcerting.

 

[ColinJ]

I chased the same rider down the descent of Holme Moss towards Woodhead on two occasions.

The first time, he was riding a heavy touring bike and the second time he was on his lovely new lightweight carbon fibre racing bike. He experienced violent shimmy on the right hand bend at the bottom of the descent on both occasions!

Since the weather conditions were different on the two days and the bikes were very different, I conclude that it was due to the way he was riding the bikes. I had no problems either time. I think he carried too much speed into the bend, scared himself, tensed up and got into panic braking. I knew what the descent is like and probably began braking before he did so I was able to do it in a much more controlled fashion.

That is a very high speed descent, 95-100 kph (close to 60 mph) so it is very easy to get nervous on the way down!

 

[Twilkes]

Anecdotally, speed wobble occurs more often with lighter riders than heavier riders. No idea why.

 

[MichaelW2]

The closest I come to consciously or noticeably doing this is if I'm coming up on an unseen pothole and I quickly steer left so the wheels avoid it; in doing that, the bike has effectively leaned to the right, or gone to the left underneath me, so I have to steer right to get the wheels back underneath me again. In combination with the lean, it turns me a little to the right so I have to reverse the lean and then turn the wheels back to get them back underneath me again. It's like a little shimmy, and I'd imagine that taking a bend at speed is a very slow, very subtle version of that.

As above, rather than counter steering one should think of it as leaning first and then steering to stop you falling off. And if you're ever in trouble on a bend you can generally lean a lot further than people think they can, which has the added benefit of scrubbing speed off as braking on a bend is generally not a good idea, it tends to send you straight on.

The pothole dodge results in "wobbling out" into the road for several cycles of balance/steer. If you start close to curb you leave yourself no choice except to wobble out. If you start from the accepted primary or secondary position you have the option to take your countersteer wobble back closer to the curb if required.

 

[matticus]

Anecdotally, speed wobble occurs more often with lighter riders than heavier riders. No idea why.

I don't know why either, but it fits with the fact that I only get it with quite a lot of luggage on-board.
(i.e. the ratio of riderWeight : loadedBikeWeight may be what matters)

 

[Jon George]

I've ridden motorbikes since the mid-seventies (I include Patsy #5 The Bandit as one of my stable of bikes) and 'played' around with counter-steering in the early days to initiate a quicker bank after reading an article about it in one of the motorbike rags. Its something I haven't thought about in decades - I found it just became a habit. The main difference (apart from the speed), is that on my motorbike I'll 'hang-off' the machine to redistribute my weight as I corner. I have been known to stick out a knee whilst cornering on my bicycles, but that's more for ego, than achieving anything seemingly worthwhile.
Oh, and I'm firmly in the camp that the rider was going too fast for the conditions. Yes, being over to the middle to see as far ahead around the rock as I could, is also where I would have been. But not at that speed!

 

[ColinJ]

Oh, and I'm firmly in the camp that the rider was going too fast for the conditions. Yes, being over to the middle to see as far ahead around the rock as I could, is also where I would have been. But not at that speed!

Yes... I was cringing, waiting for something to bad to happen. We were expecting the wobble, but I was already worried about his speed and positioning!