Bicycle Stability - one for the tech types

[flecc]

On Thursday 26th at 4.30 pm, The Material World on BBC Radio 4 or through their "Listen Again" service. Here's the subtext:

"As you head out for a gentle summer's cycle ride, spare a thought for the mathematicians who have spent a century puzzling over the maths of a bike's stability. Quentin Cooper meets the experts who think they've cracked it".
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[Haku]

I'd have thought that the stability is down to the rider's own ability to balance, after all you have to learn to ride a bike, your brain/body has to learn a new way of balancing.

So what is the real answer?

 

[coops]

Tune in, to find out!...(as they say) . Thanks for the tip-off flecc I'm not clear what the puzzle is either, but then it is maths, isn't it? .

Actually, talking about gyroscopic effects of wheels etc. today in the effect of reducing rotational mass thread makes me think that bikes work in a very clever way I'd never thought about much: I think the effect of the spinning wheels tend to act to keep you upright (in addition to the rider's balance of course, like you say haku) by opposing a force to change the spin axis? I'm not sure about that, because it doesn't seem so hard to "lean" into turn on a good bike, but maybe its because we're used to it?

I'm probably completely wrong....

EDIT: It seems I am! I fell for a popular misconception...

Stuart.

 

[flecc]

As you say Stuart, listen to the program. It's also as relevant to motorcycles, and is virtually nothing to do with a rider's ability to balance Haku. That's a very low speed and quite separate issue.

Two wheelers in motion have intrinsic stability. Any motorcyclist knows how a bike self attempts to correct for crosswinds, in that case due to the gyroscopic forces generated by the wheels, but the situation in two wheelers is considerably complicated by lean and turn angles in conjunction. Add to that the head angle and trail, and their effect on steering speed, also affected by gyroscopic forces, and there's a real tangle to sort out.

It's that complexity of interactions that's made the situation so difficult to understand, four wheeler geometry being simple and long thoroughly understood by comparison.
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[coops]

Sounds very interesting flecc, especially if it has practical repercussions for bike design, which from what you say sounds like its been another one of the "black arts" up till now .

I love maths which actually models real-world type stuff, like complex interactions in weather models, all that non-linear stuff they've been doing for decades now on computers . This cycling stability problem must be either particularly hard to crack, or not high on the research priority list, if its only just been done .

I just realised what was jarring me:

Quentin Cooper meets the experts who think they've cracked it

Now come on, have they or haven't they, Quentin? Its maths! Surely they know? I can't bear programs which lead you along then don't fully answer the question/problem posed, leaving you guessing... I hope its not another one of those...

Stuart.

 

[flecc]

This cycling stability problem must be either particularly hard to crack, or not high on the research priority list, if its only just been done .

Particularly hard to crack is the one, tons of research on it as it's fascinated the experts for years.
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[coops]

Well, I already fell into one trap... (see my earlier amended post!).

I think the fact there is a subjective human input must make it harder to crack... how do you measure what the brain does when "learning" to ride a bike?! It is quite amazing when you think about it that cyclists stay upright at all .

 

[Haku]

But without our own ability to balance we'd all be trundling around on tricycles.

coops, staying upright might not look so difficult from the outset, people have been making self balancing robots for a long time that only need a short amount of programming code to remain upright, like: YouTube - Self-balancing Robots but it's when you get to the Segway the programming code increases because of the human variable & safety aspects.

 

[coops]

I've seen walking robots, but has a cycling robot been made? Not necessarily a pedalling one, but one that can stay upright while moving, fixed to a bike, by adaptive steering like humans do?

I think with cycling (I maybe completely wrong again, but look at that link I posted previously) its more about adaptively steering the bike which stabilises it (i.e. keeps it upright) while moving, and the design of the bike's "trail" seems a big factor in how effectively that can be learned. I think a sense of balance is obviously involved, but its the adaptive steering response to correct the bike's motion when it becomes unstable which is critical to learning to ride a bike.

At first, people tend to over compensate the steering & wobble the bars all over the place till they learn the right amount by trial and error & become more confident, after which it gradually becomes "second nature" & can be done automatically, with very little or no concentration.

Stuart.

 

[flecc]

In motorcycle racing huge numbers of combinations are tried, typically a front wheel diameter might be changed for one race to one an inch smaller diameter.

That affects rotating mass, gyroscopic forces, head angle, trail, steering speed and wheelbase length!

See how complicated it is.
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[flecc]

But without our own ability to balance we'd all be trundling around on tricycles.

and oddly, once we can ride a two wheeler, a trike can be very difficult to ride. The instinct to lean into corners is strong to overpowering, and leads to the uncomfortable sensation that the trike is toppling outwards, when it's doing nothing of the kind.

That feeling can be so strong that some cyclists cannot ride a trike at all, no matter how hard they try.
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[flecc]

The program I posted about is on at 4.30 today!
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[Miles]

Thanks for the reminder, flecc

 

[flecc]

Wasn't really worth it Miles, mostly waffle, few facts, and the conclusion being they still don't understand.

So much for radio's coverage of the subject.
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[Miles]

Yes, disappointing.

Worth a look here, though:
Bicycle Mechanics - Overview, Papers, and External Links

 

[Ian]

That feeling can be so strong that some cyclists cannot ride a trike at all, no matter how hard they try.
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Many years ago when I was a motorcyclist I was invited to try a combination (motorcycle + sidecar for the benefit of any youngsters), the advice given was to take my left hand off the handlebars and rest it on my knee except when operating the clutch. This advice was very effective at the time and I would imagine is just as effective when converting to a pedal trike. (except the bit about the clutch )

 

[coops]

Still, its been an eyeopener to an interesting subject I've never thought about, oddly enough .

 

[Ian]

Still, its been an eyeopener to an interesting subject I've never thought about, oddly enough .

If you start thinking about it you fall off.

 

[coops]

For some reason, usually my own hairbrained ebike "scheme" ideas, I'm often reminded of Baldrick's ideas & "cunning plans" (from Blackadder - anyone remember that?!)... in this case "if you think too much, your feet fall off...!" - apparently someone he knew thought he'd cut his toenails... with a scythe... and his feet fell off...

 

[flecc]

Many years ago when I was a motorcyclist I was invited to try a combination (motorcycle + sidecar for the benefit of any youngsters), the advice given was to take my left hand off the handlebars and rest it on my knee except when operating the clutch. This advice was very effective at the time and I would imagine is just as effective when converting to a pedal trike. (except the bit about the clutch )

When I was in the trade combinations were still around in quite large numbers, and after fitting a chair we had to teach the owner how to cope. This usually happened after they'd got married and the first kid arrived, cars being beyond working peoples pockets at that time.

One easy way to adapt to combination or trike is to be a "sack of spuds", just sag into the saddle and treat the handlebars as a rudder initially, just pulling and pushing, and we also advised using one hand when practising. That method you mention would help in that, roughly what we used to say.
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