Here we go again

[JohnH]

Ever ridden on ice? A bicycle on a truly frictionless surface can't be ridden, because it can't be leaned.

Oooooh! I was a fan of the gyroscope theory until you said that, Jim.

I think you may have designed the experiment that ends the debate: if a rider finds it just as difficult to stay upright on ice at high speed as he/she does at low speed, then the higher angular velocity of the wheels on the speeding bike isn't giving much of a "gyroscopic effect". (Perhaps gluing some lead weights to the rims would help! )

 

[JohnH]

I remember watching an Open University programme where a bloke demonstrated how unstable a bike was until the forks were reversed and the rake made the bike stable enough to be pushed down a road on its own.

Could that be because reversing the forks puts the front axle behind the steering pivot (axis of the steerer tube), just like the wheels on a supermarket trolley?

 

[one-eyed_jim]

Could that be because reversing the forks puts the front axle behind the steering pivot (axis of the steerer tube), just like the wheels on a supermarket trolley?

The important point isn't the front axle, but the contact patch of the front tyre. That's already behind the steering axis (because of the slope of the head tube) but reversing the fork increases the distance between the contact patch and the steering axis.

That has two effects. When the bike is upright there's a speed-dependent force that tends to centre the steering, like the wheels on a supermarket trolley. When the bike is tilted, the reaction force at the contact patch steers the front wheel in the direction of the lean.

 

[Neil]

Think about when we learn to ride a bike, or teach a child to ride a bike. One of the key things is them cycling at a good enough speed for the gyroscopic effect to make it all easy for them, rather than the whole struggling for balance and trying to correct with steering.

That's a circular argument...

How so?

Surely it's a valid point - when learning to ride a bike, it is something to exploit, because the more difficult corrective slower speed skill comes with practice, learning and conditioning.

I'll repeat, a bike - or for that matter, motorcycle - travelling with sufficient speed, wants to (notwithstanding I'm not really anthropomorphising a motorcycle or bike...) stay largely upright (assuming it's already upright and not at some odd angle or lean). Now true enough, the mass of the rider has a bearing - it just makes that speed a bit higher than an unmanned cycle / motorcycle.

Speed is important because the steering input required to correct a lean is strongly speed-dependent - because centrifugal force varies with v².

To look at it another way, when the bicycle leans to one side, it will continue to fall to that side unless something happens to put the wheels back under the rider's centre of mass. The faster the bike is moving, the smaller the change in steering angle required to accomplish that lateral correction.

Consider a racing motorcycle cornering. Leaning is used to counter the natural forces at play, and assist with and significantly reduce any steering input required. And even with such considerable angles of lean, they're not usually in any danger of falling.

Now true enough, I'm not suggesting that this is purely because of gyroscopic effect - it isn't - inertia becomes a significant factor, but all the same, the faster the speed, the more influence of the gyroscopic effect, and the more relevance of inertia, and very much reduces the need for corrective input for balance.

Point being, though, that with cycles and motorcycles, with a certain speed comes gyroscopic effect, inertia, and a natural tendency to stay upright. That isn't just because of effects of leaning being minimised - that's all chicken / egg - there will be less need to lean, or for that matter, less chance of leaning - it's just as likely there will be minimal lean, because of lack of requirement to do any corrective movements to keep it in balance. The reality is, as the speed increases, gyroscopic effect increases, the effects of inertia increase, much less corrective action is required, or desired, because if it was done, it would probably largely become unstable.

Now the ice thing has some bearing - but when I was talking about friction, I wasn't just thinking of friction between tyre and surface... What I would say, though, is that for a bike, or motorcyle, that suddenly hits a brief patch of ice, but travelling at fairly normal speeds, it doesn't necessarily mean that all balance goes out of the window and a crash ensues. If the bike / motorcycle is travelling straight and true, and the surface suddenly loses a lot of friction, briefly, the bike / motorcycle may well stay upright - it just depends on whether much in the way of corrective action was required (or happened out of panic) whilst on the very slippery surface.

Clearly were the bike / motorcycle cornering, or truly having to correct much lean - then true enough, a crash almost a certainty - but running straight and true, at a normal speed, and suddenly loses grip briefly? Not necessarily catastrophic.

 

[JohnH]

The important point isn't the front axle, but the contact patch of the front tyre.

Oh, you physics guys are SO picky...

(...but quite correct )

 

[sylus]

Oh, you physics guys are SO picky...

(...but quite correct )

I know, sometimes I think I am talking to the cast of the big bang theory instead of a bike forum

 

[one-eyed_jim]

That's a circular argument...

How so?

Because the example you give assumes what you're trying to prove.

There's a clear relationship between speed and the angular momentum of the wheel. There's a clear relationship between speed and ease of balancing. That doesn't demonstrate a causal relationship between the angular momentum of the wheel and ease of balancing.

Consider a racing motorcycle cornering. Leaning is used to counter the natural forces at play, and assist with and significantly reduce any steering input required. And even with such considerable angles of lean, they're not usually in any danger of falling.

But leaning and turning are inseparable. If you turn without leaning, you fall. If you lean without turning, you fall. All that's important is that the reaction force at the ground passes through the centre of mass of the rider. There's no gyroscopic element involved.

Think of a skater in a tight turn: the same principle applies. The higher the speed, the tighter the turn, the greater the lean required to counteract the centrifugal force (mv²/r).

http://www.youtube.com/watch?v=NDhw5TMn9jA

 

[Neil]

That's a circular argument...

How so?

Because the example you give assumes what you're trying to prove.

Yes... and?

That doesn't make it a circular argument. I'm not suggesting that ease of balance, inherently, necessarily equates to, or produces increased speed.

There's a clear relationship between speed and the angular momentum of the wheel. There's a clear relationship between speed and ease of balancing.

Which is?

That doesn't demonstrate a causal relationship between the angular momentum of the wheel and ease of balancing.

My agreement that the gyroscopic effect makes it easy for kids to learn to cycle, if they increase speed, is based on:-

1) The gyroscopic effect increases with speed
2) The gyroscopic effect helps to balance a bike

Both are true.

Some quibble about the significance or impact of 2. Which is fine, but it most certainly exists, and there's defintely an effect that with speed comes ease of balance on a bike. Feel free to speculate or hypothesise on that if you don't believe the gyroscopic effect is having any bearing.

When I was a kid, I had a gyroscope and a little pointy stand, and discovered, then, that when spinning the gyroscope fast, it balanced on it's pointy stand and resisted falling or being deflected, until it's speed of rotation decreased. So when I was slightly older, and told the same gyroscopic effect has a bearing when on a bike with two big wheels spinning fast, I can accept the logic and rationale.

edit: also worth consideration to what I've been saying.

Consider a racing motorcycle cornering. Leaning is used to counter the natural forces at play, and assist with and significantly reduce any steering input required. And even with such considerable angles of lean, they're not usually in any danger of falling.

But leaning and turning are inseparable. If you turn without leaning, you fall. If you lean without turning, you fall.

If you're cycling slowly, it's perfectly feasible to turn without leaning - same with skating. It's only when a degree of speed is added that you need to deal with cornering forces.

All that's important is that the reaction force at the ground passes through the centre of mass of the rider. There's no gyroscopic element involved.

With two large-ish wheels are spinning fast, there's always going to be some gyroscopic effect. I'll buy, though, that it's not the most significant effect to deal with when cornering fast, though.

 

[one-eyed_jim]

I'll quote this passage from Witt and Wilson's "Bicycle Science" which corresponds quite well to my understanding of the different factors involved in steering and balancing:

http://tinyurl.com/6hynkef

An examination of the simple exercise of balancing a broomstick upright in the palm of the hand can elucidate many important aspects of bicycle balancing. The key rule is that unstable balance of an unstable rigid body requires an accelerated support. Whether its support point is at rest or moving steadily, a broomstick inverted and placed on the palm of the hand is unstable and will simply fall over. (A gyroscopically stabilized top is a quite different case.) Balancing a broomstick, or a bicycle, consists in making the small support motions necessary to counter each fall as soon as it starts, by accelerating the base horizontally in the direction in which it is leaning, enough so that the acceleration reaction (the tendency of the centre of mass to get left behind) overcomes the tipping effect of unbalance. The base must be accelerated with proper timing to ensure that the rate of tipping vanishes just when the balanced condition is reached. Even more sophisticated control is needed to maintain balance near a specified position, or while moving along a specified path. Taller broomsticks fall less quickly than shorter ones (...) and so are easier to balance.

How Bicycles Balance

A rider balances a bicycle in the left-right direction by steering it while rolling forward so as to accelerate the support of the bicycle laterally. Restraining a bicycle's steering makes it unrideable, a fact that is put to good use in steering locks for deterring bicycle theft. Surprisingly, the small steering motions necessary to right a bicycle after a disturbance can take place automatically, even with no rider, as can be demonstrated by releasing a riderless bicycle to roll down a gentle hill and then bumping it.

It is widely believed that the angular (gyroscopic) momentum of a bicycle's spinning wheels somehow supports it in the manner of a spinning top. This belief is absolutely untrue. Gyroscopes can react against (i.e., resist) a tipping torque only by continuously changing heading. For example, a tilted top can resist falling only by continuously reorienting its spin axis around an imaginary cone. Locked steering on a forward-rolling bicycle does not permit any wheel reorientation, and the bicycle will fall over exactly like a bicycle at rest, no matter how fast it travels, or how much mass is in the wheels. To be sure, bicycle wheels actually are changing heading continuously whenever the steering is turned, but their mass is too small to be of importance: the resulting gyroscopic support moment is tiny compared to the "mass times acceleration times center of mass height" moment that predominantly governs bicycle balancing.

Still, there is an extremely interesting gyroscopic aspect to bicycle balance: the angular momentum of a bicycle's front wheel urges it to steer (i.e., to precess) toward the side on which the bicycle leans, as can be demonstrated by lifting a bicycle off the ground, spinning the front wheel, and briefly tilting the frame. In other words, the gyroscopic action of the front wheel is one part of a system that automatically assists the rider in balancing the bicycle. If the angular momentum of this gyroscopic action is canceled (as Jones [1970] did with an additional, counter-rotating, front wheel), considerably more skill and effort are needed for no-hands riding.

 

[one-eyed_jim]

the example you give assumes what you're trying to prove.

Yes... and?

http://en.wikipedia.org/wiki/Circular_reasoning