one-eyed_jim
Old School Grand Master
His name was David Jones.GeoffApps":26v3ugfp said:I can't remember the name of the chap who did the counter-GE experiment.
His name was David Jones.GeoffApps":26v3ugfp said:I can't remember the name of the chap who did the counter-GE experiment.
Oooooh! I was a fan of the gyroscope theory until you said that, Jim.one-eyed_jim":151u1kpd said:Ever ridden on ice? A bicycle on a truly frictionless surface can't be ridden, because it can't be leaned.
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?legrandefromage":2i8hcuf5 said: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.
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.JohnH":1oeohcul said: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?
How so?one-eyed_jim":3gq85ihh said:That's a circular argument...Neil":3gq85ihh said: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.
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.one-eyed_jim":3gq85ihh said: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.
Oh, you physics guys are SO picky...one-eyed_jim":1ycrpe2s said:The important point isn't the front axle, but the contact patch of the front tyre.
JohnH":d0gdut06 said:Oh, you physics guys are SO picky...
(...but quite correct )
Because the example you give assumes what you're trying to prove.Neil":xs4gt0sj said:How so?one-eyed_jim":xs4gt0sj said:That's a circular argument...
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.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.
Yes... and?one-eyed_jim":vnimhalv said:Because the example you give assumes what you're trying to prove.Neil":vnimhalv said:How so?one-eyed_jim":vnimhalv said:That's a circular argument...
Which is?one-eyed_jim":vnimhalv said: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.
My agreement that the gyroscopic effect makes it easy for kids to learn to cycle, if they increase speed, is based on:-one-eyed_jim":vnimhalv said:That doesn't demonstrate a causal relationship between the angular momentum of the wheel and ease of balancing.
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.one-eyed_jim":vnimhalv said:But leaning and turning are inseparable. If you turn without leaning, you fall. If you lean without turning, you fall.Neil":vnimhalv said: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.
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":vnimhalv said: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.
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.