Why can we balance a bicycle only when it's moving?

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If I'll shove a bicycle down the road, without even being on it, it will go on perfectly without crashing, until it will reach to a minimal speed and then crash. Why is that? why can't we balance an imobile bicycle? A lot say it has to do with the gyroscopic effect but my physics professor said it has nothing to do with it but he wouldn't say why...
 

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  • #2
phinds
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A fast moving bike with one one it staying up probably DOES have something to do with gyroscopic effect but for a rider It's not gyroscopic at all, it's simply that when you are riding a moving bike you can move the center of gravity to be directly under you by moving the wheel. A stationary bike or a very slow moving one will topple.
 
  • #3
A.T.
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A lot say it has to do with the gyroscopic effect but my physics professor said it has nothing to do with it but he wouldn't say why...
Gyroscopic effect does play on a role normal bike by steering the front wheel, when the lean of the bike changes. But it’s not necessary for the self stability of the bike, as other effects can achieve it too.

 
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  • #4
A.T.
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you can move the center of gravity to be directly under you
What?
 
  • #5
phinds
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What?
Yeah, I guess what I should have said was you can move the center of support directly under the center of gravity. Can't figure out a good way to say it but surely you get what I mean.
 
  • #6
See a boat. The centre of weight is much up from the buoyancy centre. But what happens if boat take slope to side?
 
  • #7
rcgldr
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To summarize the previous threads on this, self stability of a bicycle relies on a geometry that steers the front tire into the direction of a lean. The conventional method for doing this is trail, where the extended steering axis intercepts the pavement in front of the contact patch. The alternative used in some experiemental bikes locates a mass ahead of and above the front tire, with the front tire mounted so it's free to rotate about it's steering axis, resulting in a yawing torque on the frame when leaned, which ends up steering the front into the direction of lean, without requiring any trail or caster setup.

There is a small gyroscopic steering reaction due to the roll torque related to the lean angle of a bike (gravity effectively pulls down at the center of mass, pavement pushes up at the contact patches), but that reaction normally doesn't steer inwards enough to result in self stability (note the roll torque goes to zero when the bike is leaned but in a coordinated turn), so the main effect of gyroscopic reaction is to dampen the lean rate by opposing the corrective steering input due to steering geometry.

At high speed on a motorcycle, around 100 mph == 161kph or more, the gyroscopic dampening dominates and mathematically causes a bike to fall inwards at an extremely slow rate, called capsize speed, and the perception to the rider is that a motorcycle at high speed tends to hold a lean angle unless / until the rider uses countersteering to correct or change the lean angle.

On a side note, a skilled rider can balance a non-moving bicycle with trail using the conventional steering inputs, but the movement of the contact patch with respect to the frame of the bike is limited, so only small lean angles can be corrected. A rider can also shift weight and/or hop to maintain balance on a bike, and this is done in "trials" type competitions to reorient (rotate) a bike while on top of a small surface.
 
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  • #8
HallsofIvy
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The gyroscopic action you refer to refers to the fact that a rapidly spinning wheel, if tilted to one side, will turn toward that side. When the bike starts tilting to the right causes the front tire turn to the right "catching" it and preventing it from falling over.
 
  • #9
A.T.
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The gyroscopic action you refer to refers to the fact that a rapidly spinning wheel, if tilted to one side, will turn toward that side. When the bike starts tilting to the right causes the front tire turn to the right "catching" it and preventing it from falling over.
This is basically the stability mechanism of a single rolling wheel, which doesn't have all the other effects that a bike has. But this effect still plays a role for the bike.
 
  • #10
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The reason it is easier to balance a bike when it's moving vs when it's still is because the wheels have angular momentum. It requires a torque to change angular momentum, and therefore it "prefers" to keep in the plane in which it rotates.
 
  • #11
A.T.
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The reason it is easier to balance a bike when it's moving vs when it's still is because the wheels have angular momentum. It requires a torque to change angular momentum,...
And gravity provides that torque when some initial lean is introduced.

... and therefore it "prefers" to keep in the plane in which it rotates.
Lock the steering of a bicycle and see how long it will "prefer" to stay upright. The way angular momentum stabilizes the bike is by causing precession which steers the front wheel, not just by "prefering" to stay upright.
 
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  • #12
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And gravity provides that torque when some initial lean is introduced.


Lock the steering of a bicycle and see how long it will "prefer" to stay upright. The way angular momentum stabilizes the bike is by causing precession which steers the front wheel, not just by "prefering" to stay upright.
There is no torque from gravity if you and the bike are upright and balanced. If you lock the steering wheel of a bike on a perfectly flat surface in the upright position, it will stay upright. I'm not sure what you think is precessing on a bike. The only thing with angular momentum are the wheels, they don't precess if they remain in the plane of rotation...
 
  • #13
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On a side note, a skilled rider can balance a non-moving bicycle with trail using the conventional steering inputs...
Also known as a track stand ...

Lock the steering of a bicycle and see how long it will "prefer" to stay upright.
Lol... it won't "prefer" at all.
If the steering of a bike is locked, it becomes virtually impossible to balance while riding.
This subject is more complicated than a lot of people think...

For plain old awesome... look here.


http://en.wikipedia.org/wiki/Danny_MacAskill
 
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  • #14
A.T.
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There is no torque from gravity if you and the bike are upright and balanced
The whole point of dynamic stability is that the bike has to recover from a slightly leaned state.

I'm not sure what you think is precessing on a bike.
The front wheel, as already explained by me, HallsofIvy and Andy Ruina in the video.
 
  • #15
rcgldr
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The reason it is easier to balance a bike when it's moving vs when it's still is because the wheels have angular momentum. It requires a torque to change angular momentum, and therefore it "prefers" to keep in the plane in which it rotates.
The wheels of a bike could be replaced with rounded skate blades, and skated on ice while coasting (no way to propel the bike with the blades, you'd need to push it to get it moving). With good steering geometry (like trail), the ice skate bike would be self-stable without any rotating parts. "Skates" for wheels have been emulated by locating counter-rotating wheels next to the front and rear wheels so that angular momentum is virtually zero.

As mentioned in my previous post, the net result of gyroscopic related steering effects is an under correction that opposes and/or dampens the corrective steering effects related to steering geometry, depending on the speed (too fast and self correction goes away, transitioning into holding a lean angle or falling inwards at an extremely slow rate).
 
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  • #16
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To add a few things: The caster and back rake of the front fork would act to stabilize a very small wheeled bicycle. However, in the long perfected design you buy at the store, the front wheel acts like a gyroscope (as said above). The standard bicycle needs negative caster and forward rake to work properly, or the gyroscopic stability of the front wheel would make it far to difficult to initiate turn the handle bars or initiate a turn. The negative castor/forward rake act to destabilize the bicycle to compensation for the over-stability of the gyroscopic wheel.

If you replaced the front end of your bicycle with a longer fork and tiny wheel, and gave it a good push, the handle bars would immediately attempt to spin 180 degrees and push the thing over.
 
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  • #17
rcgldr
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The standard bicycle needs negative caster and forward rake to work properly.
The forward curvature in the forks on a standard bicycle and the offset of the triple clamp on a motorcycle reduce the amount of caster (trail), but they don't result in a negative amount of caster. Hold a bicycle by the seat and lean it over, the front tire steers in the direction of the lean because there's positive caster (trail).

The effort to overcome gyroscopic resistance to steering inputs and leaning is related to speed, not caster: the greater the speed, the more effort it takes (torque applied to handle bars) to counter-steer and lean a bike, mostly an issue for motorcycles. At freeway and greater speeds on a motorcycle, the rider's perception is that counter-steering involves applying a torque onto the handlebars with little actual movement since the counter-steering angle is so small, and that the steering gets stiffer as speed increases.
 
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  • #18
A.T.
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The reason it is easier to balance a bike when it's moving vs when it's still is because the wheels have angular momentum. It requires a torque to change angular momentum, and therefore it "prefers" to keep in the plane in which it rotates.
Your theory is tested in the video below:

 

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