# The Naked Scientists Forum

### Author Topic: falling off my bike  (Read 13315 times)

• Neilep Level Member
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##### Re: falling off my bike
« Reply #25 on: 20/09/2006 19:15:55 »
Thanks for all the info. I will try not to start to think of it the next time I am leaning over in a corner. [^]

What you do speaks so loudly that I cannot hear what you say.

#### lightarrow

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##### Re: falling off my bike
« Reply #26 on: 18/09/2006 20:40:46 »
quote:
Originally posted by eric l

With a motorbike you have more gyroscopic effect from the flywheel of the engine and the rotor of the alternator than from the wheels.
I don't agree. Certainly a flywheel's angular speed is much greater than a wheel's angular speed, but a wheel's moment of inertia is also much greater than that of a flywheel, not only because of the greater mass M but, mostly, because of the greater radius R: I = a*M*R^2 where a depends on the form (a = 1 for a ring, = 1/2 for a disk). Gyroscopic effects depend on angular momentum K, which depends on angular speed w and on I: K = I*w.
quote:
And your frame will tilt to the right if you turn your handlebars to the left even at standstill, when there is no question of gyroscopic effect.
At standstill, whatever you do, you will fall to the ground with a heavy moto, if it tilts more than a certain angle, but you don't fall, even at an higher angle, if you go over a certain speed (~ 30 Km/h with my Yamaha FZ750) and the greater the speed, the more difficult it is to tilt the moto. A moto (or a bicycle) running without any rider can keep going straight, until its speed lowers under the value at wich gyroscopic effects becomes less important.
« Last Edit: 18/09/2006 20:58:16 by lightarrow »

#### eric l

• Hero Member
• Posts: 514
##### Re: falling off my bike
« Reply #27 on: 19/09/2006 08:52:01 »
OK, so you are going through a turn with your TZ750 at 30 km/h, at an angle at which you would fall if you and the motorcycle would stand still.  But that is simply because turning at that angle you are in equilibrium !  In fact, if you would stay vertical (I do not use the word upright on purpose) cornering at that speed, you would fall off, but on the other side.
Watch a car going through a corner at even a moderate speed.  You can not lean it over like you lean over a bicycle or motorcycle.  And what will you see ?  Due to inertia - which in this case will be called "centrifugal force" - the car will be pushed to the outside, and the suspension will allow it to be tilted outwards (unless the car has a really sophisticated computer controlled suspension).  The stiffer suspension of a sports car will allow it to corner faster before tilting over.
Cornering on a motorcycle, you lean over to the inside, so that the resultant of centrifugal force and gravity pass through the line between the contact patches where your front and rear wheels touch the road.
And the importance of the gyroscopic effect of the engine is demonstrated in trials (I mean the sport), where the rider revs up the engine while letting the clutch slip so that he (or she) can profit from the gyroscopic effect without the wheels turning.
« Last Edit: 19/09/2006 08:59:29 by eric l »

#### lightarrow

• Neilep Level Member
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##### Re: falling off my bike
« Reply #28 on: 19/09/2006 11:55:50 »
quote:
Originally posted by eric l

OK, so you are going through a turn with your TZ750 at 30 km/h, at an angle at which you would fall if you and the motorcycle would stand still.  But that is simply because turning at that angle you are in equilibrium !  In fact, if you would stay vertical (I do not use the word upright on purpose) cornering at that speed, you would fall off, but on the other side.
Watch a car going through a corner at even a moderate speed.  You can not lean it over like you lean over a bicycle or motorcycle.  And what will you see ?  Due to inertia - which in this case will be called "centrifugal force" - the car will be pushed to the outside, and the suspension will allow it to be tilted outwards (unless the car has a really sophisticated computer controlled suspension).  The stiffer suspension of a sports car will allow it to corner faster before tilting over.
Cornering on a motorcycle, you lean over to the inside, so that the resultant of centrifugal force and gravity pass through the line between the contact patches where your front and rear wheels touch the road.
And the importance of the gyroscopic effect of the engine is demonstrated in trials (I mean the sport), where the rider revs up the engine while letting the clutch slip so that he (or she) can profit from the gyroscopic effect without the wheels turning.

I have no doubt there is centrifugal force and gravity in opposite directions, in a bend, and I have no doubt about the existence of engine's gyroscopic effect, especially in trial motorbikes, where the flywheel is big and heavy on purpose, to have a big torque at low rev.

But you can be aware of gyroscopic effect even on a straight road: you make a slight force on your handlebars to make them turn left, and the moto tilts to the right, and this effect increases with the moto's speed. You say this is because of gyroscopic effect from the engine, I say it depends more on gyroscopic effect from the wheels, over a certain moto's speed.
« Last Edit: 19/09/2006 11:59:47 by lightarrow »

#### eric l

• Hero Member
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##### Re: falling off my bike
« Reply #29 on: 19/09/2006 20:40:43 »
Well, Alberto, I think we can agree that there is not one simple explanation in this case, that at least two mechanisms have a role to play.  I doubt if there is a way to determine which effect is the most important in any possible situation here.
Of course, part of this difference of opinion started with the fact that to me a "bike" is a bicycle (if I do not have a clue to think of motorcycles in the first place).  This may be due to the fact that a son of mine has been a bicycle racer (in the youth classes only), which greatly increased my interest in the physiscs of cycling.  My experience with motorcycles is comparatively limited.
Maybe in this particular topic SoulSurfer's quotation should read "Learn, create, test and fall" - but try not to do that at high speeds.  I need discussions like these to keep on my toes.
I'm sure we'll meet again soon on an other topic.

#### lightarrow

• Neilep Level Member
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##### Re: falling off my bike
« Reply #30 on: 20/09/2006 00:54:27 »
Ok!

#### thayo

• Sr. Member
• Posts: 226
##### Re: falling off my bike
« Reply #31 on: 20/09/2006 05:42:24 »
who gives cool explanation!

lets keep trying the untried since the birth of science innovations have been like  toy but their impacts have rocked the world

• Neilep Level Member
• Posts: 2175
• Scallywag
##### Re: falling off my bike
« Reply #32 on: 20/09/2006 19:15:55 »
Thanks for all the info. I will try not to start to think of it the next time I am leaning over in a corner. [^]

What you do speaks so loudly that I cannot hear what you say.

#### The Naked Scientists Forum

##### Re: falling off my bike
« Reply #32 on: 20/09/2006 19:15:55 »