Motorcycle physics -- Countersteering and Bodysteering

[Erunanethiel]

Again the issue is that the initial push at the pegs will be a relatively small force. It's not going to do much to change the center of mass of the system. However, even if the center of mass is only shifted a small amount, then gravity will cause the system to lean in the direction of center of mass offset, which in turn will further move the center of mass of the system, but that this point rider movements could not overcome the effects of gravity and steering would be needed to prevent the system from falling on to its side. Also random disturbances would cause the system to lean, and again rider movements could not overcome gravity without counter-steering.

Yes, I said it in the OP too in the last paragraph, you would have to catch it "manually" with the handlebars through steering since there is no trail, and it would be impossible to overcome the force of gravity just through body steering. But the upside is the gravity will do most of the for the rider after there is any offset of the center of gravity of the system, as you said in your post as well. I would guess forces applied through the pegs should be enough to start the process

How much of a difference is there in terms of the rider can apply on the pegs horizontally or high up on the bike like pushing with your knee or through the handlebars horizontally?

 

[rcgldr]

How much of a difference is there in terms of the rider can apply on the pegs horizontally or high up on the bike like pushing with your knee or through the handlebars horizontally?

A lot, typically motorcycle races apply a force at the pegs and handlebars to shift off to one side and hang off the bike, so that the rider doesn't lean the "wrong" way such as applying a horizontal force to the pegs and no force anywhere else. Consider the case of standing on one leg on the ground, and pushing with the one leg horizontally on the floor, versus standing next to a wall and pushing with one leg horizontally on the floor and with one arm pushing horizontally on the wall. By using the floor and the wall, the person can shift weight and lean in the "correct" direction at the same time.

 

[Erunanethiel]

A lot, typically motorcycle races apply a force at the pegs and handlebars to shift off to one side and hang off the bike, so that the rider doesn't lean the "wrong" way such as applying a horizontal force to the pegs and no force anywhere else. Consider the case of standing on one leg on the ground, and pushing with the one leg horizontally on the floor, versus standing next to a wall and pushing with one leg horizontally on the floor and with one arm pushing horizontally on the wall. By using the floor and the wall, the person can shift weight and lean in the "correct" direction at the same time.

When the rider applies the horizontal forces through the pegs, doesn't he lean the "correct way", if we define it as leaning into the side the which the motorcycle is turning to? My reasoning is that when the rider applies the forces low on the bike and lower than the center of mass of the motorcycle, more of the motorcycle's motion that counters the rider's movement which is the opposite of motorcycles will be canceled by the friction between the tires and the road, and thus the system's combined center of gravity shifts to the side which the rider leaned in to. I might be wrong, I am definitely not sure.

 

[rcgldr]

When the rider applies the horizontal forces through the pegs, doesn't he lean the "correct way" ...

Consider this scenario, the riders feet exert a left force onto the pegs coexistent with the pegs exerting a right force onto the riders feet. That right force exerted onto the riders feet is well below the rider's center of mass, and results in counter-clockwise angular acceleration of the rider in addition to rightwards linear acceleration of the rider as viewed from behind. The rider ends up "leaning" to the left, which is the "wrong" way, since the rider is being accelerated to the right.

 

[Erunanethiel]

Consider this scenario, the riders feet exert a left force onto the pegs coexistent with the pegs exerting a right force onto the riders feet. That right force exerted onto the riders feet is well below the rider's center of mass, and results in counter-clockwise angular acceleration of the rider in addition to rightwards linear acceleration of the rider as viewed from behind. The rider ends up "leaning" to the left, which is the "wrong" way, since the rider is being accelerated to the right.

If the rider pushes to the left, the rider ends up to the right and motorcycle goes to the left a bit less than it would if it was a closed system, so the system's center of mass shifts to the right and turns that way, and right is the side the rider is leaning in to.

I can't see the problem

 

[rcgldr]

Consider this scenario, the riders feet exert a left force onto the pegs coexistent with the pegs exerting a right force onto the riders feet. That right force exerted onto the riders feet is well below the rider's center of mass, and results in counter-clockwise angular acceleration of the rider in addition to rightwards linear acceleration of the rider as viewed from behind. The rider ends up "leaning" to the left, which is the "wrong" way, since the rider is being accelerated to the right.

If the rider pushes to the left, the rider ends up to the right and motorcycle goes to the left a bit less than it would if it was a closed system, so the system's center of mass shifts to the right and turns that way, and right is the side the rider is leaning in to. I can't see the problem

Compare the situation to a uniform rod in space. A right force is applied to the bottom of the rod, it moves to the right, but rotates to the "left".

 

[Erunanethiel]

Compare the situation to a uniform rod in space. A right force is applied to the bottom of the rod, it moves to the right, but rotates to the "left".

Yes but it's center of gravity still shifts to right, no matter where you pushed it.

And about the rotation we have when walking, we somehow counteract it I think by shifting our weight forwards before we set off. Just like you said you have to push somewhere to the opposite side on the bike before you actually shift your weight on the bike

 

[rcgldr]

Compare the situation to a uniform rod in space. A right force is applied to the bottom of the rod, it moves to the right, but rotates to the "left".

Yes but it's center of gravity still shifts to right, no matter where you pushed it.

And about the rotation we have when walking, we somehow counteract it I think by shifting our weight forwards before we set off. Just like you said you have to push somewhere to the opposite side on the bike before you actually shift your weight on the bike

What I've posted is that for a normal bike with steering geometry (trail) that steers into the direction of lean, then weight shifting is an indirect way to counter-steer.

For the locked steering case, in most scenarios where the rider shifts weight, the rider ends up leaning the wrong way. If the rider attempts to "pre-shift" the weight, the center of mass is moved the "wrong" way a bit, and that is enough for gravity effects to cause the system to start falling the "wrong" way, and weight shifting will not be able to recover from this.

 

[Erunanethiel]

What I've posted is that for a normal bike with steering geometry (trail) that steers into the direction of lean, then weight shifting is an indirect way to counter-steer.

For the locked steering case, in most scenarios where the rider shifts weight, the rider ends up leaning the wrong way. If the rider attempts to "pre-shift" the weight, the center of mass is moved the "wrong" way a bit, and that is enough for gravity effects to cause the system to start falling the "wrong" way, and weight shifting will not be able to recover from this.

What method do you think is the best for shifting weight on the bike in order to horizontally accelerate the system's center of mass, providing the bike has no trail?

 

[rcgldr]

What method do you think is the best for shifting weight on the bike in order to horizontally accelerate the system's center of mass, providing the bike has no trail?

I don't think there is a reliable way to do this. Again, using the bike on a tight wire comparison, the long balance pole weighs more than the bike, in order to maintain balance on such a system.

 

[Erunanethiel]

I don't think there is a reliable way to do this. Again, using the bike on a tight wire comparison, the long balance pole weighs more than the bike, in order to maintain balance on such a system.

As long as there is friction between the road and tires, it is very unlikely that bike's movement will completely cancel out rider's movement one way or the other, the hard but I think is which way that will be

 

[rcgldr]

I don't think there is a reliable way to do this. Again, using the bike on a tight wire comparison, the long balance pole weighs more than the bike, in order to maintain balance on such a system.

As long as there is friction between the road and tires, it is very unlikely that bike's movement will completely cancel out rider's movement one way or the other, the hard but I think is which way that will be

What I meant is that the rider ends up leaning the wrong way after the initial application of force. However, once gravity takes over and causes the system to continue to fall, the rider can then lean the correct way since that won't prevent the system form continuing to fall.

 

[Erunanethiel]

What I meant is that the rider ends up leaning the wrong way after the initial application of force. However, once gravity takes over and causes the system to continue to fall, the rider can then lean the correct way since that won't prevent the system form continuing to fall.

Oh what you said is there is not a reliable way to push the bike and then the rider ending up leaning the same way the system is turning to right? The rider can shift his weight to offset the center of mass of the system horizontally with a bike with no trail, but he would end up leaning the wrong way is what you are saying, have I got it right?

 

[rcgldr]

Oh what you said is there is not a reliable way to push the bike and then the rider ending up leaning the same way the system is turning to right? The rider can shift his weight to offset the center of mass of the system horizontally with a bike with no trail, but he would end up leaning the wrong way is what you are saying, have I got it right?

The rider initially ends up leaning the wrong way, but once the system is leaning, then gravity becomes the dominant factor, and the rider can then lean the correct way, since the rider's movements won't be enough to prevent the system from continuing to fall over, at which point steering inputs will be needed.

 

[Erunanethiel]

The rider initially ends up leaning the wrong way, but once the system is leaning, then gravity becomes the dominant factor, and the rider can then lean the correct way, since the rider's movements won't be enough to prevent the system from continuing to fall over, at which point steering inputs will be needed.

So the bottom line on this thread is that, it is possible to change the horizontal position of the center of mass of the system with body weight shifts on a bike with no trail geometry, but you would need to "catch" the bike through handlebar inputs to stop the bike from falling. I guess if you confirm what I said is the case, we can call it done!

 

[rcgldr]

So the bottom line on this thread is that, it is possible to change the horizontal position of the center of mass of the system with body weight shifts on a bike with no trail geometry, but you would need to "catch" the bike through handlebar inputs to stop the bike from falling. I guess if you confirm what I said is the case, we can call it done!

Correct.