Motorcycle physics -- Countersteering and Bodysteering

[rcgldr]

The Earth is much heavier than us too, why doesn't it pose a problem when we push against it when walking but it does when trying to apply a horizontal force on the pegs is what I don't get.

When walking, the person is not trying to move the Earth (it only moves a very very tiny amount), but on a bike the person is trying to lean the bike and get the tires to exert a force onto the pavement, which will result in the pavement exerting a force back onto the tires. As previously posted, in some scenarios, the forces between tire and pavement will be in the "wrong" direction, and in other scenarios, even when these forces are in the "right" direction, the rider ends up leaning the wrong way.

The system of bike and rider are a unitrack system, for a coordinated turn, the system first has to be leaned in the desired direction of turn before it actually can be turned in that direction. In some cases, an uncoordinated turn maneuver can be used to avoid potholes: say a bike is headed towards a pothole, and rather than trying to do a normal turn away from the pothole, the rider uses "pro-steer" to steer the tire contact patches around the pothole, and after passing the pothole, steers the other way to return the bike back to vertical. For example, the rider steers right to cause the tires to go around the right side of the pothole, while the bike ends up unbalanced leaning to the left. The rider delays correcting the left lean until after the bike passes the pothole. The advantage of this is that there's no delay waiting for the bike to lean to the right before turning to the right.

 

[Erunanethiel]

When walking, the person is not trying to move the Earth (it only moves a very very tiny amount), but on a bike the person is trying to lean the bike and get the tires to exert a force onto the pavement, which will result in the pavement exerting a force back onto the tires. As previously posted, in some scenarios, the forces between tire and pavement will be in the "wrong" direction, and in other scenarios, even when these forces are in the "right" direction, the rider ends up leaning the wrong way.

The system of bike and rider are a unitrack system, for a coordinated turn, the system first has to be leaned in the desired direction of turn before it actually can be turned in that direction. In some cases, an uncoordinated turn maneuver can be used to avoid potholes: say a bike is headed towards a pothole, and rather than trying to do a normal turn away from the pothole, the rider uses "pro-steer" to steer the tire contact patches around the pothole, and after passing the pothole, steers the other way to return the bike back to vertical. For example, the rider steers right to cause the tires to go around the right side of the pothole, while the bike ends up unbalanced leaning to the left. The rider delays correcting the left lean until after the bike passes the pothole. The advantage of this is that there's no delay waiting for the bike to lean to the right before turning to the right.

I think we should not try to "move" the bike, but keep it as still as possible, that's why I assumed it's better to apply the forces on the pegs, low down on the bike, so the tires will resist the bikes movement and if the bike doesn't move much to the way the rider pushed it to, the less it cancels the rider's movement and more the combined center of gravity of the system shifts to the side the rider leans in to.

 

[rcgldr]

I think we should not try to "move" the bike, but keep it as still as possible ...

Which requires counter-steering if the rider is going to shift weight at the same time. Take a look at this chase video of a motorcycle racer. Note that body movements are done in advance of turning movements, using counter-steering to keep the bike from turning while the rider hangs off to setup for a turn. Also note that lean angle adjustments are made mid-turn without anybody movements. He also transitions from a right to left turn, both shifting from the right side of the bike to the left side of the bike at the same time the bike changes lean angle from right to left, by counter-steering to the right to change the lean of the system of bike and rider.



Maybe you missed this video also from post #5, showing a bike that is particular resistant to "body steering". Skip to 2:15 into the video:

 

[Erunanethiel]

Which requires counter-steering if the rider is going to shift weight at the same time. Take a look at this chase video of a motorcycle racer. Note that body movements are done in advance of turning movements, using counter-steering to keep the bike from turning while the rider hangs off to setup for a turn. Also note that lean angle adjustments are made mid-turn without anybody movements. He also transitions from a right to left turn, both shifting from the right side of the bike to the left side of the bike at the same time the bike changes lean angle from right to left, by counter-steering to the right to change the lean of the system of bike and rider.



Maybe you missed this video also from post #5, showing a bike that is particular resistant to "body steering". Skip to 2:15 into the video:

If the rider doesn't counter steer at the same time he shifts weight, assuming the bike doesn't have any trail, it still would cause the center of gravity of the system to change horizontally, which is what I am trying to say.

 

[rcgldr]

If the rider doesn't counter steer at the same time he shifts weight, assuming the bike doesn't have any trail, it still would cause the center of gravity of the system to change horizontally, which is what I am trying to say.

The point I'm trying to make is both the bike and the rider shift weight, the rider one way, the bike the other way, and which direction the force from the pavement and the center of gravity changes horizontally depends on the scenarios I mentioned in my prior posts.

 

[Erunanethiel]

The point I'm trying to make is both the bike and the rider shift weight, the rider one way, the bike the other way, and which direction the force from the pavement and the center of gravity changes horizontally depends on the scenarios I mentioned in my prior posts.

The part I don't understand is if we can walk on the earth, shouldn't we be able to use horizontal forces through the pegs? Why the pegs being under our center of gravity would unable us? If we push to the other way first through the pegs, and then we push the way we actually wanted to push, I don't see anything that would make it not possible.

 

[rcgldr]

The part I don't understand is if we can walk on the earth, shouldn't we be able to use horizontal forces through the pegs? Why the pegs being under our center of gravity would unable us? If we push to the other way first through the pegs, and then we push the way we actually wanted to push, I don't see anything that would make it not possible.

My point was that the initial horizontal force applied to the pegs would be relatively small. Why focus on the horizontal forces only and not the vertical forces as well? Once the rider's center of mass is offset from the pegs, the rider can lift off the seat, with the pegs pushing up and gravity pulling down, creating a torque on the rider to cause the rider to lean.

Assuming you're still wondering about a bike with no trail, then that's almost like having the steering locked, and I've already gone through a close analogy of a bike on a tight wire, and what it takes to keep that system balanced (use torque on balance pole to generate a Newton third law pair of forces between wheels and wire).

 

[Erunanethiel]

My point was that the initial horizontal force applied to the pegs would be relatively small. Why focus on the horizontal forces only and not the vertical forces as well? Once the rider's center of mass is offset from the pegs, the rider can lift off the seat, with the pegs pushing up and gravity pulling down, creating a torque on the rider to cause the rider to lean.

Assuming you're still wondering about a bike with no trail, then that's almost like having the steering locked, and I've already gone through a close analogy of a bike on a tight wire, and what it takes to keep that system balanced (use torque on balance pole to generate a Newton third law pair of forces between wheels and wire).

Why would the forces applied to the pegs be small? We constantly apply forces to the floor pushing the Earth and it is enough for making us walk or even run. I also don't understand why you say we have to bend at the waist in order to generate a force on the pegs, can't we just use our legs directly to apply the forces?The reason I am focusing on horizontal forces on the pegs rather than vertical forces is because I think making most of the friction between tires and the road in order to lessen the bikes movement to the other way, so it would cancel less of the riders movement, shifting the center of gravity of the system more to the side where the rider leans in to

 

[rcgldr]

Why would the forces applied to the pegs be small?

because the forces are being applies so far below the center of mass of the rider.

We constantly apply forces to the floor pushing the Earth and it is enough for making us walk or even run.

Other than the initial start (acceleration), the average net horizontal force while walking or running is zero (constant velocity).

The reason I am focusing on horizontal forces on the pegs rather than vertical forces is because I think making most of the friction between tires and the road in order to lessen the bikes movement to the other way, so it would cancel less of the riders movement, shifting the center of gravity of the system more to the side where the rider leans in to

If a rider is going to lean and shift to the left to "hang off" to the left on a bike, the rider needs to apply a force to the right at the handlebars, pegs, and possibly the seat.

 

[Erunanethiel]

because the forces are being applies so far below the center of mass of the rider.

Other than the initial start (acceleration), the average net horizontal force while walking or running is zero (constant velocity).

If a rider is going to lean and shift to the left to "hang off" to the left on a bike, the rider needs to apply a force to the right at the handlebars, pegs, and possibly the seat.

As long us we apply a little force to the opposite side to counter the rotation of applying a force (and being applied) below the center of mass of the rider, I don't think it should matter. Like pushing a bit to the right on somewhere on the bike to transfer to weight to the left first. As I think you said in the previous thread

 

[rcgldr]

As long us we apply a little force to the opposite side to counter the rotation of applying a force (and being applied) below the center of mass of the rider, I don't think it should matter. Like pushing a bit to the right on somewhere on the bike to transfer to weight to the left first.

Say the rider leans to the left at the waist, exerting a right force onto the seat, and a left force onto the pegs. The opposing forces reduce the net force in either direction, and mostly combine to create a Newton third law pair of torques, where the rider leans left and the bike leans right. There won't be much horizontal force between tires and pavement in this situation.

I'm not sure of the point of this. Weight shifting is just an indirect method of counter-steering that relies on the steering geometry, and on some bikes such as the Keith Code green bike, the bike's setup is such that the net effect of weight shifting is only producing shallow lean angles.Hanging off to the inside in a racing situation can be helpful, as noted in post #4. In a street situation, when not at the limits, it isn't needed. I use counter-steering similar to how ailerons are used on an aircraft, with the main difference being the control input is reversed (right torque on the handlebars results in a left lean and vice versa).

 

[Erunanethiel]

Say the rider leans to the left at the waist, exerting a right force onto the seat, and a left force onto the pegs. The opposing forces reduce the net force in either direction, and mostly combine to create a Newton third law pair of torques, where the rider leans left and the bike leans right. There won't be much horizontal force between tires and pavement in this situation.

I'm not sure of the point of this. Weight shifting is just an indirect method of counter-steering that relies on the steering geometry, and on some bikes such as the Keith Code green bike, the bike's setup is such that the net effect of weight shifting is only producing shallow lean angles.Hanging off to the inside in a racing situation can be helpful, as noted in post #4. In a street situation, when not at the limits, it isn't needed. I use counter-steering similar to how ailerons are used on an aircraft, with the main difference being the control input is reversed (right torque on the handlebars results in a left lean and vice versa).

Do we have to bend at the waist? Can't we just use our legs and push to the other side first as said in my previous post?

There has to be a way to get the center of gravity of the system to change one side or the other without countersteer (direct or indirect) since the system is not a closed one and the tires are touching the ground, so body movements horizontally should cause acceleration to one way.

 

[rcgldr]

Do we have to bend at the waist? Can't we just use our legs and push to the other side first as said in my previous post?

Push with the legs against the seat or with the feet against the pegs?

There has to be a way to get the center of gravity of the system to change one side or the other without countersteer (direct or indirect) since the system is not a closed one and the tires are touching the ground, so body movements horizontally should cause acceleration to one way.

The issue is that both bike and rider move in response to a Newton third law pair of forces and/or torques applied between bike and rider. The net force has to be applied low enough that the bikes angular momentum about the roll (lean) axis resists the lean reaction to the force, resulting in a force between tires and pavement that accelerates the center of mass of the system into the direction that the rider shifts. The problem is that the rider's center of mass is too high and the rider's relative angular momentum is too low to be able to generate much horizontal force at the pegs.

Again, note that moving the center of mass without countersteer is exactly what happens in the case of a bike on a tight wire, and in that situation, a long balance pole that weighs more than the bike is used to maintain balance, with the rider exerting "counter-torque" on the balance pole to shift the center of mass of the system to keep it centered above the wire.

 

[Erunanethiel]

Push with the legs against the seat or with the feet against the pegs?

The issue is that both bike and rider move in response to a Newton third law pair of forces and/or torques applied between bike and rider. The net force has to be applied low enough that the bikes angular momentum about the roll (lean) axis resists the lean reaction to the force, resulting in a force between tires and pavement that accelerates the center of mass of the system into the direction that the rider shifts. The problem is that the rider's center of mass is too high and the rider's relative angular momentum is too low to be able to generate much horizontal force at the pegs.

Again, note that moving the center of mass without countersteer is exactly what happens in the case of a bike on a tight wire, and in that situation, a long balance pole that weighs more than the bike is used to maintain balance, with the rider exerting "counter-torque" on the balance pole to shift the center of mass of the system to keep it centered above the wire.

I can't quotes separately since I am on mobile so I have to write like this, sorry.

Pushing against the pegs or where the vertical bit where the peg attaches to the bike is what I am saying. After the initial push to the bike, the rider can apply the force necessary to stop him falling can be applied high on the bike, in order to increase the amount that center of gravity of the system can change more. So to start the movement, apply a force as low as possible, and to secure the position, apply a force as high on the bike as possible, to achieve the most change in the horizontal position of the center of gravity of the system.

 

[rcgldr]

Pushing against the pegs or where the vertical bit where the peg attaches to the bike is what I am saying. After the initial push to the bike, the rider can apply the force necessary to stop him falling can be applied high on the bike, in order to increase the amount that center of gravity of the system can change more. So to start the movement, apply a force as low as possible, and to secure the position, apply a force as high on the bike as possible, to achieve the most change in the horizontal position of the center of gravity of the system.

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.

 

[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.