ABS system on a two-wheeled vehicle - Conservation of Angular Momentun

[engineerslovephysics]

Hello,

The question I have pertains to conservation of Angular momentum on a motorcycle. I know that the dynamic friction is less than the static friction, so when you are braking on a (say a motorcycle) and the wheels lock up, the bike is bound to fall over. This is the reason ABS (Anti-lock braking) has been invented. My question is does the reason the vehicle falls over only due to the fact that Angular momentum comes to zero when the wheels angular velocity equals zero? If that is the case, doesn't ABS prevent the wheel from locking up only to keep the wheels gyroscopic ability?

This question has been bugging me for some time and I am a little confused, thanks.

 

[berkeman]

Welcome to the PF.

My question is does the reason the vehicle falls over only due to the fact that Angular momentum comes to zero when the wheels angular velocity equals zero?

No. When the front wheel locks up, you can't steer anymore, so the front end washes out and down you go. If only the back wheel locks up and you can still steer, you may be able to come to a stop without falling, but that takes a bit of skill usually.

 

[Sciencestd]

What is related to the angular momentum is the torque (force), the torque will keep you balanced on the track...
let's say you slow down with your bike until zero of velocity so the torque will decrease and then the bike fall down.
The concept of ABS is not exactly what you mentioned but to prevent the sudden change in the system so instead to convert the inertia force heavily on the breaks, you reduce it slightly as much as possible in short time intervals, so the reaction of the system (Bike in this case) will be smooth.

 

[Cutter Ketch]

We are told that a bicycle wheel has angular momentum and gyroscopic torque keeps the bike upright. Well, it does, but that isn’t the whole story. For example, we can ride bikes at slow speed when the angular momentum is very small, or circus performers can ride tiny bikes with tiny wheels that have negligible angular momentum.

What really keeps us upright most of the time is our own feed back. Think about how at slow speed you often are working the handlebars quite a lot. If the bike starts to tip to the right, we steer to the right. The rolling wheel means that the frictional force in the plane of rotation doesn’t get transferred to the bike. The component of the frictional force perpendicular to the plane pushes the bottom of the wheel to the right standing the bike back up.

However, if the front wheel skids, the rolling no longer negates the force of friction in the plane. The sliding friction points opposite the direction of motion no matter which way we turn the front wheel and we lose our ability to correct tilts by steering.

 

[A.T.]

We are told that a bicycle wheel has angular momentum and gyroscopic torque keeps the bike upright.

When hands free, the gyroscopic torque on the front wheel can steer a bicycle into the lean, which contributes to self stability. But this is not the whole story.

 

[engineerslovephysics]

What really keeps us upright most of the time is our own feed back. Think about how at slow speed you often are working the handlebars quite a lot. If the bike starts to tip to the right, we steer to the right. The rolling wheel means that the frictional force in the plane of rotation doesn’t get transferred to the bike. The component of the frictional force perpendicular to the plane pushes the bottom of the wheel to the right standing the bike back up.

Welcome to the PF.
No. When the front wheel locks up, you can't steer anymore, so the front end washes out and down you go. If only the back wheel locks up and you can still steer, you may be able to come to a stop without falling, but that takes a bit of skill usually.

Thanks!

Aah, which is where counter-steering comes into play. When I am on my motorcycle, I always wonder how counter steering works (hey it works, who am I to complain to?) and why I have to lean into corners when turning at high speeds (what you see in Moto GP races). When counter-steering, does the change in the center of gravity relative to the wheels cause the bike to go where you want it to go? Obviously, traditional steering will not work at high speeds and will prompt you to go the opposite way you intended to steer (hence counter-steer). If one could elaborate on this, I'd appreciate it much.

Thanks,

 

[berkeman]

When I am on my motorcycle, I always wonder how counter steering works

Okay, if your question about locking up the brakes and crashing is answered, your next question about how to steer a motorcycle at speed is really a separate question. Let me find a couple threads where we have discussed this in the past, and paste the links in here to end this thread. If you still have questions about countersteering (and bodysteering) after reading those threads, you can post in those threads if they are still open, or start a new thread about countersteering.

 

[berkeman]

Some reading on countersteering and body steering:

https://www.physicsforums.com/threads/is-countersteering-a-necessity.396828/

https://www.physicsforums.com/threads/motorcycle-physics-countersteering-and-bodysteering.927832/

https://www.physicsforums.com/threads/a-rider-cornering-at-maximum-g.934706/