Motorbikes not falling down

1. Jan 24, 2016

black hole 123

when a motorbike makes a turn to the left, it can lean quite a bit to the left without falling down. also its impossible to maintain balance when its on rest, but the faster it moves the easier it's to balance. why?

2. Jan 24, 2016

Staff: Mentor

When going around a curve, a bike must lean so that the centrifugal force balances out gravity. If you just turn the handlebars without leaning, the centrifugal force will cause you to lean the opposite direction of the turn and you'll fall over. When still, there is no centrifugal force so the bike cannot lean without falling over.

Now, the question of why it's difficult to maintain balance on a stationary bike while easy on a moving bike has a different explanation. The basic idea is that when moving, the bike auto-corrects itself when it leans, forcing it back upright even without rider intervention. That's why you see bikes continuing on for hundreds of feet sometimes after their rider falls off. But when stationary, the bike does nothing to correct itself in a lean, leaving it all to the rider. I'll have to leave it to someone else to explain the details though.

3. Jan 24, 2016

gennarakis

Hi! Great question! I had that question a few years ago and I had found the answer but can't remember it very well to explain you in full detail at the moment...but it has to do with the angular momentum of the wheels (and maybe the friction on the wheel) A wheel moving wants to keep moving due to angular momentum (inertia of the particles) that's what keeps it more stable when not moving..that's my guess! Also this video from Walter Lewin might help

4. Jan 24, 2016

mfig

I think of the first question as a torque balance.

When a bike is leaning, its center of mass is not above its base of support. This means a torque is acting through the center of mass to rotate the bike towards the ground. But another torque is acting as well - the force of friction developed at the point of contact between wheel and ground pushes the bike in a circle, giving it angular acceleration. This force produces a torque that acts to rotate the bike in the opposite direction from the gravitational torque. If the bike does not fall, these torques are in balance. One could use this to find the required angle of lean for a given angular acceleration or turn radius and velocity.

The second question has to do with angular momentum. When a bike is going faster, its wheels have more angular momentum. The more angular momentum the wheels have, the more stable they are in their path.

5. Jan 24, 2016

rcgldr

As posted by spamanon, gravity pulls down, and the pavement pushes up, creating an inwards torque along the roll axis. This is opposed by the inwards force applied to the contact patches by the ground, which are below the center of mass of the bike, resulting in an outwards torque along the roll axis. If the torques are equal, then the lean angle remains the same.

For normal bicycles and motorcyles, self-stability is mostly due steering geometry, the extended steering pivot axis intercepts the pavement in front of the contact patch, so that when the bike is leaned, the upwards force from the ground behind the pivot axis steers the front tire into the direction of the lean. This geometry also adds a caster effect. This is called trail, and tends to cause a bike to return to a vertical orientation from a leaned orientation (the direction the bike is headed will have changed).

Gyroscopic related steering torque is a reaction to change in lean angle. (There's also a very small roll torque related to change in direction, a reaction to rotation about vertical axis called yaw ). For a bike in a coordinated turn (no change in lean angle), there is no gyroscopic steering torque. As a bike leans inwards, the gyrscopic steering torque tends to undecorrect, and as the steering geometry corrects the bike back to a vertical orienation, the gyroscope steeing torque opposes the steering geometry correction. At normal speeds gyroscopic torques act as a a damper, helping to prevent over-correction. At very high speeds, the gyroscopic torque dominates the steering geometry torque, and the mathematically predicted tendency is to fall inwards at a very slow rate (called capsize mode), but the rate is so slow that it's imperceptible to the rider, and/or the net result due to other factors is a bike at very high speed tends to hold the current lean angle.

Some experimental bicyles use an alternate form of steering geometry. A mass is suspended above and in front of the bike so that a lean produces a yaw torque on the bike causing a free to steer front wheel to steer into the turn, without using trail or caster geometries. Typically these bikes will use counter rotating wheels to eliminate any gyroscopic reactions.

Last edited: Jan 26, 2016
6. Jan 30, 2016

CWatters

As I recall a recent paper (2011?) ruled out the gyroscopic effect AND the castor/trail as being necessary for a bike to stay upright.

7. Jan 31, 2016

rcgldr

Several universities have build model bikes similar to what I described. Link to a 2011 artcile about such a bike:

http://bicycle.tudelft.nl/stablebicycle

Although this method works, it's not practical for a bicycle or motorcyle, so conventional methods like trail / caster effects are used.