What makes a bike balance?

Question Answered by Alana Jensen, Environmental Educator, INL ESER Program

You may think that it’s the human riding the bike that keeps the bike balanced, but that’s not entirely true. If launched properly, a bike with no rider balances itself just fine and continues on its path as if a rider were guiding it. You can try this experiment yourself. So if your bike doesn’t need a rider to keep it balanced, there must be other forces at work.

Up until a few decades ago, scientists thought a bicycle is self-balancing when in motion only because of gyroscopic effects and caster effects.
Gyroscopic motion is the tendency of a rotating object to maintain the orientation of its rotation. The front wheel of the bicycle is spinning forward quickly, acting like a gyroscope. Then when you tip the bike to the right, the gyroscope applies the torque, which turns the handlebars to the right and causes the steering, bringing the wheels back under the bicycle and holding it up. This effect is easy to see when playing with a top. The faster the top spins the more likely it is to stay upright.

The caster effect is a self-aligning effect to keep the steering straight. Look at the bottom of an office chair. If you move the chair around, the wheels re-orient themselves to follow the motion. This is because the wheel’s ground contact point is behind the chair’s steering axis; the wheel trails behind. The front wheel of the bicycle also touches the ground a little behind where the steering axis hits the ground. If the bicycle direction changes when it’s going forward, the wheel will tend to follow and bring itself back under the rider.

Recent observations by a group of scientists led by J. D. G. Kooljman and his team have shown that while the gyroscopic and caster effects may contribute to the bicycle’s balance, they are not the root cause. The root cause is a front-loaded steering geometry. A front-loaded steering geometry means that the steering shape of the front wheel and frame on a bicycle is constructed such that the front of the bike falls faster than the back. If the bike starts to tilt to the left after hitting a bump and succumbing to gravity, the front wheel falls to the left faster than the rest of the bike. As a result, the bike turns left. The amazing part is that turning the front wheel to the left causes the momentum of the bike to snap to the right because of centrifugal force (just like you are thrown to the right side of your car when making a quick left turn). The right lurch of the bike compensates for the initial fall to the left and the bike ends up straight again. The fall becomes self-correcting because of the front-loaded steering geometry.

Centrifugal and Centripetal Forces
Centrifugal and centripetal forces are often confused with each other.
Centripetal force is the force needed to make something move in a circle. A constant magnitude centripetal force that is always perpendicular to the direction of motion will make the object move in a circle. For instance, suppose you swing a rock around your head with a string. Centripetal force is acting on the rock to make it move in a circle.

If an object moving in a circle and experiences an outward force than this force is called the centrifugal force. It is not a real force, but instead comes from an object’s inertia, which is the force that keeps stationary objects at rest and moving objects in motion at a constant velocity.

Let’s experiment with centripetal and centrifugal force.
Materials: Empty bucket with handle, water, your arm, outdoor area so you don’t make a mess

  1. Practice the motion you will use for this experiment. Hold the handle of the empty bucket and do a complete circular swing up from the ground to behind your back. Practice until you can do this in one smooth motion.
  2. Fill the bucket part way with water.
  3. Do Step 1 again, this time with water in the bucket.

What happened?

When you swing the bucket back and forth, you can feel the bucket and the water inside pulling on your arm because it wants to continue to move in a straight line and you want to make it go in a circle. As you swing higher and higher, the water pushes harder on the bottom of the pail. But when you swing the bucket upside down, gravity tries to pull the water out of the bucket. Whichever force, the gravity or the centripetal, is stronger wins. As long as you swing the bucket fast enough, the force of the water pushing on the bottom of the bucket (centripetal force) will be stronger than gravity and the water will stay in the pail. Of course, if you swing too slowly, you’ll get wet.

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