A Fast Physics Lesson on Two Wheels

A motorcycle is a perfect “vehicle” to describe many natural forces of physics. Aside from the pun, the inline two-wheel design of a motorcycle makes it uniquely qualified as a laboratory of physical qualities. We can use a motorcycle to describe torque, angular momentum, torque precession, gyroscopic effects just looking at the wheel stability of this vehicle. Motorcycles have motors, but so do a lot of other machines. While exploring the physics involved within a motorcycle, we will start with what makes a motorcycle unique in the first place, its two wheels. Because of the large variety of physical elements working within a moving motorcycle, we will narrow the focus to the motorcycles spinning wheels, turning those wheels and a small safety lesson, since we’re talking motorcycles.

If a rider forgets to put the kickstand down before leaving his bike, an expensive collision between bike and ground will occur. All bikes lack lateral stability when motionless and can only remain upright when moving. At rest, the motorcycle is unstable because it has no base of support. An object's contact points with the ground are what gives it a base of support. A car has four contact points formed by its rectangle shape where the four tires touch the ground. As long as an object's center of gravity (the effective location of its weight) is above this base of support, the object is statically stable.

Stability also relates to the objects increasing stored energy as it tips sideways. Tipping a statically stable object sideways raises its center of gravity and gravitational potential energy, so that it naturally accelerates back toward its upright position. Since a motorcycle has only two contact points with the ground, the base of support is a line segment and the bicycle can't have static stability. Motorcycles moving forward will automatically steers its wheels underneath its center of gravity, adding stability (How Everything Works). Just as the Harlem Globetrotters spin a basketball on one finger, they have to keep their hand moving under the balls center of gravity to keep it upright. A motorcycle can balance if it keeps moving its wheels under its center of gravity. This auto-steering effect has to do with another effect: gyroscopic precession, which we will deal with later. (Hackworth) For now it’s enough to establish that a spinning wheel is stable. Now let us talk about how you change direction on a spinning wheel.

Torque is one of the main physical concepts that work on a motorcycle. Mathematically, torque is a product of a lever-arm distance and force, which tends to produce rotation. Torque can be thought of as a turning force. An easy visual example is the idea of a wrench attached to, and turning, a nut or bolt. A longer the wrench equals a longer lever-arm. The length of the lever arm is an especially important element in determining the amount of torque applied to an object. Torque has three elements: 1) Force applied; 2) Length of lever arm; and 3) Angle between lever arm and the force vector. You must have a change to an objects angular momentum to call it torque (Hackworth). Now, how do we apply torque to a motorcycle? There are many places torque is useful on a motorcycle. Spinning wheels, crankshafts, gears all possess angular momentum and require torque to be influenced on a motorcycle. We will focus here, again, on the two wheels of a motorcycle. Torque is required to change the direction, lean or speed of a spinning wheel. The greater the mass, radius and speed of the spinning wheel, the greater the angular momentum and the more torque will be needed to change the direction of the spinning wheels. Precession changes the orientation of the rotational axis of a rotating body. There are two kinds of precession when we are talking physics: torque-free and torque induced (Hackworth). We will focus on the later on our motorcycle. Torque applied to a rotational wheel along a perpendicular axis to that wheel, like when steering a motorcycle, has a gyroscopic effect influencing the movement.

Any inline two-wheel vehicle must lean to turn. There are two ways to get a motorcycle to turn. The first is for the rider to shift their weight to the fore-aft axis of the bike; the second is to use a technique called counter-steering. We will focus on the later. Counter steering derives its name from the act of using input (force) in the handlebars to initiate a directional change in the front wheel, but opposite the direction of the handlebar you are pushing on. Here torque and handlebar length have an affect in motorcycle design. A longer handlebar (lever-arm) reduces the force needed to begin the turn. The reason you push the opposite handlebar is, as the front wheel begins to turn away from the centerline (into the opposite direction of the turn), the bike will lean since the front and rear wheels are trying to travel in different directions (Hackworth). The bike will lean right.

Camber thrust assists the tire in continuing its turn, especially at slower speeds. The term camber thrust is used to describe part of the movement of a tire as it rolls. The roundness or flatness of the tire affects camber thrust. Camber thrust is the movement of a tire perpendicular to the direction the tire is rolling. The force acts in the direction of a tire’s sidewall. If your pushing the left handlebar down, the front tire has pressure left, the back tire will be traveling slightly right of the front tire and the bike leans right. The front tire will rotate to its right side, the camber, or curve of the tire will assist in the tire now laying on its right side, the bike will lean right and continue the turn to the right. Camber thrust is often a minor factor at higher speeds. With some tires at low speed camber thrust can have a dramatic effect. Tires with a rounded profile progressively increase the camber thrust with more of a lean angle. Tires with a less rounded profile with a sharp sidewall angle do not help much with camber thrust (Foale). Counter steering at slow speeds with round profile towers is much easier. Sportier motorcycle designs like those manufactured by the Honda Motor Company usually incorporate this tire profile.

Physics has a lot to say about collisions, and motorcycle riders feel the affects of a collision more readily than the drivers of automobiles. When talking physics, change in momentum, (p), is equal to the product of mass x velocity, or (p=mv). The force generated in a collision is equal to momentum change (high speeds to a stop is a larger change in momentum), during the collision, divided by the time interval of the collision. If force = (f), the equation would be F=P/T. By the way, momentum at the conclusion of the collision is generally close to zero. A combination of high pre-collision speed, and short collision time, results in an enormous value of collision force. Motorcycle helmets use layers of cushioning to extend the time interval over which the collision occurs, and reduce accelerations experienced by your brain in the collision. (Hackworth)  Due to the risk of Post-concussive Syndrome and Traumatic Brain Injury, a snug, well fitting helmet is crucial. Borrowing a friend’s larger helmet can result in two collisions occurring; the original impact and the second one, where your brain is hitting the cushion that is not snuggly against your skull.

So just touching on a portion of motorcycle physics, namely forces relating to stability and turning of spinning wheels and a big of motorcycle safety, incorporates many common terms found in physics: torque, gyroscopic effect, precession, force, velocity and mass to name a few. As we learn new concepts of physics found in the world around us, it is always interesting to apply them to areas we find interesting in our own lives.

Works Cited

Foale, Tony. "Tony Foale Designs, article on motorcycle tyre characteristics and how they work.." Tony Foale Designs, article on motorcycle tyre characteristics and how they work.. N.p., n.d. Web. 31 Oct. 2013. <http://www.tonyfoale.com/Articles/Tyres/TYRES.htm.

Hackworth, Martin. "MotorcycleJazz - Physics." MotorcycleJazz - Physics. N.p., n.d. Web. 1 Nov. 2013. <http://www.motorcyclejazz.com/motorcycle_physics.htm.

"How Everything Works - Question 1502." How Everything Works. N.p., n.d. Web. 30 Oct. 2013. <http://www.howeverythingworks.org/page1.php?QNum=1502.