Speed 101: Motorcycle Racing as Real-World Physics Lab

A grand prix racing motorcycle is many things: most impressively, a marvel of engineering that costs hundreds of thousands of dollars to develop and build, and one of the fastest machines on wheels, capable of speeds in excess of 210 miles per hour and able to retain a grip on the road at lean angles of 60 degrees or more.

But looked at scientifically, a racing bike is nothing less than a kinetic demonstration of the laws of physics. Freddie Spencer, a legendary grand prix champion of the eighties and now "dean" of Freddie Spencer's High Performance Riding School [1] in Las Vegas, puts it this way: "Motorcycle racing is a real-world physics lab where the penalty for wrong answers is a lot more dramatic than a bad grade."

1. Gravity:

The rider shifts weight into the turn to help the motorcycle change direction and lower its center of gravity.

2. Kinetic Energy:

At speed on a straightaway, a motorcycle’s energy is directed forward.

3. First Law of Motion:

Newton stated that a body in motion persists in a straight line unless compelled to change.

4. Thermodynamics:

Slowing the motorcycle from high speed for tight turns causes heat buildup in its brakes and can diminish effectiveness.

5. Centrifugal Force:

In fast turns, lean angle and forward motion counteract the powerful pull toward the outer edge of the track.

6. Friction:

A special compound in these rounded tires allows traction on asphalt even at lean angles of 60 degrees and more.

Credit: Fiat Yamaha Team

According to Charles Falco, the University of Arizona's chair of condensed-matter physics and co-curator of the Guggenheim Museum's The Art of the Motorcycle [2] exhibition, the initial physics lesson to be learned watching a racing bike hurtle into a tight turn is Newton's first law of motion: "Every object persists in its state of rest or uniform motion in a straight line unless it is compelled to change that state by forces impressed on it," explains Falco. To a rider, this means that the faster a motorcycle is going, the less it wants to turn.

Converting a bike's kinetic energy from straight ahead to turning requires a negotiation with physics in a couple of ways. First, a rider pushes the handlebars slightly away from the direction of the turn. Because the wheels act as gyroscopes, this countersteering leans the bike in the opposite direction (into the turn), which puts the tires at an angle, narrowing what engineers call the contact patch and making the bike easier to turn.

At the same time, the rider moves off the bike in the direction of the turn. The lean angle of the motorcycle shifts the center of gravity to the side, causing the bike to turn, while the weight redistribution lets the machine stay slightly more upright. At the point of maximum lean required to get through a turn at the highest possible speed, centrifugal force wants to pull the bike machine off the track, and the rider uses traction, gravity, and momentum to stay in the game.

To explain why the machine moves at all, Falco invokes Newton's second law of motion: A force applied to an object will cause it to accelerate. "This will happen until the rider runs out of track, or other forces become nonnegligible, such as wind resistance," says Falco.

On some tracks, grand prix motorcycles approaching tight turns must slow from more than 200 mph to around 40 mph. Friction on the brakes (primarily the front brakes) makes this possible. "All that excess energy has to be dissipated by the brakes in the form of heat," Falco says, thus bringing up the law of conservation of matter and energy. Some of this heat is transferred to the hydraulic-brake fluid, which can cause brakes to lose stopping power, with potentially disastrous consequences. Engineers use space age ceramic materials to avoid this problem, and riders become skilled at getting on and off the brakes quickly.

Successful race riding is a lot like paying taxes: You want to push the rules as far as you can without breaking them. There is a very fine line between optimum cornering and crashing, where outward, downward, and forward forces balance precisely. But rules are rules. "Speaking on behalf of physicists everywhere," Falco declares, "nothing that ever happens on a motorcycle breaks the laws of physics. In fact, motorcycles are excellent examples of just how well physics works."