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# Speed 101: Motorcycle Racing as Real-World Physics Lab

Isaac Newton hops aboard a two-wheeled teaching tool.
Owen Edwards
Related Tags: Project-Based Learning

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 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 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."

###### Owen Edwards is a contributing editor for Edutopia and Smithsonian magazines.

Anonymous (not verified)

Corrections, once and for all, folks:

Centrifugal Force (Center-Fleeing) - The article is correct. Yes, the one that throws you from the merry-go-round. Spin a ball on a string around your head. It is the force that pushes the ball outward, keeping it taught on the string. In motorcycling, the force that wants to push you wide in a turn.

Centripetal Force (Center-Seeking) - The force that keeps you from flying off the merry-go-round - in other words, the horse and its pole connected to the deck. In the spinning ball example, it is the string. In motorcycling, it is the rubber in the tire sticking to the asphalt - preventing your motorcycle from reeling into the run-off.

http://en.wikipedia.org/wiki/Centripetal_force
http://en.wikipedia.org/wiki/Centrifugal_force_(fictitious)
http://en.wikipedia.org/wiki/Reactive_centrifugal_force - Very good explanation using a diagram with a car going around a turn. Centrifugal pushes you into the door. Centripetal is the tires sticking to the road, preventing the car from skidding into the woods.

Anonymous (not verified)

Which kind of wheel would be ultimately better for a motorcycle ?

In my mind the lighter the wheel the better (acceleration,braking,cornering) and for suspension too. If carbon fiber is 1/2 the mass of the forged, that would make it a superior hardware for a bike.

But some say it's too light, and not suitable for a motorcycle handling and cornering.
Reason is that it would not create enough gyroscopic effect and make the motorcycle unstable in a turn.

Am i crazy or is that person doesn't know what they are talking about?

Anonymous (not verified)

Which kind of wheel would be ultimately better for a motorcycle ?

In my mind the lighter the wheel the better (acceleration,braking,cornering) and for suspension too. If carbon fiber is 1/2 the mass of the forged, that would make it a superior hardware for a bike.

But some say it's too light, and not suitable for a motorcycle handling and cornering.
Reason is that it would not create enough gyroscopic effect and make the motorcycle unstable in a turn.

Am i crazy or is that person doesn't know what they are talking about?

Anonymous (not verified)

I'm a teacher and a former bicycle racer.

When cornering on a bicycle or moto, you have a number of different forces working on you and your point of contact.

First, terms to be familiar with:

motion-changing your position causes changes in inertia and vectors.

inertia-the tendency of a body of mass to remain in its current state (at rest, or in motion).

vector-the direction you (or other bodies of mass) are being moved.

force-the amount of energy pushing you (mass) in any given vector.

Gravity holds you down, but faster cornering pulls (or pushes) you across the road surface at an angle tangential to the direction you're aiming your wheel. This is why going around a corner too fast in a car will cause it to spin out. Why many sports cars are mid-engine to keep the center of mass between the axles. Why the Corvair was taken off the roads-it's rear engine skidded out and caused accidents. It's also why you have banking on race tracks. Velodromes (bicycle tracks) in some places have banking up to 30o incline.

What else helps hold the road? Tyre pressure. Contact area. Rubber compound (softer's better). Weight. Suppleness (A tyre with lots of fine threads per inch performs better than one with fewer, thicker threads, even if the second one weighs less).

Short answer: The faster you go, the more force pushing you out in comparison to the gravity pulling you down and creating friction with the road surface. If you're talking about dirt (I only raced road, never MTN) I would think fast would be even worse since the dirt's coefficient of friction would be lower as the particles of dirt move against each other, too.

Wow, long answer. I hope it gives you several angles to approach your conversation.

Anonymous (not verified)

I know this is not applicable to sportbikes, but what about the case of taking a corner on a motorcycle with handlebars at shoulder height vs. taking the same corner at the same speed on the same bike but with tall ape hangers.

I understand your comment about a longer handlebar acting as a lever, but in the case of cornering a bike with ape hangers, does the longer handlebars elongate the rider and push his/her center of gravity (point of maximized force against centrifugal force) so high above the center of gravity of the bike (and centrifugal force forcing the bike over) that the longer bars work against them?

What do you think? And thanks in advance for any help.

Anonymous (not verified)

The wiki was not a reference source but rather a definition reference.

Its been a long time since I needed to discuss science with anyone but I think the references in these replies to cintrifugal force are not being used properly. Cintrifugal force (real or not- gimme another term if you dont like that one) says that a spinning object tends to stay upright the faster it spins. This applies to motorcycles leaning but not to merry go rounds flinging you off. As ANY motorcylist knows the slower you go the more balance is required to keep the motorcyle upright. The faster you go the more effort it takes to force the motorcyle to lean into a turn. Wich is why techniques like body shift and counter steering help to overcome that force (cintrifugal or WHATEVER you want to call it) that is keeping the motorcyle upright because of the spinning of the wheels.

This also why you can fall or jump off of a motorcycle or bycle and it keeps on going upright on it's own with no rider on it. It's also why it's easy to ride with no hands. Just takes a bit of faith to let the controls go. Cintrifugal force or not, it (something) works!

Anonymous (not verified)

It seems like some very distinct things are being mashed together. In a hope to help clarify, I'd offer the following:

1. There is usually no force throwing you off a circular track. The reason you are on a circular path is usually due to a combination of your inertia or acceleration along a straight line, and some force pushing or pulling you off that straight path. This force could be you holding on to a merry-go-round or your tires pushing against the ground, which by the 3rd law will push you back toward toward the center (centripital). If you break this force, your inertia will take over and send you on a tangential path out of the circle.

2. The other force that comes into play in the "light vs. heavy wheel" debate is the gyroscopic stabilization of a rotating body. All the old bikers I know will tell you that heavy wheels are for road bikes and light wheel are for drag bikes. Heavy wheels tend to help hold you steady on the road and light wheels have less rotational inertia to allow for better sprinting speed. Since drag bikes don't do a lot of cornering, they don't need the extra stabilization.

I hope this adds clarity rather than more confusion, but if not, we'll keep going til we get there...

Anonymous (not verified)

To put it short and sweet, the amount of "grip" would be thermal, and friction based.

Does your tire grip more in a high speed turn? Yes, but not in the same thinking.

The friction between your tire and the pavement heats up the tire, and causes the rubber to grip more. If you were to try a skid pad that is completely flat, that would be the only thing changing the amount of grip.

If you are in a banked turn, you get more grip because the lateral force is in fact pushing you into the pavement instead of just gravity, and increasing the amount of friction the tire is applying to the ground(or vice versa).

Hehe, my physics is being useful today!

Anonymous (not verified)

So im a biologist and remember some of the physics I had to take. Any way on this discussion forum people think that using a car tire (on a sport bike, well standard bike) will aid in cornering and stopping power. Now the little bit of physic I can remember has some problems with that.
1. tire composition
2. weight of motorcycle vs weight of car (divided by 4 of course) affecting the friction coefficient of rubber comp.
3. Structure of the tire, IE, sidewall construction and properties, thread patterns,
4. goes along with structure but what about deformation do to speed or force due to cornering.

Now being a scientist I want to know how and why this would work.. or what could be the physically limits to a car tire on a motorcycle (If you are familiar with bikes we are using the suzuski bandit 1200s as our base) Any insight you can give would be wonderful. I'll tell you right now that your input will not affect my decision to keep the proper H-preformance motorcycle tires on my bike.

motorcycle racing refers to the premier category of motorcycle road racing that employs modified production motorcycles,do we need to have special training for racing.