Chapter 3.
TWO WHEELED PHYSICS

Now let's take a look at the correct way to ride a single-track, tandem-wheeled vehicle. For 100 years, this is how pedal- and motor-driven bicycles have mysteriously worked. Engineering experts have long enjoyed insight into this phenomenon. It's time for the American public to learn this valuable secret, too.

CAMBER THRUST:
GETTING SOMETHING FOR NOTHING

The most beautiful thing we can experience is the mysterious. It is the source of all true art and science. —Albert Einstein

As everyone knows, motorcycles lean into turns, using rounded-tread tires, while automobiles stay relatively flat in turns, with flat-treaded tires. This obvious difference also means that two entirely different principles apply to their operations. "Camber thrust" is a side force due to the tire leaning against the asphalt, with the turning force compressing the tire into the ground. This simulates what happens when a vehicle is traveling through a positive camber curve, and explains why a motorcycle's suspension tends to compress while turning. In other words, any turning motorcycle is capable of generating a downward g-force in excess of 1.5 times the pull of gravity, while a typical passenger will maintain1g of downward force while cornering.

A cone will roll in a circle. A leaned over tire will rotate in a circular motion just exactly like a cone will.

When turning, camber thrust is what produces the majority of the tire's sidewall friction against the roadway for two wheeled vehicles, while "slip angle" determines the majority of tire tread friction for automobiles. Camber thrust is like a pencil eraser that is tilted away from the direction it is being pushed on a flat surface—giving maximum resistance, while slip angle is like standing in tennis shoes on a flat surface while trying to do "the twist." Motorcycle tires are round and operate at ultra-high negative camber angles ("good" camber), while car tires are flat on the bottom. Outward centrifugal forces try and 'tip over' the flat tires forcing those tires to work at positive camber angles ("bad" camber). On a motorcycle at highway speeds, maximum cornering force can be generated even when the front wheel is pointed virtually straight ahead, rather than when it is turned. Lean angle takes the place of steering effort, and since the inner edge of the tire has a lesser diameter than the center of the tire, it also has lesser circumference, therefore it will carry you less distance down the roadway for each rotation of the tire than if you were traveling upright, on the center portion of the tire. This causes the bike to turn into the direction of the lean, even though the front steering wheel is not turned. In the illustration of the rolling cone above shows how the the flat surface of the cone may be pointed straight ahead, but when rolling, it turns in a circle. This is why the surface of a motorcycle tire feel can feel "powdery" after a series of turns, unlike a car tire which just heats up.

The photos below illustrate the difference between two-wheel turning and four-wheel turning.

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Notice the motorcycle in a right hand turn has tires tilted to the right and even as the tire is pointed straight ahead, the motorcycle is turning to the right. The rounded tires on the motorcycle allow the G forces created by the bike's curving motion to be transferred downward, pushing the rider downward on the seat, compressing the suspension, pushing the tires harder into the surface of the road and increasing the friction of the tires on the asphalt.


By contrast, the car in a right hand turn has tires which are straight up and down, or slightly tilted out to the left, and the front tires are pointed in the direction of the turn. All the G forces created by the curving motion of the automobile are pulling the car outward toward the left side of the road.

A myth exists that leaning a motorcycle reduces the cornering grip (friction) of the tires. Notice that the mass of the vehicle does not change—if it weighed 600 lbs. on a straightaway, it still weighs 600 lbs. in a curve. However, gravity works in corners to help us out. Friction is actually increased when g-forces come into play. Camber thrust and total grip is also affected by centrifugal force as the motorcycle leans when going around a corner. Cornering causes centrifugal force to press the tires downward into the asphalt, compressing both the front and rear suspension springs, reducing ground clearance. Although reduced ground clearance can possibly cause the foot peg (or some other part) to scrape the ground. As long as there is tire tread touching the ground, without some other part of the bike disrupting tire contact, then the motorbike will still produce full cornering grip. The harder a rubber tire is squished into the ground, the more grip it produces. Camber thrust literally compresses the motorcycle tire as the rubber tries to roll under the wheel rim. Cars can't do this trick since the inside tires lose as much grip as the outside tires gain. Traditional car suspensions lose camber thrust completely in turns. The faster a motorbike rider goes through the same turn, the more grip he has to work with (until the bike drags). At a 45 degree lean angle, a motorcycle has nearly 50% more "weight" pressing the tires into the pavement, and thus benefits from nearly 50% more grip than it does when it is vertical, thanks to centrifugal force and camber thrust.

Obviously, motorcycles do not behave in an identical manner when compared to an automobile. It is a myth that motorcycles are initially steered by turning "normally" (steer left to turn left) and shifting weight toward the turn (leaning the body) to get the bike to initially lean into a curve. How can a motorcycle be steered in the same direction as a car when it leans in the opposite direction compared to a car? Well, it just can't be.

Return to Home Page

Chapter 1. Let's Look at Some Data

Chapter 2. Risk Management

Chapter 3. Two Wheeled Physics

Chapter 4. Countersteering: Cornering Techniques

Chapter 5. Gravity Is a Good Thing

Chapter 6. Gyroscopic Precession: Nature's Power Steering

Chapter 7. Braking: Weight Transfer and Maximum Performance

Chapter 8. Controlling Slides and Tank Slappers: Mind Over Matter

Chapter 9. Group Riding

Chapter 10. Riding Etiquette

Chapter 11. MSF Courses- Editorial