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Richard Barsotti

Jun. 23rd 2009


Rolling Friction: Fleeting Moments

A motorcycle, being a single-track vehicle, is not vertically balanced and only remains stable at low speeds due to rider input on the handlebars, and at higher speeds by the gyroscopic effect of both wheels--especially the front wheel. Whenever the wheels stop rolling for more than half of a second, the motorcycle will suddenly become unstable. When the tire skids the rider loses directional control because lateral forces generated by rolling friction between the tire and road surface no longer act.

Friction, like many common occurrences, upon close examination is surprisingly complex. While gas-liquid, liquid-solid friction are also very pertinent to motion, this article will be limited to dry, solid-solid friction. Mathematical descriptions of this type of friction come from experimental data that are difficult to reproduce. Unless these experiments are conducted under pristine, vacuum conditions, contaminants such as metal oxides, adsorbed gases, microscopic oil or grease films will cause erratic results.

An object at rest, sliding, or rolling on a surface experiences friction. The contact forces are represented by components exerted by the surface on the object. The perpendicular component is called the normal force from the Latin, norma for carpenter's square. The force component parallel to the surface is friction. Friction always opposes the relative motion of two objects in contact. Three types of friction; static (no relative motion), kinetic (a sliding object), and rolling (combination of static and kinetic) are approximately proportional to the magnitude of the normal force. Under certain conditions of equilibrium, namely, an object at rest or in motion at constant speed on a horizontal surface, the normal force is equal to the object's weight. Intutitive physics is at work here; heavy objects produce large friction forces.

Maximum static friction occurs just before a stationary object begins to slide. Once sliding begins, kinetic friction opposes its motion. Kinetic friction perfoms work on the object, slowing it. Some of this work is converted into thermal energy--the two surfaces become warmer as energy is dispersed to agitate and disorder their molecules. Fortunately, roller bearings, ball bearings, and wheels under ideal conditions do not experience kinetic friction. The surface momentarily in contact with a bearing or wheel produces static friction. However since static friction produces no thermal energy, all the work performed goes into motion. Bearings and wheels roll because of static friction; a fleeting adhesion between molecules until rotation peels them apart. Static friction thus exerts torques on wheels and bearings initiating rotation. Also under ideal conditions, the normal force resulting from this adhesion is aligned with the pivot point of the wheel or bearing, thereby producing no counter-torque and hence, no sliding or wheel hop occurs.

In reality deformations of the tire and surface spread the contact forces; there is no longer an idealized point of contact. The contact forces act over an area. These forces concentrate at the front of the rolling object. As a result, the normal force acting at the front edge of the wheel exerts a counter-torque that opposes rotation and initiates some sliding. Kinetic friction then dissipates work into thermal energy heating the surfaces. One must also consider the effects of the relative motion between the wheel's hub and axle. The axle is fixed, as the wheel turns, the inside of the hub rubs and slides against the outside of the axle. So the wheel must work against kinetic friction in the hub. Mechanical energy is converted to thermal energy; surfaces warm. The sliding also exerts a small counter-torque which slows the wheel's rotation. Of course ball or roller bearings placed between the hub and axle greatly reduce the conversion of work to thermal energy. However, bearings rotate in races or cages. The bearing is hardened, polished steel and close to being perfectly round. The race distorts at the contact point with the bearing. Elastic forces in the metal restore the race but after countless encounters between race and bearing, this restorative memory fades. In effect, at the molecular level, the race surface piles up in front of the bearing and it begins to ride in a shallow trough. The race's surface flakes off, eventually forming pits and grooves deep enough to reach softer metal and bearing failure. What has been described so far is freely turning wheel with axle bearings. The front wheel of a motorcycle being an obvious example.

The rear wheel of a bike is powered via the drivetrain by the engine. The torque produced by the engine has two final effects: rotate the wheel and ironically, its second effect always acts to make the wheel skid! It is only the static friction (of rotation) between the road and tire thread that opposes this skidding effect. If the engine's torque is too great or the road's static friction too weak, the wheel skids; kinetic friction produces heat, loss of power and control. Discuss friction further by joining

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