SKATEBOARDING PHYSICS



Rolling




Rolling is a combination of rotation (of a radially symmetric object) and translation of that object with respect to a surface (either one or the other moves),
Such that the two are in contact with each other without sliding. This is achieved by a rotational speed at the cylinder or circle of contact which is equal to the translational speed.

Rolling of a round object typically requires less energy than sliding, therefore such an object will more easily move, if it experiences a force with a component along the surface, for instance gravity on a tilted surface;

  • wind
  • pushing
  • pulling
  • an engine

Objects with corners, such as dice, roll by successive rotations about the edge or corner which is in contact with the surface.
One of the most practical applications of rolling objects is the use of ball bearings in rotating devices. Made of a smooth metal substance, the spherical bearings are usually encased between two rings that can rotate independently of each other.
In most mechanisms, the inner ring is attached to a stationary shaft (or axle). Thus, while the inner ring is stationary, the outer ring is free to move with very little friction. This is the basis for which almost all motors (such as those found in ceiling fans, cars, drills, etc) rely on to operate. The amount of friction on the mechanism's parts depends on the quality of the ball bearings and how much lubrication is in the mechanism.

Rolling objects are also frequently used as tools for transportation.
One of the most basic ways is by placing a (usually flat) object on a series of lined-up rollers, or wheels.
The object on the wheels can be moved along them in a straight line, as long as the wheels are continuously replaced in the front (see history of bearings). This method of primitive transportation is efficient when no other electrical machinery is available. Today, the most practical application of objects on wheels are cars, trains, and other human transportation vehicles.

Rolling resistance



Rolling resistance, sometimes called rolling friction or rolling drag, is the resistance that occurs when a round object such as a ball or tire rolls on a surface.

It is caused by the deformation of the object, the deformation of the surface, or both. Additional contributing sources include surface adhesion and relative micro-sliding between the surface of contact.
It depends very much on the material of the wheel or tire and the sort of ground. Additional factors include wheel radius, and forward speed.

For example, rubber will give a bigger rolling resistance than steel.
Also, sand on the ground will give more rolling resistance than concrete.
A vehicle rolling will gradually slow down due to rolling resistance, but a train with steel wheels running on steel rails will roll much farther than a car or truck with rubber tires running on pavement.
The coefficient of rolling resistance is generally much smaller for tires or balls than the coefficient of sliding friction.

Primary Cause



The primary cause of rolling resistance is hysteresis:

Hysteresis -

  • A characteristic of a deformable material such that the energy of deformation is greater than the energy of recovery.
  • The rubber compound in a tire exhibits hysteresis.
  • As the tire rotates under the weight of the vehicle, it experiences repeated cycles of deformation and recovery, and it dissipates the hysteresis energy loss as heat.
  • Hysteresis is the main cause of energy loss associated with rolling resistance and is attributed to the viscoelastic characteristics of the rubber.

Factors that contribute



Several factors affect the magnitude of rolling resistance a tire generates:

Material -

  • different fillers and polymers in tire composition can improve traction while reducing hysteresis.
  • The replacement of some carbon black with higher-priced silica–silane is one common way of reducing rolling resistance.

Dimensions -

  • rolling resistance is related to the flex of sidewalls and the contact area of the tire For example,
  • at the same pressure wider bicycle tires have less flex in sidewalls and thus lower rolling resistance (although higher air resistance).

Extent of inflation -

  • Lower pressure in tires results in more flexing of sidewalls and higher rolling resistance.
  • This energy conversion in the sidewalls increases resistance and can also lead to overheating and may have played a part in the infamous Ford Explorer rollover accidents.
  • Over inflating tires (such a bicycle tires) may not lower the overall rolling resistance as the tire may skip and hop over the road surface.
  • Traction is sacrificed, and overall rolling friction may not be reduced as the wheel rotational speed changes and slippage increases.

Sidewall deflection is not a direct measurement of rolling friction.

A high quality tire with a high quality (and supple) casing will allow for more flex per energy loss than a cheap tire with a stiff sidewall. Again, on a bicycle, a quality tire with a supple casing will still roll easier than a cheap tire with a stiff casing. Similarly, as noted by Goodyear truck tires, a tire with a fuel saving casing will benefit the fuel economy through many casing lives (i.e. retreading), while a tire with a fuel saving tread design will only benefit until the tread wears down.

Tread thickness has much to do with rolling resistance. The thicker the tread, the higher the rolling resistance Thus, the fastest bicycle tires have very little tread and heavy duty trucks get the best fuel economy as the tire tread wears out.
Hard steel rails last longer but may also have lower static friction. They may also suffer fatigue cracking because the cracked area is not worn away by the passing trains.

Smaller wheels,
all else being equal, have higher rolling resistance than larger wheels in theory.
In some laboratory tests, smaller wheels appeared to have similar or lower losses than large wheels,but these tests were done rolling the wheels against a small-diameter drum, which would theoretically remove the advantage of large-diameter wheels, thus making the tests irrelevant for resolving this issue.
Virtually all world speed records have been set on relatively narrow wheels, probably because of their aerodynamic advantage at high speed, which is much less important at normal speeds.

Effects



Rolling friction generates heat and sound energy, as mechanical energy is converted to these forms of energy due to the friction.
One of the most common examples of rolling friction is the movement of motor vehicle tires on a roadway, a process which generates sound and heat as by-products.

The sound generated by automobile and truck tires as they roll (especially noticeable at highway speeds) is mostly due to the compression (and subsequent decompression) of air temporarily captured within the tire treads.
The heat generated raises the temperature of the frictional surface; moreover, this temperature increase typically increases the coefficient of friction itself.
This is why automobile racing teams preheat their tires.