Air filled tires are known as pneumatic tires, & these are the type in almost
universal use today. Pneumatic tires are made of a flexible elastomer material
such as rubber with reinforcing threads/wires inside the elastomer material. The
air compresses as the wheel goes over a bump & acts as a shock absorber. Tires
are inflated through a valve, typically a Schrader valve on automobiles & most
bicycle tires, or a Presta valve on high performance bicycles. Attempts have
been made to make various types of solid tire but none has so far met with much
success. The air in conventional pneumatic tires acts as a near constant rate
spring because the decrease in the tire's volume as the tire compresses over a
bump is minimal. "Airless" tires usually employ a type of foam or sponge like
construction which consists of a large number of small air filled cells. As a
result, compression is localised within the tire & the effective spring rate
rises sharply as the tire compresses. The result is a tire which is less
forgiving, particularly with regards to sharp transient bumps & provides poor
ride & handling characteristics. The "steering feel" of such tires is also
different from that of pneumatic tires, as their solidity does not allow the
amount of torsion that exists in the carcass of a pneumatic tire under steering
forces, & the resultant sensory feedback through the steering apparatus; as a
result they feel as if they are pivoting on bearings at the contact point. They
are more popular for bicycles than for automobiles, which have tires which are
much more robust & immune to puncture.
Motor vehicle Pneumatic tire is mounted around a steel or aluminum alloy wheel
at service stations or repair shops for vehicles using a special tire mounting
apparatus while the wheel is off the vehicle. After mounting, the tire is
inflated (pressurized) with air through the valve stem to manufacturer's
specified pressure, which is more than atmospheric pressure. The wheel & tire
assembly are then attached to the vehicle through a number of holes in the wheel
using lug nuts. Because tires are often not made with perfectly even mass all
around the tire, a special tire-balancing apparatus at a repair shop spins the
wheel with the tire to determine where small weights should be attached to the
outer edge of the rim to balance out the wheel. Such tire balancing with these
kind of weights avoids vibration when the vehicle is driven at higher speeds.
With the introduction of radial tires, however, it was found that some
vibrations could not be cured by adding balance weights. This was because the
structure & manufacture of a radial tire lends itself to the problems of
variation in stiffnes around the tire. These variations are measured as Radial
Force Variation & Lateral Force Variation, which are measured on a Force
Variation Machine at the end of the manufacturing process. Tires outside the
specified limits for RFV & LFV are rejected. This is known in general throughout
the industry as Tire Uniformity.
