Wheels & Tires

Wheel types & sizes	Wheels	Wheels are usually made from pressed steel or cast aluminum alloy. They should be lightweight, yet strong enough to withstand normal operational forces.
	Rim sizes & designations	Rim width is the distance across the rim flanges at the bead seat. Its diameter is the distance across the center of the rim from bead seat to bead seat.
	Types of wheels	Passenger cars normally use rims which are of well based, or drop-center design. The drop-center is used for mounting and demounting the tire onto the rim.

Tire types & characteristics	Tires	Tires are mainly made from synthetic materials. They can be tubed or tubeless, with different types of construction, profile and speed rating.
	Radial ply tires	Radial ply tires use 2 or more layers of casing plies, with the cord loops running radially from bead to bead.
	Radial ply tire sidewalls	A tire-wheel assembly must be balanced. As the wheel rotates, centrifugal force acts outwards. Any heavier part will vibrate vertically, and slap the road surface with each wheel revolution.
	Tire pressure monitoring systems	Automated Tire Pressure Monitoring Systems or TPMS provide a means of reliable and continuous monitoring of the vehicle tire pressure and are designed to increase safety, decrease fuel consumption and improve vehicle performance.
	Runflat tires	The major safety benefit of Run Flat technology or RFT is that it enables a driver to maintain control if a vehicle in motion suffers a rapid loss of tire pressure.
	Space-saver tires	Space-saver spares are for emergency use only, they're designed to get you to a service center where you can have the regular tire fixed or buy a new one.
	Tire distortion	During cornering, tire distortion makes a vehicle move at an angle to the direction of the wheels, called slip angle. Cornering force returns the tire to normal position. As cornering force increases, so does slip angle.
	Center of gravity	The center of gravity is the balance point of the entire vehicle. Its position is determined by the load on the front and rear wheels. It is always above the road, and between the tires.

Tire construction	Tire construction	The bead is a rigid hoop of steel braid, with the inner free edges of the plies wrapped around it. It must be rigid, to keep the tire on the rim, but soft enough to seal the air in the tire.
	Types of tire construction	A cross-ply tire has a latticed, criss-crossed structure, with alternate plies crossing over each other, and laid with the cord angles in opposite directions.
	Tire materials	Having fewer plies makes a tire more flexible. Higher numbers of plies make its response to bumps harsher.
	Hysteresis	The more you subject a tire to flexing and deformation the more heat will build up within the tire.
	Tire sizes & designations	Aspect ratio is the ratio of a tire’s height to its width. The lower the aspect ratio, the wider the tire is in relation to its height.
	Tire information	Aspect ratio is included in the sidewall marking, with the type of construction, and the speed rating. Metric-diameter rims cannot be fitted with imperial-diameter tires, or vice-versa.
	Tire tread designs	Tire treads can be directional, non-directional, symmetric and asymmetric.
	Tire ratings for temperature & traction	A Uniform Tire Quality Grading or UTQG grade provides information on tread wear, traction and temperature.

Wheel alignment fundamentals	Basic principles of wheel alignment	If wheels are properly aligned, the steering wheel will tend to stay in a straight-ahead position on straight, level roads. Similarly, little effort should be needed to turn the vehicle into curves.
	Caster	Caster is the angle between the steering axis centerline, and a vertical line through the center of the wheel. Backward tilt from the vertical line is positive caster. Forward tilt is negative caster.
	Camber	Viewed from vehicle front, camber is the angle of tilt of the wheel from the vertical. A wheel leaning away from the vehicle at the top has positive camber. Leaning towards the vehicle, it has negative camber.
	Scrub radius	The effect of scrub radius is to provide a turning moment which attempts to turn the wheel away from the central position, when the vehicle is in motion.
	Steering axis inclination	As the wheel assembly swivels, it turns around the steering axis, on a line drawn through the upper and lower pivot points of the suspension assembly. Seen from the front of the car, it tilts inward.
	Toe-in & toe out	Toe angles ensure that wheels are in line with the direction of travel. In the straight-ahead position, the wheels may have toe-in or toe-out, depending on manufacturer’s specifications.
	Toe-out on turns	Toe-out on turns is the relative toe setting of the front wheels as they turn to the left or right.
	Turning radius	The turning radius is a measure of the vehicles turning circle when the steering wheel is turned to its limit.
	Thrust angle & centerlines	The thrust angle refers to all four wheels and their relationship to each other and to an imaginary centerline that runs from each pair of wheels down the center of the vehicle.

Wheels & tires procedures	Using a tire pressure gauge	Pocket type tire pressure gauges are inexpensive and more accurate than the gauges provided by service stations. Service station gauges are often damaged by weather, misuse or being run over.
	Checking & adjusting tire pressure	Tires get hotter as they are being driven. Driving on under-inflated tires can cause serious damage to the tires.
	Checking for tire wear patterns	Some manufacturers supply an emergency rim assembly instead of a full-size spare.
	Rotating tires	The majority of tires can be mounted and run in either direction.
	Removing a tire	Keep your hands clear of the bead breaker when it is operating. It applies great force to the tire that will cause you a severe injury if your hand is trapped.
	Fitting a tire	Some tires are directional. That is, they are designed to turn in one direction only. An arrow on the sidewall indicates the direction of rotation.
	Dynamic balancing a tire	Embedded objects may fly off the tire when it is spinning on the wheel balancer, so wear safety glasses.