Wheels & Tires: 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 ratings.
	Radial ply tires	Radial ply tires use two 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	Run Flat technology or RFT tires enable 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	Manufacturers use a sophisticated selection process to create combinations that provide the required performance characteristics and 'hysteresis level' of the tire. Having fewer plies makes a tire more flexible. Higher numbers of plies make its response to bumps harsher.
	Hysteresis	Hysteresis is the energy lost when a section of vulcanized rubber is deformed in a regular manner.
	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. The term 'thrust line' refers to the direction in which the rear wheels are pointing.

Wheels & tires procedures	Using a tire pressure gauge	There are two main types of tire pressure gauges: fixed workshop gauges and portable pocket size gauges. The objective of this procedure is to show you how to use a tire pressure gauge and interpret the readings correctly.
	Checking & adjusting tire pressure	The tire pressures should be checked when the tires are cold. The objective of this procedure is to show you how to check and adjust tire pressures to correct levels. Check the owner's manual or the tire decal usually located on the driver's side door jam for correct pressures.
	Checking for tire wear patterns	The objective of this procedure is to show you how to check tire for correct wear pattern and ensure there are no embedded foreign objects in the tread. Remember to check the spare or emergency rim as well as the four fitted tires.
	Rotating tires	The objective of this procedure is to show you how to rotate tires to meet the manufacturer's requirements. Some tires are directional; they are designed to turn in one direction only. The majority of tires can be mounted and run in either direction.
	Removing a tire	An inflated tire is a pressure vessel that must be treated with care and respect. Always fully deflate the tire before performing any repair tasks. The objective of this procedure is to show you how to safely remove a vehicle tire from a rim.
	Fitting a tire	The objective of this procedure is to show you how to safely fit a vehicle tire to a rim. Always check with specifications on the tire machine you are using before proceeding with the mounting of the tire to avoid damage to the rim.
	Dynamic balancing a tire	The objective of this procedure is to show you how to dynamically balance a tire and wheel rim assembly. The first step is the prepare the vehicle and remove the wheels. Mark the wheel in relation to its location on the vehicle so you can replace it correctly.

Topic group	Topic	Summary