Wheels must be strong enough to support the vehicle, and withstand the forces caused by normal operation. At the same time, they must be as light as possible, to help keep unsprung weight to a minimum.
Wheels can be made from pressed steel, in 2 sections - the flange or disc that is drilled for the wheel fasteners, and the rim.
Wheels can also be made from cast aluminum alloy. Alloy wheels are popular because of their appearance and because they are lighter than similar steel wheels. Aluminum is a better conductor of heat, so alloy wheels can dissipate heat from brakes and tires more effectively than steel ones. These are often called mag or magnesium wheels, but wheels made of magnesium are rarely used on road vehicles.
Most wheels have ventilation holes in the flange, so air can circulate to the brakes.
Most passenger car wheels are of well, or drop-centre design. This design allows for tire removal and fitting.
Tires for heavy goods vehicles are much stiffer but they can also be fitted to a well-based rim or to a flat-base detachable flange rim, depending on construction
To ensure correct fit between a tire and rim, all manufacturers of wheels and tires comply with standard dimensions, as recommended by tire and rim associations.
The width of the rim 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.
The shape of well-based rims is provided by a letter code, such as J, K, JJ, and KK.
The width of the rim and the diameter is traditionally stated in inches.
A rim designated 7 JJ by 14 would refer to a rim measuring 7 inches across the rim flanges, and 14 inches in diameter from bead seat to bead seat, with the profile conforming to a JJ code.
The rim width and diameter can also be stated in millimeters. Metric rims are not interchangeable with Imperial rims.
The tire must be an exact fit on the rim, to fulfill a number of functions.
- It ensures that the narrow contact area between the beads of the tire and the rim will seal the air in a tubeless tire.
- It transfers all the forces between the tire and the wheel, without slipping or chafing.
- It ensures the friction between the tire and the rim prevents the tire turning on the rim.
Wheels must be strong enough to carry the mass of the vehicle, and withstand the forces that are generated during use.
Some are made from steel. They are pressed in 2 sections - the wheel center, with a flange or disc that is drilled for the wheel fasteners, and the rim. They are then welded together.
Others are made from cast aluminum alloy. Alloy wheels are lighter than similar steel wheels, and since aluminum is a better heat conductor than steel, alloy wheels dissipate heat from brakes and tires more quickly than steel wheels.
The wheel center must accurately locate the wheel rim centrally on the axle. It must also provide the required distance from the centerline of the wheel, to the face of the mounting flange. This is called offset.
Offset is important because it brings the tire centerline into close alignment with the larger inner hub bearing, and reduces load on the stub axle. This allows the inside of the wheel center to be shaped to provide space for the brake assembly, usually located inside the wheel.
Ventilation slots allow air to circulate around the brakes.
In some cases wheels are directional to assist the airflow.
The rim must be accurately shaped, and dimensioned, and strong enough to support the tire under the load of the vehicle and the forces generated by the motion of the vehicle.
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.
When inflated, the tire is locked to the rim by tapering the bead seat towards the flange, or by safety ridges or humps, close to the flange.
In the event of sudden deflation, or blowout, safety ridges prevent the tire moving down into the well. This helps maintain control of the vehicle while it is being braked.
Well-based rims can also be used on heavy commercial vehicles for tubeless tires.
The rims are referred to as 15-degree drop-center rims, because the bead seats are inclined at 15 degrees towards the flange.
The taper gives a good grip, and an airtight seal between the tire beads, and the rim.
The low flanges and drop-center allow the special size, flexible, tubeless truck tires to be mounted and demounted in a similar manner to that used on smaller passenger car tires.
The stiff sidewalls of larger cross-ply tires mean they cannot be mounted and demounted in this way, and many 4-wheel-drive and commercial vehicles use a flat-base, demountable flange rim.
When all of the air is removed from the tire, one flange can be removed so the tire can be demounted.
Wheels are fastened to the hubs by wheel studs and nuts.
They’re highly stressed by loads from the weight of the vehicles, and the forces generated by its motion, and they’re made from heat-treated, high-grade alloy-steel.
The threads between the studs and nuts are close fitting and accurately-sized.
All wheel nuts must be tightened to the correct torque, otherwise the wheel could break free from the hub.
