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.
