A wheel flange is a round flat adapter hub that allows a wheel to be attached to an axle on a vehicle. The wheel is held tight to a wheel flange with nuts that thread onto lug bolts protruding from the mounting face of the flange. The lugs then protrude through tapered holes in the wheel as it is mounted to the flange. The nuts, being tapered themselves where they contact the wheel, serve to precisely center and lock the wheel. Wheel flanges and lug nuts are the customary mounting method for wheels on all motor vehicles.
In the case of drive wheels on motor vehicles, wheel flanges are mechanically locked to one end of the drive axles or spindles. Spindles are short axles that are specific to only one wheel, on one side of the vehicle, while an axle can, and usually does, involve more than one wheel, and both sides of the vehicle. The other end of the axle is connected to or is part of the drive line of the motor vehicle. This is either through a series of gears, belts, chains, or shafts. With driving wheels, the bearings that resolve the rotary motion of the wheel to the vehicle are between the vehicle and the axle.
Free spinning wheels that do not drive a vehicle usually have their wheel bearings located between the wheel flange and the axle. The axles do not turn, but the wheels and wheel flanges as one assembly turn on them. Most trailers, such as boat trailers or truck trailers, use free spinning wheels. Wheel flanges also function as universal mounting adapters, in that they allow the same type of tire and wheel to be used for either driving or free spinning positions. They will also work with a variety of vehicle strategies, except where proprietary wheel flange designs, such as between car makers, restrict this freedom.
Modern braking systems place an individual disk or drum brake at each wheel of the vehicle to provide the best stopping performance. Many brake drums or disks are sandwiched between wheel flange and the wheel, and held tightly between them when the lug nuts are tightened. The stopping torque of the brake is transferred to the wheel through the circle of lugs. Other motor vehicle manufacturers integrate the wheel flange function into the brake disc. This allows either a drive spline or bearing hub to complete the final assembly and define the ultimate application as either a driving or free spinning wheel, respectively.
In a different context, a wheel flange can also refer to the beveled flat surfaces on the inboard side of train wheels that keep each axle centered on railroad tracks. The flanges contact the inner surface of each track. The flange on the right wheel would keep the train car from sliding to the right, and the flange on the left wheel would keep the train from sliding to the left. Similarly, wheel flanges can keep wheeled conveyors in industrial plants on their respective rails as they course their way through many turns around a building.