A wheel is made up of a rim and center member, known as a disc or spider. The rim supports the tire and the spider (disc) connects the vehicle with the rim.
Wheels are usually of two types-the Drop Center (DC) and the Semi-Drop Center (SDC). Drop center wheels are used on all cars and light trucks; semi-drop center wheels are usually only used with large multi-ply, heavy-duty tires on over-the-road trucks. The SDC wheel has a removable outer ring that allows easier installation and higher inflation pressure. Above six plys, tires would be extremely rigid in the bead and be very difficult to mount on a single-piece wheel without damaging the bead.
Most passenger vehicle wheels fall into two types. The all-steel wheel is the type found on many vehicles as original equipment from the factory. Custom or "MAG" wheels were named for their resemblance to magnesium racing wheels. True magnesium wheels are too porous to hold the air pressure of a street tire and never should be used on the street. Custom wheels are a cast aluminum alloy, a steel rim with cast aluminum alloy spider or a two-piece steel wheel.
|Fig. 1: Two types of wheel construction. The semi-drop center has removable flanges and does not need the severe drop in the center of the rim. These wheels are used on heavy equipment.|