Axle Pad or Hub Flange: The area of a wheel where it contacts a car.
Bolt pattern or lug pattern or bolt circle: These refer to the number of fasteners (lug studs) and diameter of the circle on which they are positioned. (5x4.5” would be “five by four and one-half.”)
Barrel: The outermost part of a wheel. It is the area where a tire is mounted.
Center Cap: The wheel part that covers the center bore of the wheel on the outer face of the wheel.
Center Disc: The portion of a wheel that consists of the axle mounting pad and the spokes.
Drop Center: Is the smallest inside diameter area of the outer barrel of a wheel. It is used to facilitate the installation of a tire onto a wheel. Additionally, it is used as an area for placing a center disc for some 2-piece wheels during the manufacturing process.
Back Space or Rear Space: The distance from the axle pad to the inner edge plane (inboard flange).
Front Space: The distance from the axle pad to the outer edge plane (outboard flange).
Hub Diameter or Center Bore: This is the size of the hole in the center of a wheel.
Offset: The distance +/- from wheel centerline, indicated in millimeters.
Negative Offset: When the axle pad of the wheel is closer to the inner (car underside) edge plane.
Positive Offset: When the axle pad of the wheel is closer to the outer (street side) edge plane.
Zero Offset: A situation when both the back spacing and front spacing are equal. The axle flange is centered between the outer edge plane and inner edge plane.
Wheel Stud or Lug Stud or Axle Bolt: The “BOLT” portion of the wheel-fastening device. It is what the “lug nut” threads onto.
Lug Nut or Wheel Nut: The “NUT” portion of the wheel-fastening device. It treads onto a lug stud.
Tire Mounting Size: The potion of the wheel that reflects what size tire it can accomidate. Not the overall size of the wheel. Usually used when talking about a wheel size. (15x7”, 16x8”, 17x7”, Etc.)