Glossary of Wheel Terms

Backspace:  The distance in inches from the edge of the rear flange to the mounting pad.    


Bead Seat:  The point where the tire mounts and seals against the rim. 


Bolt Pattern:  Sometimes referred to as the PCD (Pitch Circle Diameter), the bolt pattern refers to the number of lug holes and the diameter of the imaginary circle they create. 


Cast Wheels:  Produced by pouring molten aluminum into a mold.  Cast wheels are cost effective, lightweight and strong.  Most wheels sold today are cast aluminum. 


Center Bore:  The opening that allows the wheel to fit on the axle. When you buy aftermarket wheels, you must ensure that the center bore is at least the size of the OEM wheel.  


Center Cap:  The removable part in the center of the wheel’s design that covers the vehicle’s hub.  Some center caps cover the hub and the lug nuts as part of the style. 


Centerline:  The centerline is the imaginary line midway between the bead seats.  The centerline is the reference point from where offset is measured. 


Drop Well:  The lowered area where the tire bead drops during tire mounting. Having this drop makes it easier to maneuver the tire during mounting.  It is the smallest inside diameter of the barrel of the wheel. 


Dual Drill:  Also called Dual Fit, some wheels have more than one bolt pattern.  This allows a wheel to fit more vehicle applications.  


Dually:  Some trucks have dual rear wheels to increase safety and stability when towing.  These trucks are referred to as a dually or DRW (dual rear wheel). 


Forged Alloy Wheels:  Made from heated billet alloy. Pressurized machines are used to shape blank forgings from the hot material.  This process creates a dense, strong and light wheel. 


Flow Form Wheels:  Produced by combining the casting process with a stretching process.  High pressure rollers are used on cast material to stretch and form the barrel of the wheel.  This process reduces overall weight and the heat applied to create the barrel makes a stronger wheel. 


Front Mount Wheel:  A wheel where the drop center is close to the front face of the wheel.  The tire is mounted with the wheel facing up on the mounting machine.  


Hub-centric:  Refers to the fitment on the vehicle.  The center bore of the wheel matches the diameter of the vehicle hub which fits snugly and provides support along with the lugs. 


Lug-centric Refers to the fitment on the vehicle.  The wheels are centered on the hub using only the lugs with no support from the hub.  Most aftermarket wheels are lug-centric.  


Lug Holes:  The openings where the lug nuts are inserted and fastened to the vehicle studs during wheel installation. 


Lug Nuts:  The accessories used to fasten the wheel to the vehicle’s wheel studs. 


Monoblock Wheel:  A wheel made from a single forging. 


Mounting Pad:  The flat surface on the back side of the wheel which contains the holes for bolts which tighten the wheel to the car's axle pad. 


Negative Offset:  The mounting pad is behind the wheel centerline (towards the vehicle). 


Offset:  The distance in millimeters from the wheel’s imaginary centerline to the mounting pad. 


Positive Offset:  The mounting pad is forward of the centerline (towards the street). 


Reverse Mount Wheel:  A wheel where the drop enter is close to the back face of the wheel.  The tire is mounted with the wheel facing down on the mounting machine. 


Rim:  Also known as the barrel, the rim is the section of the wheel that holds the tire and air. 


Rim Width:  The width of the wheel as measured from the front bead seat to the rear bead seat. 


Rim Diameter:  Rim diameter is the distance from the base of one flange to the base of the opposite flange.  The measurement does not include the raised portion of either flange. 


Safety Hump:  The safety hump provides support to the tire’s mounting bead in the event of a loss of air pressure. Without it tires may not be able to provide adequate vehicle support for handling and control. 


Staggered Offset:  Refers to running two different offsets on the same vehicle.  This is typically required when running a wider wheel in the rear of the vehicle. 


Steel Wheels:  Produced using a steel rim and a stamped steel center.  They are heavy, offer limited styling options and are prone to rust. 


Three Piece Wheel:  A wheel made from three components.  The center component is attached to two rim components (an inner and an outer). 


TPMS:  Short for "Tire Pressure Monitoring System", these valve stems are part of an electronic system in your tire that monitors the pressure in your tires.  It activates a warning light on your dashboard to let you know if your tires are over or underinflated. 


Two Piece Wheel:  A wheel made from two components.  The center component is attached to a single rim component. 


Zero Offset:  The mounting pad is on the wheel centerline. 

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