I. Wheel & Tire fitment calculator
II. Tire Stretch Database
III. Wheel size
Look inside almost any wheel and you should find various markings which give you pertinent information on the wheel. Just for conversation sake, we will take into consideration a wheel which is marked "18x8J ET38". The first number is quite obvious and represents that the wheel is 18 inches in diameter, while "8" is the width of the wheel and measured from the distance between the flanges that support the bead. The "J" refers to the shape of that flange. Easier to understand if you imagine a steel wheel on which the lip is rolled over like a J. "ET" is the abbreviation of einpress tief, German that translates literally as "pushed in depth", or offset. The "38" is the offset measurement "38 mm".
IV. Bolt Pattern (PCD)
The P.C.D. (Pitch Circle Diameter), often referred to as the "bolt pattern", refers to the pitch circle diameter of the bolt holes. It is the diameter of an imaginary circle formed by the centers of the wheel lugs/bolts. The number of bolts depending on the vehicle type, usually four, five or six for passenger cars. Some aluminum wheels available on the market are multi-hole wheels, such as the nine-hole type that has four plus five bolt holes. A bolt circle of 4x100 would indicate a four –lug pattern on a circle with a diameter of 100mm.
V. Definition of offset
The offset of a wheel is the distance from its hub mounting surface to the centerline of the wheel. The offset can be one of three types (measured in millimeters).*
The hub mounting surface is even with the centerline of the wheel.
The hub mounting surface is toward the front or wheel side of the wheel. Positive offset wheels are generally found on front wheel drive cars and newer rear drive cars.
The hub mounting surface is toward the back or brake side of the wheels centerline. "Deep dish" wheels are typically a negative offset.
If the offset of the wheel is not correct for the car, the handling can be adversely affected. When the width of the wheel changes, the offset also changes numerically. If the offset were to stay the same while you added width, the additional width would be split evenly between the inside and outside. For most cars, this won't work correctly. We have test fitted thousands of different vehicles for proper fitment. Our extensive database allows our sales staff to offer you the perfect fit for your vehicle.
*Backspacing, similar to offset, is the distance from the hub mounting surface to the inside lip of the wheel (measured in inches).
VI. Center Bore
The other element that affects directly whether a wheel can be bolted onto a car is hubcentricity. The centerbore of a wheel is the size of the hole in the back of the wheel that centers over the mounting hub of the car. Factory wheels are all machined to fit their specific application exactly, and some of the better after-market wheels are, too. However, many after-market wheels rely on hub centric rings. This means that instead of machining wheels specifically for each O.E. centering hole diameter, the wheel manufacturer machines all wheels to one size, and then uses inserts to give a centering surface of the diameter required for each application.
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