Whether the goal is to simply adapt tires that differ in size from the original equipment or to perform a radical change to the overall track width and stance, it is critical to ensure that adequate fender well clearance is maintained. Preserving the airspace in between the rolling stock and the inner fender, suspension components and outer sheet metal is not only important from the perspective of avoiding damage caused by interference but also for safety considerations. A tire/wheel combination that is poorly conceived can adversely affect steering radius, suspension travel and stress on the steering and suspension components. One key to avoiding the pitfalls of poor a wheel/tire setup is to understand and know how to determine wheel backspacing and offset.
Backspace is really nothing more than the dimension from the inner edge of the wheel rim to the inner face of the wheel that bolts to the hub or flange (refer to diagram A). In other words, backspace is how far back towards the center of the vehicle the wheel projects from the flange.
Offset on the other hand is the dimension that represents the difference between the exact centerline of the wheel and that of the inner wheel face that bolts to the flange (where the inner wheel face is moved inward or outward with respect to where the it attaches to the inside of the wheel rim, as shown in diagram B). Youll here the term positive offset in cases where the inner wheel face is moved away from the wheel centerline and toward the outside of the vehicle (diagram C), and the term negative offset to refer to wheels where the inner wheel face is moved toward the center of the vehicle again with respect to the wheel centerline (diagram D).
It is a little more difficult to use direct measurement to determine offset than it is to measure backspace. The calculation for determining the offset of a wheel is actually very simple. First, measure the overall width of the rim at the outer edge and divide that figure by two to get the wheel centerline. Next, measure the backspace and subtract it from the wheel centerline figure. If the backspace figure is greater than the centerline figure it is expressed as negative offset. If the centerline figure is larger than the backspace, the calculation will result in a positive offset. Heres an example:
Wheel Width at Outer Rim: 8.5
Calculate Centerline: 8.5 / 2 = 4.25
Measure Backspace: 3.75
Calculate Offset: 4.25 - 3.75 = .50
The wheel in the above example has a ½ positive offset. So now that we understand how to figure backspace and offset, how do they apply to the practical matter of determining what will fit? The question of how much tire can I stuff into that fender well? is probably the most frequent one heard. Coming up with the correct answer is a matter of taking a number of careful measurements in order to validate that clearance exists in the maximum limits of suspension travel and within the maximum limits of the vehicle steering geometry. Prior to making any deviation from the OEM-specified wheel/tire recommendations, and in the interest of safety, it is highly suggested that the OE manufacturer be consulted to verify the safety of the proposed modifications.
Calculating the proper backspace, offset, overall tire height and width can be greatly facilitated through the use of an inexpensive handheld laser-level. The laser can be held against the hub mounting flange and the flange rotated to project a moving beam against the inner fender well. This path may then be traced to determine the true projected centerline. Measurements can be taken from the hub center to various points on the line to determine overall permissible wheel/tire diameter. Measuring to each side of the line allows the clearance for the width of the tire to be measured. Perhaps more clearance can be gained by moving the center of the tire toward the inside of outside of the wheel well (dictating a change in wheel backspacing). The same concept applies to determining clearance on the steering wheels. In this case, cycle the steering from lock-to-lock while projecting and tracing the laser beam path at various points along the inner fender well and in relation to steering and suspension components that may be affected. With these measurements and simple calculation results in hand, the correct wheels can be specified with confidence.