Selecting new wheels involves more than simply matching the bolt pattern and diameter. Wheel fitment determines how the wheel and tire assembly sits within the fender well and interacts with suspension components. Backspacing is a fundamental measurement that dictates the wheel’s depth and its ultimate position relative to the vehicle’s chassis. Understanding this single dimension is paramount for anyone looking to change their vehicle’s wheels, as it directly influences both safety and performance.
Defining Wheel Backspacing and Measurement
Wheel backspacing is defined as the distance from the wheel’s mounting surface to the inner, rear edge of the wheel rim. This measurement quantifies how far the wheel extends inward toward the vehicle’s suspension and frame. It is almost universally measured in inches, providing a straightforward metric for determining the wheel’s inward projection. A higher backspacing number indicates that the wheel sits further inward, while a lower number means the wheel is positioned further outward.
To determine the backspacing of an unmounted wheel, a simple two-step method can be employed using common tools. A straight edge, such as a ruler, is placed across the entire back lip of the wheel, spanning the full diameter, resting flush against the rim’s edge. The measurement is taken from the underside of this straight edge down to the wheel’s mounting pad, which is the flat surface that bolts against the vehicle’s hub. This measurement must be taken from the actual rim lip to ensure accuracy, and the resulting distance is the wheel’s true backspacing dimension.
Backspacing vs. Offset: Clarifying the Difference
Backspacing is often confused with wheel offset, though they describe the same positional relationship. Offset is defined as the distance from the wheel’s mounting surface to the wheel’s true centerline. Unlike backspacing, offset is typically measured in millimeters and can be positive, negative, or zero.
A positive offset means the mounting surface is positioned outward from the centerline, pulling the wheel inward. Conversely, a negative offset means the mounting surface is inward from the centerline, pushing the wheel outward. The fundamental distinction is that backspacing measures the distance from the mounting surface to the back of the wheel, while offset measures the distance to the wheel’s center.
Despite their different reference points, backspacing and offset are mathematically related and can be converted from one to the other. The relationship is expressed by the formula: Backspacing equals half the wheel width plus the offset. Understanding this conversion allows a user to select a wheel based on either specification, provided the wheel width is known.
The Impact of Backspacing on Wheel Fitment
The selection of backspacing affects the physical fitment and dynamic performance of the vehicle. This dimension is a direct determinant of both static clearance and the long-term mechanical health of the vehicle’s running gear.
Too Much Backspacing (High Value)
Choosing a wheel with a higher backspacing value moves the wheel further inward toward the vehicle’s center. This inward shift increases the risk of the tire or inner wheel barrel contacting stationary suspension components. Specific areas of concern include the brake calipers, the steering knuckle, and the main body of the strut or shock absorber.
Too Little Backspacing (Low Value)
Selecting a wheel with a lower backspacing value pushes the wheel and tire assembly further outward from the vehicle. This outward movement can cause the tire to rub against the fender lip or inner fender liner, especially when the suspension is compressed or the steering wheel is turned.
Impact on Handling
Moving the wheel outward increases the scrub radius, which is the distance between the tire’s center line and the steering axis intersection point on the ground. A larger scrub radius can lead to heavier steering effort and increased torque steer under acceleration. The increased leverage created by an outward wheel also places additional stress on the wheel bearings and suspension components, accelerating their wear rate.
Choosing the Right Backspacing for Your Vehicle
The most reliable starting point for determining the correct backspacing is the Original Equipment Manufacturer (OEM) specification. Factory backspacing is engineered to optimize handling, component longevity, and clearance for the stock wheel width and tire size. Any deviation from this specification must be carefully calculated to maintain proper fitment.
If changing the wheel width or tire size, the backspacing must be recalculated to ensure the new wheel’s inner and outer edges remain within acceptable clearance zones. For instance, a wider wheel will require a corresponding adjustment in backspacing to prevent the inner edge from hitting the suspension.
It is recommended to physically measure the available space between the hub and the nearest suspension or fender component before purchasing. This measurement provides a precise reference point for the maximum allowable inward and outward wheel position. For complex setups, such as those involving suspension lifts or custom brake systems, consulting a wheel and tire specialist is advisable, as they can accurately predict the required backspacing.