The perfect set of wheels is an essential element of any classic car build, but choosing wheels isn't always as easy as it might seem. There are many technical variables that must be considered, including bolt pattern, diameter, width, backspacing, and offset. Once you've established the wheel specs you need, you'll have to shop for an appropriate set of wheels with an appealing design and finish. In this article, we'll provide some info and diagrams to help you answer the question "what are wheel backspacing and offset?"
Wheel Backspacing, Offset, & Other Terms Explained
Wheel Diameter and Width
The diameter of a wheel (e.g. 15 inches) and width of a wheel (e.g. 7 inches) are typically abbreviated in Diameter x Width format (e.g. 15x7). If a wheel is far too large or wide for your vehicle, no amount of backspacing or offset adjustment will make it fit without substantial suspension and/or body modifications, so you'll need to figure out these dimensions first.
A typical set of four identical wheels is often described as square wheel setup. However, many resto-modded classic cars and muscle cars use a staggered wheel setup, meaning that the front and rear pairs are different sizes. Typically, staggered wheels are wider in the rear to allow wider tires and provide more traction — for example, 15x7 wheels up front and 15x8 wheels in the rear.
Oversized wheels will increase unsprung weight, affecting performance and handling. Read the following article for more details: Wheel Weight & Why It Matters
Wheel Bolt Pattern
Wheel bolt pattern is the configuration of the wheel bolts on the hub. It is annotated in "A x B" format, where A is the number of wheel bolts, and B is the distance between bolts (measured according to the diagram above).
For example, the '67-'69 Camaro has a wheel bolt pattern of 5 x 4.75". The Camaro has 5-lug hubs, with five wheel bolts and five lug nuts per wheel — hence the first number. The second number is obtained by measuring from the back side of the top hole to the center of the opposite hole. In this case, it results in a measurement of 4.75 inches.
Your vehicle's wheel bolt pattern must be considered before purchasing wheels. If the wheels you purchase do not match your car's wheel bolt pattern, you will not be able to install them. To check your car's wheel bolt pattern, you can either measure your current wheels using the above method, or search for "[your car's year, make, and model] wheel bolt pattern" on any web search engine.
Backspacing is the simply the distance from the bolt circle mounting surface to the inside of the wheel. If the wheels are zero-offset, the mounting surface is the along the wheel's centerline, which means the backspacing should be 1/2 the width of the wheel.
In the diagram below, backspacing is highlighted on the left side:
Wheel offset, highlighted on the right side in the diagram above, is the distance between the wheel's bolt circle mounting surface and the rim centerline. If the mounting surface sticks out farther than the centerline, the offset is positive ("high offset"). If the centerline is farther outboard than the mounting surface, the offset is negative ("low offset"). An easy way to understand this is to remember that the typical deep-dish wheel has negative offset.
Why are Backspacing and Offset Important?
It is important to consider backspacing and offset when purchasing wheels. If the backspacing is too different from the original design, you may encounter steering problems as a result of the altered geometry. In the same way, if the offset of the new wheel is significantly different than original, the load on the wheel bearings is affected and can cause them to wear out sooner. Dramatic changes to wheel offset can also cause tires to rub on body panels over bumps or during hard cornering.
Need Some New Wheels?
If you're looking for a new set of wheels for your car, Classic Industries has tons of options to choose from! Browse through the wheel and tire section in our web store, or click the button below for a free full-color restoration parts catalog for your classic car or truck.