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How Important are Offset and Backspacing?

Posted in How To: Wheels Tires on February 1, 2011 Comment (0)
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Sometimes, conversations about wheel offset and backspacing can induce glazed eyes and distant stares. Some folks would rather be forced to write a thesis on particle physics with a crayon in a regional turboprop aircraft lavatory. But the reality is that offset and backspacing really aren't that deep, and as a bonus, wheel manufacturers and suspension companies have done the majority of the work for you.

Backspacing is the distance from the rear of the wheel hub mounting surface to the outer edge of the inboard side of the wheel. Backspacing is typically measured in inches. Offset, measured in millimeters, tells you where the hub mounting surface of the wheel is located relative to the overall width of the wheel. It is the distance from the exact center of the wheel to the outer edges of the wheel. If the hub mounting surface is centered in the wheel, the offset is zero. If the hub mounting surface is inboard of the wheel's center, the offset is negative and the wheel will be pushed out from the vehicle. If the hub mounting surface is outboard of the wheels center, the offset is positive and the wheel will tuck in on the vehicle. The illustration pictured above shows this.

Your stock rig rolled off the assembly line with a set of wheels specifically designed by the manufacturer to allow for adequate clearance between the wheel and tire and things like brake calipers, tie-rod ends, and fenders. Most factory wheels are designed with a positive offset, which result in the tire and wheel being tucked in close to the vehicle.

Dick Cepek has created the Torque wheel with a positive offset for those trucks with little or no lift. This 17x8.5 wheel, as shown here on our project Power Wagon, tucks the wheel and our 35x12.50 tire close to the truck. It has a positive 6mm offset and five inches of backspacing.

When you lift your rig and install a larger tire, you obviously change things from how they were designed at the factory. Naturally, it is unacceptable for the tire and wheel to contact anything on your rig during steering maneuvers and as the suspension articulates. In most cases, you'll need to install a wheel that has a negative offset. This will move the wheel outward on the vehicle and help create room for the wider, larger tire. How do you know what to install? Well, the reality is that suspension companies do the work for you. All aftermarket suspension companies provide information on what tire and wheel combination is recommended for a given kit on a specific vehicle. Naturally, it's wise to abide by that information, as it's derived from a significant amount of research and testing.

When you install the recommended wheel, you'll not only ensure that the wheel and tire will travel freely, you'll probably also increase your vehicle's track width, which improves stability. The downside is that you may see an increased sensitivity to tracking, which may require higher steering effort. It may also contribute to an increased turning radius and create more leverage on the wheel lugs, bearings, spindles and axlehousing. This increase in width may also cause the wheel/tire combo to protrude past the fender, which is illegal in some states, requiring the installation of fender flares.

Wheels are stamped with helpful information. The ET number refers to the offset. This Dick Cepek Torque wheel has a positive 6mm offset. "ET" refers to the German word einpresstiefe, which loosely translated means "on press depth."

If you just want to install a cool set of wheels without a suspension lift, or if you've only added a leveling kit, and you don't want to install a wheel that protrudes from your fenders, there are wheel options available. One is the Torque wheel from Dick Cepek. This wheel has a positive offset so it tucks the wheel and tire close to the vehicle. Don Sneddon, advertising manager at Mickey Thompson Tires, says, "The whole reason we came out with the Torque was to fit vehicles with no lift kit or with a leveling kit so that the wheels are very close to a factory offset."

Speaking of aftermarket wheels, companies like Dick Cepek and Mickey Thompson work closely with the suspension companies when they design their wheels. Sneddon says it's not unusual for them to test-fit a number of wheels on a new suspension kit to ensure everything fits correctly. "We do a lot of test fitting, making sure our wheels fit different models, and we're updating our application guide all the time," he says.

Another example of helpful factory wheel markings is illustrated on this 15x10 Pro Comp 1079 wheel that has a negative 46mm offset.

So how important is offset and backspacing? Well, it's very important in the world of four-wheel drive modification. Fortunately, you don't have to "go it alone," thanks to the testing and expertise of the wheel and suspension manufacturers.

How To Measure Backspacing
In the four-wheel drive world, backspacing is referred to far more often than offset. Look at most suspension kit specifications and you'll see they recommend a specific wheel size with a specific backspacing. Offset is rarely mentioned, and backspacing is one of the main components used for wheel selection. It's very easy to determine your wheel's backspacing. As shown in this photo using a stock Jeep TJ factory wheel, place the wheel (without tire) face-down and rest a ruler horizontally on the wheel. Now measure the distance between the hub mounting surface of the wheel and the ruler. That figure is your backspacing.

Spaced Out
Wheel spacers can also help to create clearance. There are many sources for spacers, and they are available in a variety of thicknesses. Most are made from high-grade aluminum and have weight and tire size ratings. Adding wheel spacers will have the effect of running a wheel with a negative offset, and they create space for larger tires. The downside to wheel spacers is that they can increase stress on axle componentry just like a negative offset wheel. Also remember that because the spacers bolt onto the hub and the wheel bolts to the spacer, you now have two sets of fasteners to torque and re-torque.

When purchasing wheel spacers, carefully measure things out ahead of time so that you can purchase spacers that are no thicker than they have to be.

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