All the Measurements You Need to Know
Your wheels and tires are your rig’s feet on the dirt. Tread type, tire and wheel size, and tire track width on your vehicle all play a role in ride, handling, comfort, and traction. The factory typically installs fairly small tires to help optimize fuel economy, while keeping a very smooth street ride. As off-road enthusiasts, we often opt to steer our wheel and tire choices somewhat more towards increased dirt utility.
New treads are expensive and the choices are dizzying. Fortunately, we have a wide array to choose from. When it comes time to buy new wheels and tires, it pays to consider your options carefully, especially if you’re going bigger and wider. There’s some concern for axle and bearing wear, and the added loads to suspension and drivetrain components.
There are several dimensions used to characterize a wheel. These are: wheel diameter, wheel width, backspacing (or offset), bolt pattern, and center hub bore diameter. Bolt pattern and the center hole size must simply fit your specific vehicle. However, it’s the other dimensions where we often have to make choices.
A wider wheel will tend to fatten the contact patch of the tire, while a narrower wheel will do the opposite. The larger contact patch can increase overall traction on soft surfaces such as sand, but a wider wheel is also more vulnerable to having the tire pushed off the wheel bead surface when running low tire pressures. This is where intended off-road use of your rig plays a part in choosing your wheel and tire combo.
In recent years, wheel sizes have grown taller and taller. There are plenty of trucks running the streets with 35-inch tires mounted on 20-inch wheels. Shorter tire sidewalls translate into less flex to absorb terrain impacts, while excessively tall sidewalls make for less responsive steering, especially when using low tire pressures. Speed, terrain, and vehicle weight all play a part when choosing a wheel size for a give tire height.
One factor that can limit how small a wheel diameter you can use is the size and position of the brake caliper/rotor assembly or the rear brake drum diameter. For instance, a 15-inch wheel will usually never fit on a later model truck that was equipped with 16-inch or larger wheels from the factory. Sometimes aluminum wheels with the same diameter as the stock wheels may interfere with brake components if the casting is overly thick. We’ve also seen interference issues with clearance around the wheel mount surface and front wheel hubs with some aftermarket wheels. A check fit is always a good idea before mounting up tires, if you’re unsure of clearances.
For a given width wheel, you may have several choices for backspacing available. An increase in backspacing means the wheel is tucked further inward into the fenderwell. A wheel with excessive backspacing may cause you clearance problems with the front suspension components at the steering limits. Or, the front tires may rub on the inner fenders. A decrease in backspacing means the wheel sticks further outside the fender. This can result in clearance issues from tire to fender, or excessive tire scrub when steering.
Wheel offset is a wheel dimension sometimes used instead of backspacing. Both describe the location of the wheel mount surface within the width of the wheel, using different measure points. A wheel with zero offset has a wheel mounting surface that is centered within the wheel width. A wheel with positive offset means the wheel mount surface lies closer to the outboard rim of the wheel and the wheel tucks further into the fender. A negative offset has the wheel mount surface closer to the inboard wheel rim and causes the wheel to stick further outside the fender.
Generally, when you go with larger tires you need decreased backspacing or less (or negative) offset when compared to stock to keep the fatter tires from hitting components under the fenders. If you’re installing an aftermarket lift, most companies can advise what wheel size and spacing will work well for the application.
In any case, when choosing wheels, don’t just go for the big, blingy ones like you’re a moth to flame. Purposely choose wheels suited to your application, and you’ll appreciate the performance from right-sizing your rolling stock.