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Wheel & Tire Fitment | Measurements

Posted in How To on September 1, 2012 Comment (0)
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Wheel & Tire Fitment | Measurements

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.

Big wheels combined with a relatively small increase in tire size means the sidewall height of the tire is reduced. While this does decrease sidewall deflection on the highway, it does so at the expense of a rougher ride. In the dirt, you have less margin for keeping the rim from being damaged at low air pressures, and the rim rides that much closer to offending obstacles that can destroy a bead. Larger wheels usually add more unsprung weight on your axles and work your brakes harder.

To measure backspacing on a wheel, place a straight-edge across the outside surface of the inboard bead seat (not the inboard flange edge). Then use a ruler to measure the distance from the hub mating surface on the back of the wheel (shown here) to your straight-edge. This dimension is the wheel backspacing.

Offset is a bit trickier to measure, but can be calculated from a backspacing measurement fairly easily. It is defined as the backspacing minus the centerline. The centerline is measured as one-half of the overall outside width of the wheel. If the centerline number is greater than the backspacing number, you have a negative offset. This wheel shows it has a -6mm offset dimension. Offset specs are often expressed in millimeters, so you can convert millimeters to inches by dividing by 25.4.

A good rule of thumb when choosing tires you want use off-road is to never have a wheel that is more than half the diameter of the tire (i.e. 40s on 20s, 35s on 17s, or 30s on 15s). This allows you to keep sidewall height down to a point where the sidewall is not excessively squirmy, yet retain good flex and sidewall distance to soak up bumps that severely deflect the tire carcass.

When choosing a tire, you’ll first want to determine size, whether it be the same as the stock size or something larger. Metric passenger car tires (sometimes used on light trucks) sizes are designated with a P prefix and light-truck tire sizes designated with an ‘LT’ prefix.

This tire has a designation of LT285/70R17. The 285 number is the nominal overall tire width in millimeters. R designates radial-tire construction. The 70 designates the cross-section aspect ratio of the tire, and the 17 is the tire rim size in inches. To determine the overall tire height, we simply figure twice the sidewall height (2 x 285mm x 0.70 aspect ratio) and add that number to the rim diameter to get the approximate tire diameter. In this case, the nominal tire height turns out to be about 33 inches tall.

There are several types of lug nuts in use on wheels, and when changing wheels it may be necessary to change the lugs as well. These days you’re most likely to find wheels using acorn lug nuts with 60-degree conical seats. Some aluminum wheels do use a mag lug that is not tapered and has a shank area that fits into the wheel stud hole. These types are not interchangeable.

Another light truck tire size convention such as this lists the dimensions much more clearly. For instance, a 35x12.50R15LT tire is nominally 35 inches tall, 12.5 inches wide (overall), and mounts to a 15-inch wheel. Actual tire dimensions for any given size can vary between manufacturers. For example, one 35-inch tire may measure a true 35 inches tall, while another may be shy at 34 inches.

Wheels on IFS trucks have much more backspacing than those that originally came on straight axle trucks. Most OEM IFS type wheels won’t bolt on a straight axle due to interference with the brake calipers and steering arms. On the flip side, using wheels with much less backspacing on an IFS truck will push the tires far outside the fenders.

Wheel width can play a part of tire performance. It affects the sidewall and tread shape and how they flex against the terrain. The wheel width will also determine the amount of sidewall bulge beyond the wheel edge. In general, wider wheels will offer more footprint and flotation for the given tire as opposed to using a narrow wheel. However, narrow wheels typically allow you to run lower air pressure without pushing the tire off the wheel bead.

If you’re putting new wheels on a heavy truck or going with large tires, consider the load rating or maximum tire size rating specified for the wheels you’re considering. Some wheels that may fit on a truck are not designed to withstand off-road loading or tire sizes over about 33 inches.

If you have a set of steel wheels and are proficient with a welder, you might consider converting the steelies to beadlocks with one of the aftermarket kits such as these rings from Off Road Innovations (www.4wdsupplies.com). This is typically the most economical way to get a set of beadlocks on your rig, but the end product is often heavier than aluminum or purpose-built steel beadlock wheels.

You can mount tires on beadlocks at home yourself, and many tire stores will refuse to work on them due to liability concerns. It’s a pretty simple matter to soap up the beads a bit and slip them in place on the wheel surfaces with a small tire spoon.

Companies such as OMF Performance Products can convert your non-beadlocked wheels to a true beadlock. They machine off the outer flange and weld on an inner beadlock ring in its place. These beadlock wheels were built from Mickey Thompson Classic II cast aluminum wheels. OMF offers a variety of outer ring options to strengthen or dress up the wheels.

An alternative to the bolt-on ring beadlock is an internal beadlock. Coyote Enterprises (CoyoteEnts.com) manufactures an internal tube that fits inside your regular tire. Once inflated, it serves several purposes. It holds the inner and outer tire beads firmly against their wheel seating points. And, the inflated tube serves as a secondary tire suspension capable of absorbing greater impact loading at the tire without the tire pushing fully flat against the rim, where rim damage may occur or the tire may be cut.

One issue we deal with off-road when running low air pressures for better traction is the risk of pushing an outer tire bead off the rim seating surface. When this occurs, you rapidly lose all air in the tire. Beadlock wheels don’t rely on simple air pressure and rim friction to keep the outer tire bead in place on the wheel. A typical beadlock securely clamps the outer tire bead between an outer machined ring on the wheel and a bolt-on circular ring.

Other aluminum beadlock wheels, such as Walker Evans wheels, were designed from the start to be cast with a beadlock flange.

If you’re looking for the ultimate in strength, then forged aluminum beadlocks are the strongest designs out there and are common on race trucks. But they are considerably higher in cost too.

Steel wheel beadlock versions are available as well. Some use fairly lightweight steel outer rings, and others use heavy steel or aluminum outers. You’ll find a variety in quality and strength, so choice can depend on your vehicle weight and intended application.

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