4x4 Wheels And Tires - Wheely Basic Wheel FactsPosted in How To: Wheels Tires on November 1, 2008
As much as we love big knobby tires, they won't get you very far without a set of wheels. But wheels are much more than just pretty metal disks spinning in each corner of your truck. Wheels can affect your suspension and braking distances, they can help or hinder your off-roading performance, and they can make it easier or harder to turn.
We realize that 90 percent of the wheels sold are chosen on looks alone, but there is much more to take into account before you get your next set of rims to stuff under your 4x4. Did you know that a lightweight forged-aluminum wheel can be stronger than a heavy cast wheel and can help increase fuel economy? Or that there is a variety of different lug nuts and some are much safer to attach your wheels with? It's little details like this that most wheel buyers ignore, but that readers of 4-Wheel & Off-Road really want to know.
When looking for wheels you need to determine a few specs. The top priority is the wheel stud/lug bolt pattern of your axles. This is the number of lugs and the diameter of the circle they are placed on. For example, many 1-ton trucks run an 8-on-61/2 pattern where there are 8 wheel studs in a circle measuring 61/2 inches in diameter (current Fords are 8-on-170 mm). This is easily measured when you have an even number or studs as you simply measure from center to center of opposite wheel studs. When you measure the bolt pattern of an odd number of wheel studs you need to measure between the far edge of the first wheel stud to the center of the third wheel stud and this will give you a very close estimate.
Next determine your backspacing or mounting offset. Backspacing is the distance from the back edge of the wheel to the wheel mounting surface. Offset is the distance from the centerline of the wheel to the wheel mounting surface. Offset is considered positive if it's towards the outside of the wheel and negative when towards the back side of the wheel. The wheel in the photo has 5 inches of backspacing and is 81/2 inches wide. Because the wheel centerline would be at 41/4 inches of backspacing, this wheel has 3/4-inch of positive offset. The greater the backspacing the more the tire is tucked in under the fenders of your truck. The closer to "0" offset, the more evenly the load is spread over the wheel.
Choosing proper backspacing for your wheels involves three major considerations. One, do you like your wheels sticking outside your truck or not? Two, will wheels that are tucked in closer hit the brakes, body, frame, suspension, or steering linkage during articulation and turning? And three, what backspacing is best for your scrub radius? The first two are pretty simple to understand and can usually be answered by your local 4x4 shop or by test-fitting the wheels prior to purchase. The scrub radius question is when you draw an imaginary line through the upper and lower kingpin or ball joint of your steering knuckle. This imaginary line should hit the ground at the center of the tire contact patch for the least amount of tire scrub when turning, and this is all dependent on the angle of the joints, the diameter of the tire, and the backspacing of the wheel. A larger scrub radius puts more leverage on the steering components and forces them to work harder, and when turning you lose a certain amount of traction.
Finally, lets talk about oversized wheels such as 20-inch-and-larger. For the most part we don't recommend them. They reduce your sidewall, add more unsprung weight, and hinder braking. Unless you have massive brakes and need the clearance, we would stick to the smaller rim, and 17s seem to be the current all-around size. However, we like to use the half-tire wheel rule where the wheel shouldn't be larger than half the tire diameter. Lucky for us, the 20-inch bead locks on our UAJK are fitted with 42-inch tires.