Hopefully last month's look at the stone basics of tires helped shed some light on the four things you may abuse the most on your truck. This month we turn our attention to wheels, rims, hoops, rolling stock, mags, etc., depending on your parlance preference. We should all have them on our trucks, and those who don't will surely acquire a set or two at some point, but with all the sizes, brands, materials, styles, and finishes out there, how do you know what you're laying your hard-earned green down for, and are you buying what you need? Will they fit? Will they look right? Let's find out.
The backspace of a wheel is the distance from the brake hub or wheel-mounting surface to the outermost edge of the wheel's backside. The easiest way to measure backspace is to lay the wheel face down on the ground so the wheel's backside faces up. Take a straightedge and lay it diagonally across the wheel's inboard flange and measure the distance from where the straightedge contacts the inboard flange to the wheel's hub mounting pad. This measurement is the backspace. When building custom wheels, the manufacturer can weld the center into the wheel in a specified location that will center your custom-built rim into the fenderwell. An increase in backspace pushes your wheel inboard, and a decrease in backspace moves the rim toward the street.
WHEEL OFFSET, WIDTH, AND DIAMETER
Simply put, offset is the distance from the wheel's hub mounting surface to the wheel's centerline. Calculating the wheel's offset is a fairly easy mathematical equation that looks like this: offset = backspace - (rim width ? 2). First, measure the overall width of the wheel, and remember, just because a wheel is 15x7 does not mean the overall width is 7 inches. The wheel industry measures wheel width from bead seat to bead seat, which is on the inside of the wheel lip. This also holds true for measuring wheel diameter, but it isn't factored into measuring offset. To calculate the overall width, measure the wheel from outside to outside or flange to flange. If you measure a 15x7 wheel flange to flange, it will be approximately 8 inches. Next, divide that width by 2; this will give you the wheel's centerline, which is 4 inches. Now, subtract the last figure from the backspace measurement to find the wheel's offset. So if our 15x7 wheel has a backspace of 3 1/2 inches, the math would be: 8 ? 2 = 4 inches (centerline). Subtract that from the 3 1/2-inch backspace, and we are left with a -0.5-inch offset.