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Wednesday, December 16, 2015

Changing wheels? There are some important dimensions and specifications to do this safely.

I recently read a thread on an RV trailer forum where some were considering changing the wheel diameter so they could get an increased selection of tire brands, sizes and load capacity to address tire failures. One person said
"I am looking at some nice 16" wheel tire combos but I'm wondering if they'll fit my trailer. They came off a different tow vehicle. Do I need to expect to swap out size up the hub where the wheel bolts are?"

I had some concerns as if someone is swapping wheels there are a number of dimensions and "fit" issues that need to be considered if you want a safe vehicle.

Wheel "offset" controls the location of the tire toward or away from the center of the vehicle. Tire Rack has a good tech page with drawings and definitions "The offset of a wheel is the distance from its hub mounting surface to the centerline of the wheel. The offset can be one of three types (measured in millimeters).*"

Center bore may be critical as some wheels are "hub centric" which means the center of rotation is controlled by the fit of the wheel to the hub. Others are "lug centric" which means the fit of the lug nuts controls the wheel/tire run-out. "The centerbore of a wheel is the machined opening on the back of the wheel that centers the wheel properly on the hub of a vehicle."

Lug nut fit. Most steel wheels have a "cone fit" where the taper of the lug nut fits the taper of the wheel center. Aluminum wheels may have a simple cone fit with a straight taper or a radiused, or “ball” style. The two styles must never be mixed or you can ruin a wheel. The seat profile in the wheel dictates the nut or bolt seat style.  There is also a "mag style" that requires a large flat washer.
While it may be possible to initially use the wrong type nut the results will be a loss of torque and damage to the wheel with the potential of having a wheel come off the vehicle at speed.

Proper lug nut torque is another important number "If the vehicle is on a frame lift, and a helper is able to lock the brakes, the wheel fasteners can be final torqued while on the lift. Otherwise, lower the vehicle to the ground until the tires just begin to load (enough to prevent rotation while torquing). If the vehicle is on the ground with its full weight on the tires, you may be fighting a slight lateral load while final tightening. This is perhaps an arguable point, but final tightening before the suspension is fully loaded helps to obtain a more precise centering of the wheel fastener seats."

Improper mounting and lug nut torque can contribute to what feels like an out of balance tire.

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