Wheels and Positive Offset

The manufacturer will use terms such as offset, inset, outset to identify the styles of their wheels. It can all be very confusing until the truck shows up and you realize that things went sideways with your interpretation.

With all wheels, offset, is the relationship of mounting surface to the wheel centerline. As such, a zero-offset wheel (Example 1) will have the mounting surface at the wheel centerline. As you can imagine, this puts the least amount of stress on the components. Positive offset moves the mounting surface outboard, resulting in the tire being moved more under the vehicle. Conversely negative offset moves the mounting surface inboard of the wheel centerline with tire protruding further away from the vehicle.

An inset wheel has a positive offset (Example 2). The centerline of the wheel moves inboard as the mounting surface moves out. This results in narrower track, and often decreased turn angle. The look of this style is a shallow dish, with the mounting surface closer to the outboard lip of the wheel.

If you want a deeper dish wheel the outside edge of the tire and wheel will be moved away from the drum and toward the road. In this case the wheel appears like a shiny bowl. This will require an outset wheel, or negative offset (Example 3). This mounting style pushes the tire and wheel further out and improves turning. But the outset position comes with a caution, both overall width and track. Combining an outset wheel with a wide track axle will put the truck well over the legal 102 inches and the increased track can result in axle capacity de-rate (depending upon hub style).

When spec’ing 20K steer axles the customer will typically utilize 425 tires. Aesthetics play a large role in truck component choices here. Most customers prefer the look of deep dished (and yes, polished) front wheels.  

In this case, the Example 2 Comparison applies. The larger the offset number positive), the shallower the dish. So with Alcoa wheels, a 3.88 inch offset will have that deep dish look, similar to most rear axles, while a 5.81 inch offset will be shallower, and result in narrower overall width and track.