CENTERING & OFFSET
Get a better understanding of the wheels on your car
Offset is the location of the flat mounting surface of a wheel relative to the wheel’s centerline. Negative offset means that the mounting surface is toward the center of the car, positive offset means that it’s toward the outside of the car, or the wheel is “pulled in” toward the center. Offset affects many things other than just whether the wheel has the appearance of “sticking out” past the fender. The wrong offset can cause rubbing problems when the suspension is compressed or the wheel is turned. Offset affects the steering geometry’s
scrub radius, possibly leading to problems with torque steer or self-centering characteristics. Offset also affects the suspension’s motion ratio, which directly determines the effective spring and damper rates. Potentially, in a very heavily loaded vehicle, or with extreme changes in offsets, wheel bearing life can be affected, but this is more often talked about by truck people than by small car enthusiasts. It’s very, very important that the proper offset wheels be used. While not directly a matter of
offset, brake caliper clearance is a related issue. If you have or plan to have big brakes on your car, be sure that your wheels, or the wheels that you’re going to use will fit over the calipers. Spacers are available to solve the problem if they don’t, but it’s best to get a wheel with enough dish to meet your offset specs and still fit your brakes. Consulting the wheel and brake manufacturers ahead of time is wise. Many aftermarket brake companies even have templates of their brakes available that you can easily check against any wheel.
The other element that affects directly whether a wheel can be bolted onto a car is hubcentricity. Long ago, in the deep mists of time, wheels were located by the taper of the lug nuts or bolts. This could lead to all sorts of problems, but they can be summarized by saying centering was liable to be less than perfect, and the sheer stress on wheel bolts or studs could be enormous. We’re not aware of any passenger car wheels now made that are not hubcentric. Hubcentric wheels have a hole at their center that fits closely over a round feature on the hub, serving to center the wheel on the axis of the spindle, as well as bear the vertical weight of the vehicle. The wheel bolts or studs then serve simply to hold the wheel onto the hub, and are loaded only in tension, where they are strong. If the studs were required to absorb vertical forces, they would be loaded in single shear, the weakest arrangement for any fastener. Factory wheels are all machined to fit their specific application exactly, and some of the better aftermarket wheels are, too. However, many aftermarket wheels rely on centering rings. This means that, instead of machining wheels specifically for each OE centering hole diameter, the wheel manufacturer machines all wheels to one size, and then uses inserts to give a centering surface of the diameter required for each application. This is obviously easier to do, and makes inventorying a complete wheel line much simpler and less costly. If you buy wheels that use cen- tering rings, be sure that the rings fit snugly in the wheels. If they’re loose enough to fall out, how accurately can they be locating your wheel? Some tire shops automatically remove centering rings to balance a wheel, just to make sure that there is no slop to make their balancing inaccurate. The fact that a wheel physically bolts onto a car doesn’t necessarily mean that it “fits.” The centering surface could be too large, in which case there’s essentially no centering. Just as importantly, the offset could be wrong.