WHEEL CENTERING
All of the above factors must be taken into consideration when installing non-OEM wheels onto your vehicle, as well as the bolt patter as shown in the illustration above. The other element that affects directly whether a wheel can be bolted up is hubcentricity. All modern day cars utilize hub centric wheels. A hubcentric wheel has a hole at its 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 strongest. 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.
However many aftermarket wheels rely on centering rings. This means that instead of machining wheels specifically for each O.E. 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.
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. This would put all of the load bearing forces onto the studs, not to mention the wheels would not be centered and would most likely lead to a catastrophic failure.
OFFSET
Offset is the location of the flat mounting surface of a wheel relative to the wheel's centerline. Zero offset would mean that the mounting surface is exactly even with the centerline of the wheel. Negative offset means that the mounting surface is closer to the center of the car than the center line of the wheel. Positive offset means that the mounting surface is past the centerline toward the outside of the car. 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 one way to solve the problem if they don't, but its best to get a wheel with enough dish to meet your offset specs and will 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. |
