Get a better understanding of the wheels on you car.

From coded markings to technical descriptions, theis simple guide will give you a better understanding of the wheel and the terminology which comes along with it.

RIM MARKINGS

Look inside almost any wheel and you should find various markings which give you pertinent information on the wheel. Typicaly, they're marked on the rim and, just for conversation sake, we will take into consideration a wheel whick is marked "15x6 1/2 JJ114.3". The first number is equite obvious and represents that the wheel is 15 inches in diameter, while "6 1/2" is the width of the wheel and measured from the distance between the flanges that support the bead. The "JJ" refers to the space of that flange; easier to understand if you imagine a steel wheel on which the lip is rolled over like a JJ. "ET" may or may not be present, and if you do see it is the abbreviation of einpress tief, German that translates literally as "pushed in depth," or offset. The "33" is the offset measurement.

ENTERING

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 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 are 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 verical 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 for the better aftermarket wheels are, too.

However, many aftermarket wheels rely on centering rings. This means that, instead of machining wheels specifically for each O.E. 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 gives 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 centering rings, be sure that the rings fit snugly in the wheels. if they are loose enough to fall out, how accurately can they be locating your wheel? Some tire shops automatically remove centering rings to balance 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 therels essentiallly no centering. Just as importantly, the offset could be wrong.

OFFSET

Offset is the location of the flat mountin surface of a wheel relative to the wheel's centerline. Negative offset means that the mounting surface is toward the center of the car, postive offset means that it's toward the ouside 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 causerubbing problems when the suspension is compressed of 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 offset, wheel bearing life can be affected, but this is more often talked about by truck people than by small car enthusiasts. It is very, very important that the proper offset wheel be used.

While not directly a matter of offset, brake caliper clearance is a related issue. If you have, or plan t have big brakes on your car, be sure that you wheels that you are going to use, will fit over the calliper. Spacers are available to solve the problem if they don't, but it is best to get a wheel with enough dish to met 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.