If you spend time around them, you'll find that wheels are more than round. But let's consider the round part first-most wheels are not round! In our discussion, we will be referring to the two pieces of metal that make up the wheel: the rim and the center.
The center is stamped out in a large press. It goes through several forming stages to shape the flange, the torque ring, and the taper ring where the lug nut fits. The rim starts out as a piece of flat steel that is roll formed to shape and then welded together. There are infinite variations of these parts and the ways they can be assembled.
First, let's look at the wheel width. Many times, the maximum width is specified by a particular track's rules. This width is measured where the tire bead seats against the inside of the wheel. This should not be confused with the measurement from outside edge to outside edge of the wheel. Be sure you are measuring a wheel to take the best advantage of your rules. If you are using OE wheels in, say, a Bomber/Hobby car, be sure to check the wheel for cracks around the lug holes and damage around the rim. Under no circumstances should you use old wheels with rims riveted to the center. In these classes, you should be using racing wheels due to the stresses involved. Racing wheels will most likely be lighter, thus offering more performance.
When using spacers, be absolutely certain the spacer has a diameter large enough to cover the torque ring.
There is much to be said about the weight of a wheel. Weight is important due to several factors. One, the wheel must be accelerated just as the car is, only it has to accelerate faster. The heavier the wheel, the more power it takes to bring it up to speed. Just as important is the reaction of the wheel to the racing surface. No racing surface is glass smooth. As the wheel moves across the surface, small to large irregularities force the wheel to move vertically. The springs and shocks are there to keep the tire in contact with the surface. However, be aware that less weight in a wheel is not the end of the story. Using a 19-pound wheel on a 3,500-pound car running on a rough track could prove to be an embarrassment. You need all four wheels on the car to finish a race.
Two of the more confusing things about wheels is wheel offset and wheel backspace. They are not the same. Offset is the placement of the wheel center somewhere other than the center of the wheel rim. For instance, the wheel might be offset 1 inch from the center of the rim. This 1-inch offset could be several different backspace measurements, depending on the width of the rim. Offset is more of a designer's term when building a wheel. Wheel backspace is a racer's term and generally the only one of the two that a racer will use. Backspace is measured from the wheel center flange where it contacts the hub to the outside of the rim on the side toward the middle of the car. The less backspace, the farther the outside edge of the wheel extends away from the car. To ease confusion, unless you are a wheel designer, the racer might do well to forget the term "wheel offset." Less backspace sets the wheel farther out.
This cross section of a wheel center shows the torque ring as a bump.
An additional thing to consider with front wheels is the tire scrub radius. If the center lines of the upper and lower ball joints (this is referred to as kingpin inclination) touch the ground at the same point as the center of the tire, then the center will rotate about this point. When backspace is reduced, extending the wheel outward, the ball joint line will contact the ground inside the center of the tire. It will then have to follow a scrub line outside the pivot point (where the ball joint line touches the ground). This creates a small amount of drag and also changes the wheelbase during cornering. This effect may or may not be a desirable feature. But just knowing it is there can sometimes be helpful in car setup and adjustment.
Wheel spacers are often used to adjust the width of the track on a race car. These simple items have several features that bear looking into. The first, and perhaps most important, is the outer diameter of the spacer. It must be larger than the torque ring of the wheel. The torque ring is the circular bump stamped in the wheel center that is outside the bolt circle. This torque ring is effectively the lock washer that keeps the lug nuts tight. Your spacer should have a full flat face against the wheel. A spacer with hollows between the lug holes should be avoided. These are probably cast of a pewter/ aluminum mix commonly known as potmetal. The hollows were not put there to save weight but to make a cheaper part. If you are going to use wheel spacers, install larger-than-stock 51/48-inch studs. The OE 71/416-inch studs are dicey at best and even then too short. Aluminum spacers, if they are made from aluminum plate, work fine. Steel spacers are even better since the torque ring seat won't wear on them.
The detail of the adjustable chassis is shown here. With this, it is easy to make adjustments and comparisons.
Making ChoicesHow thick can the spacer be? Maximum thickness should be no more than 11/42 inch. If you need more than this, then it is time to call your trusty wheel distributor for a little more offset no, a little less backspace. Remember, spacing a wheel with spacers will cantilever the wheel out on the lugs. Flex here is not good.
Now that you know about the parts and duties of the wheel, let's look at the tools to use them properly.
Starting with round, know that many wheels are not round. I have measured OE wheels and found them as much as 0.045 inch out of round. Even some racing wheels are out of round. Now before we get too upset, here is what happens. Remember that the rim is rolled and welded. The last inch or so of each side at the weld can't be formed as perfectly as the rest. When a dial indicator is used, it will show a slight dip at the weld point. The tire doesn't seem to care too much at low speeds. If you want to improve on this, then you will want wheels that have been spun to shape after welding. Better wheels, but more money, too.
Here is what backspace can do for your car. Two things happen when a wheel is moved in or out. Let's take the rear wheels. If they are in line with the front wheels, then the traction developed will push straight ahead right in line with the front wheels. Now, if the right rear is moved out, power will tend to swing the rear of the car to the right. This would give the car a loose tendency anytime you have the throttle open. It would also have the opposite effect under braking. Braking force on the right rear would give the right rear some leverage to pull back outside of a line through the front, thus exerting a force trying to turn the car to the right. Remember, these are small adjustments when compared to, say, springs.
If the right-rear wheel were moved inside the line to the front wheel, the traction would push inside the front track. This would make the car stable in the straight. However, when the power is turned back on in the middle of the corner, this stability can turn to a push.
Using the above illustrations, you can apply the same scenarios when moving wheels. When rear drive wheels are moved in or out, they will push the car depending on their position.
Notice the blue and yellow arrows. Yes, these are extreme examples designed to make the point. | 
With the right rear spaced out under braking, the tendency will be to pull back (see arrows) on the right rear. | 
Our simulated chassis has been put on wheel scales. This will record the weights as we move the right-rear wheel in and out. |
Wheel backspace measurements usually change by increments of one inch at a time. Wheel spacers are usually 11/44 inch thick. Therefore, the spacer makes a finer adjustment than the coarse adjustment of backspace.
Weight shift is the other effect of moving wheels in or out. The further a wheel is spaced out in relation to the other wheels, the less weight it will carry. Conversely, if a wheel is moved in toward the center of the car, it will carry more weight. To understand this, visualize a tricycle. Imagine that both rear wheels were shifted all the way to the center to become one wheel. This wheel would carry all the weight of the rear, and it would be in the center of the width of the tricycle. This is an extreme example of moving the wheels in.
Moving the wheels in or out will also affect the weight shift during cornering. The suspension will exert less force on a wheel moved out because the chassis has less leverage against it. In a similar manner, a wheel moved in will receive more weight because the chassis has more leverage. Both of these changes can have the effect of tightening or loosening the car either under power or braking.
There are obviously many more ways to adjust a race car. Using wheel backspace and spacers is just one way. I can't tell you exactly how much wheel backspace to use or in which direction. What you have here are tools with which you can learn to use to tune your car.