Stagger is the difference...
Stagger is the difference in circumference of two tires on the same end of the car. Measure the stagger around each tire near the middle of the tread. Subtract the two measurements to find stagger, or rollout, as it is sometimes called. Always inflate to racing pressures before measuring.
It is important to take tire temperatures when evaluating the car's setup. The temperatures can indicate how well each tire is working in conjunction with the other three. We set camber and tire pressures based on tire temperature.
To take tire temperatures properly, read the temperatures in three places across the face of the tire with a tire temperature probe. This instrument has a needle-like end inserted into the rubber to get the inside temperature. It should be inserted into the rubber at a 45 degree angle. Measurements should be taken at the edges about 1 inch inside the edge of the tire and at the middle of the tire.
Give the instrument time to react to the temperature of the tire. Once the reading stabilizes, record the amount and move on to the next reading. Don't be in too much of a hurry to do this. Many decisions will be made based on the temperature readings, so they must be accurate.
Even when desired stagger amounts are known, tires can be mounted in a manner to allow changes to improve the car if there are unexpected stagger changes. Once the tires get hot, the gas expands and the bias tires stretch. By putting the correct stagger on the rear, the front stagger can be set so that the tires can be swapped on each side of the car to affect the rear stagger if one of the rear tires changes its size after being run.
To find stagger the easy way,...
To find stagger the easy way, follow this simple method and never subtract tire sizes again. Measure the larger tire first. Then remember the number.
Example: A rear stagger amount of 2 1/2 inches is needed. The right-rear (RR) tire is 86 inches in circumference, and the left-rear (LR) tire is 83.5 inches around. One of the rear tires will grow, causing either an increase or decrease in stagger, depending upon which tire changes size. If the same stagger is on the front, but at a larger size, swap either left-side or right-side tires to correct the problem.
The tire stagger must match the racetrack and groove radius. Do not correct handling problems with excess or deficient stagger. For every track, there is an optimum stagger for the rear tires. The need for correct stagger is even greater if you're using a locked-spool rear differential.
The spool will need a stagger that is an average of the radii that the tire will experience in the turns. Few driving lines result in a single, constant radius. The driving line through a turn is more like a parabola, or constantly changing radius, with the smallest radius in the middle portion of the turns.
Wrap the tape measure around...
Wrap the tape measure around the smaller tire and mark where the large tire stagger number falls near the end of the tape. Read the measurement from the end and you have the stagger.
A Detroit Locker rear differential unlocks when going into the turns and locks back up upon acceleration. For this type of rearend, match the stagger to the radius of the last third of the turn. This may be less stagger than what might have been needed at the very tightest portion of the mid-turn.
When running on a set of tires at race speeds, there are three indications of how well the tires are working. Those are tire temperatures, tire pressures, and tire wear. Pressure and wear are fairly easy to read for both asphalt and dirt teams. Dirt racers may have a difficult time getting temperature readings because of the way heat bleeds off the tire after a run.
The temperatures will indicate how well the tire footprint is working. Ideally, the heat should be nearly even across the face of the tire. The temperature is usually taken in three places. Some teams prefer to see a little more heat on the inside (toward the inside of the racetrack for this purpose) of the tire, but the progression of the heat should be increasing steadily from the outside to the inside in any event.