tire stagger

Stagger is the difference in tire circumference between the left and right side tires. Front stagger would be comparing the RF to the LF, and rear stagger would be the RR to the LR. Stagger is used on oval tracks, with the right side tires being larger than the left. Side stagger is rarely used on a road course (though having the rear bigger than the front is not uncommon). For ovals, a larger tire on the right side allows both left and right sides to rotate the same number of turns through the corner. This is especially important for cars that have locked axles. If the tires try to turn by different amounts, one side or both will lose traction and slip.

How stagger affects handling

Too little rear stagger will try to push the car to the outside wall on turn exit. To much will make the car loose under power, as one or both of the rear tires will end up slipping (since they aren't turning at the same speeds).
Too little front stagger will show up as a push into the corner. Too much will pull the car to the left hard under braking.
Don't assume that because a car is loose or tight that it needs a change in stagger. The best approach is to get the stagger as close to optimum as you can determine and then leave it alone, unless you see an issue that points specifically to the stagger.

(Thanks to Circle Track Magazine for much of this information)[1]

The formula

Correct stagger is determined by the track, tire circumference, and car wheelbase. It should not be used as a tuning tool. If changing the stagger fixes a handling problem, it is probably a crutch, and you should look for a problem elsewhere in your set.

Pi = 3.1416…
T = track (width) of car at tire centerlines
Ri = radius of turn to inside tire
Ro = radius of turn to outside tire (Ri + T)
Di = distance inside tire travels through turn (Ri * 2 * Pi / 2)
Do = distance outside tire travels through turn (Ro * 2 * Pi / 2)
Ci = circumference of inside tire (measure tire)
Co = circumference of outside tire (Ci * (Do/Di))
Stagger = Co - Ci

If you have the circumference of the outside tire, then…
Co = circumference of outside tire (measure tire)
Ci = circumference of inside tire (Co * (Di/Do))
Stagger = Co - Ci

Unfortunately, iRacing does not supply the size of the tires used in the simulator, so some guessing is involved. According to the 2010 Stafford Motor Speedway SK Modified® rulebook, the tire sizes for the SK are 26.0/13.0-15 M45 on the left side and 27.0/13-15 M46 on the right side, while the minimum right side circumference is 84" @ 20psi.

I was unable to find a specific track width for the SK Modified, but for an example, let's use 70", and for the inside tire circumference, 84". If the turn radius of a track is 235' (Stafford would be close to this), then…

T = 70"
Ri = 2820" (235 * 12)
Ro = 2890" (2820 + 70)
Di = 8859"
Do = 9079"
Ci = 84"
Co = 86"
Stagger = 2"

Another wrinkle is taking into account the tracks banking. Banking will reduce the effective radius of a turn, so the more banking a track has, the less stagger you need. Multiply the stagger value by the cosine of the bank angle. For Stafford, turns 1 & 2 are 9º, while turns 3 & 4 are 7.5º Since we can only tune for one angle, we use the average of 8.25º.
COS 8.25º = 0.9897
2" * 0.989 = 1.979"
That's close enough to 2" not to really matter, especially considering we aren't confident with our starting numbers. Even so, we might want to try the next step down from 2", which would be 1.875"

If we had higher banking, however, like Talladega's 33º, we'd have…
COS 33º = 0.8387
2" * 0.8387 = 1.68"
That's enough to make a difference, so we should try the iRacing value on either side of 1.68".

If the car has the same track with for both the front and rear tires, the same stagger should be used on both the front and rear. If they have different widths, then the stagger should be calculated for each.

1. "Circle Track Magazine - Race Cars, Dirt & Stock Car Racing, Sprint Cars & Racing Tech" Circle Track Magazine http://www.circletrack.com (Feb. 11, 2010)
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