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For drag racing enthusiasts, achieving the perfect launch is paramount to securing victory on the strip. However, one often-overlooked aspect of achieving that perfect launch is tire pressure and rollout measurements. Brands like Mickey Thompson, Hoosier, Goodyear, M&H, and Phoenix offer high-performance drag tires tailored to meet the demands of competitive racing. Understanding the nuances of tire rollout procedures, including proper measurement techniques and the significance of tire pressure, can make all the difference between a blistering start and spinning wheels at the line. In this article, we delve into the essential procedures for measuring rollout with precision, providing drag racers with the knowledge they need to optimize their tire setups and gain that crucial edge on race day.
What is Tire "Roll-Out" and how do you measure it?
Roll out (circumference) is the number of inches the tire rolls before it has traveled back to its initial starting point. A tire can be measured for circumference or roll out by using a tape measure. Start by setting the mounted and inflated tire in a vertical / rolling position. Insert the end of the tire tape measure into one of the dimples of the tire in the center. Next, roll the tire, keeping the tape measure centered. Once you reach the beginning of the tape measure, record the measurement to the nearest 1/8".
How Do You Measure The Outside Diameter Of A Tire?
Overall Diameter (O.D.) is a measurement of a tire taken from the top of the "face"/ side of the tire to the bottom "face" /side of the tire. Note: The O.D. of a tire is measured with the tire off of the vehicle, not under load, and at appropriate air pressure. The most accurate way to calculate the OD of a tire is measuring the circumference and dividing by 3.1416”. (Example: Circumference = 88.0” ; therefore 88.0 divided by 3.1416 = 28.01”) Knowing the O.D. of a tire will aid in fitment issues and can assist in gear ratio and speedometer corrections.
Tire Diameter & Circumference Calculator
Enter either the tire diameter or the tire circumference to calculate the other measurement.
Roll-Out Measurement Procedures By Manufacturer
Each tire manufacturer requires a slightly different method to determining roll-out. This is due to the material of the tire and how it is produced from the tire mold. Below are the individual procedures from to drag racing tire manufacturers.
Mickey Thompson
Mickey Thompson bias race tires (ET Streets and ET Drags) are made with a nylon construction. For this reason, it is necessary to use the following procedure for checking the rollout (circumference) of tires when they are first mounted. Always check the size before installing rim screws or putting the tires in use.
Before mounting the tires, M/T recommends checking the date codes. A natural characteristic of nylon is to shrink over an extended period of time. The dates should be within six months to help ensure close measurements. If they do not, please contact your dealer or point of purchase.
After mounting the tires, take the air pressure to 20 psi then immediately drop to the pressure you plan to use at the track (example - 12 PSI). Important Note: never let one tire sit at a higher air pressure for an extended period of time. To clarify, do not air one tire up, drop it immediately, then let the other tire sit at 20 psi for while. This will give inaccurate numbers as one will have stretched. Before the tires are run they should always have the same air pressure for the same amount of time unless you are trying to stretch it.
Tires must be matched within 1/2” maximum before they are run. If they exceed this difference, it can usually be corrected. This is done by over inflating the shorter tire and letting it sit a short period of time to allow it to stretch. It is very important to monitor this procedure to avoid stretching too much. When stretching tires always drop them back to "0" psi,, then back up to your track pressure. This will show if it has actually stretched or not. This should be done as close to running the tires as possible. Never inflate tires over 40 PSI to stretch them. If you cannot get the tires to match, please contact JEGS.
Mismatched tires exceeding 1/2” cannot be exchanged if:
This procedure has not been followed.
The tires have been put into use.
Rim screws have been installed.
Again, this procedure will only work before a tire has been used.
All Mickey Thompson® tires are listed in specification sheets with an O.D. (overall diameter) or circumference. These measurements are derived in the following manner.
M/T® Slicks:
All tires taken to 20 PSI then:
Measuring rim at 10 psi cold.
(Exception: 33" drag @ 6 psi)
Hoosier
Hoosier Drag Tire Rollout (Circumference) Measuring Procedure Roll out is the number of inches the tire rolls before it has traveled back to its initial starting point. Measurement Procedure: 1. Once the tire is mounted to the wheel inflate to 20 PSI and then start deflation. 2. Once the tire reaches race pressure (6 PSI for big tires and 12 PSI for small tires) use ¼” wide measuring tape and place it around the center of the tire. 3. If they are within 1/2" of each other the tires are a match; if you wish to get them closer take the smaller one back up to 25 PSI let sit for one minute; deflate and measure again. 4. If tires are ¾” or more off in circumference they will need to be swapped out.
Goodyear
Goodyear drag slick measuring must be done before new tire is run and before installing rim screws. Tires must be inflated to 20 psi and then down to race pressure before matching in circumference. The circumference is measured at the center of the racing tire and variance should be no greater than 1/2" inflated pressure. Note: radial tires are excluded from this roll-out procedure.
M&H
M&H Drag Tire Rollout (Circumference) Measuring Procedure Roll out is the number of inches the tire rolls before it has traveled back to its initial starting point. Measurement Procedure: 1. Once tire is mounted to wheel inflate to 20 PSI and then start deflation. 2. Once tire reaches race pressure (10 PSI for big tires and 12 PSI for small tires) use ¼” wide measuring tape and place it around center of the tire. 3. If they are within 1/2" of each other the tires are a match; if you wish to get them closer take the smaller one back up to 25 PSI let sit for one minute; deflate and measure again. 4. If tires are ¾” or more off in circumference they will need to be swapped out.
Phoenix Drag Tires
Phoenix Drag Tire Rollout (Circumference) Measuring Procedure Roll out is the number of inches the tire rolls before it has traveled back to its initial starting point. Measurement Procedure: 1. Once tire is mounted to wheel inflate to 20 PSI and then start deflation. 2. Once tire reaches race pressure (10 PSI for big tires and 12 PSI for small tires) use ¼” wide measuring tape and place it around center of the tire. 3. If they are within 1/2" of each other the tires are a match; if you wish to get them closer take the smaller one back up to 25 PSI let sit for one minute; deflate and measure again. 4. If tires are ¾” or more off in circumference they will need to be swapped out.
