What is Roll Out?
Roll out is a measurement that helps calculate gearing. It is something that is used more for foam tire setups, but can be applied to rubber setups, also.
Roll Out is defined as the distance a vehicle moves forward per revolution of the motor. It can change due to tire wear or a tire swap (on foam tires). Roll Out is measured using this equation:
Tire Circumference x Transmission Ratio x (Spur x Pinion) = Final Drive Ratio
Your Roll Out depends on your tires circumference. You can calculate the circumference by doing the math: Tire Diameter x 3.14 = Circumference. Or, you can do it the easy way and use a ruler to measure. Place a mark on the tire. Start with the mark pointing down and roll it along the edge of the ruler until the mark comes around to the bottom. This is your circumference.
Tuning with Roll Out
Roll Out is best used to match a faster racer’s gearing. Simply using the same spur/pinion combo will probably not work if he is using larger or smaller tires or his transmission ratio is different. If you know the circumference of his tires (and transmission ratio), you can use the above equation, determine his ratio, then plug in your numbers until you reach a similar ratio.
All rubber tires are basically the same circumference, and since wear isn’t that much of a problem, equal Roll Out is much easier to achieve. All that is basically needed is transmission ratio and you can find a faster drivers Roll Out.
Back to the RC Suspension Tuning Guide.