Gear ratio basics

[Plex]

Beginners and experts alike often ask "How do you determine a gear ratio?" . Well, figuring the ratio for a car is easy, figuring out what ratio to run is the hard part!


Grab a calculator, a pad of paper and a pen, there is some simple math that will allow every racer to figure out a gear ratio, overall ratio and how to adjust a ratio for tire wear.


To find the gear ratio, simply divide the spur gear by the pinion gear. As an example, a 120 spur and 20 pinion equals a 6 to 1 ratio. This means that for every 6 turns the motor rotates, the wheels rotate 1 time.


When using a car with a gearbox, simply multiply the ratio by the internal gearbox ratio to get an overall ratio. The internal ratio is usually listed in the instructions for the car or truck. Using the 6 to 1 ratio from the 120 spur and 20 pinion, if a gearbox had an internal ratio of 2.25, the overall ratio would end up being 13.5 to 1. Meaning the motor turns over 13.5 times for every 1 rotation of the wheels.


An important thing to remember, when comparing the ratio you' re using to the ratios other drivers are using, is tire size. The diameter of a tire drastically effects the choice of which ratio to run. All things being equal, tires with a larger diameter may require a smaller pinion and small diameter tires may require a larger one, to maintain the same performance.

Also, as you run a set of foam tires they naturally wear down in size. The smaller diameter means you must adjust the gear ratio to maintain the same top speed and performance.


To figure out how much of a gear ratio change is needed, simply divide the ratio by the old tire diameter then multiply by the new tire diameter and this will give you the new ratio to use. If the size of the worn tires measures 2.25, and the new tires measure 2.50, and the old ratio was 6 to 1, then figure out the new ratio like this: 6 divided by 2.50 multiplied by 2.25 equals a 5.40 ratio. (Then switch to a pinion that works with the spur to get as close to a 5.40 to 1 ratio as possible. In this case it would be a 22.)


Pretty simple stuff huh? You bet! Use a pair of calipers to measure the diameter of the tire. When using the formulas to figure things out, and the rear tires are two different sizes, use the average of the two (add them together and then divide by 2) or true them down to the smaller size.


As for figuring out which ratio to use when racing at a track for the first time, there is really no substitute for experience. after racing different tracks, and noting what ratios you use for the length of each, you' ll be able to take "an educated guess" on a ratio to start with. Then you' ll have to use lap times, and feel to get the ratio dialed in. Of course it never hurts to ask a few fast locals what they a using!

Gear ratio: Spur divided by the pinion= ratio.

Overall ratio: Gear ratio multiplied by gearbox internal ratio = overall ratio.

Ratio change for tire wear: Old ratio divided by old tire diameter multiplied by new tire diameter = new ratio.

Source: http://www.jphracing.com/Gear_Ratio_Basics_by_JPH_Racing.cfm