Understanding Gear Ratios

Adapted from an article which first appeared in Racing Lines, No. 80, November 2001. For more information on this Australian magazine, please email the editor David Smith: modmags@ozemail.com.au

o matter what car you have, or whether you have the latest batteries and motor or high performance blue-toned engine, unless you have the right gear ratio for the track you are on, you will not get the best performance from your car. Even if you are just a sports racer who wants only to run his car up and down the street or at the local track, a basic understanding of gear ratios will help you get even more fun out of your car. Gearing plays an enormous part in your car's ultimate performance. The most asked question in R/C cars is, "how can I make my car go faster?" Nearly as often are questions like, "why does my battery dump so soon?" and "why is my motor so hot?" While not the only solution to those problems, there's no doubt that getting your car's gear ratio right will go a long way to resolving them.

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We should define the term "gear ratio" right from the start. In its simplest form, it defines the relationship between any two gears. Usually this will be the "pinion" gear (found on a motor in electric cars and on the clutch bell of a nitro car) and the "spur" gear (which more often than not is found on the drive axle of a car). Pinion gears come in all shapes and sizes, as do spurs but for our example we'll say hat the pinion gear has 18 teeth and the spur gear has 90 teeth. You often see in car specification charts we publish in Racing Lines a gear ratio of 18:90. Often it is divided into its lowest common denominator which would be 5:1 after we divide the number of teeth of the pinion (18) into the number of teeth of the spur gear (90). _________________________ _________________________ 90 divided by 18 = 5. What all this means is that the pinion gear must turn five times to make the spur gear turn once.

The above example is for what we know as direct...