R/C Car Gear Ratios

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R/C Car Gear Ratios 

The gears of a car follow a simple concept but can have an enormous effect on the performance of the vehicle. The diameter of the gears should be adjusted to match the specific application in which the vehicle will be used for. This means that a different gearing will be used for a large track with sweeping turns than for a small track with tight turns. A simple explanation of gearing is necessary. Basically, a large pinion gear (the little gear attached to the motor) will offer increased top speed and decreased acceleration and a smaller pinion gear will offer decreased top speed and increased acceleration. The opposite is true for spur gears (the large gear attached to the tranny that the pinion gears runs against), a smaller spur gear will increase top speed and decrease acceleration while a larger spur gear will decrease top speed and increase acceleration. A standard way of comparing gear sizes between cars is to use Gear Ratios. 

There are 2 basic types of gear ratios for your RC car, the pinion to spur ratio and the overall drive train ratio. To equate the pinion to spur ratio, a simple formula is used: 

Pinion to Spur gear ratio = # of teeth on spur gear / # of teeth on pinion gear 

To give an example, assume you have a 25 tooth pinion gear and a 100 tooth spur gear installed on your car. The pinion to spur ratio is 100/25 = 4. Therefore; your pinion to spur gear ratio is 4 to1. This means that the motor must rotate 4 times to rotate the spur gear once. 

The other basic gear ratio is the overall drive train ratio. To equate this, the following formula is used: 

Overall Drive Train Ratio = (# teeth on spur gear / # teeth on pinion gear) x Tranny Gear Ratio 

Here is an example. Assume you have a 25 tooth pinion gear and a 100 tooth spur gear installed on your car. Your transmission gear ratio is 2.25 to 1 (the transmission gear ratio is complicated to equate but it is usually given to you in the transmission instructions). The overall drive train ratio is (100 / 25) x 2.25 = 9. Therefore; your overall drive train ratio is 9 to 1. This means that the motor must turn 9 times to fully rotate the wheels of the car once. 

Sometimes people refer to the gear ratio of the car instead of the number of teeth on the pinion and spur gears. A low gear ratio number will offer increased speed and decreased acceleration and a high gear ratio number will offer decreased top speed and increased acceleration. For example, a 3 to 1 ratio will give your car more speed and less acceleration than a 5 to 1 ratio. 

It is important to understand that there are limitations to what an electric motor can handle when it comes to gearing your car. This means that a larger pinion and a smaller spur will increase your vehicles top speed up to the point in which the motor is overloaded. When the gear ratio is too low, the motor will not have enough output to benefit, and reduced performance can be expected. The key here is that a proper gear ratio is very important. Don't just put on the largest pinion gear and smallest spur gear that you can find and expect to go fast. 

Another issue regarding gear ratios is tire size. The size of the cars tire will have an effect on the speed a vehicle reaches. A larger tire is like running a lower gear ratio and a smaller tire is like running a higher gear ratio. The formula used to calculate gear ratios that take tire size into consideration are very complex and aren't necessary for most applications. Just remember that a larger tire will increase top speed and decrease acceleration and a smaller tire will decrease top speed but increase acceleration. 

There is no strict rule for setting up gear ratios in your vehicle. Different cars, tracks, motors, batteries, driving style and many more variables all affect the size of the gears to be used. The best method of setting up your own ratios is to start with manufacturer recommendations and then experiment on your own. Use a stop watch to see if changes you make are for the better. The proper gear ratio selection can make the difference between an A main finish and no finish at all.