Tech Check: Gear Ratio

Each motor needs a specific gear ratio to run with best efficiency. Many motor manufacturers offer gear ratio tables. Below you can see an example from Yokomo. In the left column are the motors defined by turns. Depending in which type of chassis you want to use the motor, the gear ratio changes.

Motor Touring Car 2WD Offroad 4WD Offroad Drift
6.5T 1:8.5 1:10.0 1:10.5 -
7.5T 1:8.0 1:9.5 1:10.0 -
8.5T 1:7.5 1:9.0 1:9.5 -
10.5T 1:7.0 1:8.5 1:9.0 1:6.5
13.5T 1:6.5 1:8.0 1:8.5 1:6.0
15.5T 1:6.0 1:7.5 1:8.0 1:5.5

A gear ratio of 1:8.0 means that if the motor rotates eight times, the wheels spin one time. A long gear ratio would be 1:5.0 for example. With each rotation of the motor, the car moves a further distance compared to a short gear ratio (1:10.0 for example). A car with 1:5.0 gear ratio would spin it's wheels one fifth of 360 degrees while the car with 1:10.0 would spin it's wheel only one tenth of a full rotation.

wheel spinUsing a long gear ratio of 1:5, the wheel spins one fifth of it's circumference when the motor makes one complete rotation. It covers double the distance of a car with a gear ratio of 1:10.

The higher turned motors of the same manufacturers and series have more torque, yet lower RPM. On brushless motors RPMs are given by kV. kV means RPM per volt. If you use a 7.2V battery, a 5000kV motor would spin 36,000 RPM (7.2x5000).
On the table above you can see that the gear ratio for drift is longest. The drift tires slip very quickly and therefore there is not much load on the motor. High grip tires mean more stress for the motor; more torque is needed to move the wheels when they grip to the tarmac. Therefore a shorter gear ratio is used to avoid overheating of the system.

If you do not gear your motor properly, you might destroy motor and ESC. So how can we calculate the gear ratio?
First you have to know the fixed (second) gear ratio in your car. Usually you can not change the gear ratio of this pair of gears.
Divide the number of teeth of the big gear by the number of teeth of the small gear. Yokomo often uses 1:2.35 as a second gear ratio.

second gear ratioSecondary gear ratio on belt driven cars. Divide the number of teeth of the differential gear (green circle) by the number of teeth of the center pulley (red circle).

second gear ratioSecondary gear ratio on shaft driven cars. Divide the number of teeth of the differential gear (green circle) by the number of teeth of the center pulley (red circle).

So we can change the spur gear and the pinion gear to change the final gear ratio. Each chassis has it's limits on which size of gears can be used. It is best if you do not vary too much from the original spur gear included in the kit. Changing the pinion gear is quicker than changing the spur gear.

To get the final gear ratio you have to multiply the first gear ratio with the second. The first gear ratio is calculated the same way like the second. Divide the number of teeth of the spur gear by the number of teeth of the pinion gear.
To sum it up: In this example we have a spur gear with 70T, a second gear ratio of 1:2.35 and we want to use a 10.5T motor for drifting. We check the table above and see that a final gear ratio of 1:6.5 should be fine.

Final gear ratio = first gear ratio x second gear ratio

Let's put real numbers in this equation:
6.5 = 70/pinion gear x 2.35

Now we solve the equation by the pinion gear and get
pinion gear = 70/6.5 x 2.35

This gives us a pinion gear with 25.3 teeth. Since there is no pinion gear with 25.3T, we would try a 25T gear for a final gear ratio of 1:6.61. It is a little bit shorter than the recommended gear ratio. Or a 26T pinion gear for a fast track (1:6.35).

Of course the spur gear can also be changed to fine tune the gear ratio.

When you got a new motor and are not sure about the gear ratio, check the temperature of the motor and ESC after 30 seconds of driving. If they do not become hot, check again after a few minutes. If they do get hot, use a shorter gear ratio (samller pinion gear or bigger spur gear).

Gears come with different teeth sizes. Most common is 48dp (diametrical pitch). The variation with smaller teeth is 64dp. Alternatively your car might use module 0.4 or module 0.6 (0.4 has smaller teeth). Always use gears of the same pitch/module. Refer to the manual of your chassis to find out which pitch/module your car uses.

Gear Ratio Calculator

Below is a calculator which helps you to find the best gears quickly.
The text field you keep empty will be calculated. The second gear ratio will not be calculated. Please do not leave it empty. You can use the second gear ratio from the selection box or input it directly into the text field. Please use "." as a separator.
After you press the button the gears will be calculated. Also a small table with adjacent gears is generated.

Spur Gear T / Pinion Gear T x Second Gear Ratio =

Second gear ratio

Counter Steer Ratio on Street Jam OTA-R31

This form helps you to calculate the counter steer gear ratio which can be adjusted with Gear Set for Street Jam's OTA-R31 chassis.
Please note that the gear size in the illustration is just schematic.
ota-r31 gears
Front diff gear (F1) T Front belt pulley (F2) T
Center belt pulley 1 (R1) T Center belt pulley 2 (R2) T
Rear belt pulley (R3) T Rear diff gear (R4) T

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