Adjusting Gearing

Acceleration and Top Speed can be adjusted by changing the gearing in your car.  If the car needs more acceleration or more "punch" out of the corners, then drop 1-2 teeth on the pinion gear.  If the more top end speed is desired, then add 1-2 teeth on the pinion gear.  These are the first basic steps in adjusting gearing.  If you are racing, you will probably want to have a set of pinion and spur gears in your toolbox.

If the motor is getting too hot, it is a recommended practice that you drop teeth on your pinion gear.  When a motor gets too hot, it is not being used efficiently.  Energy is being wasted by creating heat instead of getting power to the drivetrain, wheels and tires.  Hot motors can also be the result of other factors in the car such as drivetrain resistance, motor problems or other electrical disturbances.

How do I find my Gear Ratio ?

The gear ratio is simply dividing the spur gear by the pinion gear.   

(Spur gear / Pinion gear ) = Gear Ratio

(122 / 35 ) = 3.486

What is a Final Drive Ratio ?

The final drive ratio (FDR) is calculated by multipling the Gear Ratio by the car's gearbox reduction ratio.  The gearbox reduction ratio is can usually be found in the kit's manual.  (Ex:  HPI RS4 Pro2 Gearbox ratio = 2.133)

(Spur gear / Pinion gear) = Gear Ratio * Gearbox = FDR

(122 / 35 ) = 3.486 * 2.133 = 7.435

The motor makes 7.435 revolutions for every single revolution of the tire.

This is a Gear Ratio Chart for the HPI RS4 Pro2.  The Pro2 kit comes with a 35 tooth pinion gear and a 122 tooth spur gear.