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Understanding RC Gearing

3/14/2012 by Gary Katzer

Copyright:© 2012 Horizon Hobby, Inc.

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      One of the most common questions we hear about RC cars and trucks is, “how fast does it go?” While this seems like a common, ordinary and easy question to answer, the truth is that there are a number of different factors that contribute to a vehicle’s speed, torque and acceleration. One of the big determining factors is the gear ratio and transmission of your vehicle. While things like installing a faster motor or higher voltage battery are easy things to understand, wrapping your head around gear ratios can be like taking an advanced course in physics, geometry, algebra and trigonometry. While there’s a lot going on, there’s some simple ways to get a basic understanding of what the gears in your transmission do, what changing them means and how things work. We’ve also put a list of commonly used terms together for you below for quick reference to help you as you progress.

Ball Differential – A differential that utilized a series of ball bearings, thrust bearings, differential rings, tensioning spring and thrust washers. When going around a corner, the tire on the inside will need to rotate more slowly while the tire on the outside will need to rotate more quickly. Ball differentials can be infinitely adjustable, but require more maintenance (distance traveled)

Clutch Bell Gear – The gear that mounts to the crankshaft of a nitro- or gasoline-powered engine that is used to drive the spur gear.

Differential Gear – The gear that drives and houses the differential components. Typically the differential gear is mounted on the bottom of a transmission case in a 2WD vehicle.

External Gear Ratio – The gear ratio of the gears that are visible or outside of a transmission case. This is the ratio of the pinion and spur gear.

Final Drive Ratio – The gear ratio of the entire drive system in a car or truck. This includes the internal gear ratio of the transmission and the external gear ratio of the pinion and spur gears. You can calculate this ratio by dividing the spur gear tooth count by the pinion gear tooth count and multiplying that total by the internal transmission ratio.

Gear Differential – A differential that uses gears instead of ball bearings and allows wheels to rotate at different speeds. When going around a corner, the tire on the inside will need to rotate more slowly while the tire on the outside will need to rotate more quickly. The differential compensates for this difference in distance traveled.

Gear Down – To install a smaller pinion gear or larger spur gear to increase the final drive ratio of your transmission, resulting in a higher gear ratio. Gearing down will generally reduce top speed but increase torque and acceleration.

Gear Mesh – The clearance between two gears as they spin against each other. A gear mesh that is overly tight can cause excessive wear and drag, causing your motor to overheat and reducing performance. A gear mesh that is too loose can cause excessive gear tooth wear or stripping of the teeth on the gears, or other potential damage.

Gear Ratio – A gear ratio is the relationship between the number of teeth on two meshed gears. This relationship is expressed mathematically. For example, if one gear with 50 teeth is driven by a gear with 20 teeth, the gear ratio is 2.5:1. You would calculate this by diving 50/20 for a total of 2.5.In an RC Car, you have 2 different ratios you'll need to work with to achieve the proper gearing for a specific application and goal. The first is the external ratio and the other is the internal ratio.

Gear Up – To install a larger pinion gear or smaller spur gear to reduce the final drive ratio of your transmission. Gearing up generally increases top speed at the expense of torque and acceleration.

Idler Gear(s) – Gears mounted inside a transmission case between the input gear and the differential gear. The purpose of the idler gear is to change the rotational direction.

Input Gear (or Top Shaft Gear) – The gear attached to the input shaft on the transmission that directly meshes with the idler gear. The spur gear mounts to the same shaft as the input gear shaft on the outside of the transmission case.

Internal Gear Ratio – The gear ratio of a vehicle's transmission gears separate from the pinion and spur gear ratios. The internal ratio is a fixed number determined by the manufacturer of your vehicle.

Module – For countries that use the metric system, Module is the equivalent of pitch. It typically refers to the pitch diameter, in millimeters, divided by the number of teeth. The higher the number on the pitch, the finer the tooth profile is. The common module types are 1.0 module and 0.6 module.

Pinion Gear – A small gear that directly attaches to the output shaft of an electric motor.

Pitch (AKA Pitch Diameter) – In countries that do not use the metric system, pitch refers to the number of teeth on a gear with a 1-inch pitch diameter. The higher the number on the pitch, the finer the tooth profile is. The finer the pitch, the more efficient the gear; however, since they have less material, they are often more prone to stripping if the mesh is not set properly. The more coarse the pitch, the less efficient the gear will be; however, they are also more durable and less susceptible to damage. 48 pitch is the most common pitch in RC, however, 64 pitch and 32 pitch can also be used.

Sipper Clutch – A device mounted to the top input shaft on off-road vehicles designed to reduce wear and tear on a vehicle's transmission gears and drivetrain over rough terrain. The slipper clutch is made up of some sort of slipper pad material, a slipper plate, spring and your spur gear. This is adjustable by changing the tension on the spring.

Spur Gear – The large gear mounted to the top shaft of your transmission in an off-road vehicle, the center differential in an 1/8-scale or 4WD Short Course Truck or the main lay shaft in an on-road vehicle. The spur gear is driven by the pinion gear or clutch bell.

Transmission – A collection of meshing gears or pulleys designed to transfer and multiply the power from the motor or engine to the differential in order to drive the wheels. A transmission is designed to multiply the torque of a motor or engine.