Motorcycle V-Twin Drivetrain - Drivetrain Basics

Understanding Your Gearing Can Lead To Improved Performance

By William Denish

Photography by William Denish, Manufacturers

With the abundance of performance options for the V-twin, it is easy to spend a small fortune on building a stout engine. Unfortunately, drivetrain gearing is often overlooked when making performance modifications, but it is crucial if your bike is to perform well during stop-and-go riding and highway jamming. Understanding a few basic principles about your bike's drivetrain will help get the engine's power to the ground where it really counts and will make stop-and-go riding or occasional bursts down the road more enjoyable.

The V-twin's drivetrain is divided into three distinct assemblies: primary drive, transmission and secondary drive. The primary drive consists of an engine output sprocket or pulley (attached to the crankshaft), clutch assembly, roller chain or belt, and housing that encloses the entire assembly. The purpose of the clutch is to engage the primary drive to the transmission. The secondary drive includes a transmission output sprocket or puley, belt or roller chain, and rear wheel sprocket or pulley.

The function of the drivetrain is primarily two-fold: transmit power from the engine to the rear wheel and to provide gear reduction. Gear reduction is a method for multiplying engine torque. That is important because it takes more torque to get a bike moving from a standing start than it does to keep it going once it is moving. For example, to get moving from a standing start, early model Harleys had to be pedaled because they had little gear reduction (due to a lack of a transmission) to multiply engine torque. With gear reduction, rotating the crankshaft three times for every one rotation of the rear wheel provides greater torque and acceleration than rotating the crank two times for every revolution of the rear wheel.

Gear Reduction
In the case of an automobile, there are two locations where gear reduction takes place: the transmission and the rearend or differential. However, in the case of the V-twin, there are three: engine sprocket or pulley, transmission output sprocket or pulley and rear wheel sprocket or pulley. While the transmission offers changeable gear reduction, the primary and secondary gear reduction remains set during operation. With this design a common and easy place to make a gear reduction change on a V-twin is at the rear wheel. However, with secondary belt drives, due to the lack of different size pulleys, gearing options can be limited. The transmission is another typical location for gear reduction changes, but again, with secondary belt drives the options are limited. Additionally, making gearing changes at the transmission on a Big Twin requires removal of the entire primary drive assembly. If very high (low numerically) gear ratios are required, changing the engine`s drive sprocket or pulley in the primary drive becomes a necessity.

Gear reduction is expressed as a ratio that represents the number of input revolutions for each output revolution. As an example, a ratio of 3.37:1 means that there are 3.37 input rotations for every output rotation. To calculate a gear ratio, divide the number of output revolutions by the number of input rotations. The primary and secondary drives along with the transmission are assigned ratios. The product of these ratios is called the final drive ratio (reference Figure 1).

Primary Drive RatioThe primary drive ratio is determined by dividing the number of teeth on the clutch sprocket or pulley (output revolutions) by the number of teeth on the engine sprocket or pulley (input revolutions). For example, if we assume a Big Twin has a 37-tooth clutch sprocket and a 24-tooth engine sprocket, that results in a 1.54:1 primary drive ratio (37 divided by 24 equals 1.54).