In 1964, the Harley-Davidson Motor Company smacked an electric starter on its big twins and launched a completely new world for motorcycle riders. The first starter motors, which were really cranking motors, were adaptations from the automotive industry. Not much has changed since 1964.
True, there has been a selection of different cranking motors installed on Harley-Davidson big twins since the first one appeared on Panheads, but the basic location of the cranking motor (above the transmission, bolted to the inner primary cover) has remained the same. Compu-fire Performance Products in Pomona, California, re-thought and redesigned the entire cranking motor system for big twins by placing the cranking motor in a new central location. Compu-fire mounted the starter motor to the inner primary plate, between the engine's output sprocket shaft and the clutch drum (which is mounted to the transmission's main shaft). This unique and innovative cranking motor configuration is available and works in conjunction with a Rivera/Primo beltdrive.
Cranking motors turn electrical energy, which is stored in the battery, into mechanical energy by powering the cranking motor, which spins the engine. Most batteries are designed to provide maximum cranking battery power to the cranking motor at 8 volts. The amount of cranking amps varies from battery to battery, depending on its construction. The amount of amperes sent to the cranking motor dictates the amount of rpm's the cranking motor's armature turns. This is the torque value of the cranking motor. The cranking motor is not large enough to turn the engine over by itself, it needs additional help. That assistance comes in the form of a gear reduction which is bolted to the primary side of the starter motor and drives the ring gear on the clutch basket.
When the starter button is depressed, electricity flows from the battery to the cranking motor, kicking-off a chain of events, which starts the engine.
Remember, the cranking motors used on big twins originated from the automotive world, where they have the advantage of much greater battery amperes and higher-torque motors. The automotive industry arrived at what it considered the optimum combination for cranking the ring gear, mounted to the engine's flywheel: A ratio of 60:1 (eight cylinders or two, it doesn't matter). Most non Compu-fire cranking motors currently available operate using a 21.6:1 ratio. The Compu-fire Gen III starter raises this ratio to 44:1 when used with the 10-84 pinion/ring gear set. The new engine-driven D2S further raises the ratio to 60:1. These ratios are dictated by the number of teeth on the armature gear combined with the number of teeth on the ring gear, mounted to the clutch basket. It can use this optimum ratio because it can match a ring gear bolted to the alternator rotor to a specific-sized pinion gear on the cranking motor. With the optimum ratio employed in this manner, battery life is extended. There are no worries about drawing too much current from the battery. Any ignition will fire and there's ample current to power any fuel-injection system.
The D2S is the answer for large-displacement engines, with high cylinder cranking pressures. They may be tough to start using conventional systems, but the power of the D2S will even kick-off the larger-displacement engines under adverse conditions of cold weather or low battery charge. The Direct Drive Starter System from Compu-fire will be available by the time you read this article.