Six-Speed Transmissions

But there is a penalty to pay for the overdrive design. High gear, regardless of whether it is fifth or sixth, should be the most efficient and quietest, because it is where you spend the most time riding. That is the reason stock Harley transmissions are designed with a direct drive high gear instead of an overdrive. With direct drive, there are no gear reductions between the main and counter shafts, resulting in less power loss and reduced noise. In other words, the input shaft and drive gear are locked together, so power enters on the input shaft and leaves immediately on the drive gear. However, with a true overdrive tranny, power for the overdrive gear flows through gearsets on both the main and countershafts, reducing power by up to 8 percent while increasing noise. This is where the direct drive six-speed comes into play.

Direct Drives
With a direct drive six-speed, high gear is not an overdrive gear. Instead, it has a 1:1 ratio like fifth gear has in a stock Harley five-speed transmission, and, of course, power exits directly through the drive gear. And since power is not flowing through two meshing gearsets, high gear noise and power losses are reduced while fuel mileage is increased. However, by using a 1:1 sixth gear, we have lost the benefit of the overdrive's rpm reduction in top gear. The dilemma is eliminated by overdriving the primary drive using a larger (more teeth) compensating sprocket on the engine. That results in a six-speed, which ratiowise is exactly like a six-speed overdrive transmission. The internal transmission ratios of first through fifth gears are also typically changed on a direct drive six-speed so that the first five gears retain the same overall ratios that they have in a stock five-speed but with a higher sixth gear, which is direct driven instead of overdriven.

Interestingly, the 2006 Dyna and 2007 and later big twin six-speed transmissions use a direct drive (called Cruise Drive) instead of an overdrive design. In contrast, the factory's complete six-speed transmission and six-speed gearsets sold for upgrading a 2006 and earlier five-speed transmission are a true overdrive and not direct drive.

There are several additional benefits to a direct drive six-speed. First, direct drive six-speed transmissions typically have more evenly spaced gears than a stock five-speed, which eliminates wide gaps between gears while providing excellent power transfer and more even upshift points in the rpm band. Another benefit is that unlike an overdrive six-speed, where the top gear robs power from the drivetrain, a direct drive's sixth gear is a power gear like the other five gears. So if you are into high performance, a direct drive six-speed is the way to go.

Straight-cut vs. Helical-cut Gears
Older big twin transmissions came supplied with only straight-cut gearsets. Starting with the 2006 Dyna and all 2007 and later big twins, both straight-cut and helical gear designs are used in big twin transmissions. Helical gears have an angled tooth pattern, which results in multiple teeth in mesh 100 percent of the time. This produces a higher contact ratio and reduces gear noise, but it also increases side loading while being slightly less efficient.

Other Gearing Considerations
To get a taller high gear for less vibration and increased fuel mileage on long-distance interstate riding, you can raise your bike's final gearing by changing sprockets or pulleys on the transmission and rear wheel instead of installing an overdrive or direct drive six-speed. However, the problem with doing this is that all the lower gears would also be made taller. That would decrease acceleration and make cruising in hilly or mountainous terrain more difficult. On the other hand, a six-speed overdrive or direct drive transmission provides the best of both worlds, because it will add a taller gear for pleasant and efficient highway cruising while retaining crisp acceleration in the lower gears.