with offset engines and wonder why they were made like that. “How crude,” we’ll say about this primitive and barbaric practice.
That time is now. Something magical has happened in the world of wide rear tires that makes offset drivelines a thing of the past. It’s all about the evolution of transmission technology that now allows the engine to be located in the center of the frame while the belt clears the fat rear tire. This is due to the genius of Bert Baker, the guru of gearboxes. Baker, after single-handedly inventing the six-speed transmission for Harleys, also created the revolutionary right-side drive gearbox. He also went on to create his DSSC, or Drop Starter Slam Clutch, which narrows and cleans up the whole primary drive system.
There’s some interesting genealogy in these iterations of transmissions. When Baker conceived the original six-speed box he elected to leave the standard transmission alone and simply add-on a pair of gears to create a sixth speed. This retained all the lower five gear ratios exactly as they were, while adding a rev-reducing sixth gear. This was a compromise that had the advantages of being easily adapted and less expensive than a complete re-design, thus providing a relatively cost-effective sixth gear to ease into the marketplace.
In this system fifth gear is still direct, but top gear is indirect. There are, of course, trade-offs: inefficiency and noise. The sixth speed involved sending the power across the secondary transmission shaft, which generates more noise and more power loss than direct-drive.
Enter the latest DD6, or Direct Drive 6-speed, a fresh design which is completely engineered from top-to-bottom gears. In this set-up, the lower five gears are all indirect drive, while the sixth is direct. This direct-drive is sometimes called a “no-load” gear because it serves as nothing more than a jackshaft, where the primary drive transfers to the final drive through a single bearing without spinning another transmission shaft and without loading other gears with drive stress.
The theoretical engineering advantages of this no-load technology are that it is quieter and more efficient, and it eliminates wear and tear on gears during long hours of top-gear touring. But for practical purposes we have to ask, is it really that much better to the rider? Having ridden Harleys with both of these overdrive systems I can tell you, yes, the DD6 gearbox is functionally superior to its predecessor, at least in terms of noise. In top gear this new DD6 is noticeably quieter to the rider. When shifted into sixth, both the engine and the driveline quiet down as they happily mutter along at a more relaxed pace—it’s less fatiguing for the rider.
We spend a lot of attention on engine performance. Some performance nuts will spend $2,000 or even $4,000 on engine modifications, then—and I am mystified by this—complain that they lost 5 mpg. But what about improving your Harley’s performance by changing the bike’s overall gearing and adding overdrive?
I haven’t amassed enough comparative mileage data between the indirect and the direct drive systems to draw any conclusions about fuel efficiency. I do know that an overdrive on a Harley seems to improve fuel mileage by at least 4 to 5 mpg. The average mileage of the 2004 EFI Heritage Softail I’ve been riding equipped with the new Screamin’ Eagle six-speed went from 38 to 42 mpg with the overdrive. At a cost of around $2,500 for a complete six-speed installation, it would take a lifetime of riding to recover the cost, so fuel mileage, while psychologically significant, has little impact on the financial balance sheet.
Baker’s latest DSSC innovation, the slammed clutch, is the final step in the world of overdrive six-speeds that brings his entire driveline development to its ultimate conclusion. His philosophical approach to the original right-side drive was the same as the premier OD gearbox: Make it easily adaptable to current Harleys. Thus, he had simply removed the final drive sprocket
from the left side of the transmission and moved it
over to the right side, vacating the gap between the primary drive and the gearbox. Which means that the
primary case still hangs out on the left like a big lump.
The DSSC, however, effectively slides (or “slams”) the primary drive and clutch over to the gearbox, filling that gap. Naturally this narrows the overall width of the drive system. It also makes the primary-drive side
of the motorcycle look incredibly clean and sleek.
When you combine the overdrive, the six speeds, the right-side drive and the slammed clutch, you have
the culmination of technology that has great potential for Harleys and for the future of wide tires. Now we can have those big, fat, wide tires, complete with overdrive and six speeds, in a finely balanced motorcycle. That’s never been possible before, but is now thanks to the vision and magic of the Wizard of Cogs. U
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