hey call it their “next generation transmission” and you’d be hard pressed to come up with a better description. Baker Drivetrain’s Direct Drive 6-Speed is, as the folks at Baker say, pretty special and it’s definitely “next generation.” There are benefits galore to this retrofit package, all making this complete transmission or Builder’s Kit gear set, your choice, a pretty desirable piece. A pretty popular one, too. And then at the opposite end of the drivetrain spectrum there’s the Baker 6-Into-4, a true overdrive 6-speed that’s a direct bolt in fit for classic Shovelhead applications. Here’s modern technology, modern metallurgy and modern design all wrapped up in a real overdrive 6-speed package–complete with a kicker. Next-generation or old-school, Baker Drivetrain isn’t missing a lick.

The husband and wife team of Bert and Lisa Baker have created a company that has opened new possibilities for the custom V-twin market that few would have thought possible a decade ago. Photo by Don Emde

     But then Baker Drivetrain never has, not since Bert and Lisa Baker took their GM engineering experience and turned it toward motorcycles. More than a decade ago they came up with that first-generation 6-speed overdrive transmission that wowed the biker world. It’s still a great choice for custom builders everywhere. But Baker hasn’t sat still over those years, satisfied with that first OD 6-speed. A good thing, they figure, can be made even better and that’s what’s behind the DD6, the “next generation” overdrive, the Direct Drive 6.
     Back in ‘97 when Bert and Lisa Baker worked up that now legendary Baker 6-speed Overdrive (the OD6) they drew on their General Motors engineering background to neatly rework the factory Harley-Davidson 1-to-1 top gear ration 5-speed transmission design into a 6-speed overdrive variant. The design was similar to what the car companies had done in the ‘60s with their 1:1 4-speeds. The original Baker overdrive-6 retained all the same 1st through 5th gear ratios found in a stock transmission but added that overdriven 6th for smooth highway cruising. It worked great and still does. It’s the transmission that made Baker Drivetrain. But Bert and Lisa knew they could do better.
     The new DD6 is a 6-speed overdrive, but this time with a 1:1 top gear, thus the name “Direct Drive” 6-speed. Why they did that is going to take some explanation. How can this be an overdrive if the top gear is 1:1, straight through? Well, Baker has accomplished this with some crafty juggling of the primary drive and the gear ratios inside the transmission, 1st through 5th. It starts by overdriving the transmission in the primary with a 28-tooth drive sprocket and then dropping the gear ratios in 1st through 5th by 14% to make up for that change. The result is that those gear ratios in 1st through 5th are pretty much as they are in a stock Harley-Davidson transmission for great low-end acceleration, and then 6th, which is now a 1:1 direct drive through the transmission, becomes, in effect, an overdrive. It’s ingenious. Why do it this way? What’s so important about having a 1:1 ratio for 6th gear? It’s a lot more efficient, that’s why, and here’s the engineering explanation…
     In any transmission the 1:1 gear, the straight-through drive, is always the most efficient. It’s typically 99% efficient. As a comparison the 6th gear in an ordinary overdrive unit is around 90% efficient. Not bad, but it’s not 99%, either. To Bert and Lisa Baker it just made sense that the gear in which you spend the most time, 6th, should be the most efficient. And the DD6 is efficient. And consider this: In a non-DD6 overdrive transmission in 6th gear there’s a power transfer between the two gears that make up the 6th gear pair and also between the two gears for the 5th gear pair. Efficiency is lost spinning two sets of gears in the transmission, four gears total, to get one output speed, 6th. Not only do you waste horsepower doing this, you burn more fuel and you get a lot more transmission noise. Furthering the efficiency, the new Baker DD6 uses smooth-meshing and quiet-running helical-cut gears in the 4th, 5th and 6th gear positions. In these cruising gears, with their correspondingly lower torque-bearing requirements, the helical-cut 4th, 5th and 6th gears provide the quietest operation. It’s almost silent. The gear meshing is a graceful, rolling motion and it’s almost vibration free.
     And make no mistake, the DD6 with its quietly efficient 1:1 top gear is a real overdrive transmission. At 65 MPH with a standard Harley-Davidson transmission in 5th gear the engine RPM is 2,945. With the Baker DD6 in that 1:1 6th the RPM drops to 2,528. At 70 the 5-speed has the engine turning 3,172, with the Baker it’s 2,723. At 75 the tachometer numbers are 3,398 for the 5-speed, 2,917 for the Baker. And at 80 MPH it’s 3,625 for the 5-speed, 3,112 for the DD6. Next generation overdrive 6-speed? Absolutely. The Baker Direct Drive 6 is an overdrive transmission in its most efficient configuration, enhancing not only the ride quality but the fuel mileage, too. And even though it includes one more gear than a standard 5-speed the DD6 is no wider then a stock Harley transmission. The trapdoor is the same width as a stock Harley-Davidson piece, meaning there are no worries about oil-spout spacers or modified exhaust systems and brackets. The DD6 also features what Baker calls its “redundant neutral system,” a design using a spring-loaded ball that runs in a groove on the shift drum to more easily locate neutral. The Direct Drive 6-Speed is available as a retrofit “Builder’s Kit” that installs in any existing 5-speed transmission case from 1990 on, or it can be had as a completely assembled unit in a brand-new Baker case in a variety of finishes.
     Forward thinking stuff? You bet. The Baker DD6 is as new as tomorrow, but that’s not all that Baker Drivetrain is about. These folks from Haslett, Michigan, can honor the past, too, and do it with both feet firmly planted in the modern-engineering present. The Baker 6-Into-4 is proof of that. Here’s a genuine Baker 6-speed overdrive packed in a four-speed case, and for one of those increasingly popular Shovelhead bobbers this, definitely, is an elegant way to go. The Baker 6-Into-4 fits all 1970 to early 1984 Shovelheads and it’s a true overdrive 6-speed. Some five-speed conversions, remember, still top out with a 1:1 5th. Not here. This Baker’s 6th is a .086 overdrive so along with that cruise-friendly RPM drop in 6th gear you get all the good, acceleration ratios in 1st through 5th to really get that bike moving–and then kick it into 6th with all the benefits of a overdrive for top gear. A direct replacement for the 1970 through 1984 4-speed, this one’s even offered with or without the Harley-classic 1936–1984 style kicker. The best of the old, the best of the new.
     Baker’s 6-Into-4 transmission bolts directly into a stock Harley frame and the stock inner primary bolts right up, as well. The case for this transmission, of course, is unique to itself and in place of the old ratchet top Baker has switched to a late-style top with the shift arm. Due to the extended length of the transmission on the kicker side, the exhaust pipe, oil tank and electric starter brackets might have to be modified in some installations, and some inner primary covers could require modification to clear the main drive nut. No big deal. And while there’s no speedo-cable provision in this tranny case there is an electronic speed sensor provision in the door and it accepts factory ‘94 and later sensors. All a small price to pay to get a real overdrive 6-speed mated up with a vintage Shovelhead. With a kicker, to boot. No pun intended.
     All of this Baker hardware, new and old, is available straight from Drag Specialties. It’s all in the FatBook, so call your rep today for more information.