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Adding an overdrive or direct drive six-speed tranny reduces vibration on the highway, making long rides less tiring and more enjoyable. A sixer also reduces engine wear and improves fuel economy. Six-Speed Transmissions Regardless of whether you're cruising the local freeways or jamming on the intestates to a major cycle event, you've probably found yourself wishing for another gear, wanting to upshift into a higher cog to reduce the relentless vibrations the V-twin engine emits at highway speeds. Raising your bike's final gearing would move you in the direction of having a higher gear, but it would also render all lower gears equally higher, reducing acceleration and making passing on hills when fully loaded more difficult. However, if you own a 2006 Dyna or a 2007 and later big twin, you found that extra gear without encountering the lethargic acceleration pitfalls: It's called sixth gear. Adding a sixth gear reduces vibration on the highway, making long rides less tiring and more enjoyable.
With the Baker six-speed overdrive design, power travels down the input mainshaft, across to the countershaft, back down the countershaft and finally to the drive gear. With a Baker direct drive DD6 (bottom), the input shaft and drive gear are meshed, so power enters via the input shaft and exits through the drive gear.If your bike doesn't already have a six-speed transmission, there are two ways to upgrade:
You can either replace your current tranny with a complete, new six-speed or install a six-speed gearset into a conventional five-speed box. Harley-Davidson and several aftermarket companies offer six-speed options for the big twin in both complete transmissions and internal gearsets. More importantly, though, the sixers are available in two critically different designs: overdrive sixth gear (OD) and direct drive sixth gear
(DD).
Let's take a look at both designs.
Overdrives
Overdrive transmissions are nothing new to the automotive world. In fact, you probably have one in your late-model grocery-getter. However, overdrives are relatively new to the Harley
world, first appearing about the mid-1990s. A big twin five speed transmission or even a Shovelhead four-speed tranny has a 1:1 internal ratio high gear. That means the rear wheel turns one revolution for each revolution of the engine. To be classified as an overdrive, a transmission must have at least one gear with an internal ratio of less than 1:1. An overdrive six-speed trans adds an overdrive sixth gear to a five-speed box. The OD sixth gear will have less than a 1:1 internal ratio, thus reducing engine rpm.
The primary drive includes the engine sprocket, clutch and primary chain. For a direct drive six-speed to function properly, the primary drive must be overdriven by installing a larger engine sprocket and longer chain. For example, big twin six-speed overdrive transmissions usually have an OD ratio of 0.80:1, 0.86:1 or 0.89:1, thereby reducing fifth-gear rpm between 20 percent and 11 percent.
Although six-speed ODs are available in several different internal ratios, a 0.86:1 internal ratio, which decreases a standard transmission's fifth gear ratio (and engine rpm) by 14 percent, is a common choice. Here's another way of looking at it: Installing a 0.86:1 overdrive sixth gear in a trans with a 1:1 ratio/3.15:1 final drive results in fifth gear remaining with 1:1 and 3.15:1 ratios, but sixth gear becomes a 2.71:1 final drive ratio (3.15 x 0.86 = 2.71). By the way, I might add that overdrives are not limited to only six-speed transmissions. Five-speed transmissions can also have an overdrive gear where fourth gear is a 1:1 ratio and fifth gear uses a less than 1:1 overdrive ratio.
Depending on the year and model of big twin, the engine is typically turning between 3000 and 3500 rpm at highway speeds. If we assume your engine is turning 3200 rpm at highway cruise with a five-speed 1:1 ratio nonoverdrive tranny, installing a 0.86:1 ratio overdrive reduces engine rpm by 14 percent, resulting in a reduction of 448 rpm or 2752 rpm at cruise speed. That is a significant reduction in rpm. It not only results in happier cruising and reduced vibration but also reduces engine wear while increasing fuel economy.
Six-Speed Transmissions
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.
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.
JIMS offers six-speed overdrive gearsets for upgrading 2006 and earlier five-speed transmissions. Other Gearing Consideration. 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.
Six-Speed Transmissions
Some six-speed manufacturers offer optional ratios for first gear and overdrive sixth gear. For example, you can often choose between a 3.24:1 and 2.94:1 first gear and either a 0.89:1, 0.86:1 or 0.80:1 overdrive sixth gear. Choose the first gear ratio based on your bike's weight and its primary drive ratio. With a heavier bike and/or lower ratio primary gearing (lower numerically), consider a higher ratio first gear (higher numerically).
Remember that many fuel-injected big twins have lower primary gearing (1.54:1 versus 1.44:1) than carbureted models, so choose accordingly. For high-torque big-inch engines, you may already have more low-speed torque than you really need, so a lower ratio first gear (lower numerically) may be in order. Additionally, a big motor can pull a taller sixth gear, so don't choose the overdrive sixth gear ratio (i.e., 0.89:1, 0.86:1 or 0.80:1) without first considering the displacement and power of your engine and highway riding style.
For four-speed Shovelhead owners looking for an extra gear or two, Baker Drivetrain offers a six-into-four gearset. It is a six-speed overdrive with a 1:1 fifth gear and a 0.86:1 sixth gear. The OD sixth gear is good for a 500-rpm reduction in cruising rpm at highway speeds.
The S&S complete direct drive six-speed transmission features helical gears and is available in several different finishes. Remember that six-speed transmissions are available as complete transmissions or gearset kits. Of course, a complete transmission is more costly, but then again you have a used transmission to sell, which can offset some of the costs. Additionally, some aftermarket complete transmissions include upgraded features like improved shift mechanisms, offering smoother, more positive shift action and reduced component wear. Such features may be worth the additional costs.
Installation Considerations
Installation of a six-speed tranny isn't a difficult procedure, but you may have to do a bit of clearancing and grinding to get everything to fit. Don't forget that you'll need a larger
engine sprocket and longer primary chain to overdrive the primary when installing a direct drive six-speed. The good news is that these items are usually included with direct drive
kits. Another consideration is speedometer accuracy after upgrading to a six-speed. You may need a speedo recalibration box if your bike uses an electronic speedometer.
For wide-tire bikes, Baker Drivetrain and JIMS Machine offer right-side drive transmissions to eliminate the need for left-side transmission offsets.
Final Thoughts
Adding a six-speed tranny, whether it is a direct drive or overdrive, reduces rpm between 11 percent and 20 percent, which makes a world of difference for highway and interstate riding. Reducing vibration makes long rides less tiring and much more enjoyable. Sixers also reduce engine wear and improve fuel mileage. And if you choose the correct first gear ratio, you
can have crisper acceleration, too. Six-speeds don't come cheap, but they provide a win-win situation when selected properly and installed correctly. Moreover, your "behind" and your
passenger will thank you handsomely for spending your hard-earned dollars wisely on the sixer of your choice.
Calculating Drive Ratios
The final drive ratio is a function of multiplying the primary drive ratio by the secondary drive ratio. The primary drive ratio is determined by dividing the number of teeth on the
clutch sprocket by the number of teeth on the engine's compensating sprocket. The secondary drive ratio is calculated by dividing the number of teeth on the rear wheel sprocket by the number of teeth on the transmission sprocket. For example, assume we have the following: 24T engine sprocket, 37T clutch sprocket, 32T transmission sprocket and 70T rear wheel sprocket.
The calculations look as follows:
37 24 = 1.54:1 Primary Ratio
70 32 = 2.19:1 secondary ratio
1.54 x 2.19 = 3.37 final drive ratio
Now, let's assume we install a six-speed overdrive with a 0.86:1 ratio to the above gearing combination. What would be the final drive ratio of the overdrive sixth gear?
3.37 x 0.86 = 2.90 overdrive sixth gear final drive ratio
writer: D. William Denish
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