Techline, June 2007, Part 2

[Buzz Kanter]

Modifying Ratios
The following is a rudimentary guide to the effects of these drive sprockets on engine performance based on overall gear ratios. First, reducing the number of engine sprocket teeth and/or transmission pulley/sprocket teeth, depending upon the year, and increasing the rear pulley/sprocket teeth will make the motorcycle faster off the line by increasing rpm and acceleration speed. Radical tooth-count changes in this direction will make the bike slower on the top end (highway). The opposite is also true: Increasing the number of engine sprocket teeth and/or transmission pulley/sprocket teeth and decreasing the rear pulley/sprocket teeth will make the motorcycle slower off the line by decreasing rpm and acceleration. Dramatic tooth changes will make the bike faster on the top end. Big cubic inches will help this second scenario, since a more powerful engine usually requires higher gearing.

Certainly, the 2007 gear ratios would not be practical without the engine increase to 96 cubic inches and closely matched internal six-speed gear ratios. If there is a secret to the '07s, then this is it! Gear ratio performancing will be crucial to big gains with these later models in future years. Installing 2006 and earlier primary ratios will be key. The rear pulleys (secondary ratios) can also be substantially changed for performancing goals. This will have to be accomplished with aftermarket parts that are yet to be developed at the time of this writing for the following reason: The compensator parts from 1989-2006 are not interchangeable with '06 Dynas and '07 Big Twins. This is due to a change on the engine sprocket shaft splines. The sprocket shaft extends into the primary, where the compensator engine sprocket is fitted. Earlier models have fewer (10) fat, wide splines, while the new bikes have 24 thinner multi-splines.

Early builders of bigger-cube 2007 engine models have noticed a more measurable loss of horsepower than if they had done the same upgrades to the 25/36 or the 24/37 primary reduction gear ratios in earlier Twin Cams. We are all attributing this to the drastic changes in primary reduction sprocket and clutch gear sizing. This also explains one major reason why the 2007 TC 96 models don’t have the expected increase in horsepower over the previous Twin Cam 88s. However, there is a dramatic and positive increase in torque with the new '07 configurations.

Transmission Parts Comparison
There are three dog rings in the H-D six-speed: two (first/second and fifth/sixth) with dogs on either side and one (third/fourth) that is pocketed. The pocketed dog ring has no dogs or lugs. Instead, it has six indents, or pockets, on either side.

The advantage of these dog rings is that there is less inertia required to move them back and forth, so they can engage with their respective gears easier than if the rider has to move a whole gear to engage another whole gear, as it is in the five-speed. In order to make the dog rings even lighter with correspondingly less resistance to movement (inertia), they ride on guide hubs. The three guide hubs, one specifically for each dog ring, are multi-splined inside and out. The inside splines ride on mating splines on their respective shafts, two on the countershaft and one on the mainshaft. The shift dog rings are also splined on the inside for mating with the splines outside of the guide hubs. Now we have a lightweight shifting mechanism that is nimble, with the least inertial resistance possible -- a huge improvement over the bulky five-speeds.

The H-D six-speed transmission offers surprises when these external gear ratios are changed to gain more horsepower. External ratios may need to be modified to coexist in a positive way with the factory six-speed when the engine is performanced. These may be combined with a much more powerful engine and will involve creative sprocket/pulley changes to achieve the desired results.

The stock six-speed has a 1:1 internal ratio in sixth gear, while the Screamin' Eagle six-speed, which is about six years late in development relative to the excellent aftermarket offerings, has an overdrive sixth gear with a ratio of 0.89:1. RevTech has offered a 0.8931:1 overdrive sixth gear ratio since around 1999. The 0.089:1 overdrive will lower rpm by about 360 rpm in sixth gear. However, it is not for use on the 2006 Dyna and all 2007 Big Twins, since they fit 1985-2006 Big Twins, except the ’06 Dynas.

With the H-D 1:1-ratio four- and five-speeds, it is important that gear ratios be kept within bounds (the powerband) to ensure good start-off power and acceptable highway cruising rpm on an all-purpose street Harley. The exception, of course, is when a specialty bike, such as a light-to-light drag racer or a touring bike that rarely sees city traffic, is being geared as we alter the powerband for usable horsepower. These are the tradeoffs with traditional 1:1 final transmission gear ratios. An advantage of the BAKER direct drive DD6 is that a higher percentage of torque is transferred through sixth gear relative to an overdrive transmission, since only two gears are involved at this point.

The 0.089:1 overdrive aftermarket six-speeds work well with more cubic inches, but there is some torque loss because four gears are required to transfer sixth gear torque. Unfortunately, the H-D transmission also utilizes four gears to transfer its sixth gear torque. Even though Harley-Davidson has done an initially satisfactory job with its 2007 transmission combined with primary drive gear ratios, I didn’t quite understand why an overdrive sixth wasn't used. I say "satisfactory" because there are changes that could be made to make it better. We will see, as this series of articles progresses, that H-D could have improved its six-speed design to reduce the transfer of torque power loss, as well as address a nagging noise issue. I asked Mr. Transmission, the venerable Bert Baker, what he thought. “Overdrive six-speeds are inherently noisy in sixth because it is virtually impossible to make a large gear on the mainshaft drive a small pinion on the countershaft quietly, which is the case on the sixth gear pair in six-speed overdrives," he says. "High-quality transmission makers like BAKER spend a lot of money on grinding the flanks of the gear teeth after heat treating to minimize this noise, whereas some lower-quality transmission makers do not. I feel that this inherent noise with six-speed overdrive designs is the reason why Harley did not opt for this route.”

Ya just gotta agree with the guy! His logic and knowledge just roll along. Why would H-D build an overdrive that is intrinsically noisier when the whole goal of the helical gears was to appease the noise cops? I feel that H-D's transmission will be improved in coming years. Its six-speed may even be in for a redesign, since Harley riders are not as sensitive to noise as the environmental folks are. If customer complaints about noise are becoming an issue, then the government agencies may not be far behind.

“Being that the H-D transmission is direct drive in sixth gear, like our DD6, there does not need to be four meshed gears, just sixth main and sixth counter meshed," says Bert. "The H-D design drives three gears and has a fourth along for the ride. It does not utilize the advantages that a 1:1 drive ratio can give you. In a traditional overdrive six-speed, the four meshed gears are needed to reduce the drive ratio below the 1:1 threshold to the 0.86:1 or 0.80:1 (0.89) ratio, depending upon the transmission. Not so with our Direct Drive.”

There are also 0.086 and 0.080 final overdrive ratio six-speeds, but they should be combined with progressively bigger-inch bikes to achieve a powerband that is satisfactory all around. The H-D, BAKER DD6, and S&S 1:1 ratio six-speeds attain their goals through another method: changing the primary or secondary sprocket/pulley ratios. Both the BAKER and the H-D transmissions feel like they are overdrives in sixth gear, even though they are not. Harley got around the overdrive issue by supercharging the primary gear ratio. The conservative S&S 1:1 six-speed uses a 34-tooth pulley, up from the stock 32-tooth one. BAKER’s DD6 1:1 comes with a previously huge 28-tooth engine sprocket and longer primary chain. BAKER has now developed a sportier 27-tooth compensating sprocket for use with its Direct Drive six-speed. H-D’s stock 1:1 six-speed comes with the really big 34-tooth engine sprocket, but the gear ratio is also modified with an increase in clutch sprocket teeth from the stock 36 to a whopping 46. When using a stock pre-2006 clutch, transmission, and rear wheel pulley/sprockets, BAKER's DD6 allows the rider to easily mimic the 2006 Dyna and 2007 Big Twin gear ratios. BAKER's final drive ratio of 2.83:1 is right in the middle of the 2006 Dyna’s 2.88:1 and the '07 Big Twin's final drive ratio of 2.78:1. The S&S six-speed comes in at 2.97:1, lower than the 1985-93 Big Twins, but higher than the 1995-2006 (except the '06 Dyna) models. The S&S transmission is different from the other manufacturers' because of turndown ratios, which we will discuss later.