HARLEY’S SIX-SPEED
Part I: Driveline gear ratios
by Donny Petersen
The new six-speed, while a wonderful improvement, has its detractors. Some say that upgraded performance packages do not give equivalent horsepower increases that would be garnered using a previous-year Big Twin tranny. Others complain about what they feel are disturbing noises emanating from the new transmissions and the primary system. Harley-Davidson, in convincing and soothing interviews, responds that all is normal. Spokespeople say that the company will explore the veracity of these concerns, but it first needs background information to fully understand the dramatic changes instituted by the factory.
Another factor is consumer mismanagement of the engine’s horsepower and torque output due to a general lack of knowledge about gear ratios. How the power is produced by the engine and then transferred to the rear wheel in usable amounts relative to the engine's rpm, and how this interrelates with the bike's speed and engine load is the true key to a functional motorcycle, whether the bike is used for comfortable touring, stoplight racing, or anything in between. We’ll be delving into this aspect of the new driveline this month and talk about the tranny itself in the next Techline.
Aftermarket Six-Speeds
The aftermarket has been producing reliable and efficient six-speed transmissions for close to a decade now, including full-size gear, American-made BAKER (
BAKER Drivetrain | The Drivetrain Authority) transmissions and the first ones produced offshore by Custom Chrome’s RevTech performance line with less beefy and lighter, but stronger, chrome moly gears, as well as many newer manufacturers' transmissions. Several variations of these transmissions are now being produced offshore in Korea and other places. The Harley rider should understand that many transmissions are being produced in reputable automotive transmission plants before they conjure up visions of cheap, unreliable products. As testament to their quality, some offshore brands come with three-year full-replacement warranties, while others have two-year warranties. I don’t know of any that do not have at least a one-year warranty. The offshore RevTech overdrive six-speed comes with a limited warranty of five years or 50,000 miles. The company used to have extended warranties on its six-speed builders kits but has reduced the warranty period because it was unable to control the quality of the installations. Installers were making mistakes that caused problems, which they naturally blamed on the transmission parts instead of their own ineptitude. BAKER’s American-made direct-drive six-speed (DD6) also comes with a limited warranty of five years or 50,000 miles. Furthermore, the price of aftermarket six-speeds has plummeted in the last few years by as much as 60 percent. American manufacturing is suffering as a result, but when was the last time that Harley-Davidson dropped prices in such a dramatic fashion?
Perhaps profit-driven, publicly-owned companies like H-D have been focusing too much on the bottom line. Investors are getting rich off H-D stocks, but the workers begin to suffer as more production moves offshore. The flip side is that share investment gives the factory the capital to reinvest in the development of a better-quality product and expanded operations. In general, Harley quality is top-notch, but has it ever offered a three-year full-replacement warranty? Not on factory-installed components, although Harley-Davidson did extend its motorcycle warranty from one to two years sometime in 2004. If you buy Screamin' Eagle performance parts, such as a complete six-speed transmission gearset, you will be hard-pressed to get a comprehensive warranty of any type, although I offer that the parts are of excellent quality and some are superbly designed.
Primary Ratios
We’re going to spend what may seem like a lot of time exploring gear ratios and their historical use on Big Twin Harley-Davidsons. We will discover that the big changes in the '06 Dynas continued with more changes in the '07 Big Twins in overall gear ratios. This is a big reason for the increased-performance seat-of-the-pants feel of these new models. The conundrum is that the increase in cubic inches doesn’t offer any more horsepower in combination with the new sixth gear without overdrive.
An overdrive sixth gear lowers rpm in that final gear. H-D achieves a similar objective in its 1:1 final-ratio six-speed. It does this by incorporating a new primary gear reduction ratio of 1.35:1 as opposed to the 2005 ratio of 1.44:1. The new ratio spins the engine 1.35 times for one revolution of the clutch pack (primary ratio) as opposed to 1.44 times (1994-2006, except the '06 Dyna) or 1.54 times (1985-93) on its predecessor five-speeds. This ratio is calculated by dividing the number of teeth on the clutch hub sprocket by the number of teeth on the engine sprocket. The formula is:
Clutch Sprocket Teeth/Engine Sprocket Teeth = Primary Gear Ratio
Therefore, 2006 Dynas and 2007 Big Twins have a primary ratio of 1.35 engine rotations for one rotation of the clutch hub and, therefore, one rotation of the attached transmission mainshaft, which the hub sits on, using a 34-tooth engine sprocket and a 46-tooth clutch hub sprocket.
Modern clutch hubs (1990 to present) have fine-splined centers that mate with an outside splined transmission mainshaft, which extends from the transmission into the inner primary cover. The clutch pack connects the primary chain, which is driven by the engine, to the transmission. On earlier H-D models, the clutch hub is attached and connected to the transmission’s mainshaft via a press-fit keyway, which did not provide as much strength or concentricity as the splined method. If the clutch hub does not sit perfectly in line with its supporting transmission mainshaft, it becomes a huge source of vibration.
Big Twins from 1994-2006 (except '06 Dynas) have a 25-tooth engine sprocket and 36-tooth clutch sprocket, which gives a primary ratio of 1.44 engine rotations for one rotation of the clutch hub/transmission mainshaft. The 1985-93 Big Twins have a 24-tooth engine sprocket and 37-tooth clutch hub sprocket for a primary ratio of 1.54 engine rotations for one rotation of the clutch hub and transmission mainshaft. The primary drive, which consists of the engine sprocket, primary chain, and clutch basket sprocket, and the transmission’s internal gear ratios are designed to best utilize the stock engine’s power and torque output.
The primary drive system transfers power from the engine’s crankshaft to the clutch and then the transmission. A primary chain connects the smaller engine sprocket to the larger clutch sprocket on the front or back of the clutch shell (depending upon the year), which encases the steel and fiber-covered clutch plates.
The front engine sprocket on the 2006 Dyna and 2007 Big Twins has been increased to a whopping 34 teeth from the previous 25-tooth one used from 1994-2006. The duplex (double-row) primary chain still connects the engine sprocket to the clutch sprocket. However, the clutch sprocket has been increased to 46 teeth from the previous 37-tooth one. The sprocket, which has always been on the back of the clutch hub on previous Big Twins, is now inboard and in front of the starter ring gear on the new primary system. The primary chain is now automatically kept at the proper tension by the new chain adjuster. The new outer primary cover no longer has an inspection hole for checking primary chain adjustment.
Overall Gear Ratios
There are other gearing changes in the new transmission, as well as on the transmission pulley and rear wheel pulley. The total reduction, also known as overall (final) gear ratio, indicates the number of engine revolutions required to drive the rear wheel one revolution. For example, a 1993 FLHT engine turns 3.37 times in fifth gear in order to turn the rear wheel one full revolution (360 degrees). This is the USA ratio. International models use smaller rear wheel pulleys for better highway cruising, which is especially important in countries where the speed limits are much higher. Some European highways, such as the famed autobahns in Germany, have no upper speed limits at all.
The 2005 USA-model engine turns 3.15 revolutions in fifth gear to turn the rear wheel one revolution. This difference is due to a change in sprocket sizes in the primary, since the tranny’s internal gear ratios are the same, as are the transmission pulley at 32 teeth/nubs and the rear pulley at 70 teeth. The pre-1993 five-speed models have a 24-tooth engine sprocket and 37 teeth on the clutch sprocket. The 1994 and later five-speed models have 25 teeth on the engine sprocket and reduce the clutch sprocket to 36 teeth. The engine sprocket has more of an impact on gear ratio than the clutch sprocket on a per-tooth basis, while the transmission sprocket has a bigger impact than the rear wheel sprocket. When performancing these at Heavy Duty, we will routinely replace the 25-tooth engine sprocket with a 24-tooth one for better off-the-line get-up-and-go. This gives a primary ratio of 1.50:1 (24/36 = 1.50), which means the engine turns 1.5 times for every clutch hub/transmission mainshaft revolution versus 1.44 times when using the stock 25-tooth engine sprocket.
For those who only care about stoplight-to-stoplight drag racing, Screamin' Eagle borrowed a page, so to speak, from BAKER Drivetrain, the initial developers of the 21-tooth engine sprocket (#155-56S for 1985-2006 FLT-FXR or #155-56L for 1985-2006 FXST/Dyna), along with a mating correct-length primary chain. BAKER also has 22-tooth (#156-56S for 1985-2006 FLT-FXR or #156-56L for 1985-2006 FXST/Dyna), and 23-tooth kits (#157-56S for 1985-2006 FLT-FXR or #157-56L for 1985-2006 FXST/Dyna), which also come complete with correct-length primary chain.
To be fully accurate, early Harley FL police bikes from the Pan and Shovel eras sometimes used 21-tooth engine sprockets for faster get-up-and-go in city traffic, although the engines just screamed on the highway. Highway police bikes of the era had engine sprockets with up to 23 teeth for better highway speeds and lower engine rpm. The Screamin' Eagle offering (#40331-02 for 1994 and later FXST/Dyna or #40200-02 for 1994-2006 FLHT) is also available in its original form from BAKER. This sprocket changes the primary ratio from the stock 1.44:1 to 1.7:1. The engine spins at higher rpm at any speed with the 1.7:1 primary drive ratio that rotates 1.7 times versus 1.44 times as a baseline. The politically correct Screamin' Eagle catalog blurb says it, “will help you get the hole shot at the racetrack.” A peppier '06 Dyna/'07 Big Twin off the line can now be achieved by replacing the stock 32-tooth transmission pulley (#40659-06) with a BAKER 31- or 30-tooth replacement.
I mention these overall ratios to better demonstrate a major change that occurred with the six-speed 2006 Dyna and all subsequent 2007 Big Twins. The 2006 Dyna’s six-speed and primary gear ratios were carried over into all the 2007 Big Twins. The 2006 Dyna engine turns over only 2.96 times per rear wheel revolution. The 2006 Dyna and all '07 Big Twins have an unprecedented 34-tooth engine sprocket compared to either a lazy 25 (at least, that's what we thought) or a better-performance 24-tooth one in previous years. They also have a big clutch sprocket change from previous years with sizing of either a lazier 36 or a better-performance 37-tooth to a whopping 46-tooth sprocket. The 2007 Big Twins turn over even less at 2.79 engine revolutions per one rear wheel revolution. Although they have the same oversized primary sprockets, the rear pulley is reduced to 66 teeth in 2007 from 68 teeth on the '06 Dyna.
Techline June 2007, Part 1 
Linear Mode
