In the past, our tech department has been asked to explain the differences between stock Harley transmissions, overdrive transmissions and final drive changes. So we’re going to get your gears spinning on this topic and discuss the history of Harley-Davidson Transmissions and the advent of aftermarket replacements.
In 1936, Mother Harley introduced one of the benchmark motorcycles of all time —the OHV 61-cubic-inch EL. This beautifully styled bike is affectionately known as the Knucklehead. What’s truly amazing about this bike is the design of the transmission, which remained in service until 1986 — an incredible half-century run with the same basic configuration.
There were, of course, different variations available in the early years, like the standard 4-speed, a 3-speed with reverse for sidecar use, and hand shift or foot shift. The original design was extremely bulletproof. Knuckles and Pans shared the same outer case. The only exception was the introduction of the electric start with the 1965 Pan. The transmission case grew a couple of ears to support the new aluminum inner primary to which the electric start was bolted, and the main shaft in the transmission got a quarter-inch inch longer to accommodate these new goodies.
Things pretty much remained the same until the introduction of the alternator. Adding an alternator affected the transmission. The primary had to be moved out to accommodate the alternator. As a result, the main shaft grew again. These minor changes went on until the demise of the 4-speed in 1986. The biggest complaint about the 4-speed was universal throughout the years — most everybody thought the bike needed another gear.
In 1980, Harley introduced the FLT series motorcycle, a fairly revolutionary bike in Harley history. A rubber-mounted engine with a 5-speed gearbox was the major design change. For many years, riders thought an additional gear in the transmission would be ideal. Harley listened, but what many riders didn’t understand is that the final drive ratio of the 4-speed and the 5-speed was the same 1 to 1. Many erroneously believed that the 5th gear was an overdrive. The lower gear ratios in 1st, 2nd, & 3rd allowed the engineers to raise the overall gear ratio to achieve the same off-the-line performance with a higher top speed.
Today’s higher road speeds have prompted requests for yet another gear. Harley introduced its 6-speed cruise drive transmission in 2006. The changes in the overall gear ratio are what made this unit a positive upgrade to the Harley gearbox. However, it’s not as refined as some of the aftermarket transmissions that are available today. Baker, Jim’s Machine, RevTech and Milwaukee Twins all produce overdrive gearboxes. These new transmissions feature a high gear that is .86 to 1 or .80 to 1 — less than the stock 1 to 1, resulting in a reduction in engine rpm at a given speed, or a faster speed at the same rpm.
For example, a mid-’90s Harley Softail would have an overall ratio of 3.36 to 1. This is determined by the number of teeth on each of the gears or pulleys in the primary drive and the secondary drive.
The problem with any overdrive type system is that if you slow down too much in that overdrive gear, the engine can lug when you attempt to accelerate. This can be detrimental to the engine. The solution would be to downshift to accelerate.
An alternative to adding an overdrive is to change the overall ratio of the bike to reduce engine rpm at a given speed. When motorcycles had a chain drive secondary, this was very easy to do. Chains are easy to obtain in various lengths to accommodate the differences a larger or smaller sprocket would require. With the advent of the belt secondary, things got more complicated. Due to the fact that belts are fixed in application we have some serious limitations in what we can do, not to mention removal and reinstallation of all the components.
In summary, using a 6-speed overdrive transmission, or modifying your stock drive ratios are fine ways to lower overall engine rpm. Hopefully we’ve given you a little history lesson and enough information to understand both Harley and aftermarket transmissions. As always, if you have questions or need assistance, don’t hesitate to chat with a J&P technician via Live Chat. Or call our technical support staff at (800) 397-4844.