Overdrive Transmission vs. A Pulley Change?

This past month month I had a customer request a discussion on gear ratios & belt drives. More specifically "what are the pros and cons of an overdrive transmission or a pulley change?" I thought it was a great idea. But first lets discuss the history of Harley- Davidson® Transmissions.

In 1936 Mother Harley® introduced one of the benchmark motorcycles of all time with the OHV 61 cubic inch EL. This beautifully styled bike has become affectionately known as the Knucklehead. What many people fail to realize is the design of the transmission first used here would remain in service until 1986, an incredible run of 50 years with the same basic configuration. There where of course different variations available in the early years, for example the standard 4 speed, a 3-speed w/reverse for sidecar use, hand shift or foot shift. The original design was extremely bulletproof. Knuckles and Pans shared the same outer case. The only exception was the 1965 Pan with the introduction of the electric start. The tranny case grew a couple of ears to support the new aluminum inner primary that the electric start bolted to and the mainshaft in the tranny got 1 /4 inch longer to accommodate these new goodies. Things remained the same until the introduction of the alternator. Adding an alternator affected the tranny. The primary had to be moved out to accommodate the alternator. So the mainshaft grew again. These minor changes went on until the demise of the 4 speed in 1986. The biggest complaint of the 4 speed was pretty universal throughout the years. Many felt that the bike simply should have had another gear.

In 1980 Harley® introduced the FLT series motorcycle. Once again a pretty revolutionary bike in Harley® history. A rubber mounted engine with a 5-speed gearbox was the major design change. This was in response to public opinion. What many did not understand was the final drive ratio of the 4 speed & the 5 speed was the same 1 to 1. Many mistakenly thought 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 raised the requests for another gear once again. While Harley® hasn't listened yet, the aftermarket has. There are a variety of 6 speed transmissions available these days. Baker, Jim's Machine®, RevTech® & Milwaukee Twins™ all produce overdrive gearboxes. These new tranny's feature a high gear that is .86 to 1 or .80 to 1, less than the stock 1 to 1. This results in a reduction in engine RPM at a given speed, or a faster speed at the same RPM. For example; a mid 90's softail would have an overall ratio of 3.36 to one. This is determined by the number of teeth on each of the gears or pulleys in the primary drive and the secondary drive. (see figure 1 for the changes in RPM) The problem with any overdrive type system is that if you slow down too much in that overdrive gear the engine will lug when you attempt to accelerate. This is detrimental to the engine. The solution? 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. In our example Softail™ the 3.36 to 1 ratio was achieved with a primary ratio of 1.54 to 1 (24/37 tooth combo) and a secondary ratio of 2.1875 to 1 (32/70). A change to a 65 tooth rear pulley would make the secondary ratio 2.03 and the overall ratio of 3.12. At 70 MPH engine RPM would drop 235 revs.

When making a change like this the physical difference in pulley diameter is of serious concern. Our example Softail™ has a 132 tooth belt & a pulley center to center distance of 22.07 inches with the stock 32/70 pulley combo. If we change to the 65 and use the same 132 tooth belt, the new center to center distance will need to be 22.82 inches, or 3/4 of an inch farther apart. It is unlikely for us to have that much room in the axle adjusting slots. We do have the choice of changing the belt to a new length of 130 teeth to get a center to center of 22.27 which will be much more likely to work in the slots. One other important note is the fact that upper & lower belt guards that work for one combination will not satisfy the needs of another.

In summary, using a six speed overdrive transmission is a fine way to lower engine RPM. Changing pulley ratio can get similar results as long as you understand all the variables to make it work. For further info on the content in this article you can call any of the J&P Cycles® Tech staff to help you.