Motorcycle V-Twin Drivetrain - Drivetrain Basics

Secondary Drive Ratio
In order to calculate the secondary drive ratio, you divide the number of teeth on the rear wheel sprocket or pulley (output revolutions) by the number of teeth on the transmission sprocket or pulley (input revolutions). Some Big Twins have a 70-tooth rear wheel pulley and a 32-tooth transmission pulley. Dividing 70 by 32 results in a 2.19:1 ratio.

Final Drive Ratio
Multiplying the primary ratio by the secondary ratio yields the final drive ratio, which is a term commonly provided in specification charts. By multiplying 1.54 by 2.19, we end up with a 3.37:1 final drive ratio. To keep things simple, the transmission's internal ratio was not factored in, but the high gear internal transmission ratio for most Harley models (without overdrive) is 1:1. This means that the transmission's input shaft is turning at the same speed as its output shaft, therefore all gear reduction is done at the primary or secondary drives.

So What Do All The Numbers Mean?
All that theory sounds terrific, but what does it mean for a bike ridden on the street or racetrack. Let's compare two different models of Big Twins. Starting in 1995, all Evolution Softail Big Twins were shipped from the factory with a 2.92:1 final drive ratio (a 1.44 primary ratio multiplied by a 2.03 secondary ratio equals 2.92) while all other Evo Big Twins shipped with a 3.15:1 final drive ratio (a 1.44 primary ratio multiplied by a 2.19 secondary ratio equals a 3.15). Most 1993 and earlier Evo Big Twins came with a 3.37:1 final drive ratio. The pattern we see here is that 1995 and later Softails have a higher (lower numerically) final drive ratio (2.92:1) than the other models. This means the engine's crankshaft turns 2.92 revolutions for every revolution of the rear wheel. All things being equal, this gives the Softail less acceleration than the other models but requires less rpm at highway speeds for smoother running.

When changing gear ratios, everything works on a sliding scale -- when you gain on one side, you lose on the other. In general, if you change gearing to improve acceleration, high-rpm performance can suffer. On the other hand, increasing top-end performance often hurts acceleration. The trick is to end up with a happy balance between the two because there is no such thing as a free lunch. To select the proper gearing, you need to set your priorities based on the type of riding you do and your engine combination. In other words, gearing should be selected based on the total application: engine combo and riding style.

Examples
If you do mostly highway riding, you would generally be better off with a higher final drive ratio (lower numerically) because that would require less rpm for a given mph, resulting in more top speed and typically less engine vibration on those long-distance trips. Of course, the tradeoff would be slower acceleration. On the other hand, for maximum acceleration, a lower final drive ratio (higher numerically) would be in order, but the result will be less speed for a given rpm.

Let's calculate a few gearing examples. If you want to calculate accurate ratios for your specific model of V-twin, just replace the numbers used below with values specified in your service manual or sales brochure. For example, let's assume a bike has a 2.03:1 secondary ratio, which is the result of a 65-tooth rear wheel pulley and a 32-tooth transmission pulley (65 divided by 32 = 2.03). By changing to a larger 70-tooth rear wheel pulley, which is found on many models, the bike's secondary ratio would change to 2.19:1 (70 divided by 32 = 2.19:1), giving a 3.15:1 final drive ratio (1.44 x 2.19 = 3.15). With a 3.15:1 final drive ratio, the bike would have quicker acceleration, but less mph for a given rpm, which could make long distance riding more taxing due to the engine's higher rpm at highway speeds.

What About Overdrive Transmissions?
You have probably found yourself wishing for another gear while cruising on the freeways. Raising your bike's final drive gearing (lower numerically) would reduce engine rpm and have the effect of adding a higher gear, but it would also render all lower gears equally higher, thus reducing acceleration and making passing on hills when fully loaded more difficult. This is where an overdrive (OD) gear helps out because an overdrive gear (for example, an OD Sixth gear) would not render all lower gears equally higher, which means the bike would maintain normal acceleration while high-speed rpm would be reduced.

A Big Twin 5-speed transmission tranny has a 1:1 internal ratio high gear, which means the engine crankshaft turns one revolution for each revolution of the rear wheel. An overdrive V-twin 6-speed trans adds an overdriven Sixth gear to a 5-speed gearbox. Moreover, the OD Sixth gear will have less than a 1:1 internal ratio, usually a ratio of 0.80:1, 0.86:1 or 0.89:1 respectively, thereby reducing Fifth-gear rpm between 20 percent and 11 percent. For example, installing a 0.86:1 overdrive Sixth gear in a trans with 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). The transmission retains its 1:1 internal ratio Fifth gear but now also has a 0.86:1 ratio 6th gear.