Motorcycle V-Twin Drivetrain - Drivetrain Basics

So, what benefits would a 2.71 final drive gearing offer when cruising down the highway? Depending on the year and model of your Big Twin, the engine is typically turning between 3,000 and 3,500 rpm at highway speeds. If we assume the engine is turning 3,200 rpm at highway cruise with a 5-speed 1:1 ratio non-overdrive transmission, installing an 0.86:1 ratio overdrive reduces engine rpm by 14 percent, resulting in a reduction of 448 rpm or 2,752 rpm at cruise speed. That is a significant reduction in rpm. It not only results in happier cruising and reduced vibration but also reduces engine wear while increasing fuel economy. And don't forget, the ratios for gears 1-5 are not affected, so acceleration in those gears is not reduced.

Nevertheless, there is no free lunch with an OD transmission because there is a penalty to pay for the OD design. Here's why. The reason that transmissions are designed with a direct drive high gear (1:1 ratio) instead of an overdrive is because a 1:1 ratio is most efficient while an OD trans suffers some power loss. With direct drive, there are no gear reductions between the transmission's main and counter shafts, resulting in less power loss and reduced noise. As such, the input shaft and drive gear are locked together, so power enters on the input shaft and leaves immediately on the drive gear. However, with an overdrive trans (where top gear has less than a 1:1 ratio), power flows through gear sets on both the main and counter shafts, reducing power by up to 8 percent while increasing noise.

This is where the direct drive 6-speed comes into play. A direct drive 6-speed tranny has six gears similar to an OD 6-speed, but the direct drive's top gear has a 1:1 ratio where an OD's Sixth gear is less than a 1:1 ratio. This means that a direct drive 6-speed trans is more efficient than an OD 6-speed. That is a subtle but significant difference. Nevertheless, adding a 6-speed tranny, whether it is a direct drive or overdrive, reduces rpm between 11 percent and 20 percent, which can make an enormous difference during highway riding.

Concluding Thoughts
For most highway riding with a 5-speed, a 2.92 to 3.15 final drive ratio (assuming a non-overdrive tranny) works well because less rpm is required for a given mph. Where your priorities dictate a combination of highway riding and acceleration, a 3.37 final drive ratio (non-overdrive trans) offers a good compromise. With a 6-speed transmission (either direct drive or overdrive), top gear will be 11-20 percent higher (lower numerically) for reduced rpm. And for maximum acceleration, a Big Twin generally requires a final drive ratio between 3.70 and 4.15, but that would result in poor highway performance and additional wear because it would require extremely high engine rpm.

5-Speed Drive Ratios

Table 1 Listed are the drive ratios for an early model Twin Cam 5-speed. Note that Fifth gear has a 1:1 internal ratio, which means that the engine's crankshaft turns one revolution for each revolution of the rear wheel. Now, refer to Table 2.

6-Speed - Direct Drive Vs. Overdrive

Table 2 On the left side are Twin Cam 6-speed direct drive ratios. Notice that top gear, Sixth, has an internal ratio of 1:1, which means that the engine's crankshaft turns one revolution for each revolution of the rear wheel. On the right side are listed the ratios for a SE 6-speed overdrive transmission. Notice that its top gear, Sixth, has an internal ratio of 0.89 while gears first-fifth mimic the 5-speed ratios listed in Table 1. This means that the rear wheel is overdriven by 11 percent. In other words, the engine is turning only 89 percent of the rear wheel revolutions. Or, one could say that the rear wheel is turning 11 percent faster than the crankshaft. An engine is most efficient at a 1:1 ratio, so overdriving the rear wheel reduces efficiency. And that is one benefit of a direct drive 6-speed over an overdrive 6-speed. A second benefit of a direct drive is that gears First through Fifth can be bunched closer together, making the gearbox more of a close-ratio box, which can result in less rpm drop during up-shifts.

Other factors that play a major part in gear ratio selection include engine displacement and power band, total weight of bike and rider, rear tire diameter and chassis setup. Higher gearing can improve highway cruising, but will generally decrease acceleration. Large displacement engines can handle lower numerical gear ratios than smaller engines. Keep in mind that secondary belt drives may offer less gearing options than chain drives, so for serious racing, choose a rear chain drive and change gearing at the rear wheel. Additionally, keep in mind that a higher gear ratio (lower numerically) places greater loads on the engine, making it more prone to detonation.