Deep Overdrive and the Ultimate Drivetrain Performance Package

A few years back we introduced the 21 tooth compensating sprocket for 1990-2006 Big Twins which delivers 19% more torque to the rear wheel by increasing the overall drive ratio. Hands down, it is the easiest and most inexpensive way to increase performance with no change in dependability. Compared to stock, this change makes the power hit like a 2-stroke 250cc motocross bike in 1^st and 2^nd gears. Coincidentally, Harley also introduced a 21 tooth compensating sprocket. The down side of this drivetrain modification is that the cruising RPM on the highway increases 19%.

The fact that the 19% increase in torque comes at a cost of increasing the cruising RPM by the same amount is called engineering compromise. To get one thing, you must give up another. However, engineering compromise can be cheated by installing a 6-speed because it gives you an extra gear and offsets the increase in cruising RPM from the 21 tooth sprocket. But standard 6^th gear ratios on all 6-speeds, including the BAKER OD6, are not deep enough to bring the cruising RPM back to the stock level or below. So we developed the deep overdrive option with our OD6 (Overdrive 6-speed).

With the BAKER OD6, two 6^th gear ratios are available; .86:1(standard Overdrive) and .80:1 (deep Overdrive). For overall drive ratios less than 3.55, we recommend the standard .86:1 overdrive ratio for most applications. The deep Overdrive .80:1 is recommended for overall ratios greater than 3.55.

The overall drive ratio of a Big Twin is calculated by multiplying the primary reduction (primary drive) by the secondary reduction (drive to the rear wheel). Most Big Twins from 1994-2006 have a 36/25 primary drive ratio and 70/32 secondary drive which translates to an overall ratio of 3.15 ([36/25]X[70/32]). This number is equivalent to the rear axle ratio in a rear wheel drive car. Replace the stock 25 tooth compensating sprocket with a 21 tooth sprocket and the overall drive ratio jumps up to 3.75 ([36/21]X[70/32]).

Actual calculated numbers tell the complete story. By using the RPM and Speed calculator from our website the cruising RPM at 75MPH for stock, BAKER OD6, Screamin Eagle 6-speed, and BAKER deep overdrive are derived: 

RPMat 75 MPH for the stock 1994-2006 5-speed configuration:

[75X70X36X1X88]/[6.54X32X25] = 3178 RPM

RPM at 75 MPH for a stock 5-speed with 21 tooth compensating sprocket installed:

[75X70X36X1X88]/[6.54X32X21] = 3784 RPM

RPM at 75 MPH with a Screamin Eagle 6-speed with .895 6^th gear ratio with 21 tooth compensating sprocket installed:

[75X70X36X.895X88]/[6.54X32X21] = 3387 RPM

RPM at 75 MPH with a BAKER OD6 with standard .86 6^th gear ratio with 21 tooth compensating sprocket installed:

[75X70X36X.86X88]/[6.54X32X21] = 3254 RPM

RPM at 75 MPH with an OD6 with .80 deep overdrive 6^th gear ratio with 21 tooth compensating sprocket installed:

[75X70X36X.80X88]/[6.54X32X21] = 3027 RPM

The first and last calculation spells out, in black and white numbers, a stock bike vs. running deep overdrive with a 21 tooth sprocket. We consider the 21 tooth compensating sprocket and deep overdrive 6-speed the ultimate drivetrain performance package because this combination yields 19% more torque to the rear wheel and a 4.8% reduction in cruising RPM on the highway with no compromise in engine durability. For more information on your particular bike or application give our tech line a call.