Some of our riding buddies complain that their five-speed transmissions require high-motor rpms to maintain highway speeds more than 65 mph. They also complain that these high rpm's create excessive, uncomfortable vibration and kills their fuel mileage. They want to know how to reduce engine rpm at cruising speed. Since drive ratios govern the relationship between engine speed and bike speed, one popular cure is to add another gear by installing a six-speed transmission. However, that is an expensive and extensive conversion. Another option is to use a larger transmission pulley or a smaller rear wheel pulley. But these require changing the drivebelt, another nasty job.
To provide a less expensive and easier-to-install solution, Illusion Motorsports offers its 27 Tooth Motor Sprocket Kits. Illusion has three kits: for '90-06 Softail and Dyna (except '06 Dyna), for '94-06 FXR and FLT, FLH, and for '93 down using a 26-tooth motor sprocket (it's best to contact Illusion for proper fitment). Each kit contains a 27-tooth (i.e. larger-than-stock 25-tooth) motor/compensating sprocket and a new custom-length primary chain (MSRP $122.09). Changing the compensating sprocket and primary chain only takes a couple hours.
The compensating sprocket is the front sprocket in the primary drive. Installing this larger compensating sprocket will change the primary and final drive ratios and result in higher speed at a given rpm, or conversely, lowers rpm at a given speed. Illusion claims its kit will increase highway speed by about 8 percent . That amounts to about a 5-mph increase in speed or a drop of about 200 rpm for a given cruising speed. To learn more about primary, secondary, and final drive ratios, check out "Drivetrain Basics" in the tech section of hotbikeweb.com.
Illusion sent us its Softail kit because our friend, Steven, wanted to smooth out the highway cruising on his '05 Softail Heritage Classic. We hustled over to his garage where he did the complete install in less than two hours. Prior to that, Steven made a data run and noted his rpm's at 50, 60, and 70 mph. After the install, he made the same data run.
Here are the results:
|2,300 rpm||50 mph||54 mph||8%|
|2,700 rpm||60 mph||65 mph||8%|
|3,000 rpm||70 mph||75 mph||7%|
This data to confirms Illusion's 8 percent claim.
All drive ratio changes are trade-offs and the downside here is a slight reduction in low-end torque. However, Steven reported no noticeable loss on his testrides.