Final drive improvements for Harley-Davidson Sportster models

Gear shifting is used on motorcycles to adapt the engine's torque and power delivery to specific driving and speed needs.
Just like when pedaling a bicycle, the combination of the output sprocket and the wheel's gear will determine how easily we can take advantage of the available power to climb a hill or increase our speed.

RPM and gear shifting
Transmission ratios are usually explained through not very complicated but not very intuitive graphs and numerical calculations.

We will try to do it in a simple and easy-to-understand way.
On a Harley-Davidson there are 3 places that intervene in the transmission ratio:

1. The primary transmission, which links the crankshaft output to the clutch

1. The gearbox, where this ratio can be varied with the shift pedal

1. The final drive pulleys, which connect the gearbox to the wheel

Only in the gearbox can we choose different variations of this ratio (4, 5, 6, or 7 depending on the number of gears). If we wanted to change, for example, the top speed in the last gear, we would have to physically replace one of the three previously mentioned components.
With a long final drive ratio, the engine revolutions (RPM) will be lower, and we will gain top speed but lose acceleration (engine torque).
On the other hand, if the ratio is short, we will lose a little top speed but will gain acceleration, and we will shift gears with the engine revving higher.
From all this, we deduce that choosing the optimal final drive ratio is always a compromise, where we must decide the best balance between RPM, top speed, and usability.

A practical case: increasing engine displacement
Let's take, for example, an 883 Sportster engine with a Big Bore kit installed to increase the displacement to 1200cc. Suddenly, the engine has more torque and power, and we want to optimize how this power is transferred to the road. One of the key points to consider is the vehicle's primary use and the type of roads we'll be driving on, to find the best compromise.
In this case, we'll be using the vehicle for urban use during the week and long trips on the weekend. Since we have greater acceleration due to the increased torque, we'll take advantage of this opportunity to lower the RPMs from our cruising speed, slightly "lengthening" the gears. This will achieve three main objectives:

• Greater ride comfort (less vibration) at high speed
• Lower fuel consumption
• Longer engine life (less stress and wear)

Let's get to work
To achieve our goal in a simple and cost-effective way, we opted to replace the transmission output pulley with one with two more teeth. This way, we can continue using the original belt (a change in the drive pulley has a greater effect than a change in the wheel pulley, since the smaller drive pulley changes the percentage of change.)
On our motorcycle we will replace the original 28-tooth pulley with a larger 30-tooth pulley .
We begin by loosening the drive belt, then removing the rear exhaust and pulley cover. With the pulley exposed, we remove the bolts and the retaining nut locking plate. Next, we need to use a specific impact socket wrench and a locking device to loosen the nut. Importantly, it is left-handed, meaning it loosens clockwise.

At this point we can remove the original pulley and replace it with the new one, which, as we can see, is slightly larger in diameter.

We continue the assembly guided as always by the workshop manual, taking into account the specific tightening of the retaining nut, where we will apply strong screw locking, a tightening torque of 67.8 Nm and a final tightening marked by an angle between 30º and 45º.

Final adjustment
After assembling the rest of the components, all that remains is to correctly tension the belt following the steps described in the manual.

We'll then manually rotate the wheel, checking that there's no friction along the entire length of the pulleys and belt. All that's left to do is electronically adjust the speed indicated on the odometer, as otherwise, it would always show a slightly lower speed than the actual speed due to the modified final drive ratio.

During the road test, we'll find the motorcycle responds smoothly and without jerks, and we'll enjoy greater smoothness at high speeds. If the road permits, we can also perform a top speed test, which will now be slightly higher.