Final Drive Improvements on Harley-Davidson Sportster Models

Gear shifting is used on motorcycles to tailor the engine's power and torque delivery to specific driving and speed needs.
As when pedaling with a bicycle, the combination between the output pinion and the wheel will determine that it is easier to take advantage of the available power to climb a hill or to increase the speed at which we circulate.

RPM and gear shifting
Generally, the gear ratio is explained through graphs and numerical calculations that are not very complicated, but not very intuitive.

We will try to do it in a simple and easy to understand way.
There are 3 places on a Harley-Davidson that go into the gear ratio:

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

1. The gearbox, where you can vary this relationship with the gear pedal

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

Only in the gearbox we can choose different variations of this ratio (4, 5, 6 or 7 depending on the number of gears). If we want to modify for example the final speed in the last gear we would have to physically replace some element of these 3 previously mentioned places.
With a long final transmission ratio the engine revolutions (RPM) will be lower, and we will gain final speed but we will lose acceleration (engine torque).
On the contrary, if the relationship or "ratio" is short, we will lose a bit of top speed but we will gain acceleration, and we will change with the engine more revved.
From all this we deduce that the choice of the optimal final transmission ratio is always a compromise, where we have to decide the best balance between RPM, top speed and usability.

A practical case: displacement increase
Take, for example, a Sportster 883 engine in which a Big bore kit to increase the displacement to 1200cc. Suddenly the engine has more torque and power, and we want to optimize the way this power is transferred to the road. One of the main points to take into account is the main use that we are going to make of the vehicle, and the type of roads on which we will circulate, to obtain the compromise that best suits us.
In this case we will use the vehicle for urban use during the week and long trips during the weekend. Given that we have more acceleration due to the increase in torque, we are going to take advantage of it to lower the RPM of our cruising speed, slightly "lengthening" the gears, with which we will achieve 3 main objectives:

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

Let's do it
To achieve our objective in a simple and economical way, we opted to replace the transmission output pulley with another with 2 more teeth. In this way we can continue to use the original belt (a change in the transmission pulley has more effect than a change in the wheel pulley, since the transmission pulley is smaller, the altered percentage is greater ...)
On our motorcycle we will replace the original 28 tooth pulley with another bigger than 30 teeth.
We start by loosening the transmission belt, then removing the rear exhaust and the pulley cover. With the pulley in sight, we remove the screws and the security plate from the retaining nut. Next we have to use a specific impact socket wrench together with a lock to loosen the nut which, very important data, is to the left, which means that it is loosened in a clockwise direction.

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

We follow 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-fixers, 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 under the steps described in the manual.

We will then turn the wheel manually, checking that there is no friction along the entire route of the pulleys and the belt. Now we only have to electronically adjust the speed indicated on the odometer, since otherwise it would always mark a speed slightly lower than the real one due to having modified the final transmission ratio.

During the road test we will check that the motorcycle responds smoothly and without jerks and we will enjoy greater smoothness at high speed. If the road allows it, we can also carry out a top speed test, which will now be slightly higher.