Improvements in the final transmission of Harley-Davidson Sportster models

The gear change is used in motorcycles to adapt the delivery of the torque and power of the engine to the specific conduction and speed needs.
As when we pedal with a bicycle, the combination between the output pinion and that of the wheel will determine that it is easier to take advantage of the power available to raise a slope or to increase the speed at which we circulate.

RPM and gearshift
Generally, the transmission ratio is usually explained through graphics and numerical calculations not very complicated, but not very intuitive.

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

1. The primary transmission, which links the crankshaft exit with the clutch

1. The gearbox, where this relationship with the change pedal can be varied

1. Final transmission pulleys, which connect the gearbox with the wheel

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

A practical case: cylinder rise
Let's put for example an 883 Sportster engine in which a Big Bore Kit to climb 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 we will circulate, to obtain the commitment that best suits us.
In this case we will use the vehicle for urban use during the week and long trips during the weekend. Since we have greater acceleration due to the increase of the motor torque we will take the opportunity to lower the RPM of our cruise speed, "slightly" extending "the marches, with which we will achieve 3 main objectives:

• Greater march comfort (less vibrations) at high speed
• Lower fuel consumption
• Longer motor duration (lower stress and wear)

Hands to work
To achieve our goal in a simple and economical way we choose to replace the pulley of the transmission output with another with 2 more teeth. In this way we can continue using the original belt (a change in the transmission pulley has more effect than a change in the wheel pulley, since being smaller the transmission pulley the altered percentage is greater ...)
On our motorcycle we will replace the original 28 -teeth pulley with Another larger than 30 teeth.
We start by wearing the transmission strap, then removing the rear exhaust and pulley lid. With the pulley in sight we remove the screws and the safety plate of the retention nut. Then we have to use a specific impact tube key next to a blocker to loosen the nut that, very important, is left, which means that it loosen in the direction of the clock needles.

At this point we can already extract the original pulley and replace it with the new one that as we can check is slightly larger in diameter.

We follow the guided assembly as usual by the workshop manual, taking into account the specific tightening of the retention nut, where we will apply strong fixed-tornillos, a 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, the strap is correctly tightened under the steps described in the manual.

We will turn the wheel manually checking that there is no rubbing throughout the pulleys and strap. We only have to adjust electronically the speed indicated in the accountckilometers, since otherwise it would always mark a slightly lower speed than the real one for having modified the final transmission ratio.

During the road test we will verify that the motorcycle responds gently and without pulling and we will enjoy greater soft speed. If the route allows it, we can also perform a tip speed test, which will now be slightly higher.