(image for) Warning Black-Week-Sale up to 50%

Which primary gearbox for Vespa PK 50 S/XL2 and V50?

When installing a tuning cylinder, the primary gear of the PK 50 S-XL2 and V50 should be adjusted. This addresses the question of which gear enables which theoretical maximum speed. Theoretical because a 50cc cylinder of a Vespa PK 50 does not provide enough power to "pull" a primary with a long gear ratio (e.g., the 2.34) in the last gear to the maximum RPM. If the gear ratio is too long, the RPM in the highest gear will actually decrease because the engine lacks the power to overcome rolling and wind resistance.

The original PK 50 S-XL2 gearbox is ideal for the original cylinder and its engine power. However, when using a DR 75 cylinder, for example, the increased displacement and modified channel design provide significantly more torque, imposing different requirements on the gearbox ratio. If the original gearbox ratio is used in such a case, the engine reaches its maximum RPM very early in the highest gear and does not go faster due to RPM-based limitation - even if it has the power to do so. Driving at 7000 RPM for an extended period increases wear and is also uncomfortably loud.

Desirable is a usable power band across a wide range of RPM.

An optimal gearbox ratio enables:

  1. Good acceleration
  2. High top speed
  3. Long service life
  4. Low fuel consumption

Definition of Primary Gear / Secondary Gear

A primary gear with a modified gear ratio lengthens or shortens the total gear ratio of an engine.
The secondary gear is responsible for gear shifting and the gear ratio of the gears to each other.

Data Basis

The calculation is based on the data from the RPM calculator by "nukes." The values for the division of the secondary gear are based on the information provided by GSF user "bobcat."

Justification for Selection

The common primary gears were included. The 4.25 gear from the Vespa 50 produced for the Italian market with 9" rims is not included for reasons of clarity, as the differences in top speed compared to the depicted 4.3125 primary gear ratio are very minimal and would make the illustration cluttered.

The same applies to the 2.54 primary gear from the Vespa 125 Primavera. The original helical 2.54 is often replaced with the 2.56 primary gear because it has less power loss due to straight teeth, and therefore, it is often installed in sporty engines with Polini 133 and Malossi 136 cylinders (whereas the helical teeth of the 2.54 provide the sound of an original Vespa engine). Therefore, only the 2.56 primary gear was included in the selection due to the small difference in speed and the advantage of better readability.

Explanation of Diagrams

The diagrams show the speed reached at a certain RPM. Diagram 1 shows the RPM from standstill to reaching fourth gear with seamless gear changes.
In Figure 1, the RPM in all gears is aligned to a maximum RPM of 7200 rpm, which is roughly the RPM of the popular DR 75 cylinder. The maximum RPM varies, of course, from cylinder to cylinder.

The x-axis of the diagram shows the 4 gears, the y-axis shows the speed. The indicated speed requires a correspondingly strong engine to actually achieve the theoretical top speed on the road.
After a theoretically lossless gear change from 3rd gear to 4th gear, the RPM is 5414 rpm.

Max. RPM = 7200 RPM

Vespa Primary Gear Ratios Compared

Note on the 4.3125 Primary Gear

The depicted primary gear ratio of 4.3125 is achieved by replacing the 15-tooth pinion of the original 4.60 gear with a 16-tooth primary pinion, which changes the primary ratio without the need to change the primary basket. The conversion only requires removing the clutch cover. Splitting the engine is not necessary.

Notes on Altered Wheel Diameter

In the case of the Italian 3-speed model of the Vespa V50, the 9" tires must be taken into account. The top speed can be compared to the speed of the German Vespa "50N" as follows.

  • The 1st gear of the Italian Vespa V50 with 9" tires corresponds to a factor of 0.967x to the 1st gear of the German Vespa 50N with 4-speed gearbox
  • The 2nd gear of the Italian Vespa V50 with 9" tires corresponds to a factor of 1.143x to the 2nd gear and a factor of 0.823x to the 3rd gear of the German Vespa 50N with 4-speed gearbox
  • The 3rd gear of the Italian Vespa V50 with 9" tires corresponds to a factor of 0.979x to the 4th gear of the German Vespa 50N with 4-speed gearbox

-> to the RPM calculator