DRT Vespa Smallframe 28 Tooth Primary Gear

 

The sprocket offered here from the manufacturer DRT is one of three that can be used with a standard 29/68 (2.34) ratio in the Vespa small frame motors.

 

An original 125cc Vespa with a small frame (like a Primavera or PK) originally uses a primary reduction ratio in the gearbox of 24/61 (2.54).
Therefore the 29/68 (2.34) is already much longer and only really usable in very few engines. This has to do with the power which is necessary to overcome the increasing air resistance with increasing speed. Since this increases by the square, i.e. double speed means quadruple air resistance. A small-frame motor can, with the appropriate tuning, deliver a lot of power, but of course it does not do this at the level of a serial motor. Often the full power is only available at far more than 8000 rpm.

This means that even with the standard gear ratio it would already become much faster, because it can turn much further. On the other hand, a strong engine has the full power only in the upper rpm range, in the speed cellar this is usually missing.

If you extend the final ratio of such an engine as drastically as with a 29/68 (2.34) primary, this often causes problems with the gear connection in the upper gears or it can happen that the engine cannot reach its full speed in fourth gear at all, because it is braked by the driving resistors and cannot reach the full power range.

DRT therefore offers three sprockets for the 29/68 (2,34) reduction:

• 27 teeth (2.52)
• 28 teeth (2.43)
• 30 teeth (2.26)

With the first two, the reduction can be shortened, whereas the 30 tooth sprocket considerably extends the reduction again.

This can be useful if, for example, an extremely short stepped gearbox is used with the aid of the so-called matrix auxiliary shaft and the 47-tooth Parma 4th gear. In this case, the ratio of the Matrix auxiliary shaft and the Parma gear wheel would result in a secondary transmission ratio of 18/47 (2.61). In combination with a primary 29/68 (2.34) gear, which was changed to 30/68 (2.26) with the help of the DRT 30 tooth pinion, the overall transmission ratio would be the same as the length of the standard gearbox and 29/68 (2.34) primary gear.

Now, however, with the huge advantage of the closely spaced gear steps due to the Matrix auxiliary shaft and Parma pinion. This increases the chance that an engine can really pull such a long gear considerably, because it no longer falls out of the so-called resonance range in which the exhaust fully supports the engine.