Racing crankshafts - Reed Valve
for classic Vespa
Racing Crankshafts - Reed Valves are the ultimate in engine tuning for a classic Vespa and a must-have if you want to achieve outputs of 25 hp and more. A reed shaft enables higher engine speeds. The Engine becomes more lively and agile. The result is better acceleration and a higher top speed. The crank webs are aerodynamically optimised and often polished. Depending on the design of the crank webs, we distinguish between Bell-Shaped Crankshafts, Flowed Crankshafts and Full Circle Crankshafts. The intake on the Crankcase must be adapted for all diaphragm shafts. There is no going back to the Disc Valve. But who wants to go back?
Crankshafts whose design does not allow the original rotary valve intake to be used are referred to as diaphragm shafts. A different type of intake control, usually a Reed Valve, must be used. A Reed Valve controls the fresh gas supply according to demand and the operating situation. The negative pressure in the Crankcase opens the diaphragm; once the pressure difference is equalised, the diaphragm closes again. In this way, the Reed Valve automatically regulates the gas supply at the ideal control time. The control time of a rotary valve inlet, on the other hand, is rigid and cannot adapt to the respective engine speeds or operating situations. This means that the intake timing is either too long or too short in many speed ranges. In the case of the Vespa, the rotary valve intake is also relatively small. If a large Carburettor is installed for tuning purposes and the Engine speed is increased, the cross-section quickly reaches its limits and acts as a throttle.
Diaphragm shafts are available in different versions.
TheBell-Shaped Crankshaftor mushroom shaft, is a Crankshaft whose shape resembles a bell or mushroom. It also offers the mixture as little resistance as possible. It is not quite as uncompromising as the Flowed Crankshaft, as its symmetrical shape is not quite as ideally suited to the asymmetrical intake times of the Engine. However, a very large cross-section is available, especially in the phases with the greatest vacuum. As a result, the difference in performance compared to the Flowed Crankshaft is minimal. On the other hand, the symmetrical shape has the advantage that uniform balancing with an ideal balancing factor can be achieved more easily and effectively.
Flowed Crankshaftsare flow-optimised reed shafts that harmonise very well with reed intake manifolds. On PX/T5/Cosa Engines, the intake is located above the crank cheek on the clutch side. A particularly flow-optimised machining of this crank web often resembles the shape of a lip.
Full Circle Crankshaftshave, as the name suggests, two completely closed cheeks. A large diaphragm inlet is required here. The use of a Full Circle Crankshaft only really makes sense in conjunction with a direct inlet into the Cylinder. These shafts are also available with a long stroke. The basic idea behind the Full Circle Crankshaft is better pre-compression, which is achieved via the completely closed crank webs. However, in engines controlled via the Casing, even large milled diaphragm inlets with boost ports cannot prevent the mixture from being unfavourably obstructed by the cheek standing in the way.
The next decision criterion is the choice of the right engine type.
Each engine type has its own Racing Crankshaft
The range of smallframes and largeframes is so large that we also offer these models the racing crankshafts inDisc ValveandDiaphragm shaftsfor the Vespa scooters, we basically differentiate between the models according to their frame widths:
Smallframe - Vespa 50-125/PV/ET3/PK/XL
Largeframe - Vespa 125/150/Rally `57-
Largeframe - Vespa PX/T5/Cosa
It is therefore advisable to select the appropriate "Vehicle type" in the field under "Classic Vespa" to see all Crankshafts that are suitable for this.
More stroke = more cc = more power
Standard stroke: In addition to the cylinder diameters, the original engine types also differ in terms of stroke, as we have listed in the engine classes.
Long stroke: If you want more torque, you can opt for long-stroke variants. When looking for long-stroke variants, you need to pay attention to the stroke lengths of the diaphragm shafts.
Note: A thicker cylinder base gasket or cylinder head gasket is required. More stroke is usually compensated for by a head gasket with a thickness equal to half the additional stroke. If a base gasket is used for equalisation, the exhaust should also be machined. This makes particular sense for higher tuning levels.
The subtle difference lies in the Conrod
A longer Conrod does not change the stroke, but the Cylinder must still be raised accordingly. The longer Conrod has the advantage that it is less inclined at half the stroke and the piston therefore exerts less lateral force on the cylinder wall, resulting in less friction. A longer Conrod also reduces the maximum piston speed and therefore also the change in speed of the Piston during one revolution. This reduces vibrations.
The Conrod is one of the most heavily loaded parts in an Engine. It has to transmit the force of the explosion, withstand extreme acceleration and endure great heat. In addition, two bearings run in the running surfaces of the Conrod, one at engine speed, so a high-quality Conrod is suitable and important to prepare an Engine for Elevation loads or to increase its stability.
There are a large number of features and combinations of the realisation of properties on a Conrod. To make the selection a little clearer, we distinguish between three types of Conrod:
Standard: A good Conrod that has been manufactured to standard specifications.
Sport: An improved Conrod
Race: An elaborately manufactured Conrod
The better the Crankshaft, the more effective the balancing and the smoother the Engine runs
Strong vibrations not only have a negative effect on riding comfort, they also affect the durability of the individual components on a scooter and, last but not least, the performance of an engine. This can be remedied by "finely balanced" Crankshafts. "Finely balanced" is not actually a clear technical term. It is more of a colloquial term for a particularly well-balanced Crankshaft. Nevertheless, we have decided to use this term to describe a Crankshaft.
Our top brands for racing crankshafts - Membrane for Vespa Classic
SIP PERFORMANCE: As SIP Performance version with polished forged Conrod or in the Premium version with a high-quality bronze Conrod.
MAZZUCCHELLIthe classic Racing Crankshaft for Largeframe. Made in Italy. Good and favourable or as K2D version also suitable for more powerful tuning.
POLINI: The Italians have been offering Crankshafts for Vespa Models again for some years now.
Technology tips
If you want to delve even deeper into Crankshaft technology, we recommend our Technology blogs.
Customers ask - SIP answers
"What other parts do I need for a crankshaft replacement?"
Often only cent items, but annoying if you don't replace them:
WoodruffKeys - don't put an old Woodruff Key in a new groove
Gasket Set Engine - if you don't change it, it will drip
Crankshaft bearings - always replace LIMA as well as CULU side
Oil Seal Crankshaft - replace both sides here too
NutLock Cap - if you have a Castle Nut on the clutch, you need it
