Crankshafts for Vespa and Lambretta

Created by Jesco at 11:01 on January 12, 2022

Crankshafts: General

More power through more torque and/or more speed? Power is equal to the torque times the speed at which that torque is delivered. So how do racing engines achieve such high power? It can only be done by increasing the torque or the speed at which the torque is applied.

P=Md * (r)

P = Power (kW)
Md
= Torque (Nm)
(r) = Speed (rad/s)

If you want to recalculate this formula using your scooter, you still have to bring the units of measurement to a common denominator. The formula is valid when the speed is given in the unconventional dimension rad/s. One has to know that one minute is one minute. You have to know that one minute is 60 seconds and one revolution is 6.28 rad. If we assume the maximum torque of 15 Nm at 6000 rpm, the power here is:

P = 15 Nm * 6000 rpm * 1 min/60 s * 6.28 = 9420 Nm/s

1 Nm/s = 1 Watt is, 1000 Watt = 1 Kilowatt, so the Vespa engine described here has a power of 9.4 kW *1.36 = 12.8 HP at 6000 rpm.

The torque results more or less from the displacement. There are fixed upper limits to the torque, because at best the complete cylinder volume can be filled with fresh petrol-air mixture and burnt per piston stroke (turbocharging, laughing gas or similar aside). All we can do is make sure that all the components are well matched to fill the cylinder as much as possible and that the displacement is as large as possible. And that can only be done with more stroke and more bore. More stroke comes from long-stroke shafts, more bore from tuning cylinders. The speed of the torque output is the other variable that can be used to increase the power. If this is doubled, the power is also doubled. Racing engines rev so high because they are supposed to burn more cylinder charges in the same time.
It is important to make the engine rev as high as possible by using lighter weights and harder counterpressure springs, because you will hardly find any power in the rev range of a high-speed engine.

The crank webs - the right type

For optimal cylinder filling at high engine speeds, the mixture must flow through the overflow ports as quickly as possible, and this works best if the pressure in the crankcase is as high as possible when the overflow ports open. Since this pressure is higher the smaller the ratio of the volume of the crankcase between the top and bottom piston positions, one tries to keep the dead volume (air volume in the crankcase) as small as possible. This is achieved, among other things, by crankshafts with full crank webs that fill the crankcase as much as possible. However, it is possible to overdo the game of eliminating dead space, because there is an optimum level of pre-compression. If this is exceeded, negative effects prevail and the engine's performance is reduced instead of increased. In addition to the solid shaft, there are many other types of shafts available for tuning, each of which is optimised for a certain purpose.

Flow-optimised lip and bell (or mushroom) shafts - as a further development of the solid cheek shaft - are suitable for engines with direct or diaphragm intake. The shape of the cheeks does not unnecessarily obstruct the inflowing gas flow and creates more usable housing volume. The pre-compression is thus slightly reduced, more fresh gas can flow in and a higher degree of filling is achieved. This interaction enables a wider rpm band and the achievable power increases.

Crankshafts and rotary valves

Rotary valve drivers, on the other hand, like to use racing lip shafts or long-stroke shafts. In addition to a significantly longer intake time, which has a positive effect on the degree of filling, racing shafts also have streamlined and polished crank webs that reduce turbulence and stalls. The newer racing lip shafts for rotary vane engines also have the same advantages as diaphragm lip shafts.

High-quality shafts are characterised by a fine balancing (see below "Full the balance") and high concentricity - i.e. no "knock" in one direction - (= directional shaft). This makes them particularly smooth-running.

Polished cheeks or connecting rods offer less flow resistance.

For high-end smallframe engines, crankshafts with a diameter of 87 mm (instead of the original 82 mm) are offered. These offer more security against twisting, as there is more material over the pin and thus the press dimension can be selected higher. However, the crankcase must be spindled to 88 mm.

Full balance
When we use the term "balanced", it means "12 o'clock balanced" or "statically balanced". The connecting rod eye swings out at 12 o'clock (top) when the shaft is freely and horizontally suspended. All rotating masses are 100 % balanced by means of balancing holes. Pistons, rings, pins, etc. are the "reciprocating" translational masses. Unfortunately, the entire system can never be 100% balanced, which is why there are no vibration-free 1-cylinder engines. This has to do with acceleration and deceleration at the dead centres and with the eccentricity of the masses in between.
Dynamic balancing would mean that the entire system, including pistons and shafts, would be balanced. Unfortunately, the effort is not feasible in practice. In fact, one tries to balance piston and shaft well, described by the "balance factor". It describes the weight ratio of rotating to oscillating (translatory) masses. Experience shows that a value of about 40 % gives good results.
The NIK balancing stand is a good tool for checking and adjusting the balance.

NIK balancing stand.

The stroke

Longer stroke is achieved by positioning the crank pin further away from the crankshaft's axis of rotation. This effect can also be achieved with an eccentric pin, for example. 1 mm more distance from the axis means 2 mm more stroke. The piston then moves 1 mm beyond top dead centre and 1 mm below bottom dead centre. To prevent the piston from colliding with the head, a 1 mm thicker foot or head gasket must be fitted. In addition, the piston skirt should be shortened slightly at the bottom so that it does not collide with the housing at bottom dead centre. The timing will change in any case.

Compensation by foot seal: exhaust times become longer, overflow times become significantly longer. If the exhaust is also raised slightly by milling, the engine is optimised for higher speeds.

Compensation by head gasket: Exhaust and overflow times are only slightly extended - whereby the overflow time changes more than the exhaust time (=less pre-discharge). Optimisation for more torque at medium engine speeds.

Unterschied zwischen einer original Hubwelle und einer Langhubwelle (z. B. Vespa PX200): Der Pleuelbolzen sitzt um 1.5 mm weiter außen. OT und UT zusammengerechnet ergibt das 3 mm mehr Hub für die Vespa. Ausgeglichen wird mit passenden Fuß- oder Kopfdichtungen. Dadurch ändern sich aber auch die Steuerzeiten, was berücksichtigt werden muss.
Unterschied zwischen einer original Hubwelle und einer Langhubwelle (z. B. Vespa PX200): Der Pleuelbolzen sitzt um 1.5 mm weiter außen. OT und UT zusammengerechnet ergibt das 3 mm mehr Hub für die Vespa. Ausgeglichen wird mit passenden Fuß- oder Kopfdichtungen. Dadurch ändern sich aber auch die Steuerzeiten, was berücksichtigt werden muss.
Den Hub einer Kurbelwelle kann man ganz einfach ausmessen: Man misst den Abstand Pleuelbolzen außen zu Lagersitz außen. Davon zieht man je die Hälfte von Ø Pleuelbolzen und Lagersitz ab und hat so den schlecht messbaren Abstand Mittelpunkt zu Mittelpunkt. Der gemessene Wert *2 ergibt den Hub.
Den Hub einer Kurbelwelle kann man ganz einfach ausmessen: Man misst den Abstand Pleuelbolzen außen zu Lagersitz außen. Davon zieht man je die Hälfte von Ø Pleuelbolzen und Lagersitz ab und hat so den schlecht messbaren Abstand Mittelpunkt zu Mittelpunkt. Der gemessene Wert *2 ergibt den Hub.

Crankshafts: More bore or more stroke?

Long stroke is the more expensive but, in terms of increased power, also the better method of expanding the displacement, because 5% more displacement (achievable by 5% more stroke) means 5% more square millimetres of window area if the timing were left unchanged. So displacement and window area increase in the same proportion. In contrast, with 5% more displacement by changing the bore (achievable with 2.46% more bore because the bore is squared in the displacement calculation) you only achieve 2.46% more window area. This is also the reason why Grand Prix high-performance two-strokes are mostly quadrathubers (stroke-bore ratio roughly the same).

We scooter riders, however, have to tune on the basis of standard engines and cannot freely choose the displacement and the stroke-bore ratio. More bore is easily and inexpensively achieved by swapping cylinders, which is why this is the most common method. Even if, unfortunately, this only results in a short-stroke engine.

However, very long exhaust and overflow timing, and thus high power, can only be achieved by increasing the piston travel. However, the screw connection around the crankcase sets narrow limits to the possible additional stroke and the housing must be machined, i.e. spindled out, to provide sufficient space in the diameter for a long-stroke shaft. Since the critical piston speed of 20 m/s can be reached with eccentric long-stroke shafts, the carburettor and ignition settings should also be adjusted with extreme precision. However, if you take the trouble and also adjust the cylinder's timing optimally, you will be rewarded with a very potent engine and power outputs of over 40 hp/40 Nm are quite possible.

What does a low-vibration crankshaft need?

The crankshaft is significantly involved in how strongly an engine vibrates. Strong vibrations not only have a negative effect on riding comfort, they also influence the durability of the individual components on a scooter and, last but not least, the performance of an engine.

Various factors are important for low-vibration running of the crankshaft:

  1. Alignment of the crankshaft: A crankshaft should always run completely straight between the bearings. The two crank webs are connected to each other via the crankpin. If the crankshaft is twisted at this connection, vibrations will occur.
    Particularly high-quality crankshafts, such as SIP Performance shafts, are very precisely aligned by hand.

  1. Balancing position: A crankshaft should have a counterweight to the oscillating masses (pistons etc.). This means that an unassembled crankshaft resting on its bearing surfaces will oscillate around this counterweight. The heaviest point of the crankshaft points downwards. For this centre of gravity to be a useful counterweight to the piston movement, the crankpin should be at 12 o'clock or (if the crankshaft is rotating clockwise) at 1 o'clock.
    Many racing crankshafts for Vespa used to be roughly machined to provide more space for the fresh gas in the intake area. This often led to a skewing of the counterweight. A good modern shaft compensates for this skew by the shape of the machining or balancing weights.

  1. Balance factor: The counterweight that the crankshaft exerts on the piston should be matched to the weight of the piston. The ideal factor of this weight is called the balance factor in percent. What the ideal balance factor is for an engine depends again on many other factors. For example, the cylinder position.
    The problem is: there is no one absolutely correct balance factor. Every manufacturer has his own philosophy. In addition, each manufacturer uses a different piston weight to determine the balance factor. Usually that of their own products for which the crankshaft was designed.

What does "finely balanced" mean?
Finely balanced
is actually not a clear technical term. It is rather a colloquial expression for a particularly well-balanced crankshaft. Nevertheless, we have decided to use this term as information about a crankshaft.

We use the term "finely balanced" to describe crankshafts that fulfil all three of the above points:

  • They are particularly precisely aligned.

  • They balance at 12 or 1 o'clock.

  • The manufacturer has implemented a certain balance factor in the design.

Unfortunately, this is no guarantee that the balancing of the respective shaft fits the intended engine perfectly. But it is a clear quality feature, which greatly increases the probability of a smooth-running motor.

The connecting rod

A longer connecting rod does not change the stroke, but the cylinder must still be raised accordingly. The longer connecting rod has the advantage that it is less inclined at half stroke and the piston thus exerts less lateral force on the cylinder wall, resulting in less friction. However, this advantage comes at the price of a drastically higher crankcase volume: It increases by the value of the area enclosed by the inner edge of the foot seal, times its height.

The timing also changes with a long connecting rod. However, only slightly, because the piston lift curve changes slightly.

Since the crank webs are held together by the connecting rod pin, it is desirable to have as high a compression dimension as possible to prevent the two webs from twisting. The connecting rod pin, on which the connecting rod and bearing sit, is pressed into the two cheeks of the shaft with high pressure. If the holes in the cheeks are too large or the diameter of the pin too small, the press-fit connection cannot build up sufficient strength and gives way. This can lead to serious engine damage. For this reason, the connecting rod journals of more powerful engines are often welded to the cheeks to prevent twisting from the outset.

A distinction is made between standard connecting rods and polished blade connecting rods, which are flow-optimised.

The connecting rod length is always measured from centre eye to centre eye. For high-end smallframe engines, there are, for example, special shafts from POLINI with a connecting rod length of 102 mm instead of 97 mm.

On older Vespa models, brass bushings were used as connecting rod bearings to minimise friction. Due to the high oil content in the mixture and very low engine speeds, it was possible to ride reliably in the past. However, as modern engines reach higher speeds and less oil is added, needle bearings are nowadays usually used at the top and bottom of the connecting rod. These can withstand higher speeds and, in the case of racing shafts, are also supplied with mixture through additional lubrication holes or slots. This reliably prevents a bearing from running "dry", getting too hot and seizing on the piston pin or connecting rod pin. In high-quality silver bearings, the bearing cage is silver-plated, which leads to less friction and wear and thus a longer service life.

Beispielbild
Messer Pleuel
Messer Pleuel
Standard Pleuel
Standard Pleuel
Pleuelsatz inkl. Pleuelzapfen und Anlaufscheiben
Pleuelsatz inkl. Pleuelzapfen und Anlaufscheiben

Smallframe: Cone, bearing and oil seal seat

Smallframe crankshafts are available in three different versions:

The older smallframe models (V50/PV/ET3) are equipped as standard with a crankshaft with a 'pointed' cone (oil seal Ø 19 mm, bearing seat Ø 20 mm) on which the fan wheel sits. However, this is not particularly hard-wearing and can therefore shear off even with slight tuning.

In contrast, the cone of the successor model is much more stable: The crankshaft of the PK XL has a blunter cone with a 20 mm oil seal seat (the bearing seat Ø remains the same), which is not only optimally suited for all tuning purposes, but also represents the ideal basis for a conversion to electronic ignition. The shafts with 51 mm stroke (originally installed on PK125XL/ETS) have the most stable design with a further reinforced oil seal seat (24 mm) and a 25 mm bearing seat. Perfect for ambitious tuning projects. However, they can only be installed in original V50 and PK50 housings in conjunction with a conversion bearing and oil seal.

In short, there are three sizes of oil seal seats and two bearing seat sizes on smallframe Vespas.

Crankshafts and Lambretta

The standard shafts of the series 1-3 LIS/SX/TV DL/GP each have a stroke of 58 mm and a 107 connecting rod. Only the TV175 comes with a 116 mm connecting rod as standard.

It is possible to install a crankshaft with a 110 mm connecting rod instead of the 107 mm connecting rod. This should result in a smoother running engine. In this case, the longer connecting rod should be compensated with a 3 mm foot seal. However, the difference between 107 and 110 mm is very small. It is better to convert to a crankshaft with a 116 mm connecting rod. The longer connecting rod can be ideally compensated by a piston with a lower compression height. For most Lambretta cylinders, pistons with a compression height of 30 mm instead of the original 39 mm are available. In this way, a real improvement in terms of smoothness is noticeable.

In the tuning sector, a distinction can be made between racing shafts and long-stroke shafts. Since Lambretta engines have a direct intake into the cylinder, the shape of the crankshaft is not influenced by the requirements of an intake into the crankcase. Racing shafts are characterised by particularly resilient materials, high-quality bearings, special connecting rods, special balancing or weight. Long-stroke shafts are available in a wide range of variations. 60 mm strokes are by far the most popular, as 60 mm shafts can normally be installed without modifications to the housing. Long-stroke shafts with more than 60 mm stroke require a crankcase with a larger diameter.

In contrast to the Vespa, the Lambretta cone is only available in two versions, which are interchangeable. The large cone with 25 mm (measured directly after the oil seal) was originally only in the engine of the DL/GP. The small cone of all other models tapers to 21 mm directly after the oil seal seat. Under maximum load or at high speeds, however, enormous forces act on the entire cone, which the narrow tip of the 21 mm cone cannot always reliably withstand. Basically, the same applies here: The thicker the cone, the more stable and the more suitable for tuning. Depending on the cone, the corresponding fan wheels are of course required.

Jesco
Jesco

Jesco is a technician and product developer at SIP Scootershop. Many people know him from videos on the SIP TV Tutorial Channel, where Jesco explains complicated stuff in a simple way.