The clutch for Vespa & Lambretta

Created by Jesco at 10:03 on March 14, 2023
Diagram of a Smallframe clutch

In this blog we will give you an insight into the function of the clutches, which are a very crucial part for every Vespa and Lambretta. If you have any further questions, please contact our telephone customer service or ask for advice directly at our flagship store in Landsberg. Our experts will help you with words and deeds, because the clutch should not be the cause of failure.

What does the Vespa clutch do?

On a Vespa, the driving force of the crankshaft is transmitted to the gearbox by a wet clutch system. The clutch has several tasks: Firstly - and this is important for a smooth shifting process - it separates the drive (i.e.: the rotating crankshaft) from the gearbox, so that no torque is transmitted to the rear wheel when the clutch lever is pulled. On the other hand, the clutch can be used to regulate the torque transmission when starting off. The driver uses the clutch progressively ("lets it come") and doses the power that is actually transmitted to the rear wheel by how far he pulls the clutch lever. The difference in speed between the friction discs times the torque is, physically speaking, work that is converted into frictional heat. This is initially stored in the clutch and gradually distributed to its surroundings, including the transmission oil. The advantages of such a wet clutch compared to a dry clutch (e.g. DUCATI) include better heat dissipation and good protection against external influences such as dirt and water.

Scheme "Cosa" coupling

Function of the clutch

In the friction pack, the friction discs are toothed alternately internally and externally. The internally toothed steel discs engage positively in the hub (also clutch gear, inner basket), the externally toothed ones (steel plate with lining) in the basket. On Wideframe Vespas, the lining discs are still internally toothed. The friction package is compressed into a compact sandwich by means of one or more clutch springs. More pressure means more friction and therefore more torque. At full spring force, the clutch must transmit more torque than the engine can provide, otherwise the clutch will slip when it is not supposed to. If you pull the clutch lever, the force of the springs on the friction pack is cancelled. The friction plates can then move independently of each other and no longer transmit power to the gearbox.

Clutch plates

The half-life of a clutch depends on two factors: Engine power and use. If the clutch in an original scooter is not overused and the gear oil is changed regularly, it can last a very long time. However, sprint starts, old, wrong or too little oil or heavily tuned engines quickly bring a standard clutch to its limits. This is usually noticeable when the friction discs overheat and the brown cork coating turns black. Whether and how you upgrade your clutch depends mainly on the engine's power and rpm level and how you want to use it. Most lightly tuned engines can still be run with a standard clutch as long as you use high quality individual components. Another upgrade is to use a kit with more friction discs.

Each friction pair can transmit the same amount of power. This means that a clutch with six discs can transmit twice as much power as a clutch with three discs. Provided diameter, lining material and spring force remain the same. In order to fit more discs in original clutches, the larger friction surface and higher load capacity of the clutch is bought by thinner plates, whose thin steel flanks hammer nasty pawls into the cheeks of the clutch basket. The result is poor clutch control, as the discs can no longer be moved freely in the clutch basket. If you try to fit more pads than intended in a small space, another problem often arises: the remaining disengagement travel is too small for the clutch to separate completely when the lever is pulled. Alternatively, there are now friction discs for the Vespa whose pads do not contain cork but carbon and resin compounds. Compared to standard cork linings, these new linings are designed for higher temperatures and are therefore better suited for high loads (e.g. use in racing). These modern pads also have specific grip levels and sometimes require harder springs and special gear oil to work optimally.

Clutch springs


If the clutch "slips" on a tuned engine, the first step should be to install harder springs. In most cases this is already very effective. With older clutch models, however, this often makes the clutch lever harder to pull. Modern PIAGGIO clutches such as the "Cosa" clutch (LF) and the clutch of the PK XL2 (SF), on the other hand, are very smooth-running due to a modified lever ratio and can also be pulled easily with harder springs. With smallframe clutches, a distinction is made between the version with a central spring and the multi-spring version of the more modern PK XL2 clutch. More springs naturally distribute the spring force more evenly over the lining surface and ensure better power transmission and smoother engagement.

The hardness of a clutch spring is influenced by the thickness of the spring wire, the number of coils, the material used and the overall length. We specify the hardness of clutch springs either descriptively, e.g. with "normal" or "hard", or in gradations such as "L", "XL".

Clutch basket

The clutch basket is exposed to the highest loads. Particularly with tuned engines, the baskets can knock out or bend open due to high engine speeds. The market now offers a wide range of remedies. Baskets with a ring effectively prevent the clutch from "coming undone". Reinforced or nitrided baskets are hardened again and therefore more resistant. High-end baskets are CNC-milled with extreme precision, have reduced weight and forged inner parts with reinforced rivets and space for up to 16 springs.

A question of oil?

With standard cork linings, it is recommended to follow PIAGGIO's specifications and fill in an appropriate amount of SAE30. It is essential to measure the required amount of oil beforehand and not to rely on the filler plug. Especially if the angle of the engine has been changed by a different chassis or the scooter is not completely level. Especially in racing, many riders who use carbon/sintered linings in their clutch use SAE80 gear oil. (Attention: engine oil and gear oil are graded differently!) They also often fill the clutch with 25-50% more oil. However, the limit is set by the bleeder screw on the clutch cover, from which the oil leaks out while riding if the level is too high. In the manuals of the new Vespa PX '98/2011, SAE80 gear oil is also approved, but this is mainly because the classic SAE30 oil is hardly used any more. To avoid clutch problems caused by the additives of the modern SAE80 oil, we still recommend our SAE30 gear oil for everyday use.

However, these problems are a thing of the past if you decide to take one of the numerous complete clutches available on the aftermarket. The new design of all parts offers the possibility to build perfectly functioning clutches that transmit even very high engine power without any problems.

Clutch gear and primary transmission

Regardless of whether it is a Vespa or a Lambretta, the clutch is always directly connected to the primary gear ratio. This means that the choice of the right clutch is always linked to the choice of the right gear ratio. Depending on the power of the engine and the purpose of use, there can be very different requirements here. Of course, the whole thing must then fit the gearbox used. Fortunately, almost any gear ratio is freely selectable in this sector today.

Another important final decision, at least in the Vespa sector, is the shape of the gearing: straight and helical gearing have different characteristics. Helical gears run very quietly and have little wear. Straight gears can transmit higher forces and have less loss. Therefore, straight-toothed primary ratios are ideal for sporting use, while helical-toothed ratios are best suited for everyday road use.

Further upgrades - High Tech clutches

In the case of smallframes, development has progressed even further and completely redesigned clutches are now available with more discs, more springs and a wide variety of facings (from motorbikes or other scooter models). With racing engines reliably delivering over 30 hp, the only limit now seems to be the thickness of one's wallet.

Product presentation: SIP Supersport clutch for VESPA


d = pitch circle diameter
d =pitch circle diameter
dk= tip circle diameter
df = root circle diameter
hk = tipheight tooth
hf = root height tooth
p =distance tooth centre - tooth centre

Gear pair in mesh. The toothing can be straight, i.e. parallel to the axis, or helical (helical toothing). The size of the toothing determines the module. The mating gear must have teeth of the same module. The module "m" is an important gear dimension for gears and is defined as the quotient of pitch circle diameter "d" (in mm) and number of teeth "z", or of pitch "p" and circle number "π":


This is also the reason why you cannot build a 23-tooth clutch gear into an original PX125 primary, the module does not fit. DRT has filled this gap and offers, among other things, a 23Z clutch gear with a FIT module, for a simple extension of the transmission ratio.

More videos about the clutch

Tutorial: Assembly Vespa clutch SIP SUPERSPORT

Product presentation: SIP BFA clutch for Vespa Largeframe

Tutorial: Installation of clutch & crankshaft for VESPA 180 SS

Tutorial: Function and assembly of a Vespa clutch

Tutorial: Clutch for Vespa Largeframe

Tutorial: Clutch for a 40 hp Vespa


Jesco is our product manager and technical editor. Many people know him from his videos on SIP Scootershop TV, where he presents tutorials, stories and products.