DIFFERENCES IN THE LUBRICATION OF PASSENGER CARS AND MOTORBIKES

If we focus on a passenger car, the engine has a dedicated oil sump, typically wet (i.e. the volume of oil is stored in the lower pan), with a closed circuit that focuses on lubricating the moving parts meant for this power-generating element. This oil does not have to withstand high loads, but it must protect parts that move at very high speed and generate heat; pistons, piston rings, cams... Wear is a factor to be considered, and it is typical to add additives called "friction modifiers", which increase the lubricant's ability to separate metal surfaces that would otherwise come into contact. 

On the other hand, there is the transmission, which has a separate, dedicated sump. Although temperatures are not as high as in the engine, the loads transferred are enormous, and the lubricant must be formulated in a much more specific way so that both the lubricant bases and the additives can withstand them adequately. It is common to include so-called "extreme pressure" or EP additives in its composition which, at a certain pressure and temperature, generate a sacrificial film to prevent damage to transmission components.

Finally, the clutch can be dry or wet, and this establishes a fundamental point between the lubrication of passenger cars and motorbikes. In passenger cars, the clutch is usually dry and does not require any lubrication. Motorbikes, however, are usually fitted with wet clutches (except in racing or specific off-road applications). This clutch requires oil immersion which promotes cooling and aids in smoother, quieter operation. 

The other fundamental difference in lubrication between passenger cars and motorbikes is the oil circuit. In a passenger car, as mentioned above, both the engine and transmission are fitted with separate, dedicated sumps, which can store completely different oils. This means that an ideal product can be used for each application, maximising the qualities of the oil that help to protect the hostile environments of each component (extreme temperatures, high load transfer...). Hence there are different engine and transmission oils for passenger cars, as well as specific solutions for axles and differentials (which, again, have a separate sump). 

This issue is much more complex on motorbikes. In general, on a motorbike, a single oil sump is used for all components. This means that the same circuit drives an oil that will lubricate the engine, transmission and wet clutch. At this point, lubrication engineers are faced with a triangle of opposing characteristics:

- Engine performance and protection, for which the oil requires anti-wear additives and friction modifiers.

- Transmission protection, which involves the addition of EP additives to improve shear stability and other high load factors.

- Cooling and behaviour of the wet clutch, which needs a certain degree of friction to engage the discs properly and prevent them from slipping against each other.

If a motorbike were to use an oil with a high load of anti-wear additives, friction modifiers and EP, engine and transmission operation would be ideal. However, because of the shared circuit, the friction modifiers would stick to the clutch discs and, when they tried to engage with each other, they would slip. This would result in erratic clutch behavior and an inability to efficiently couple the power generated from the engine to the wheels (commonly called clutch slippage).

In the case of using a regular car engine oil in a motorbike, it would provide adequate protection to the engine, although the higher engine speeds and temperatures involved would require special attention to the drain interval. However, transmission protection would be severely compromised (as the oil would not be able to withstand the loads on the gears transmitting the power due to the absence of the mentioned additives) and the clutch would slip (due to the large amount of friction modifiers added).

What are the implications of this casuistry? After much research effort and extensive collaboration between regulatory bodies (mainly JASO, the Japanese Automotive Standards Organization) and private organisations, there are a number of specifications that limit the addition of friction modifiers in motorbike oils. This limitation involves a delicate balance in formulating products that ensure the performance of all components, including the clutch. The clutch must provide a reasonable friction zone, which provides the user with sufficient comfort while riding the motorbike, while ensuring engine and transmission protection.

The relevant exception is scooters. In general, a scooter has a dedicated lubrication circuit for the engine (separate sump, as in passenger cars) and a CVT (Continuously Variable Transmission) oil bath in the belt type transmission. The clutch is typically a dry, centrifugal type, which does not require lubrication. In other words, there are two separate sumps (engine and transmission) and a dry clutch; very similar to a passenger car! In fact, some manufacturers recommend oils with specifications traditionally associated with passenger cars for some scooters with these characteristics. 

However, a specific recommendation for motorbikes without a wet clutch is the norm and the most common choice in the industry. These are scooter specific oils. They are formulated to improve the ability to cope with oxidative stress caused by high temperatures, which are common in scooters due to the reduced cooling they suffer by their design. 

Companies that formulate and produce high quality motorbike and scooter oils, such as Castrol, adhere to the standards set by the industry and maximise product compatibility, efficiency and protection at all levels. It is also important to note the significant test facility which are racing teams, where innovations in formulations can be properly tested. The performance of the oil technology is then transferred to our Power1 range of products.