ET-CLUTCHES ARE WIDELY USED in motorcycle applications.
In a wet-clutch design, the oil acts as a
heat-transfer medium, reducing operating temperatures and minimizing varnish
and lacquer formation that can lead to slippage and increased heat. The oil
also minimizes build-up of wear debris on the frictional plates and provides
lubricity to components and wear areas within the clutch, including the
bearings and the points of contact between the outer tabs of the frictional
plates and the clutch basket.
Wet clutches can be found in three different
configurations. One features a separate oil reservoir for the clutch (isolated
from the engine and transmission), another features a shared oil reservoir for
the transmission and clutch, and the third features a shared oil reservoir for
the engine, transmission and clutch. The different configurations place unique
demands on the lubricants that protect them. In the first example, the oil must
handle the lubrication and frictional demands of the clutch and possibly a
roller chain or single gear set. In the second example, the oil must handle
both clutch and transmission lubrication. In the third example, the oil must
handle clutch, transmission and engine lubrication, requiring a dynamic fluid
capable of meeting a variety of needs.
Good wet-clutch performance is extremely important
to ensure satisfactory drivability. Frictional resistance is separated into two
types: static and dynamic friction. The force required to begin movement of a
box across the floor is an example of static friction, while the force required
to keep it in motion is an example of dynamic friction. It takes less effort to
keep the box moving than it does to break it loose. In motorcycles, static
friction is the force that keeps the frictional plates and steel plates locked
together and prevents them from slipping when the clutch is engaged. Dynamic
friction comes into play as the clutch is engaged and the plates begin to
contact each other. Dynamic friction begins the rotation of the steel plates.
When there is enough contact and the forces of static friction are overcome,
the steel plates rotate at the same speed as the clutch and become locked
The surface condition of the plates affects the amount of friction generated
during lock-up. There are significantly different surfaces in a clutch: the
rough frictional plate and the smooth steel plate. The resulting force required
for the two different plates to grab and lock-up is called the coefficient of
friction. A rough plate will lock-up quicker than a smooth plate.
The graph displays a typical friction profile. As the clutch is engaged,
spring pressure forces the rotating frictional plates up against the
non-rotating steel plates. Dynamic friction between the two plate-types
increases rapidly, causing the steel plates to begin rotating. The level of
dynamic friction remains relatively constant until both plate-types are
rotating at the same speed. Once rotation speed is equalized, undesirable
slippage between the two plate-types is minimized by the resistance provided by
static friction. The ability to minimize slippage when the clutch is engaged
and locked is depicted on the right-hand side of the graph. Static friction is
highest just prior to the plates breaking away or slipping. Once slipping, the
resistance force is reduced as dynamic friction takes over.
Dynamic friction should have a high and relatively flat trace, providing a
shorter time between clutch engagement and lock-up, and resulting in faster
shifting. The level of dynamic friction should decrease slightly as the plate
rotation speeds equalize, providing a smooth shift feel. If there is too much
dynamic friction, the shift feels abrupt and harsh. If there is not enough, the
shift is elongated and increases the potential for excessive plate slippage.
High static friction is also desirable as it provides good clutch holding power
and the ability to transfer the maximum design capacity through the clutch.
Not all lubricants are suitable for use in wet-clutch
applications. Frictional properties, cleanliness, clutch material/oil
compatibility, anti-foaming properties, shear stability and high-temperature
stability are all important to maintain the integrity and performance of a
Properly selected synthetic base oils perform very well in wet-clutch
applications and can improve performance and longevity; however, additive
chemistry has a far greater impact on performance. Friction modifiers can
decrease the coefficient of friction within the clutch pack and result in
excessive plate slippage, while extreme-pressure additives commonly used in
gear lubricants can cause excessive clutch slippage and related damage.
AMSOIL Synthetic Motorcycle Oil is multifunctional and provides outstanding
protection for wet clutches. It is shear stable and resists thinning from
mechanical activity, performing like a gear lube without the negative effects
of extreme-pressure additives. AMSOIL Synthetic Motorcycle Oil contains no
friction modifiers and promotes smooth shifting and positive clutch engagement.
It controls heat and helps prevent slippage and glazing, while its high TBN
helps improve clutch life by resisting the acids that can degrade clutch