Written By: Castrol Motor Oils
A modern four stroke motorcycle is one of the toughest applications for a lubricant. Today’s complex multi-cylinder engines are amazingly high revving, with some road going units now routinely approaching 16,000 rpm. Just for the sake of comparison, your typical passenger car will operate at roughly 1,800 rpm. The oil operating temperatures in a modern motorcycle engine are 20-30°C higher than in cars, with air cooled engines running as much as 50-60°C hotter. On the other hand, a motorcycle sump is considerably smaller than what would be found in the average passenger car. Most motorcycles have a common sump, which means the same fluid that lubricates the engine must also provide lubrication for the clutch and gearbox.
Join us on a journey through the critical zones of your engine and see how motorcycle oil is formulated to protect and ensure proper functioning of each vital component.
Oil first enters your engine through the sump, which is a chamber that holds the fluid. Motorcycle oils must have a detergent, dispersant and antioxidant additive system designed to cope with very high sump temperatures. It’s then splashed onto the cam and lifters where in provides a protective barrier between these two surfaces preventing metal to metal contact. It then encounters the piston and rings, where it clings to the surfaces to lubricate yet maintain compression – despite the forces that are trying to tear it apart. Oils with insufficient high temperature deposit control and antioxidant performance can rapidly lead to severe engine problems due to stuck piston rings and excessive thickening that makes oil flow through the engine difficult. This will lead to deposit build up, increased wear, overheating and ultimately, engine failure.
Lubrication of the wet clutch is a very critical. The clutch systems of modern motorcycle engines are designed like the engines – very compact, high performance and highly stressed. They consist of a series of individual clutch plates, separated by rings with springs squeezing them together. When the clutch is activated, the rings are pulled apart allowing them to slip, breaking the power connection between the engine and gearbox and enable shifting. Once released, the plates spring back together generating an enormous amount of heat. The engine oil must have a balanced friction coefficient in order to guarantee the correct grip between clutch plates and avoid slippage – and also allow smooth engagement / disengagement of the clutch without “shudder” or “stick slip”.
The last critical zone encountered by your motorcycle oil is the gearbox – where high precision meets high stress. Lubrication of the gearbox is based on different principles with respect to the engine. Extremely high loads between the gear teeth can shear the oil, reducing its viscosity, if it is not shear stable. The same oil that has been put to torture tests in the engine and clutch now must cling to the surface of the gear teeth, providing a protective barrier and preventing metal to metal contact of meshing gears. Direct contact of gear teeth can lead to scuffing or scoring of the surfaces, while vibrations typical of high revving engines promote fatigue-induced damage like pitting. All of these conditions will impact the performance of your bike and cause significant damage to your gear box.
Formulating motorcycle engine oils means finding optimum balance between conflicting requirements.
1 .From the engine zone, the high stresses of mechanical loading, high piston speeds and combustion temperature – as well as the need to maintain high oil flow under all conditions.
2. From the clutch zone, the need for cooling – as well as the ideal balance between slip and grip for smooth & reliable operation.
And 3, from the gearbox zone, the need to fortify the oil to combat shearing and sliding forces – and protect precision components against wear.
All engine oils have a difficult task. But, motorcycle engines oils have by far the most difficult.
For more information about Castrol’s premium line of high performance engine lubricants and bike care products, please visit Castrol.com.
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