David Redszus wrote: ↑Tue Jan 28, 2020 11:47 am
The term "slippery" refers to a reduction in the coefficient of friction; a result of oil viscosity and oil film thickness.
Viscosity changes with temperature and oil film thickness varies with clearance. For engine oils, an oil
cannot be too "slippery" unless that oil has other reduced properties.
For a wet clutch (or wet brake) system, things change. If the oil viscosity is too high, the surfaces will slip. If the
viscosity is too low, the surfaces will grab. Taken together we have a condition called stick-slip.
Wet clutch oils have an additives that controls the slippage ramp; it allows sufficient slippage to avoid stick but allows enough grab to transfer power. Synchro rings are affected similarly.
While an engine can use either oil, wet clutches cannot. The additive is consumed with use and must be
replaced.
Standing start karts and bikes gained a great advantage off the line, but needed a clutch oil change every four race weekends.
Bike clutch issues started about 20 or so yeas ago when motor oil first got the API Starburst label.Yes, about the same time as flat tappet cams failing .Before than time there were no clutch sticking or slipping problems when using any motor oil .The Japanese bike manufacturers appealed to the API and got a JASO rating on oil suitable for wet clutches.These oils generally, but not always, carry a SG or SH rating rather than the current SL or SN. And generally but not always, say JASO on the oil container
What is not in bike oils is Moly type friction modifiers as listed on oil manufacturer information.If anything is added, I have no idea of what...
From my own experience I have found that oil with high ZDDP levels seem to cancel out the non wanted friction modifiers...Some Diesel oils lhave a JASO rating for whatever reasons, Rotella T4 is popular for use in bikes.
Many bikers have noticed from experience ,that bikes having a common oil supply for engine and transmission/clutch, the shifting feel can change over time with some brands of oil. The oil ,in my hillbilly language, gets used up from shear caused by the tranny gears. Or by products of engine operatin are affecting the oil...
This is a lab anaylsis I had done of a non used bike oil having a solid repuation with racers.....The lab obviously does not test for every addtive...but you see other than ZDDP and calcium( I believe is a detergent?) there not much in it...
Motorcycle land speed racing... wearing animal hides and clinging to vibrating oily machines propelled by fire