Power loss vs. viscosity

[mechtech]

Motorcycles require thicker oil and more frequent changes becaue they usually share the engine oil sump with the transmission, which shears the oil to a thinner viscosity quickly.
I worked for Honda, and the early/mid 750s used 10-40 or 20-50.
Anything thinner gave problems. I suppose changing it at 500 miles would be OK, though, with a thin oil.

 

[1sttruck]

Thinking back on the very few motorcycle gearboxes that I've been into, they were spur gears on shafts with some selector forks, not fancy high pressure gears like hypoids and such. In a gearbox loads will be higher and rpms maybe lower than something like on a timing chain, but the shearing might be similar. Some V twins as I recall have seperate engine and gear oil, and they tend to use heavier oils in both units.

 

[CATERHAM]

Motorcycles use heavier oils for one very simply reason;to offset the thinning affect of higher oil temperatures than what you typically see in a much lower reving passenger cars.
A Honda CBR1000 running 10W40 probabily has a lower kinematic viscosity after a good run than a Honda Civic sedan with 5W20 in the sump after a highway run.

Peter

 

[1sttruck]

"an armchair philosopher would have a field day with the number of invalid arguements you make."

The reason that I made the statements was for people to do exactly that, if they so desire. From where I sit it appears that thinner oils are used primarily in a couple of makes of US passenger vehicles, in order to increase fleet mileage so that lower mileage trucks and SUVs can still be sold. Well, I'll grant that Honda is jus ttrying to be a good corporate citizen. These makers tend to use heavier oils outside of the US. In all other vehicles with 4 stroke engines, in the US and around the world, heavier oils are used; passenger vehicles, motorcycles, passenger car/light truck/heavy duty truck diesels, etc., and heavier oils are typically used in marine engines, generators, piston engined aircraft, etc.

With no CAFE pressure thinner oils would have very little use outside of 'disposable engine' racing.

 

[1sttruck]

"Motorcycles require thicker oil and more frequent changes becaue they usually share the engine oil sump with the transmission, which shears the oil to a thinner viscosity quickly."

An argument that 5W20 advocates make is that it's very shear resistant. With sport bikes being so competitive for power output why aren't makers using 5W20 Motorcraft ?

 

[mechtech]

Gears crunch dino oils to a thinner viscosity.