Quote:
Originally Posted by dcb
are you including the transmission in the bike figure?
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I'm not sure who this was directed to, but if it was to me, then no, the transmission is included in neither of the figures I gave. These are figures from engine labs where there is nothing between the engine and the dyno: typically just a straight coupling connecting engine and a hydraulic pump or generator.
Transmission losses depend upon the exact organization of the transmission. In the old days of car transmissions, top gear (third or fourth) was "straight through," so there were no loaded gear meshes in the transmission. A little oil was flung around, and there was some bearing drag, but the transmission was very efficient in top gear. In the other gears, power flowed through at least two loaded meshes, (typically jumping from the input shaft to a layshaft and back to the output shaft, so efficiency was lower.)
Overdrive typically adds a gear mesh, and the consequent losses. However the increased loading and speed reduction on the engine (and lower BSFC) more than compensates for the gear losses.
Motorcycle transmissions can be in some ways efficient: some use straight cut gears which are noisy but efficient. But Franks point is correct, that many motorcycles use a lot of gear meshes that are absent in a car transmission. His Gold Wing is incredibly lossy in this respect. (Where this gets almost ridiculous is in bikes that have the crankshaft oriented transversly, but have shaft drive: this entails two lossy right-angle turns.) Most old style bikes mount the sprocket driving the chain to the rear wheel on a counter shaft, which means that there are always two loaded gear meshes in play.
There were a few bikes that in top gear had from the engine a oil bathed chain primary (about 98% efficient) a straight thru gearbox (99% eff, with losses mainly from bearings and oil flinging) and a chain final (98%, if perfectly clean and well lubed.) No conventional car beats this, because the losses in a hypoid final drive alone are the same (or slightly higher) than these.
But there are all sorts of exceptions. The transmission (and rear end if there is one) are separate issues about which there are few applicable generalities. Even in cars, there are chain final drives (highly efficient) helical gear final drives (quite efficient) and hypoid gears (lossy, in comparison). (But lossy means only 5% or 6% loss, often.)
Then there are weight issues... but that gets outside the simple answer the OP sought, I suppose.