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That is the interesting, both vehicles with charging systems working properly should be at the same voltage (i know 14.4 - 13.8) Also jumper cables aren't as thick as the cables from the motorbike battery to the starter motor, so have often wonder where this adage comes from.
Do you attach the the earth to the battery or to the engine?
lynx-
Think amps not volts. It's the amppage of the car alternator that could be an issue. Why risk it when no need, just use the battery, without car running. 99/100 you might have no issue. But there is no need to have the car running, so why risk it? Just use the battery.
Earth the frame is safest. On the donor vehicle.
skinny
Short answer is that you're bypassing the bikes regulator. And without knowing the car in question, it could put more voltage (engine running) into the bike than the bike can handle. It might not. But given that the bike will start just off the car battery, there is no need to run the car and risk anything.
So it may be fine, it may not. Why risk it when there is no need? If you know the voltage regulation of both circuit, then I'm sure you know enough to know what to do.