sump
noun: sump; plural noun: sumps
1. a pit or hollow in which liquid collects
From the 1890s until the mid 1920s, almost all motorcycle engines were lubricated by dripping or pumping (by hand or by mechanical pump) oil into the crankcase of the engine. The oil that made it in was splashed around by the flywheels or crankshaft until it was burned or leaked onto the road. We now call this "total loss" - I don't think this term was used in the day, nor was the term "wet sump".
Most engines (not Douglas!) used internal flywheels, so oil inside the crankcase caused drag as the flywheels pushed through the puddle of oil at the bottom and threw it around everywhere. You'll know what this feels like if you've ever over-oiled a total-loss bike!
Lots of bikes in the twenties had mechanical oil pumps, but these were more like "mechanical oil meters" as they simply delivered oil in regular quantities to the engine. The oiling was still "total loss".
It wasn't until the mid 1920s that a few bikes - mostly racers - started using the "dry sump" system in which any extra oil in the crank case was removed back to the oil tank (instead of ending up as smoke or on the road). On conventional motors, there was a scavenge pump to pick up excess oil from the bottom of the crank case and send it back to the oil tank, but Douglas had enough room to put the oil tank underneath the engine so that oil could drain directly back into the "tank".
Dry sump engines had less drag so they ran better. The also recirculated the oil.
So is the thing under the engine of some Douglas models an "oil tank" or an "oil sump"?
For some models - the RA - it's just a tank. Oil gets pumped out of it, but never returns. The engine is "total loss".
On other models - like the TT, and the B29 I think? - it's a sump. Oil gets pumped out to the engine, but there is an opening in the bottom of the crankcase through which the oil drains back into this tank/sump. .
By 1930 all but the cheapest models were using the "dry sump" system.
Cheers
Leon
