Originally Posted by
Duragrouch
What it comes down to is this: Do you want minimally adequate lube, or good (better) lube?
I've used WD40 in a pinch when I had nothing else. It's better than dry. But it's not best, and fairly close to the bottom.
Bike chains don't operate at high speeds or high heat, but there is a very small bearing area on modern bike chain links ("bushing-less"), and coupled with pedal forces, the unit pressure at the pivot points is high enough to get rapid wear (and thus chain "stretch") quickly without lube. Lubes designed for that kind of pressure, with bigger molecules, work better than WD-40.
If you are in the middle of nowhere and have nothing but WD40, go for it. Otherwise, I would recommend something better (and not expensive, everything from motor oil to gear lube would be better, and cheap).
Your point might be valid if oils provided longer chain life. However chain wear seems to be independent of what kind of lubrication you use. There is a chart above showing wear vs lubricant but I can’t find anything on where it comes from. Even taking that chart at face value, the amount of wear between the minimum and maximum is pretty small. Micrometers squared is a really tiny amount of wear.
As to molecular size, that’s not necessarily a measure of lubricity. The oil in WD40 is in the 50 carbon range which is similar to what motor oil has. Thick oils with high molecular size are ideal for mechanisms under heavy loads with rough gears while light oil is good for light loads and polished gears. The latter certainly describes bicycles. The only load on the bike is what load that can be generated by the rider’s weight. There is no need for a heavier oil for protection due to “heavy” loads.