40 results in more noise and vibration over small sprockets, and small sprockets are necessary with a mini bike style drive system. 35 has more pitches per foot of chain than 40/41 so naturally the higher number of wear points will allow in more "strech" per foot. The saveing grace with most mini bike chain drive systems is the insertion of a sentrifigal clutch whitch tends to eliminate "shock loads". In other words the "service factor" of the chain can be classified as rather gentle in comparison to a chain, for instance, in a motorcycle that is often shifted without a clutch. My opinion? 35 chain is far stronger than required for almost any drive system likely to be found on a mini bike. The chain killer is the horrible enviornment and spotty lubrication that a chain, of any size, is subjected to in this application. High powered go karts seem to have trouble with 35 chain but with that fact well known to all, Why do they continue to use it? It takes less HP to drive it.