I'm gathering parts for a 700cc engine build and will need to select a crank sometime in the near future. I have been advised by several people that a bis crank is the best option for a tuned engine can anyone explain why?
- If using bis pistons, the bis crank makes sense because the weight is matched. (heavier pistons = heavier crank counterweight)
- The centre of mass is closer to the axis but this is proportional to the mass used. E.g. 500g mass at 2" from centre will have the same moment as 1Kg mass at 1" from centre. so this seems like a moot point.
- The profile design means that the lowest point of the crank is very narrow. I believe this is for barrel clearance on the bis but the effect could be similar to knife edging, although I haven't read anything conclusive about knife edging since there is very little oil in the bottom of the sump to cut through while the engine is running. SO this may also be a moot point.