[ERothfus]

The main SprocketR developer is deep in finals this week, so I thought I'd go ahead and respond. I'm one of the mentors working with the Chap Research team on SprocketR.

>> 1. The chain sizes are in inches and the c2c distance is in mm .
>> What's the reasoning behind that? Can you make an option in inches?

We don't have a lot of options in SprocketR yet. Things are rather hardwired to mm right now. We intend, though, to make "units" an option in the future. For chain, we just used the "industry" designation of #25 and #35, which aren't really in inches.

>> 2. When would you want slack in your chain? I've heard that some people add a
>> little bit of c2c distance to get tighter chain.

There is an "best" amount of slack for a chain that promotes the best wear on the teeth on the sprocket...which is particularly important for SprocketR given that the resultant teeth are made of plastic. Further, a bit of slack allows the chain to position itself on the sprocket during uptake, where an absolutely tight chain may bind, again, particularly on a plastic sprocket which isn't as rigid as a metal sprocket and may not be perfectly flat under a load.

SprocketR bases calculations on the "perfect" or "ideal" chain length, which is not attainable since a real chain's length is a multiple of the length of a single chain link. So SprocketR rounds-up to the nearest chain link length taking into account the desired slack.

Having said all of that, you can absolutely specify zero slack if you want, though you're likely to get just a bit of slack anyway since the chain length will likely be longer than the "ideal length" given that it must round up to the nearest whole link length. We have found that 1% is an appropriate target chain length, which is the program's default.