I got a bike recently with kind of a weird set up. It has only one chainring, and a 5 speed freewheel, making it a five speed. I've grown to like this set up and not having to worry about a front derailer, but it has raised some chain length questions for me. I'm pretty sure it's too long right now (since it occasionaly falls off one of the derailer pulleys), and because I took off the chain today to clean the drivetrain, and when I put it back on, I checked with the big/big method. There were about 3 inches of overlap, but I'd run out of time at my community workshop and had to go. Most methods for determining this that I've found sacrifice either the big/big combination, or the small/small combination, neither of which I really want to do.

If I use the "largest cog to the chainring without running through the derailer" method, will I have enough length to run on the big cog? Should i add two extra links instead of one? I have a chain tool and a quick link, so I can do it at home, but I wanted to get some opinions first.

Derailer is a Shimano Altus-LT
Chainring is a 42 tooth piece of crap
freewheel is old and not sure of the tooth count

Thanks

The chain must allow big-big for safety purposes and if the rear derailleur has enough wrap capacity, small-small is perfectly usable too.

Set the chain length by wrapping it around the largest cog and the single chainring without going through the rear derailleur. Overlap the ends and allow 1" (2 half-links) of extra length and cut the chain at that point being sure to properly match the ends (male-female if using a joining pin and male-male if using a master link). Add a single half-link if needed to get the proper match.

That will provide a safe chain length to use big-big and you should be able to use the smallest cog with no difficulty.

parktools have a section of calculating chain length, using 3 different methods.

Since you only have one chainring, there is no small-small set up to sacrifice (unless you plan to put on a smaller chainring in the near future).

If you don't plan to go to a different chainring setup, the method HillRider outlined above usually works brilliantly. You can double check it by seeing if the desired length looks good through the derailleur when on the big-big and the small-small (not applicable in your current case). I can't run small-small on any of my bikes without some type of rubbing, so I never worry about that personally.