I recently had a multi-speed bike built up from a frameset that has forward facing dropouts. I've been interested in trying a single-speed for some time so I'm considering making the investment in new parts to make my bike a part-time SS.

For economic reasons, I would go the route of removing my 9-cog cassette and using spacers and a single cog on the rear. How much of the "performance" aspect of a SS will I be giving up by going this route as opposed to a dedicated SS wheel? I know it will weigh slightl more but will be the only real penalty?

Also, are there SS cranksets that use an external bearing BB? This would greatly simply things.

Would a conversion be as simple as replacing the casette as described above, replacing the crankset and stripping off the parts that are no longer needed? I would have my LBS do the actual work since they've been building SS bikes for close to 20 years but I want to do my research before getting that far into the process.

There isn't really a performance aspect of singlespeed.

There are external BB singlespeed cranks, SRAM S300 for example. You could also use your current cranks with a single chainring, you'd just need to buy singlespeed chainring bolts.

Only other thing that you'd need to do is either shorten your chain, or buy another chain (probably the better option if you plan on going back to a geared setup in the future).

When I was thinking about buying a singlespeed bike I just left my Trek in one gear.

Use a gear ratio calculator: https://home.earthlink.net/~mike.sherman/shift.html

Count your teeth, probably on your big sprocket, then enter your numbers----chainring and cog teeth. Pick a gear that's somewhere between 60-70 inches and leave it there.

A few days riding in that gear will give you a good indication of how you will like life with a singlespeed.