I love my single-speed. It's an old Raleigh Record that's been converted. I tried riding it fixed for a while, but it hurt my old knees.

I've been doing a lot of riding recently and I'm having knee issues again. I'm not running it fixed, but I'm beginning to think I need a bike with gears for commuting. Sure, I can ride one of my other bikes, but I really like this one.

What I don't have is an IGH based bike. So, I'm considering converting this old Raleigh to a 3 or 8 speed IGH. It has horizontal dropouts. Any pitfalls I should look out for?

I would suggest moving your seat around ever so slightly- up/down, fore/aft, maybe a quarter inch, and see if that changes anything.

This only makes sense if your knees hurt because of the size of the gear. Can't you just try a smaller cog first?

I feel your pain, For the most part I had to give up the SS because of an MCL tear in my knee that doesn't want to heal. My knee does fine as long as I can spin at a farily high cadence, with SS and my route that's not always possible so I ended up mashing too often and it aggravated the knee. So I bought an Afline 8 rear wheel, and it works really well. The nice thing is I can can switch back to single speed if I want or need to on a moments notice becuase the chain length is the same and the horizontal dropouts. Just zip tie up the shifter cable and go.

I like the 8 speed, a 3 speed might not be as good choice for me because of my need to spin the higher cadence, and the ratios on those are pretty far apart., but it might work for you. I can't really think of any pitfalls. They are heavier, but low maintenance. You can't shift under load, but you can shift at a stop. You need to get the right lock washers for the dropouts, I use the yellow ones for the horizontal dropouts. I don't know what kind of bar set up you have but the Jtek bar-end style shifter for the Alfine/Nexus 8 is really great. I also have the regular Nexus 8 twist shifter that works perfectly even though it's way ugly and overbuilt. I say go for it.

Should be reasonably simple just a torque reaction washer in the dropouts under the axle nuts.
and the shift cable to hook up..
, 2 ... 3 speed options AW3 is the oldest .. benefit: 100% of the replacement parts are available
1:1 and a reduction gear .75 and an overdrive 1.3x

and the new fixed S3x but you can screw a single speed freewheel on it,
1 is the top, .75 and .63 are the other ratios

essentially its the 5 speed . minus the top 2 gears.. , so the 5 speed makes more sense..
except they did up a range of colors for the flash on the other one.