We had a thread on knee pain and it has been known to be a common complaint for fixie/ss riders. I believe i have isolated the cause of my knee pain. I had some pain, switched to my fixed and the pain went away. So i was trying to figure out what was different between my roadbike and my mountain bike, the old mountain bike was causing me the pain when i went back to it.

It was all fit. Even if you have knee pain and cannot eliminate it, if you spend a lot of time working on your fit you can minimize the pain. At least i could. What worked for me was seat height and seat position (for/aft) and Generally i was too far back. The mountain bike saddle had shorter rails than my roadbike which forced me to sit too far back. So I started swapping parts, took the peddles with toe clips and put em on my mountain bike which forced me to get more exact with my position on the bike. Then i tried to duplicate the full, smooth arc that my fixie had, finding the proper height. Then the final thing that worked was too put the road seat on and move my seat forward, almost too far forward. The perfect height with a saddle that was extremely forward removed a ton of pressure on my knees. I was spinning like mad and it was no problem at all. The cadence was circular, pushing down and even a little back. The narrow seat allowed me to have my knees closer together and made them more in line, straight up and down.

OK, I'm sort of self taught on this stuff and this may be super obvious or irrelevant to some or all of you, but it helped me a ton and if it can help someone else ride more and have less knee pain then so be it.