This question has a couple angles. So i've been having knee problems. One of my bikes has a fixed gear. It used to be 49x15, then 46x15, and now 44x15 progressively because of my knees. I even just got some speedplays (haven't arrived yet) to try and help. So i really like fixed gear but i realize i might have to abandon it. so my question is: what specifically causes the knee problems? is it the mashing or the fact that it is fixed, as in, would getting a freewheel help?

on the same note, i feel like riding fixed has made me a lot stronger of a rider. would i lose a lot of that benefit from converting to a freewheel?

thanks.

Mashing will cause patella over-use syndrome(runner's knee). Very common. If you can't spin most of the time then you've chosen the wrong gearing. If you want todo leg presses goto a gym, if you want to ride a bicycle for 40 minutes then you'll have to ease up on the mashing. Anyways without details about your knee problems who knows? Does fixed gear or single speed make you a strong rider? No, however it is cheaper to maintain.

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I led a fixed/SS ride series one winter. We all got good at climbing 10% grades at low cadences. Didn't help our geared riding a bit. People with bad knees were nervous about trying it, but surprisingly no one had to drop out, though these were all experienced mega-mile riders, so that was probably already sorted. Bum knees are very likely a combination of incorrect position and incorrect technique. Surprisingly, riding fixed exacerbates poor technique. If you can ride geared at any cadence without your upper body moving at all, it's probably OK to ride fixed, except for winding up in the ditch with a broken collarbone.

You shouldn't have knee pain from riding fixed. Have you looked at your fit? My guess is your saddle is too high or too low. You also have a rather big gear you are pushing. Try something in the high 60's or low 70's.

For the record, I tore my ACL and my medial meniscus and ride fixed at 46x17 and rarely have knee issues.

FG or SS isn't the issue. The issue is pushing too high a gear and not spinning enough. In fact, I need slightly LESS effort pushing an identical gear in FG than SS due to the carried momentum of pedals from the turning of the wheels.

Install a rear brake on the FG and keep riding it. Gear down between 66-72 gear inches and you should feel better.

You might want to switch from fixed to free, or at least do not use your legs to slow you down at all on fixed, rely only on your brakes. Your knee isn't meant to pull and resist the forward motion.