Some input from my end. How is your seat height and position fore/aft over the BB between the two bikes. If you used the exact static measurements between the two, since the FS rear sags it will not be exact that same as the hardtail once you sit on the FS. You may have found the sweat spot on your FS, but those exact measurement to the hard tail may not translate to ideal. Of course the shorter cranks would make things easier, but I would probably really look at bike fit first. Also what type of pain are you experiencing in your knees, front, side, back? I see you are on flat pedals, do you use flats on the FS also. Also are the cranks between the two bikes the same spindle width. One problem I have run into if switch between bikes with different spindle length or pedals and just go by look on the pedal to place my feet, the width my feet are apart will not be the same. If my feet are further apart then ideal I will get knee pain. Therefore on the bike with wider spindle/pedal interface I have to be conscious that I put by feet more inward on the pedals.Any of the single speed crowd try really short cranks in order to reduce knee stress? I use 155mm Canfields on my trail bike with a Wolftooth oval and I LOVE that setup. But that's a bike with a fairly active suspension and work great for sitting and spinning. So much easier on my knees while sitting and spinning.
I have 170s on my single speed and the very nature of single speeding is rough on the knees. I'm not sure going to 155's would be a significant benefit for my knees in a single speed scenario? I'm afraid going from 170 to 155's would just throw off my gearing and not really give any benefit to ease the effort on my knees. A hard effort ride on my single speed and I can definitely feel it walking up the steps to my office the next morning. If I didn't have an oval on my 155's I'd swap them over for a couple of rides to try out. It uses a zero offset direct mount chainring and at minimum I would have to find and but a new chainring just to try them out. Trying to decide if it's worth the time and effort to test out.
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