Equivalent saddle positions on different bikes

[mailbox13]

Hi... I have an interesing bike fit problem.

I have two bikes. The old one, an 80s steel touring setup with clips + straps, I've ridden for a few years. I've found moderately comfortable saddle positions on it, though even in the best I seem to start getting a bit of pain in one knee or the other after 100km or so (but I've managed a 200km+ day and survived with stretching and careful pedal strokes, kind of).

My new bike is a racer with clipless SPD pedals. By luck, I seemed to find quite a good saddle position straight off and I think I can do about 100km without real discomfort (haven't tried anything longer yet).

I still ride the touring bike around in the city, and take it camping occasionally, so I'd really like to find a saddle position that is equivalent to what I've got on the road bike. Most especially because I don't want to strain my knees as I switch between the two of them.

The racer has 172.5mm cranks; the tourer has 170s. And of course the foot position with clips and straps is a little different (probably further forward) than what I get with clipless shoes. The tourer has a slightly bigger, squishier, saddle. Anyone have a good idea of how to go about calculating the closest thing to a biomechanical equivalent position on the tourer?

 

[mailbox13]

I guess I need to know two things: (1) which parts of the pedal stroke matter the most and (2) what aspects of leg position are most relevant at those moments?

 

[artmichalek]

I guess I need to know two things: (1) which parts of the pedal stroke matter the most and (2) what aspects of leg position are most relevant at those moments?

The two parameters that you probably want to match are knee relative to the pedal spindle when the crank is horizontal (baseline is directly over it, but most people are comfortable slightly fore or aft) and leg extension at the bottom of the stroke.

 

[mailbox13]

The two parameters that you probably want to match are knee relative to the pedal spindle when the crank is horizontal (baseline is directly over it, but most people are comfortable slightly fore or aft) and leg extension at the bottom of the stroke.

Interesting that it's horizontal and bottom positions. Naively, I would have guessed that the point furthest from the saddle, and the point 90 degrees before that, would be the most relevant. But then the leg isn't symmetrical, so I guess this is all about heuristics.