Crank lengths

Mike M. asked me about modifying position with crank lengths so I'll give you my rambling thoughts as of this moment (void in Mississippi). Anyhow if you're changing crank lengths you have to change leg extension and fore-aft saddle position to adjust for crank length change. For example if you were going up from say 172.5 mm to 175 mm you would move the saddle forward ~2 mm (to get the same knee over pedal at 3 o'clock) and move the saddle down approximately ~2 mm. Why not 2.5 mm? Well if you move the saddle forward say 1 cm you effectively lower the saddle height ~3 mm, so if you move the saddle forward ~2mm you lower the saddle ~0.6 mm (I'm laughing as I type this) so by moving the saddle 2 mm forward and 2 mm down you effectively lower the saddle 2.6 mm. You should therefore probably only lower the saddle 1.9 mm, now to do this properly get out a micrometer and ...

No seriously you get the point. However it isn't quite that simple. Usually when it's a small change in crank length, like 2.5 mm, it's either moving the saddle forward or backward 2 mm and the saddle down or up 2 mm at the same time and done. However bigger changes in crank length don't always correlate to the same fore - aft and up down saddle changes. For instance I can ride 165's with a saddle extension that is 3 cm lower than say 180's. I have to do this almost on the short cranks or my body gets "wacked" out. For 172.5 to 175 and vice versa I can pretty much make the small changes without a huge effect, depending on the weather I have to ride a bit lower on the 170's than the extension on the 172.5's and if I go up to 177.5 / 180 I have to ride a higher saddle. For some people though these changes could be more or less profound. For instance if you are 6'5" and change 5 mm in cranks you may not notice it at all whereas if you are 5'7" the 5 mm could make a huge difference in feel, due to the proportionally larger change for the smaller rider. Anyways these are the thoughts as of now.