Originally Posted by
rosinante25
rowan, i checked out the crank lengths and found that the secteur's are 172.5 and the moto's 175. this may be the solution. previously i had my lbs mirror the two bikes in fit and altho the problems diminished they were still there with the moto. i had knees and achilles injuries in the past from sports so maybe i just need the easier cranks. i'll take the bike in to the shop & c if i need to change the gearing as well along with the cranks. thanx so much guys. i hope this is it.
oh & yeah i guess geometry isn't the culprit except maybe in school.
I dunno how big a project you want right now, but I'd start with the small one of making sure the pedal to saddle top distance is the same for the two bikes, with the crank arms aligned with the seat tube. This would have you raising the saddle 2.5 mm for the bike with 172.5's. At the same time the two bikes need the same setback.
The consequences are that with the longer crank, your leg will rise an extra 5 mm at top center, with a tighter hip closure angle resulting. This can be uncomfortable, or perhaps you have enough flexibility that it's no problem.
Essentially, geometry can cause injury, but only if it requires a compromised fit - in other words, if the frame is just plain wrong for you.
Chainring, I dunno.
BTW, I've found the difference between a low saddle that causes knee pain and high enough not to cause it, can be less than 5 mm.