New answers tagged weight
Individual "ideal cadence" is probably more closely related to muscle composition and comfort than it is to size. While someone could probably "maths out" an ideal cadence for a rider based on all the variables necessary (height, weight, power, muscle composition, etc), that won't matter one whit if the rider has spent the 2500 hours over the last 3 months ...
I suspect you'll be buying a whole new crankset. You may also need to upgrade the bottom bracket as there's not much choice of upgrades if you're starting with square taper parts. My hybrid had a very similar drivetrain, including suntour cranks/chainrings which were riveted together. You've already got a fairly wide ratio cassette for an 8 speed (I run ...
The thicknesses of 8 and 9 speed Shimano cogs differ by 0.02mm (http://www.sheldonbrown.com/cribsheet-spacing.html). I would say that the same chainwheel is compatible with 8 and 9 speed chains.
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