R20/SXRF 8 speed Gearing Question
I'm considering upgrading the BB and cranks on my R20 and would like to know how reasonably small a chain ring I can use. I have bad knees, like to spin uphill (actually all the time) and find a bit lower gear would benefit me greatly. I have the 305% XRF8 hub and never use 8th gear and only occasionally 7th. Will a 42 or 44 tooth ring be enough of a change or will it throw off the entire ratio through the rest of the gears? I purchased a UN72 73 X 113 BB and will probably go with the Phil rings when I have the time and cash to do the rest. Also, any crank recommendations? Thanks for any advice. I'm looking forward to getting back on the bike after a long winter of eating.
I have a 40-tooth sprocket and a 25 tooth cog (standard) on my 406-wheeled R20 with an S/A 8-speed hub. It has a 29.9 inch low and a 91 inch high gear, which allows me to spin up pretty steep stuff. Sheldon, rest his soul, made a gear calculator so you can compare things: http://sheldonbrown.com/gears/internal.html
I use 165 mm cranks, but that's just what the chainring came with. I've heard that shorter cranks spin better and longer cranks are better for people with longer femurs, but I really can't tell the difference much. Certainly, a shorter crank, 165mm or less, is better for avoiding petal strike, which may be a problem on a small-wheeled bike. Me, I don't go fast enough around corners to make it a problem