a bit sidewise....if you are carrying the 15 lb pack all the time.....I would look to getting a rack/bag combo also. Much easier on the body than using a backpack on the bike. even one of the racks that attache to the seat post may work
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Life is too short not to ride the best bike you have, as much as you can
(looking for Torpado Super light frame/fork or for Raleigh International frame fork 58cm)