Aside from the usual 'lose weight' and "improve the engine", all of which will be the best upgrade BTW, the single best thing i've done to upgrade my bike to climb better is buy a 12-27 cassette. I may not be the first to the top among my racing peers, but with that 27 tooth in the back, I never have to get off the bike, no matter what the gradient is. I have a compact crank too, but I havnt used it yet this year. I've gotten my wieght down to a point where I do just fine on any hill with a 39x27 as my tallest gear.
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John
'09 Cannondale CAAD9 - Team Latitude/ABRT Special.
'04 Lemond Victorie Ti
'98 IF Crown Jewel (dead)
'92 Trek2100 (TT)
'50 something Gino Bartali (fixer)
'02 Ducati ST4s (Moto-Ref mount)
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