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Mate of mine in Stoke is after a Cannondale supersix 105. He wants a 2012 model in the black and white. He's visited 3 shops so far including Brian Rourkes and Evans in Manchester. He's so far been told that he's a 54cm in 2 shop and 56 in another. Never bought a roadbike so does he go smaller or get the larger frame. He feels comfortable on both of them. Any other good Cannondale dealers round the area?
Hard to say really, personally if I were between two sizes I'd go for the smaller one and get a slightly longer stem later if I needed to but then I'm not especially flexible.
"buy in haste, repent at leisure" as my old Granny used to say. If 56 is right, then 56 is right. Cannondale website should give a list of dealers, most would ship to a UK address.