Question: For winter riding I'm looking at a 57-cm LeMond cyclocross bike. My road bike is 55 cm. Will this size difference cause problems? -- Mark F.
Coach Fred Matheny Replies: It's tough to say without seeing you on your 55-cm bike.
If that bike is marginally small and equipped with a long stem and seatpost to make it fit you, then the 57, with a shorter stem and lower saddle, might be fine. But if the 55 is right on or slightly large, then the 57 will be too big.
An important consideration is stand-over height, especially if you want to use the bike for cyclocross as well as road training. When there's a chance of unplanned dismounts (as in 'cross events), you'll benefit from the increased stand-over height afforded by a smaller frame.
Can you set the 57 to your position measurements to see how it fits? A test ride would help you decide.
Even better would be to get a professional fit on the 57. A good bike fitter at a shop could tell you if that bike's frame size can work for you. While a pro fit is fairly costly, it might help you avoid an even more pricey purchase of a bike that doesn't fit.