JUST A LITTLE DIFFERENT FROM MY OTHER BIKE...
Well, yes and no.
As I have discussed (at length) before - there is a lot of good and bad info out there about bike fitting and selecting the right size frame. One of the more common topics I run into this time of year is how to select the right size of Cyclocross bike.
A common guideline I have heard is that you ought to size-down from your road bike frame size to obtain the right 'cross frame. As a universal guideline, this couldn't be moreWRONG! There are instances where this may apply, but I would say that they are rare now. It is a unique bike and just like you wouldn't adjust your road bike a "little different from your road bike"; treat your cyclocross bike like and individual and get it right.
A BRIEF HISTORY OF THE BICYCLE...
To explain my stance - first a little history. In the (seemingly) long-gone days of"traditional" (horizontal top tube) geometry, one of the big differences between a road frame and a 'cross frame was a higher bottom bracket shell (where the cranks pass through the frame) to allow for clearance over obstacles and terrain. Depending on what road frame geometry you were comparing this to (criterium, stage race, touring etc...) this could result in a bottom bracket that is 10, 15, or 20mm higher on a 'cross frame than the comparable road frame. Therefore, to get the same top tube length for proper fit, you would subtract that 10, 15, or 20mm from the road frame's height to arrive at your 'cross frame size. For example, I fit nicely on a lot of "traditional" 58cm and 59 cm frames. So, I would ride a 56 or 57cm 'cross frame from the same manufacturer (if it had a higher bottom bracket) to get the same extension to the handlebars.
Since then, a few things have changed. (o.k., a lot of things - but only a few in the world of bike geometry.) But these few items may have a large effect on what frame size you ride.
It's complicated for sure - but your best bet is to look for a frame that will give you the following:
Velonews.com also has a great article on the topic here:http://velonews.competitor.com/2010/08/news/velonews-training-center-how-should-a-cyclocross-bike-fit_136539
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