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 more WRONG! 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.
- Sloping top tubes and taller head tubes have changed how we size and fit bikes
- The same top-tube slope has introduced more variability into "numbered" frame sizing (i.e. 53, 54, 56, 59 cm, etc...) for road and cyclocross frames
- Many frame builders have moved away from the "Euro" higher bottom bracket to a more "American" lower bottom bracket for cyclocross - although, not all of them.
- Integrated headsets "inflate" head-tube dimensions and must be accounted for versus traditional "press-in" headsets when determining front-end height
It's complicated for sure - but your best bet is to look for a frame that will give you the following:
- The same leg extension in the saddle with the same knee position relative to your pedals. This will provide the same power and efficiency you should be enjoying on your road bike.
- A similar distance from your saddle to your handlebars as your road bike to ensure good weight distribution for good, confident handling with proper muscle engagement from the gleuts, hamstrings, and upper body
- Possibly a slightly higher handlebar position relative to the saddle (consider 2-3 cm depending on your current drop to the bars on your road bike). This delivers a slightly more relaxed position for better access to the drops of the handlebars for powering up steep climbs, more ability to use your elbows for "suspension" over rough terrain, and easier re-mounting after clearing barriers.
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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