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Join Date: Dec 2003
Location: San Francisco East Bay
Posts: 247
Bikes: 2016 Tom Kellogg steel Spectrum all-road, '89 Eisentraut Rainbow Traut, '81 Marinoni Special, 2018 Ritchey Road Logic, 2006 Ritchey Breakaway Cross, 2009 custom Joe Wells alu Tsunami CX, '71 Favorit (Czech Rep) Special, 2012 Co-Motion Tandem
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The common wisdom to size a cross bike 0-2cm smaller than a road bike is because-
1) Higher BB on cross-specific design means shorter seat tube, so effective size is the same, although the BB to toptube measure is shorter than an equivalent road frame.
2) You MIGHT want to ride a little more upright than a racing position on a road bike
If you can straddle a few cross bikes in a shop before you buy, check out the standover height (floor to top tube) and top tube length. Different cross bike makers determine "frame size" differently so its a little less obvious what you need than it would be in selecting a road or MTB hardtail frame. Knowing your standover height and top tube length should get you pretty close to the right size, you can play with the stem length and saddle position from there.