Low car diet
Join Date: Aug 2007
Location: Corvallis, OR, USA
Bikes: 2006 Windsor Dover w/105, 2007 GT Avalanche w/XT, 1995 Trek 820 setup for touring, 201? Yeah single-speed folder, 199? Huffy tandem.
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Always use the effective/horizontal length if the top tube is not parallel to the ground (i.e., sloping)--it is still measured cnter of head tube to center of seat tube. If the top tube is parallel to the ground, then the center-to-center top tube length and horizontal length are the same.
The new frame is 4mm longer, so all things being equal you'd want a 4mm shorter stem (effective length of the stem, parallel to the ground). Things that do not make this "all things being equal" include but is not limited to: head and seat tube angles; saddle to handlebar drop; reach, drop and width of handlebars...you get the point. 4mm is not that big of a difference.