Bicycle Mechanics - Top tube question

When considering reach (stem and handlebar length) do you use the horizontal top tube length (i.e., parallel to the ground) or the center-to-center top tube length?

For my current bike (which fits well), the horizontal tt is 545mm and the c-c is 542.5mm and I use a 90mm stem.

I am looking at another frame with a horizontal tt of 549 and c-c of 531. What size stem would elicit the same reach?

Thank you!

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.

Thanks for your response, very helpful! I was thinking along the same lines, but needed another opinion.

For my current bike (which fits well), the horizontal tt is 545mm and the c-c is 542.5mm and I use a 90mm stem.


A 90mm stem is pretty short, sounds like you need a shorter top tube, not longer.

Al,