Just measured mine. 17" on both mt. bikes and my "touring bike." They certainly looked longer on the mt. bikes. According to this article by Bruce Gordon, 17" (43.2cm) is marginal. http://www.bgcycles.com/bruce.html

Still, it's what I'm going to use. And the only tour I've ever done was with front low-rider panniers, and the sleeping bag on top of the rear rack - no rear paniers at all. Next time, I'm planning on the same setup with a small set of rear panniers. I already have these, so I know my heels will clear. But I see why most people would want/need longer chainstays.