The problem is that none of the stiff ones mentioned are actually folding stems (but detachable stems).
Stiffest built still is a single piece fork, plus ahead stem.
Out of the folding stems the one's from Dahon are comparatively stiff, due to their conic shape with larger diameter below.
Originally Posted by
BruceMetras
...in a folding post, the Birdy (left short) is one of the best.. Dahon's one piece short handleposts (like on some Jetstreams) are also good....
I like the short one from the Jetstream XP with the Syntace VRO most. Fairly stiff. And gives a perfect geometry for me (186cm tall).
The Birdy hinges are not as nicely integrated as the Dahon "Fusion" ones, and also I would not recommend any telescoping and clamping in a handlepost.