It's not the length of the rods that matters. It is the angles formed by the clevis and horn (and the servo wheel at the other end) Have a look at some of these and instead of wheels think control surfaces.

http://users.ids.net/~bdfelice/ackerman.html

http://www.muller.net/mullermachine/docs/ackerman.html

http://www.rctek.com/handling/ackerm...principle.html