Algodoo

I've made at least two other walkers with a system like this one, but for some reason I never used the two-spring-and-a-ball anywhere else. I realised just now how useful it could be, as it's basically an extremely simple piston with an extra degree of freedom.

HOW IT WORKS:
The Kinematic Module, the body of the piston, has some local variables: (ignore _angle, it isn't meant to be there but I'm not redoing all the work to remove it)
_footpos: position of the foot of the piston, relative to the center of the KM
_jointApos: position of the left spring's attachment to the KM, relative to the KM
_jointBpos: position of the right spring's attachment to the KM, relative to the KM
_lengthA: length of the left spring
_lengthB: length of the right spring

HOW TO USE IT:
There is a spring attached to the piston off to the side of the walker. Attaching the loose end to any geometry with a _footpos variable will update the piston's own _footpos. With some coding to modulate _footpos in said geometry, all kinds of movement are possible.

Controls: AD to move the walker (speeds up and slows down instead of starting and stopping), S to brake