I believe the "support points" of the "stability triangle" as often used in training programs are still the same (front wheels and center of rear axle), however in the case of a bendi, the triangle changes shape as the front end is turned (from an isosceles triangle to a scalene triangle).

But to be honest, I've never quite understood why a "stability triangle" is generically used in training for lift trucks. I expect a 3-wheel truck would have a stability triangle and a 4-wheel truck would have a stability rectangle (whereby, in the case of a 4-wheel bendi, would change to some type of non-square quadrangle). Any engineers out there have some input on this one?

The trickier part is demonstrating where the center of gravity is as the truck goes through the various stages of placing and retrieving loads. Instead of moving forward and back within the triangle (or quadrangle), it is also moving towards the side.

But again, I guess I come back to the fact that the engineers already figured out the maximum size and weight load that will keep the center of gravity within the support points (whether it be a triangle, rectangle, or whatever) for that specific truck under recommended operating procedures, and put this information on the placard on the truck. The stability triangle concept is used to help communicate where this information on the placard comes from (for example, why the capacity changes as the load is raised), but the information on the placard itself is the critical piece of information the lift truck operator absolutely needs to understand.

I think it's great if the operators have a thorough understanding of the physics, but I'm not sure I want to start bringing up scalene triangles and non-square quadrangles in a lift truck training session (I can just imagine the nervous stares I would get). So I guess I'm ok with providing a basic understanding of the physics combined with a thorough understanding of the placard and safe operating procedures. I would think your 35 minute presentation on support points and stability is still more than adequate for training with these other trucks provided you add that there are a few additional factors unique to these specific vehicles and just breifly explain the additional factors (such as the front axle turning on a bendi, or the mast swinging and shifting on a swing mast, etc).