Hey all, did some more CAD (this time cardboard aided design) to try and get the geometry and lengths right. Found that the arms need to be a bit shorter than I had hoped. With the design below, in the full down position it is still about 10" above ground. To try and keep ground clearance I did it like this. This will require a small flip up ramp on the end of the platform so we can roll the motorcycle/tire off the platform. The cardboard platform is 30" wide in this picture, just got back home where the motorbike is and now think I can trim the platform down to 24". The tire/tires (still not decided on 1 or 2 spares) will load first with the motorbike aft.
In this mock-up the arms will be made out of 2"x4" 3/16" wall rectangular A513 steel. I am thinking that the pivot points that the arms fit into will be 3" wide 3/8" wall rectangular tube with a face cut off. This should allow some washers to fit and have a little play. I am also thinking 1" shoulder bolts for the arms. The triangular holes would be water jet cut locally. Not sure if it is worth the expense but would save a bit of weight.
So, now I need to work out the lifting mechanism. One plan, and the one fairly easy to figure out would be a fixed frame in between the chassis rails that goes up the back of the habitat. The fix a hoisting winch (bi-directional) with blocks to raise and lower. The other one that I have not figured out the needed geometry would be a hydraulic ram. I am guessing it is way simpler to do the winch.