Tri-wheel stair climber MkII
After the MkI chassis bellied out on the stairs I rebuilt the chassis to give an extra 55mm (about 2inches) clearance in the center. The new chassis is also 50mm wider and a little bit shorter to help improve turning on the spot.
Unfortunately the result was almost the same as the wheels seemed unable to get enough traction. I think a big part of the problem is the 28 steel gears in the tri-wheels which combined weigh about 1Kg (2.2lb).
We took the chassis outside and found it could handle smaller steps and rough terrain ok. Looks like I just need bigger wheels for bigger steps. Looks like Wild Thumper tires are the next thing to try. The diameter is about 64% bigger and the knobs on the tires should grip the carpeted steps nicely.