Tool, I think your concept is sound...but we don't know what clearance is under the scooter in order to make it doable.
On another note I looked online and saw a scissor lift for motorcycles and did find one with swivel casters that sat collapsed down to 4-1/2" and sold at Home Depot.com. But it was not wide enough (around 15") to fit under his 21" frame supports on the scooter.
I looked at the photos. I called the Palmer Scooter company to ask if their scooters had reverse (they do!) and learned that they can handle uneven pavement. I thought up the design while I was in the shower this morning. After I posted my drawing and the text, I looked at the photo again and realized that it may not fit as drawn, so I added the "P.S." weasel words at the end to say that some design adjustments may be needed to fit under the seat of the scooter. In addition, I would want a few thin pieces of scrap on hand as shims to adjust the plywood height above the floor.
It would take me less than an hour to build. I would make it "small" and "thin" to keep the weight and storage needs down, but use thick wood appropriately for strength to support the plywood "cradle", the casters, and weight of the scooter.
When I first saw the thread, I looked at lever lift designs. I have a very heavy metal one for a motorcycle repair stand (Handy Lift). The wheel axle serves as the fulcrum. You press down on the long handle, and the small protrusion raises the lift with the heavy motorcycle on it. It allows you to wheel it around despite the extreme weight (well over 1000 pounds). The reason I rejected this design is the space required to operate the long handle. A shorter handle might be viable, but then @konjur1
would have had to bend over or be on his/her knees while the scooter was raised and moved. One advantage I saw for this design was reduced storage requirements, but I rejected it for other reasons.
Another design I considered and abandoned was some type of jack lift, like the motorcycle scissors lift you suggested. It would have a plywood base with casters, a screw post, and a plywood platform on top. Roll the gadget under the back of the scooter, and then give it a few turns to raise the post and lift the back of the scooter. A rubber pad on top might hold it against the scooter bottom as you rotate the caster base to raise it; something like that. I abandoned that idea. It was too complex.
The "scooter carpet dolly" is dead simple. It has no moving parts, other than the casters. It is cheap and easy to design, build, and operate, and it meets all the requirements well. It is relatively small, relatively light, and easy to move. It doesn't need additional space around the scooter when it is in use.
I believe that my design is viable and may be the simplest and best solution. I hope that @konjur1
looks at my crude drawing and gives the basic design due consideration.
My true hope is that someone here can take my design and run with it, or suggest something better.
P.S. Does everybody "get" the design? The scooter wheels rest in the "cradle" gap between the plywood boards on each side. The dolly "frame" does not have to be actual 4x4s and 2x4s; they are examples of the "beefiness" needed for strength to support the weight of the scooter, and sort-of, relative size.
I can think of easy refinements, like:
* Chamfering (angle) the end of the plywood as a short "ramp" to make it easier to push or back the scooter onto the dolly.
* Change the design so that the plywood strips are somewhat behind the middle of the dolly, so that the dolly is less likely to move when you roll the scooter onto it.
Do I need to make a better drawing?