quite popular in places, home made versions are common as muck in europe, especially fastened to bicycles, but I would DEFINITELY want a pneumatic tyre on the axle not that thing that looks like it would pass every jolt to your kidneys
Wear that on a British cobbled street or rural path or old track bed and you would be pissing blood for a month. To me the wheels are far to small for off road use they are going to drop into every crack or undulation, you need something like a 21 or 26 inch mountain bike pneumatic tyre .
Look at it this way what type and size of wheel & tyre do mountain bikers use, Big rugged and pneumatic or small and solid ?
it does look like it'd jolt the kidneys,and who know what else..and like raider says..it's because of the wheel n tire.nothing less the a 20 inch air up tire and tube..and it attaches to low on the harness.somewhere between middle of the back,and up to the shoulders..