An ”old” design I made around 1990 (art- work Huibert Groenendijk). In the meantime- 26 years- some 2 wheeled skateboards have been made available but none af them as well functioning is this one is ( in my dreams :-)
My problem now is: Once you have a rowingbike at your disposal, where do you need the 2 wheeled skateboard for…..

Besides that, prototypes of this machine ( i made a few)

might have childhood diseases and even without that it will be an apparatus where one will fall of accidentally.  As easy as falling has always been for me, once over 50 years of age you do want to be more careful as bones are not getting more flexible.