Interesting, The self propelled feature reminds me of the older Lawn-Boy Self Propelled method.. The system itself is called a "Capstan Drive" in which a rotating roller of some sort is pushed against the wheel causing the machine do drive..

On the lawn-Boys they worked exceptionally well.. Hardly wearing the wheels down at all..

This is the best picture I could find, I will take some better pictures when it's not -30c Outside..

[Linked Image from hunt101.com]

Kori laugh