I am an experienced bike-user....it seems to me that the World Bike design would be more effective with ATB (commonly known as "mountain bike") wheels. Wheels that size are stronger, have lower axles and use wider tires. The wider tires (not necessarily knobby) work better in soft materials but are also fine on pavement and absorb more shock on rough surfaces. I know you have many other considerations that I may not be aware of, but I could not help but wonder this upon viewing the project.