Wow, thanks for all the replies, particularly the calculations and concerns. I recall that the trike could go a little faster than walking (low expectation), so 7km/h seems about right, but I will have my kid confirm with a GPS. Is this fast enough to maintain balance? If not, then a trike it must remain. I am more worried about the pedal clearance (safety) while in a turn, and may just skip pedaling. If this hack works out, I can look into a 20in or larger version. Regarding traction, the frame will need to be longer, which may defeat the compact folding purpose. The geared hub concept seems tough, so not sure when I will get there... On a different matter, if the rain eases up this weekend, I will make a second attempt riding my python bike that I abandoned after 10min of learning, in the rain with my welding gloves, which I should keep dry (excuse). If all goes well, I will set up a Wiki page with the training diary. Cheers, Vi