> four wheels would double the rolling resistance and thus the effort required to push the cart

Rolling resistance = Crr * N where N = the normal force a.k.a. the weight bearing down on the wheel for a slow moving cart. If you use the same tires but double the axles you reduce the weight on each wheel by half leading to about the same rolling resistance for the vehicle as a whole.

> its narrow wheels minimize rolling resistance

This is a myth that has gripped the bicycle industry for decades but has been slowly dying over the last two. Fat tires have slightly lower rolling resistance at the same pressure and a smoother ride at the same rolling resistance. It takes more energy to lift a hard wheel over a small bump than for a softer wheel to deform and lift less so the difference becomes greater as soon as the road isn't perfectly smooth.

The main reason road racing tires only got a little fatter and aren't as fat as say e-bike tires is aerodynamics which is hardly applicable to hand carts.

> Unlike a van or a car, my handcart doesn’t need gasoline, electricity, or batteries, making it entirely independent from energy infrastructures.

TANSTAAFL - you're going to need more calories to compensate for the additional effort you're expending to move your handcart. Human food is far from the cheapest fuel and few people are able to obtain it "entirely independently from energy infrastructure."

"Many people have asked us why we didn’t build a four-wheeled cart that wouldn’t need to be balanced. However, four wheels would double the rolling resistance and thus the effort required to push the cart. Furthermore, a four-wheeled cart is less maneuverable and more difficult to drive on uneven terrain. You also need to get two extra wheels, and you need to build a steering mechanism. "

I would have asked them why they didn't make a 3 wheeled version. Keep the two big wheels on the side, add a smaller freely rotating one in the front, like a lot of strollers have. They could make it just a tiny bit too low, so it's easy to roll it with both 2 or 3 wheels.

Stroller can be seen has specialised handcart.

There is a guy ( Nicolò Guarrera ) who completed a world tour by walk and his tool of choice to transport stuffs was a trekking stroller.

As the article points out these things are lovely until you have a hill. Fortunately the original designer built it for use in the Netherlands, a country purpose built for human powered vehicles due to its flatness. I do question its utility in any environment even remotely less flat than NL. I guess you could take it to Denmark too?

I'll see your handcart and raise you one yatai.

https://youtu.be/FEgTg_IyHwQ

This is the site that runs their blog on solar power, and was an inspiration for me to do the same (now my setup powers my home too). Love the site and all the various concepts about sustainability.