"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.
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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.
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.
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.