Originally Posted by
MadCityCyclist
No, it's simple physics. If you brake the front wheel more than the rear, the rear wheel moves faster than the front wheel and try to move around it, especially if there is more weight on the back. It's like a car braking on ice and then fishtailing, or a rear wheel drive car going up a hill on ice and stepping on the gas, which results in fishtailing. Trust me on this, I'm an engineer. If you know anyone who is a professional truck driver they will tell you basically the same thing. With the big trucks, they cannot even maintain the same speed going up a hill on ice because they will fishtail (although moving at the same speed, power is applied to the rear wheels going up a hill to maintain a constant speed, and the power that the rear wheels have but the front wheels don't have means that the rear wheels try to move in front of the front wheels). On ice, big trucks have to use the momentum of the downhill run to carry them up the next hill so they don't have to step on the gas. It's basically the same principle with stopping a cargo bike, in less dire circumstances of course. The rear wheel can't have more energy than the front wheel or it will find a way to move ahead of the front wheel, which isn't good.
In the situation described in your owner's manual, I don't think they were saying the Mundo would flip over your head if that is what you were thinking. But there is a good chance that it will flip to your left or right side in the right set of circumstances. In less extreme circumstances you will likely experience this as the bike doing a "shimmy" from side to side when you stop.
I don't really agree with your assessment MadCity. Braking can't be compared to a car or truck driving on ice or up a hill; the physics are completely different. Braking results in a greater normal force on the front wheel and a lower normal force on the rear as a result of a moment about the center of gravity of the rider and bike. As a rider you can feel this when you brake; you'll feel your body pushing forward on the handlebars which translates down to the front wheel contact patch. This results in significantly higher traction on the front tire than the rear. As a result, the front wheel has significantly more stopping power then the rear which is why motorcycles and sports cars have such huge disk brake rotors on the front and relatively small ones on the rear - so they can take advantage of this stopping power. And this is why you can endo if you get on the front brake too hard. I'm not sure you could lift the rear wheel off the ground with a Mundo but it would be interesting to try. It would be more difficult on a Mundo than on a single bike because the center of gravity is located farther back from the contact point of the front wheel mainly because the bike itself weighs so much. If the bike was loaded up with cargo then the CG moves back even farther and it would certainly be impossible to endo since there would not be enough traction on the front wheel. Of course this all assumes that the bike is vertical (not turning) and the rider doesn't turn the handlbars; braking under those circumstances would certainly cause different issues.
By the way, later in the Mundo manual they do mention that braking too hard w/ the front brake could cause the rear wheel to lift off the ground so I think when they warn of flipping the bike they are referring to endo'ing.
Disclaimer: I too am an engineer (or enginerd depending on your perspective).