[Gary Dillard]

Consider these numbers:

Mass "m" located 60 inches high, 17 inches aft of the front wheels (assuming 4 inch wheels).

Summing the moments about the front wheels where they contact the carpet:

(60)(m)(a) = (17)(m)(g), where "a" is either the acceleration where you start driving or the decelleration where you run into something or someone. This says that you will begin to tip over at .283 g's acceleration or deceleration - that's about 9 ft/sec/sec. Most robots will get to full speed (>9 ft/sec) in less than a second, so their peak acceleration must be higher than their average acceleration of 9 ft/sec/sec. And when you run into someone (or they run into you) you will stop considerably faster. I would say you are looking for disaster here.

One interesting note (maybe a little too technical to explain in a simple post) is that this assumes rigid body motion. If your arm / mechanism is flexible it will actually accelerate or decelerate slower than your chassis because of the energy that gets stored in the spring rate. So if you have a tall rigid structure it could be more susceptible to tipping than a soft structure, assuming the decrease in acceleration of the cg more than compensates for the change in downward moment arm to the cg.