I've been wanting to design one for quite some time, but it's the cost and complexity of the suspension + the pedal chain twisting under flex that is the issue. To make one motored and pedal isn't easy if you want it to lean. I have found on a regular trike, or even a bike, if you lean your body off the bike while standing on the pedals, leaning in towards the curve you are taking, you won't tip over.