I just returned from my maiden voyage towing the Smart after having the Roadmaster base plate installed by Camping World a few weeks ago. Before this trip I had only towed the car a couple of blocks after the installation and had no problems. However, after we got hooked up and ready to pull out for a 200 mile trip to the coast, I used the extra key to lock the car (as I had always done with my previous toad -- a Land Rover Discovery). Immediately when I started out of the parking lot, the wheels went crazy wildly flying back and forth. Also, the alarm system started sounding. I tried again with the same results, so I pulled over and called the SmartUSA road service number. The person on the other end was very sympathetic but did not have a clue. Together we read the manual which states that the key should be left in position "O" when flat towing; however, I could not get the key to go to that position with the gear shift in neutral. He decided to call the service rep at my dealer in Charleston, and he too could not get the key in any of his cars to go to position "O" with the shift in neutral. We concluded that the manual was incorrect.
While waiting for the service rep to call me back after doing research on the problem, I found in the manual that the car's security system has a tow-prevention feature, and that this feature has to be deactivated when flat towing. After I deactivated the system, the car towed just fine.
When I got ready for my return trip I tried again to tow without deactivitating the tow-prevention feature and again had problems with the wheels tracking, but after deactivating that feature I had no problems.
Other posts suggest that the tracking problem is inherent in the Smart car design. Maybe it was just coincidental that the car towed just fine with the tow-prevention system deactivated, and otherwise would not track correctly.
I am now eager to find out how it works the next time out. Of course, I will always deactivate the tow-prevention feature and will not attempt to lock the car when towing it.