We got a used Arriva and my wife was telling me the bike was leaning left on the first ride. I went over it and found out the stoker bars were off center by about 2cm to the right.

The deal was, they were on center as far as where marking grooves went into the stem, but the markings were not centered on the bars. So I actually centered the bar ends onto the center of the pilot's seat and it was much better on the next ride.

We're probably going to change over to drop bars soon anyway, but at least it's right for now.