By side-side motion I'm guessing you mean a tipping force - and the the answer would be that he's pretty hard to shove over, at the limit of his lean he's stable, 10 degrees tilt and he'll fall. From fully upright, you need a bit more of a shove - having just pushed one over. Sandy says they can tip themselves if you wobble fast enough.

We're guessing you were thinking of something like this --> http://i.imgur.com/Cmmwu5A.jpg