The big trailer-mounted wheelie machines have dyno drums that probably add a lot of 'synthetic momentum', for lack of a better term (I use a spring to simulate it, still working on the rate & preload but getting there). My setup has almost no lateral play in it - took a while to get it that way, didn't want to worry about it falling over while vertical - so I won't know how badly I'm off left-to-right until the driveway phase starts. Doug Domokis used a tiny front hub motor, apparently it made a real improvement in lateral stability.