On hard ground at slow speed, I don't see how it would have rocked the driver so much since that contraption was fitted through a shaft which had to be articulated (hopefully on both side), keeping the rear relatively level. Of course, that is only at very slow speed. The pointed end could have "paddled" its way on really soft soil. At best, I would guess this offered an advantage only in very soft soil (over the same setup with round wheels). In any case, it seems to me that the greatest difficulty would have been to turn (unless the support shaft was locked in middle position on both sides, which would have then rocked the driver, or maybe there was a way to drive each side independently or maybe you only turned at a speed higher than very slow...). Now, given that both wheels on the same side were driven at the same very slow speed by a tight chain and going forever in a straight line, would there have been interactive tire wear between the front and rear tire of a single side ?