Oversteer, when you hit bumps

A stiff rear can skip over ripples, but I don't think that is usually the whole reason it steps out. What decides it is extra steering load while the inside rear is already light. The common mistake is to reach for the bar or the ride height first, while the kart is still being rotated across the bump. Get the rotation done earlier so the wheel is closer to straight when the ripples hit. Then the chassis is not adding more load through the tires at the same moment the rear is being unloaded. If it still hops after that, drop the torsion bar before you change ride height. Height moves geometry and rear stiffness together and is easy to take too far. Leave lifting until last in a fast open bend, because you give up exit speed. Softening the rear too much also costs drive out of the same corner, so move the bar in small steps rather than treating it as a complete cure.