newbuilder wrote:Why could I not use a sprung axle with it?
The Ultralight uses the strength of the rubber torsion axle as part of its structure. If you wanted to use a leaf-spring axle, then you have to replace that strength. You would need to lengthen the angles on the side so that they pick up the two ends of each spring and then you would need to add a cross member at each end to hold the axle brackets rigidly in place.
This would add quite a bit of extra weight and complexity to the chassis, so I never considered that alternative - it kinda stops being an ultralight and becomes a light-ish.
Andrew