The Urchin is a thin oversized model vased on the A-RT Orphan, but with a slightly smaller gap.
It has a thinner gap, (1.8mm) and no drilled in response system. It relies purely on the friction on the inner walls. This makes for a smoother experience, but requires a little more breaking in and personal tweaking to get it where you want it.
It has a ~1/4" walnut hand turned axle. Take care of it to ensure you don’t snap it by sitting on it in your pocket.
