These feets use 50 durometer, 30 and 40mm half-spheres made out of the visco-elastic urethane rubber "Sorbothane" to reduce the amount of conducted vibrations by a lot, without the problems of self-induced oscillation of more springy solutions. The front use 30mm half-spheres, where the rear uses the 40mm half-spheres. This is because the Ultimakers are a bit back heavy and the Sorbothane half-spheres needs a certain amount of loading to perform well. They work extremely well compared to most other solutions i have tried through the years, and makes the printer appear much more silent than before.