I've got a model with 6 tyres contacting the ground. I start the analysis just above the ground to let it settle in position. When I add gravity to a motion study I cannot get the smaller ones to not fall through the ground. The ground is solid and the tyres have a fair amount of thickness. I've tried different solvers, different max step sizes, different materials (rubber etc), different frames, individual and grouped contacts but I can't get it to just sit on the ground on ALL tyres.

any ideas what might I be missing?