If you mean spares for the track I would recomend front upper and lower arms, and front knuckles. In a season and a half, the only chassis parts I have broken were a front lower arm (it was new at the time), a front shock end (same crash) and a rear outer hinge pin. When you see somebody with a broken Mugen there is always 2 or 3 more people standing next to him in stunned in disbelief.