Reifeneinlage Pit 24mm medium Graupner 96375 The new GM Shimizu shaped foam inserts increase the useful life of our tyres, and at the same time make the tyres more true-running, with improved lateral stiffness. Generally we advise the use of inserts on the front axle which are one grade harder than those on the rear axle. The higher the grip offered by the race track surface, the harder the inserts which should be used.