Every radio control car has an axle and bearing setup of one kind or another. The tolerances of these setups usually warrant the need for shims to eliminate excessive slop back and forth within the axle. Take some time and be sure to shim these areas properly. You basically want to remove as much end-play as possible without side-loading your bearings. Trial and error is the best system here so just keep adding shims and testing the slop until you have minimized it. Always remember to leave a tiny bit of back and forth play as to keep everything rolling freely but reduce the slop as much as you can to increase chassis responsiveness and help keep your car planted on the ground with more authority. It is also a wise idea to revisit axle shimming from time to time to adjust for parts wear.