Bearing can get dirty after few battery pack run. Yeah Racing produced high quality ball bearing set for TT01 type E chassis at low price. Why not buy second set of ball bearings, which can directly swap the dirty bearings during the chassis maintenance. The dirty bearings can be cleaned and stored for next bearings set change during maintenance. Anyway, ball bearings set is number 1 list of chassis upgrades. This bearings set is recommended for RC hobbyists who like to upgrade on a budget. The only difference of this bearing set compared to TT01 is the type E set have four extra 850 bearing used for steering bell cranks.