There are many shafts that go through until the power of the motor is transmitted to the tire, but these bearings are made of special resin plastic bearings.
Replacing this with a ball bearing is the first step in improving performance and you can expect less resistance to increase speed and riding time.
Semi-Trailer Optional Parts
Fits: 40 Container (3 Axis), Japanese Postage 40 Container Semi Trailer (3 Axis), Reefa Semitrailer, Stadium Blitzer, Blitzer Beetle.
Set Contents: 1150 Ball Bearings x 12
(From Amazon.co.jp)