Just one more thing (no i ain`t columbo
) To gain real noticeable speed from just inserting bearings is quite a tall order it would be very minute if noticable at all to the average racer.Unless the bearings you have are utter junk!! In reality you would be better of learning how to balance your wheels and tyres and checking them after every race more so if you crash.
Balanced wheels puts less strain on the drivetrain and especially the shocks.
The shocks have a major job keeping the wheels producing traction you certainly don`t want to be over stressing them with wheels that are well off balance.
Just think of your real car when the wheels are out of balance,its like trying to shake your fillings out your teeth.
These toy cars produce so so much more power to scale than a real car, so think of how much stress those shocks and the drivetrain are under when your wheels are out of balance
Anyway just a thought!!