The softer the wheel, or lower Shore “D” hardness number, the more the grip and slightly less roll. The harder the wheel, or higher Shore “D” number, the more the roll, with slightly less grip.

It is almost always recommended to use spacers in the wheels, when using standard 7mm bearings. Long ago, before the use of Lock Nuts (the nuts that have the Nylon Plastic inserts) to hold the wheels on the axles, Jam Nuts were used, but when the spacer was not as wide as the web of the wheel, or the wheel was not machined accurately, the bearings would lock up, the nut had to be tight, and could not be “backed-off”. This method, of locking the Axle Nut down, did promote proper bearing alignment and did increase the diameter of the axle from 7mm to approximately 14mm, because everything was locked down, which also saved the problems of bent axles. Today we do use Lock Nuts. It is still very advisable to lock everything down (especially for FreeStyle, Jam, and Roller Derby Skaters). Roll-Line does provide Spacers for Roll-Line Wheels with each pair of Frames or they can be purchased separately. Properly locking the Wheels down when using standard bearing types, will also promote longer bearing life.

The Roll-Line 7mm SpeedRace & SpeedMax Bearings , are specifically made for very High Level Competitive FreeStyle Roller Skating and are a very Durable Bearing with lots of ROLL. The SpeedRace & SpeedMax  Bearings are Free Rolling with minimal Radial Play which promotes and very evenly distributes the load across the 7 or 8 (of the SpeedMax) balls for optimal Roll and a good amount of Axial Play or Thrust, which is the side movement of the inner race in relation to the outer race. This movement is very important because when Skaters push, we always put side load on the skate to get power in the push or stroke. If the Bearing does not allow for this, it will bind and LOOSE operating speed or ROLL. These provisions in the Race Curvature and Contact Angle allow the Roll-Line SpeedRace & SpeedMax Bearings to ROLL very freely 
The Roll-Line SpeedRace Bearings have 7 balls and a rubber sealant Shield and the SpeedMax have 8 balls and a metal Shield and are ONLY available for 7mm Axles. The Roll-Line SpeedRace & SpeedMax Bearings are skated on by most of the World level FreeStyle Skaters and Champions.