SKF doube side steel (ZZ) shielded bearings has contact between the seal and the outer only, and Shielded bearings are primarily intended for applications where the inner ring rotates. If the outer ring rotates, there is a risk that the grease will leak from the bearing at high speeds. However, the frictional losses are lower than rubber sealed bearings where the seal contacts both the inner and outer race.

While not ideal use on the road or in other harsh environments, they can be fitted in conjunction with dust seals, where they will require lower maintenance than open bearings.

We use these bearings without seals in our race bikes, where they perform well but get cleaned frequently and changed regularly.