Your bearings are as only as good as your spacers. The job of the spacer is to keep the bearings from pulling into the core of your wheel and slowing you down. If the core of your wheel doesn't exactly match the size of your spacer then you're not achieving full efficiency. These spacers measure exactly 10.35mm in 'depth', allowing you to dial in your ride to the fraction of a millimetre. 

Sold individually