I’m often asked what adapter spacers to use when mounting different road cassettes. The answer depends on your freehub and cassette. If you’re a Campagnolo user, the good news is no spacers are required if you’re using 9, 10 or 11 speed cassettes.
Shimano is more complicated. Shimano-compatible freehubs utilize the same design for 9 speed and 10 speed (ignoring the special outlier case of Dura-Ace 7800 hubs). If you want to run 9 speed with such a hub, no spacers are required. If you want to run 10 speed, a single 1mm spacer is installed behind the cassette. This spacer is packaged with 10 speed cassettes.
When using 11 speed Shimano-compatible hubs with 11 speed cassettes, no spacers are required. 11 speed hubs are backwards compatible with 9 speed when a single 1.85mm spacer is installed behind the cassette. To run 10 speed, install the 1.85mm spacer in combination with the 1mm spacer (2.85mm total). The 1.85mm adapter spacer is usually packaged with 11 speed hubs — if you’re missing this spacer and need one for compatibility, email and I’ll sort you out.