8 speed shimano claris cassette on Campagnolo Calima C17
A road 11 speed compatible wheelset will have a freehub body that is 36.75mm wide. A 8/9/10 speed compatible wheelset will have a freehub body that is 34.95mm wide - 1.8mm narrower.

You can fit 8, 9 and 10 speed cassettes on an 11 speed freehub body with the addition of a 1.8mm spacer.