When to use spacer on rear cassette

[GeorgeHu]

I have a mavic crossride and I am putting on a Shimano HG70 8 speed. the rear wheel came with a spacer. Does anyone know if I need to install the spacer with the cassette? What does the spacer do?

[rlouder]

The freehub body on the mavics are longer than others to allow the use of both regular mtn cassettes and a wider one (campy?). Just stick the spacer behind the cassette. Otherwise, it won't tighten up.

[voodoo5]

yep i ran a 9 speed cassette on the crossrides and needed the spacer...

[FBinNY]

You use the spacer if and when you need it to get the front edge of the cassette out beyond the edge of the freewheel body. If the cassette doesn't overhang the body, the lockring will bottom out before compressing the cassette.

It's analogous to how how steerer tubes have to end below the top of the stem for threadless headsets to be tightened.