How do I use spacers on a cassette?

[Bert1220]

I purchased new wheels and it's Shimano 7 8 9 and 10 speed compatible but it says 7 speed cassettes will need a spacer. Where does this spacer go and why do i need it? Would my local bike shop have this?

Thanks! I've never really bought new wheels before.

[cyclerUK]

The spacer goes on the freehub first, followed by the cassette.
An 8 speed cassette is fractionally wider than a 7 speed (about 4mm?).
All Shimano freehubs are a standard width to take 8 & 9 speeds without any spacers.
Funnily enough fitting a 10 speed may also require a very thin spacer .
You can't fit 8/9/10 speed on a 7 speed freehub as it's not wide enough.

Your LBS should have spacers.

http://www.spacycles.co.uk/products.php?plid=m2b0s150p1331