It's been done a lot, both TT and off the road do a set of excel rims and spokes in 18" to go on the standard rear hub, you will need a decent wheel builder for the assembly. Off the road also do an 18" excel rim with a talon hub/cush drive combo, but this is a grand for the rear alone. Not sure where you live, but this may leave you with a problem with your insurance and passing roadworthy tests as the tyre size may be in the bikes homologation (EU certificate of conformity) document. I wanted to do this, but my insurers wouldn't allow it.

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