I had read somewhere that the 11speed was in fact slightly narrower than the 10speed, but I might have got my wires crossed?
Yes, crossed wires. The 10-speed Shimano cassettes (apart from Junior gearing versions) are 1mm narrower on the rotor than the 9-speed ones, so you have to put a 1mm spacer on before the cassette.
Yes, crossed wires. The 10-speed Shimano cassettes (apart from Junior gearing versions) are 1mm narrower on the rotor than the 9-speed ones, so you have to put a 1mm spacer on before the cassette.