Actually I'm afraid both previous are slightly incorrect.

Yes,you have to change the knuckle/hub, but if you fit the ST170 there is no need to change driveshafts, suspension, etc. - the same as a complete ST170 300mm conversion.

You need a 5mm spacer behind the RS Disc to give clearance from the lower arm/ball joint. Then a conversion plate for the calliper and RS brake hose.

All described in a How To in Focus Owners Forum over a year ago.

Martin