As I understand it (apologies if wrong) the "solution" is to fit loose sleeves to bolts that hold adaptor to bolt on wheel hub to better match adaptor hole size. Not a spacer as such therefore.
No ...
but ... if, as I understand it, the splined hub adaptors involve bolting a plate/disc (whatever it is that carries the splined hub) to the face of the existing wheel fixing/hub, then, by definition this must be spacing the wheel further out - just like boy-racer wheel spacers of yore? This would then place an "off-centre" load on the wheel bearings, would it not? Just saying ...
