As having my first ASR-Vespa, I'd been noting some hesitation in steady speeds, like if driving in the city 30km/h it just does feel it does not like it. A bit searching and those asr-rings they change for more speed and better speedo reality. (and yes I know not for new ones like mine) But, as ASR wants both wheel driving on same speed, how it then could do that, as that new rear disk reads different speeds all the time? So: why you not have to change both?
Anyway if push the asr off, it is still the same hesitation on steady speeds. So it is not that at all.
Lastly, why the speedo ring is the rear one, why not the leading one?