Complex sentence: Vintage motocross for machines up to 1974 (with a few exceptions allowed to 1976 if the bike didn't change) which is us usually run on the Saturday of events, and Post-Vintage motocross which is for machines from 1975 through to the mid 80's.
Split into simpler sentences: Vintage motocross for machines up to 1974 (with a few exceptions allowed to 1976 if the bike didn't change) which is us usually run on the Saturday of events. The second is Post-Vintage motocross which is for machines from 1975 through to the mid 80's.