| Cant see why you cant, just means you will be down on power etc. but be careful of any mods eg adding adjustable forks as then it wont go into B class. Just need to have a white number board and chnage to yellow when back in B
Tbh starting in A or B as a first class isnt an issue, only that B bike are usually cheaper to buy being older etc so nothing stopping the missus racing an A bike, not like jumping off 100 hp to 180 | 
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