Just a thought but could you expand your range and consider car and van tyres as well as bike?? That would give you way more potentual customers
I understand that start up costs can be high but I am sure you could sorce most equipment second hand at a modest price given a bit time.
Also you will have an big advantage over some of the established tyre companies, they have rent, rates building insurances all related to a fixed premisess.

Good Luck