12mm - 15mm will be fine & 20mm is doable but possibe rub with arch liner when hitting dips at speed or cornering hard with full tank - 2 passengers .
Visually on the rear E89 arches there appears masses of clearance but look & feel the arch liner & you will see they have a pronounced lip / edge that protrudes from the actual inside of wing . why it would be designed this way is anyones guess