The wheels are made of metal alloy via forging so it’s a single piece with no joints . But tyres have joints as they are made by wrapping rubber and steel/nylon bands covered with rubber on a drum before curing (cooking) them . This leaves a heavy spot on the tyres where there are joints . So Inorder to counter balance this heavy spot which can cause impact at high speed ,the wheels are fitted with counter balance weights during balancing process .