When inspecting drive chain tension on a standard street motorcycle, what is the typical recommended vertical free play (slack) measured midway between the sprockets?

1. 0 mm (tightly stretched with zero slack)
2. 50 mm to 75 mm of loose slack
3. 15 mm to 25 mm (约15至25毫米，或参照制造商标牌规定)
4. Chain tension cannot be measured without removing the rear swingarm