Rigid front axle structure is simple so that there are few faults. Main faults are: axle head is overheating, which generally appears after the repair and maintenance. When the hub is assembled, bearing pre-pressing is excessive so that bearing matching is too tight. The hub bearing is damaged or has corrosive pitting so that overheating is caused and also there is noise. Hub bearing is lack of oil, which can also make axle head hot.