The wagon itself is made of various kinds of wood, with the rims of the wheels being covered with tyres of iron, and since the middle of the 19th century the axles have also been made of iron. The back wheels are usually substantially larger than the front ones and rigidly connected to the tray of the vehicle. The front wheels are usually greater in diameter than the clearance under the tray of the vehicle so that the steering axle could not turn far under the tray. This makes little difference to the turning circle of the wagon because of the oxen drawing it (see above) and it makes the front of the wagon much more stable because the track is never much less than the width of the tray. It also allowed a much more robust connection between the hauling traces of the oxen and the rear axle of the wagon (usually iron chain or rods) that is necessary for heavy haulage.