A danger with off-roading is damage to the vehicle from hitting rocks or other hard surfaces on very uneven terrain. A typical solution would be to install skid plates (sometimes also called bash plates), which are thick metal plates protecting vulnerable parts (such as the transfer/gear box or engine oil sump). Some manufacturers install skid plates as standard equipment on some of their vehicles. For many others this additional protection is available as an after-market accessory. Skid plates may be simple flat plates, but they may also be formed (by stamping or by welding multiple pieces) to protect shaped items like differentials. Fuel tank skid plates are a common factory option. In addition to skid plates, many off-roaders install differential covers that will not bend or break if they are smashed on rocks or other large objects. Since the axles on an off-road vehicle typically are the lowest hanging part of the vehicle making them susceptible to damage.