Springs. The springs or suspension assemblies shall be of ample resiliency under all load conditions and of adequate strength to sustain the loaded vehicle without evidence of overload. Springs or suspension assemblies shall be designed to carry their proportional share of gross vehicle weight. If rear springs are used, they shall be of a progressive type. If leaf type front springs are used, the stationary eyes shall be protected by a full wrapped leaf in addition to the main leaf. Shock Absorbers. Two front and two rear double acting shock absorbers shall be provided and be compatible with the manufacturer's rated axle capacity. Air Ride Suspension. An air ride suspension may be installed, which shall: Meet all of the manufacturer's requirements; Have a stop block to prevent the body from hitting the tire in case of an air bag failure; Permit the vehicle to be driven off the roadway if the air bag fails; and Have a manual drain valve. The air ride suspension system may be equipped with an automatic, electrically heated drain system.