Air Ride Equipment
Trucks are designed to carry several times their own weight. An empty semi-trailer's tandem axles weigh roughly 10,000 lbs on a scale. But loaded to the legal max, they're at 34,000. So in the old days they had leaf springs that were stiff enough to handle 34,000 going down the road. But, when the trailer was empty, it bounced all over the place affecting both keeping it on the road and reducing structural fatigue.
Air ride was invented to solve that problem. Rubber air bags replace the leaf springs, and a control valve automatically adjusts the amount of air pressure in the bags. When the load is great, air pressure in the bags is increased. When the load is less, the air pressure in the bags is reduced, thereby softening the ride.
Today, air ride suspension is considered standard equipment on Class 8 trucks and their trailers.
Prior to becoming standard equipment, carriers would advertise "air ride service" and charge additional per mile fees for the use of such equipment.