Riding on Air

When the Visicoach rolled out of the Loudenville, Ohio factory, it was riding on four leaf springs. The leaf spring has been a part of vehicles almost as long as the wheel itself, having supported everything from horse drawn wagons to pony cars. Even modern Corvettes still use the technology.

But if you've ever ridden in the back of a school bus, you probably understand there is some potential for improvement on ye olde leaf spring. A modern air suspension can provide both a gentler ride down the road and significant improvements in handling.

The differences between metal springs and air springs boil down to a couple of key elements. Leaf springs create a mechanical connection between the chassis and the axle, providing a fantastic conduit for communicating every bump on the road right up into the passenger compartment. Air springs are more like heavy balloons, with a cushion of air that effectively separates the chassis from the axle and running gear.

And for the purposes of this comparison, leaf springs are essentially consistent in their spring rate, meaning that regardless of the movement of the bus or the wheels, the springs are going to give the same level of flex and resistance. Air springs, on the other hand, are dynamic. With valves, pressure regulators and a compressor, the rate of an air spring can change all the time.

Consider taking a fast corner in a tall vehicle like the bus equipped with leaf springs. Very quickly, a great deal of weight will shift to the outside of the turn and cause the bus to lean over to the outside, thrilling its occupants. In contrast, air suspension can automatically compensate as the weight shifts, increasing pressure on the outside air bags to keep the bus level through the corner. The bus is easier to drive and passengers have a more tranquil ride in the back.

That's a long way of saying we wanted to take advantage of the donor's air ride on the old bus. Simple in theory, challenging in practice.

First, the leaf springs and hangers had to come off the Flxible, along with all the running gear: wheels, axles, brakes and steering. We also had to flatten out the existing frame rails, which had arcs to provide space for up and down axle movement. The silver line that goes across horizontally is the seam where the existing frame has been extended with a new piece of steel to make it flat to support the geometry of the new air suspension.  You can also see the air bag installed on the trailing end of the link, just behind the blue and yellow of the new (old) shock absorber.

The new trailing link suspension mounts were cut to fit the Visicoach frame, then welded into place. Then we bolted the Freightliner trailing link arms into place to form the forward pivot for the suspension.  The air bags mount in back, along with a horizontal link that keeps the two sides in line. The scale of the Meritor rear-axle is still astounding to me.  There is just a lot of metal there. You can see the two air bags with the cross-link installed, looking through from the still-empty engine bay.

After the big metal work was complete, we installed leveling valves at each corner. With a simple linkage, the valves open or close depending on the movement of the axle relative to the chassis, regulating the air pressure at each corner.  The leveling valve is connected to the little arm in the top right of the photograph and then directly to the suspension link at the bottom. It is a remarkably simple technology that transforms the way the bus will handle on the road.  The last step is to install tanks to hold compressed air in reserve and keep the compressor from running all the time.

The bus requires a lot of plumbing, and the first components have been the air hoses. They'll supply both the suspension and eventually the brakes. With the axles mounted, we'll move onto the fitting and installation of the Cummins 5.9 turbodiesel and Allison transmission combination.

.andrew