Ambulance A & P: Steering, Suspension and Tires

There was a time, not so long ago, when ambulance drivers, working five days a week for $1.25 a hour and getting paid for only 13 out of 24, couldn't even buy respect from anyone but each other. But we dispensed the utmost care for our patients-and our equipment-and took great pride in being who we were. We kept our patients warm, handled them gently, used their names, respected their families and knew everything there was to know about a Miller Meteor high-top Cadillac. We dreamed of a day when we could become EMTs and be welcomed into medicine. But we couldn't even imagine facing a family after a mechanical failure and explaining to them why we hadn't checked our equipment. Could you?

If you want to become an EMT today, you enroll in a course and get a certificate. Few of us develop an understanding of how an ambulance works (or fails). Why is that? Even though ambulances have become much more complex during EMS's 37-year lifetime, we're still teaching our crews the same thing about their ambulances that we taught them 37 years ago: nothing.

This series of articles discusses the systems in your ambulance. Our goal is for you and your colleagues to understand how your ambulance works as well as you understand your patients' anatomy and physiology. Start. Steer. Stop. Stay running. Every ambulance must execute predictably and reliably for you to complete each mission and come home safely. In earlier issues, we looked at the electrical system that starts your ambulance's motor and discussed the motor itself. We also described the braking system that controls speed, brings you to a stop and lets you park on scenes. In this issue, we cover steering and suspension systems, and the tires that protect so many lives every time you run a call.

Steering and Suspension Systems Suspension System

Your vehicle has a steel frame that is shaped a bit like a capital letter A lying on its face with the top of the A pointing forward. This allows space for the front wheels to pivot when you steer. The suspension system is a set of attachment points, springs and hydraulic shock absorbers that enable the frame and axles to absorb irregularities in the road, and to keep the ambulance stable during starts, turns and stops.

The rear axle assembly is comprised of a gearbox penetrated by two steel tubes that emerge from it sideways, and a solid steel shaft pointing forward (the driveshaft). Those tubes also contain heavy steel shafts called axles, whose outboard (lateral) ends are connected to the hubs that the rear wheels are bolted to. Inside the gearbox, or differential is the planetary gear, a ring-shaped gear attached to the driveshaft; a bunch of roller bearings; and a pair of beveled or "tilted" gears attached to the inboard ends of the axles. The driveshaft is attached to the rear end of the transmission by means of a telescoping tube called the drive­line. The driveline telescopes because the rear axle's distance from the transmission changes slightly when you go over a bump. The rear surface of the differential, also called the banjo housing, is removable, and the whole assembly normally contains a fluid level of 90-weight oil that is specifically designed to bathe its contents: the gears inside it, the axle rods and all of the bearings from hub to hub. The outside surfaces of the banjo housing and axles should never be greasy.

The rear axle is fastened to a pair of leaf spring assemblies comprised of long, layered flat leaves made of very hard spring steel. It's important to know that they're only clamped together, so if you hit something really hard with one of the rear tires, you can affect the alignment of the rear axle. (You may also bend a wheel, which can result in a wheel bounce and/or sudden deflation of that tire without warning.) Your fleet staff knows that when you log a lot of miles, stuff happens. When it does, let them know about it.