Ambulance A & P: Steering, Suspension and Tires

Your ambulance doesn't really have a front axle, unless you drive a big truck. Ford uses a front suspension system, called a "twin I-beam" design, that resembles the rear end of an old Volkswagen. Each of the front wheel spindles (you remember them from the section on brakes) is attached by means of a pair of ball joints to the outboard end of a tapered steel beam shaped like an I-beam. A ball joint is shaped like a baseball held in the grip of a closed greasy fist. The baseball is attached to something that needs to move, and the fist is also attached to something that needs to move, but they need to stay attached to one another.

The front end of a van-type ambulance has either coil springs or torsion bars instead of leaf springs. A torsion bar is a rod made of spring steel, about four feet long and mounted parallel to the frame. The rear end is firmly attached to the frame, and the front end is attached to the inboard end of what is called a control arm. When the front wheels bear weight or bounce, the torsion bar is forced to twist. It resists that with great force, thus behaving like a spring. Ford uses a coil spring that looks like the spring in a ballpoint pen, only much larger.

One way or another, each wheel is also attached to a hydraulic shock absorber-a simple piston in a sealed tube full of hydraulic fluid. Shock absorbers keep the ambulance stable when it encounters a bump, when it corners and when its speed changes suddenly.

The Steering System

When the steering wheel is turned, it rotates a shaft that enters a hydraulic pump. The pump is full of hydraulic fluid, like the kind in your automatic transmission, and it's powered by a drive belt attached to a pulley on the front of the crankshaft. The pump's job is to multiply the force of your hand movements many times, so your most casual effort results in a force of many hundreds of pounds. That force is transmitted by means of a second shaft that emerges from the pump. It's attached to a steel lever (called a Pittman arm), which in turn is connected to some rods (steering links) that pivot the front wheels and steer your ambulance.

The geometry that enables you to turn into a driveway without ripping your tires apart is anything but simple. That's because your wheels all need to be able to adjust to changing surfaces, while the inner wheels must make a tighter turn than the outer ones. So, your steering system is comprised of hundreds of parts. Many of them are only clamped together to allow adjustments for each front wheel. Camber, caster and toe are three important adjustments.

Camber is the amount of inward (-) or outward (+) tilt of a wheel in degrees from vertical. Caster is the amount of forward (-) or backward (+) tilt of the steering axis, which, like the axis of the earth, is the pivoting axis of a wheel. It needs to vary for each of the front wheels, between a positive value when you're driving straight down the freeway and a negative one when you're making a hard turn. Toe, or toe-in, determines whether the front wheels are parallel to one another with the steering centered.

Your steering geometry does a lot more than protect your tires. It affects the behavior of your ambulance when you execute a panic maneuver as simple as stepping on the brakes. The vehicle communicates to you constantly about its steering geometry, if you pay attention. We'll go through some of the signs later. Most important, numerous operator errors can affect one or more of those adjustments-like hitting a curb or a gutter at speed. No matter how careful you are, nobody can expect you to be perfect. But when you do misjudge something, it's important to understand the risks to your equipment. If you're concerned about an occurrence, communicate the circumstances to your fleet staff.

The Tires

There is no feature of an emergency vehicle that directly impacts your safety, or the public's, the way your tires do. If your agency skimps on its tires or neglects them, you need to leave as soon as possible. And if you don't examine and pressure-check your tires faithfully at the beginning of every shift, start doing that today.

Three factors affect the reliability of even the best tires: a proper match between inflation and load; the condition of the steering and suspension systems; and wear (including damage). Fortunately, the most important factor-proper match between inflation and load-is the easiest to check. Inflation pressure varies from vehicle to vehicle. You can get that information from the fleet staff beginning on your first day with a new vehicle. Overinflation usually results in excessive wear in the center of the tire tread, while underinflation or overloading produces excessive wear on both outside edges of the tread simultaneously. Any of these conditions can produce a high frequency of blowouts. (In a well-maintained fleet equipped with good tires, blowouts should be rare.)