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.

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.)

The condition of the steering and suspension systems has a profound impact on the effectiveness of the tires and produces characteristic wear patterns. Uneven wear (excessive wear on an inside or outside edge of one or both front tires) usually indicates misalignment of the steering system. Cupping of the tire tread is usually a sign of wheel bounce related to an imbalanced wheel, a defective shock absorber or a loose component somewhere in the suspension system. We'll discuss wheel bounce later, but you should be able to detect it long before it damages a tire.

Unpatterned wear, meaning the tires are excessively but evenly worn, warrants taking your vehicle out of service, especially in bad weather. This shouldn't happen in a well-run fleet, but when it does, it is always the eventual responsibility of a vehicle's operator to determine its safety. Driving on bald tires is about the worst thing an emergency vehicle operator can do, next to not checking them in the first place.

In addition, inspect the inner surfaces of the rear tires. If you ever notice that one of them is covered with oil, there may be a leaky axle seal, which will eventually affect the rear brakes, producing a brake pull toward the opposite side.

What Is Your Vehicle Telling You?

Your ambulance should steer straight and true on a flat surface, and respond promptly when you turn the wheel, accelerate or depress the brake pedal. You should not feel or hear any road noise or vibration during coasting on smooth pavement, although a coarse tire tread or a snow tire will produce noise. Anytime you hear or feel something unusual, try to determine whether it's related to road speed or engine speed. For instance, a wheel bounce related to an out-of-round tire or a bubble in a tire might become noticeable at a road speed of 55 mph and get worse as the road speed increases. Wheel imbalance also produces wheel bounce, but it might show up at 55 and again at 68 mph. Remember, the steering and suspension systems are complex. You don't have the tools or training to diagnose problems involving them. Instead, try to understand the kinds of problems that can threaten your safety, then describe your observations to the fleet staff like the professional communicator you are.

Other unusual behaviors include:

1. Drift. This is not the same as brake pull, as discussed in the brake section. Drift is not related to brake use. Drift is when you're cruising down a straight road and notice that when you let go of the steering wheel, the ambulance persistently tends to drift left or right. Drift to the right is normal on some roads. All roads are designed with a crown, or high center, to allow water to drain toward the gutters. But the amount of crown is variable from road to road, and you should notice the ambulance wanting to drift more on some roads than others. It's abnormal if the vehicle wants to drift left, or if it drifts in either direction so much that you feel the steering wheel tugging in that direction. Abnormal drift can be caused by misalignment, but it is also a warning that you may have a low tire on the side of the drift.

2. Wheel bounce feels like something is vibrating or chattering under the ambulance. It's unusual at speeds under 40 and is always related to road speed. It always produces damage, at least to the tires.

3. Tire squeal during turns is common when you're driving at parking speeds on smooth concrete or painted surfaces. It's not normal at any other time, unless your speed is excessive for conditions. Persistent tire squealing when you are not stressing the steering system should always be taken as a sign of misalignment, and is a warning that you are damaging the tires.

4. Poor tracking occurs when the rear axle is mounted improperly. Normally, the rear wheels should follow the front wheels precisely. If the rear axle housing is misaligned when you look in the side mirrors, it will appear as though the ambulance is driv­ing down the road crooked, because that's exactly what it's doing. The rear axle can be dislodged in an impact, and sometimes a body shop forgets to check it. The problem is important, because in the event of a panic stop, the rear end may come around on you.

5. Excessive bouncing of the ambulance itself results from worn or inadequate shocks. When you traverse a speed bump, it's normal for each end of the ambulance to bounce one cycle (up once, down once, then return to normal). Of course, that can be exaggerated by speed. (Tip: Try to take the bump with only one side of the ambulance, preferably the right side if there's a patient aboard. It will bounce a lot less, and the patient won't feel it so much.)

6. A roaring sound that starts at about 25 mph and gets worse with speed can be a sign of a worn or damaged wheel bearing. It can be hard to localize, but if you can do that, it may prove helpful to your fleet staff. It's uncommon in newer vehicles.

7. Excessive steering play can result from a number of causes, ranging from a low tire to a misaligned or damaged steering system. Remember, the ambulance needs to start, steer, stop and stay running. Impaired steering always warrants taking the equipment out of service.

8. Vibration in the steering wheel is always abnormal. One of the things that can cause it is damage or wear to a special horizontal shock absorber called a steering damper. Let your fleet staff decide if that's the problem and if it's serious enough to bring you in.

9. Chirping or chattering noise that occurs during steering maneuvers, especially during parking, can be caused by a loose or worn drive belt or a seized power steering pump. It's never a good sign, although it may be a quick fix for your mechanic if it's just a belt.

10. A loud noise of variable quality that occurs under the conditions listed in the preceding item can result from air in the power steering lines-most often caused by a low power steering fluid level. Check your fluid level and look for leaks, then give your fleet tech a call.

11. Excessive steering effort is never acceptable, and is usually traceable to the power steering pump. The only time you can fix it is if it's related to low power steering fluid level, but even that should make you want to look for a leak somewhere.

Conclusion

The steering and suspension systems and the tires you ride on impact your safety every day, whether you're aware of them or not. Staying aware of them is just as important as your situational awareness on a scene. As a professional, you can't afford to be a passive observer or a passive driver. Make sure you come home safe at the end of every shift by remaining as alert and informed about your equipment and observant of its behavior as possible. You're worth it, don't you think?

Cap (Tony) Unrein worked as an EMT for five years with Children's Hospital of Denver and Colorado's Ambulance Service Company. He now owns EVMARS, a comprehensive fleet service company that has maintained several Denver-area EMS fleets since 1981. He has an ASE (Automotive Service Excellence) certificate and is also a certified Emergency Vehicle Technician (EVT).

Thom Dick has been involved in EMS for 40 years, 23 of them as a full-time EMT and paramedic in San Diego County. He is the quality care coordinator for Platte Valley Ambulance Service, a community-owned, hospital-based 9-1-1 provider in Brighton, CO. Thom is also a member of the EMS World editorial advisory board. E-mail boxcar_414@yahoo.com.