Buying Tomorrow’s Buses Today: Part 2: Structures and Suspension Systems

Ned Einstein (Transportation Alternatives, New York, NY)
Anil V. Khadilkar, Ph. D (AVK Engineering, Huntington, Beach, CA)

A quick glimpse at the best and the worst of Today’s buses reveals stark differences at the ends of the spectrum: (a) bare-bones body-on-chassis conversions with leaf-spring suspension systems lasting seven years in a schoolbus duty cycle versus (b) lush, integral motorcoaches with pneumatic suspension systems lasting between one and two million miles (notwithstanding a power train replacement). Nothing is, of course, this simple: Vehicles wear out more from slow-speed, stop-and-go duty cycles. And vehicles deployed fewer hours decline in value, to a greater degree, as a factor of their mere age (compared to just their mileage). As one might expect, those vehicles providing five to seven times the mileage cost five to seven times as much – as a starting point. In terms of true costs, of course (see “Life Cycle Costing” in National Bus Trader, December, 2004), they cost pretty much the same.

Summarizing bus and coach structures and suspension systems would be a snap if only these two extreme formats (body-on-chassis and integral) existed. This is, of course, not remotely the case. The variations between them have been intertwined historically and commercially. More recently, the distinctions have been further blurred by an extraordinary array of niche market products, a striking variation in materials (e.g., composites) and structures (massive low-floor chassis), and the increasing application of “bridge technologies” (like enhanced suspension systems) to models of all types and sizes. Regardless, the better a vehicle’s structure and suspension system, the safer it is likely to be, and the greater its value – particularly when distinctions in safety are exaggerated by the spectre of liability.

Concepts and Trade-Offs

A critical factor defining a vehicle’s safety is how well it absorbs the impact forces generated by a collision. This concept is known as the vehicle’s ‘crashworthiness.’ A vehicle’s crashworthiness reflects its ability to manage the “loads” and “load paths” generated by a collision to lower the g-forces encountered as they intrude into the vehicle’s structure. In simple engineering terms, the most crashworthy vehicle is the one that provides the best paths for distributing these loads.

Because crashworthiness is largely a function of design, engineering and construction, it follows that the more crashworthy the vehicle is, the more it will cost, at least initially. Of course, vehicles that are more crashworthy generally last longer. So, in terms of safety, it would seem that one gets what one pays for. However, this is becoming less and less true because the notion of liability has exaggerated the distinctions between vehicles with respect to their comparative safety. When one factors in the costs of damage awards, one often finds that the more crashworthy a vehicle is, the less it costs.

Vehicle Structure and Safety

The enormous variation in vehicle structures is earlier to understand if one first examines the extremes of design and construction – body-on-chassis and integral. One useful way to view this distinction is to examine how well these formats reflect the engineering concept known as “elegant design.” In automotive engineering terms, an “elegant design” is one in which all the structural members share close to equal participation in bearing the loads.

A body-on-chassis vehicle is comprised of two distinct sections effectively clamped together:

  • A chassis consisting of two massive, longitudinal “frame rails”
  • A body constructed of frame members of significantly lesser mass, to which a skin or shell is attached.

All running gear, suspension and steering are attached to the chassis (and, in fact, it can even be driven without the body), while the driver and passengers ride inside the body.

Some body-on-chassis vehicles are obviously more crashworthy than others. As an example, welding the two sections together is generally better than riveting or bolting them together. However, the method of attachment is of somewhat minor importance since the two sections consist of such different masses. (Picture gluing an egg to a watermelon and dropping the unit from a tower.) Nor does it matter whether the “skin” consists of small body panels or, for example, a single, full-side body panel. The central point is that, with the differential in mass between these two units, the two central frame rails of the chassis provide the primary path for carrying the loads, instead of spreading them out among more numerous and more similar frame members.

In rare, catastrophic collisions between schoolbuses and other large vehicles (a large truck in the 1993 Palm Springs accident, a freight train in the 1995 Fox River Grove accident), the buses’ bodies separated completely from their chassis, and came to rest several yards away. Similarly, in a series of crash tests conducted by Transport Canada in 1984, the bodies of the three schoolbuses crashed into a cement barrier (@ 30 mph) shifted forward on the chassis – in one case by more than three feet.

These examples do not mean that body-on-chassis design and construction are unsafe, or even less safe in all accident scenarios. One intriguing curiosity about the Transport Canada tests was NHTSA’s conjecture that the separation of the body and chassis may have actually absorbed some of the crash forces. This hypothesis has never been tested. Further, the Altoona testing program has demonstrated that some body-on-chassis vehicles are surprisingly durable. Regardless, the essential point about their crashworthiness is that body-on-chassis vehicles depart dramatically from the principle of “elegant design.”

Historically, it is also important to remember that body-on-chassis vehicles evolved from trucks. Even today, many body-on-chassis “conversions” (the term applied when anything but a truck is built on a truck chassis) are constructed on the same chassis designed and produced specifically to accommodate truck bodies. Only recently, and largely as a coincidence of commercial consolidation, have schoolbus manufacturers begun to manufacturer their own proprietary chassis – although they are still chassis. As recently as a decade ago, many schoolbus bodies and chassis were largely interchangeable.

Lying at the opposite end of the spectrum is are integral vehicles. Unlike “body-on-chassis,” however, “integral” is far more difficult to define. In simple terms, an integral vehicle is one in which the design elements, materials and joints share the responsibility for carrying the loads, and which provides more and better paths for distributing the loads equally among them. In order to accomplish these goals, the mass of the elements and joints, and the type and quality of their materials, must be far more similar than those of a body-on-chassis vehicle.

The best integral vehicles employ tubular frame members of the same or similar mass and strength throughout the entire structure. In contrast, the frame rails of a body-on-chassis vehicle’s chassis may contain 30 to 50 times the mass of its body frame-members. In fact, the body frame members are often not even tubular (e.g., providing a stronger, “closed” cross-section). Instead, their cross-sections are “open” (e.g., “C-channels”) As an engineering matter, the strength-to-mass ratio of a tube (or other closed profiles such as a “D-channel”) is geometrically greater than that of a frame member with an “open” cross-section constructed of the same mass and material.

In contrast to the truck-industry origins of body-on-chassis vehicles, integral vehicles evolved from aircraft design – a design referred to as ‘monocoque,’ since the “vehicle envelope” is comprised of a continuous skin forming a circular tube, reinforced by a lattice-like structure, or continuous hoops, spaced longitudinally at regular intervals. (Because of the high speeds involved, the circular cross-section is critical to minimizing aerodynamic resistance.) However, because of the strength-to-mass ratio needed to propel the vehicle into the air, the skin – not the structure – supports far more of the load. Further enhancing the skin’s ability to perform this role are the envelope’s relatively small windows.

As these aeronautical concepts were transferred to ground vehicles, the degree to which the skin could bear the loads was necessarily altered, largely for two reasons: The rectangular profile, and the need for larger windows. The rectangular profile transferred much of the loads on the skin to four corners, where their integration had to be reinforced (thus the emergence and importance of joints). Further, the larger windows reduced the surface area of the skin. As a consequence, much of the load had to be shifted to the structure. For this reason, integral vehicles are not truly monocoque – even though the term is often applied to them. Instead, they are essentially ‘semi-monocoque.’

Regardless of the definitions, what distinguishes an integral bus or coach from a body-on-chassis bus or coach is its ability to provide more and better load paths. The sections of the vehicle cannot come apart, by definition, because there are simply no sections. Similarly, an integral vehicle does not have a body and a chassis at all: It is simply a vehicle “envelope.” But regardless of the distinctions, the better the integration, the better the crashworthiness.

While these distinctions are complex, they become exponentially more complex when one tries to describe vehicles lying between these extremes – particularly when the distinction is compounded by the inclusion of either pneumatic or enhanced suspension systems. Vehicles in between these extremes are differentiated by a multitude of differences in their design, construction, materials and assembly processes. At the extremes of the vehicle spectrum, the soundest structures are still packaged with the best suspension systems. However, with the emergence of enhanced suspension systems, this alignment has become less clear for the array of vehicles lying in between.

Suspension Systems and Safety

A vehicle’s suspension system enables it to “counter-mirror” the contours and irregularities of the road surface, or horizontal plane, over which it moves. To accomplish this, the suspension system elements minimize the transfer of forces from these contours and irregularities to the vehicle’s occupants. Thus, the primary purpose of a suspension system is to absorb and dissipate vertical forces or loads.

Regardless of the type, most vehicles’ suspension systems contain the same elements: shock absorbers, tires, wheels, wheel bearings, axles, trailing arms (some buses), and – most importantly – springs or air bags. One immediate difference between leaf springs (the most common format on body-on-chassis buses) and air bags (the most common format on transit buses and motorcoaches) is that air bags provide better “damping” of the loads. This is largely because these two approaches provide this damping differently:

  • The “compression rate” of leaf springs is constant and linear. It can be set at various levels. But once it is set, the variation needed to offset differences in loads (e.g., full or overloaded vehicles versus empty or near-empty ones) and roadway conditions (surface composition, topography, bumps, dips, potholes, banking, curvature, etc.) is extremely limited.
  • In contrast, when properly designed, air bags can provide far more flexibility, and permit the spring rate to vary to better match and mirror operating conditions. While the variations are not geometric, the spring rate is still “load sensitive.” This capability yields better ride comfort, better control and better directional stability, delivering a superior combination of safety and comfort that justifies its increased capital costs.

In the simplest terms, air bags – or pneumatic suspension – permit the ride to more closely simulate the qualities it would experience as the vehicle moves across a flat surface.

Regardless of the type or format, a suspension system is designed to enhance three vehicle characteristics:

  1. Control
  2. Directional stability (roll, pitch and yaw)
  3. Ride comfort

How well various suspension systems enhance these three characteristics distinguishes the various approaches and brands from one another. However, because each design involves some trade-off among these characteristics, the ideal suspension system provides the best balance of the three. Because of the greater variation in their compression rates, pneumatic suspension systems generally provide the most optimum mix of these characteristics. In contrast, enhanced suspension systems tend to optimize one extreme of ride comfort versus control-and/or-stability more than the others.

While the paramount role of a suspension system is to offset vertical forces, it is also noteworthy that, to a limited degree, it also offsets longitudinal and lateral forces, particularly in extreme acceleration and/or deceleration moments, when the suspension system reaches its limits or “overcomes its margins.” Once the edge of this envelope has been reached, the system’s ability to offset these forces no longer exists. When this happens during severe braking, for example, the vehicle’s front end tends to dip down, and its rear end tends to rise up. Similarly, when it corners sharply to the right, the right side descends and the left side rises (or the opposite for sharp left turns). These events pitch the occupants upward, but also pitch then slightly forward or sideways, exaggerating the change in inertial or centrifugal forces they would otherwise experience from the interruption of their longitudinal or lateral movement. While one must recognize that all suspension systems have their limits, it is important to note that spring suspension systems reach them sooner.

To fully understand why bus suspension systems are so important, one need only compare a bus to a car. In a typical passenger car (not including an SUV), the center of gravity passes pretty much through the seated passengers’ navels. On a bus, the passengers are positioned further above the vehicle’s center-of-gravity, compromising their stability with respect to interruptions of inertial and centrifugal forces. This instability is further compounded – if not greatly exaggerated – by the fact that they often ride standing, or worse, walking. Worse still, the walking often occurs during the most vulnerable moments: When the vehicle is pulling out (accelerating and cornering) or pulling in (decelerating/braking and cornering). Particularly when the vehicle is overloaded, the tendency for standees to topple over during such moments provides the most important justification for optimizing the vehicle’s suspension system. Because it is “load-sensitive,” a properly-designed pneumatic suspension system provides a greater “margin” for accommodating this deflection.

There is one more curious difference between air bags, leaf springs and enhanced suspension systems – a distinction that effects both suspension system design and costs. In simplest terms, columns of air provide more cushioning than layers of steel. As a result, the air bags operate somewhat as auxiliary shock absorbers, and as noted, provide a better “damping” of vertical forces. However, air bags by themselves provide no means of keeping the vehicle from swaying laterally – as leaf springs do as a consequence of their dual role as both the vehicle’s suspension system and part of its structure. As a result, the reliance on air bags for damping necessitates the need for additional structures to augment the air bags’ positioning over or alongside the axles (combinations of bags located in front of and behind the axles are referred to as lying “outboard”).

Buses and coaches designed initially to include pneumatic suspension systems contain these additional structures. In contrast, “hybrid” or “enhanced” suspension systems rely largely on existing OEM leaf spring elements or their modification (ergo the term ‘enhanced’) to control this lateral movement. Some variations include actual air bags for this damping, while others (e.g., Mor-Ryde) employ rubber pads. Regardless, enhanced suspension systems provide interesting and measurable improvements in ride comfort. However, where they also improve directional stability and control, they also enhance safety.

While enhanced suspension systems can be added at the conversion level or retrofitted, and while air bags can be “shoehorned” into body-on-chassis vehicles designed to contain leaf springs, it is important to remember that the rest of the vehicle’s structure – particularly the other components of the suspension system – was not designed to operate with such a system. This notion should not be over-exaggerated. Similarly, the match between the two is rarely dangerous. And the designers of enhanced suspension systems take the relationship between structure and suspension system into account in their designs, elements, construction and materials. So too do the vehicle manufacturers who offer them as options. Just the same, given the complexity of suspension systems, and the challenge they are designed to overcome (particularly in worst-case situations like traversing a speed bump at high speeds), a vehicle purchaser must recognize the difference between a suspension system designed for and with a specific vehicle versus one designed to improve an existing vehicle (or more commonly, a range of existing vehicles).

Evolution and Choice

Vehicle structures have evolved radically over the past decade and a half. A short-list of the most dramatic changes include:

  • Low-floor designs (providing easier entry and egress, particularly for disabled and elderly passengers)
  • Heavy-mass, low-floor chassis like Workhorse and Freightliner (providing better protection of the passengers’ feet and legs from side-impact intrusion into the passenger compartment)
  • Drop axles (accommodating continuous low floors throughout the bus rather than the “split-level” configurations that subject upper-deck passengers to potential slip-and-fall accidents while changing levels)
  • Expansions in “forward control” configurations (i.e., transit-style drivers’ compartments and front caps built either over the cowl-positioned engines, or with engines inside the passenger compartment alongside the driver)
  • Raised floors (created at the OEM level to “hide” the wheel wells and expand the usable space for wheelchair securement)
  • Raised roofs (many of which meet the rollover requirements of FMVSS #220, to address the added risks from also raising the vehicles’ centers-of-gravity)

Paralleling these advancements were a substantial pair of improvements in suspension systems, including:

  • Kneeling features (which permit the vehicle’s curb-side and/or front-right corner to descend to facilitate boarding and alighting)
  • Enhanced suspension systems (available as both OEM options and for retrofit purposes) accommodating vehicle conversions of virtually every size, make and model.

Diversity and Drooling

This remarkable evolution has produced a startling array of choices, within a vast spectrum of price ranges, for accommodating a vehicle operator’s specific needs with remarkable precision. At the same time, these choices have greatly complicated the vehicle selection process.

The dramatic array of choices in structures and suspension systems are also critical to an operators’ efforts to optimize safety and limit liability exposure. As bus connoisseurs and non-connoisseurs alike have probably noticed, motorcoach sales have recently risen for the fourth quarter in a row. Further, with the sales figures of four quarters now in, it is clear that bus and coach sales have also risen significantly this entire past year. We finally have a trend we can lean on to justify and support our investment in new and better equipment.

As noted, the spectre of liability exaggerates the importance of safety, and exaggerates the importance of deploying the most crashworthy vehicle. Given the current litigation environment, purchasing the most crashworthy vehicle is among the most critical safety-related decisions a vehicle operator can make.

The most ignorant historian can begin a speech about any period in history by exclaiming, “It was a period of great change.” Qualitatively and quantitatively, the motorcoach industry is roiling in opportunity. The winners may someday look back on this period and exclaim, “It was a period of great planning. It was a period of great investing.” The losers can only sigh, “If only I had made a better investment.”

Publications: National Bus Trader.