First completed: October 12 1997
Last revision: November 9 1997

Introduction

There is now a growing chorus of rebellious voices against the SUV, mostly based on the SUV's hostility towards green issues and other road users. Many however, are unaware of how technically inferior SUV's are, strictly from an engineering point of view. This would be far less outrageous if not for their high asking prices and huge profit margins. My motivation for writing this article is to reveal the truth of what people are actually getting for the large sums of money they are paying for a SUV. I would like to see good products succeed in the marketplace and bad ones fail, not the other way around.

The question posed by the title of this article is answered in detail in a question and answer format... specifically:

Q1: Why do SUVs ride so rough?

A: Ancient suspension technology

• Leaf springs

  Many SUVs use cart springs (more formally known as semi elliptic leaf springs) which were in use before the invention of the internal combustion engine! Remember those cowboy flicks with horse drawn wagons? Observe the springing used in those wagons and see how similar they are to the springing used on a typical SUV. There is no difference at all!

  In those days, this springing arrangement held an important property: the deformation of the leaf spring meant that the individual "leaves" rubbed against one another thus providing a crude form of damping action. In any kind of suspension system, the compression of a spring requires a damping force so that when the spring rebounds it will not oscillate indefinitely. Friction based damping is crude because it is difficult to control the damping properties of the system. In those days (over 100 years ago, that is) damping was provided by the friction inherent in the leaf spring design since there were no other means to do so.

  The invention of the familiar telescopic damper (or "shock absorber") has rendered friction based damping completely obsolete. In fact, friction is regarded as a totally undesirable property in suspension design today and great pains are taken to eliminate friction completely. The ubiquitous coil spring can be easily seen to have practically no friction in its action and is ideally suited to working cooperatively with the telescopic damper as part of a modern suspension system. The coil spring can also be designed to work with far greater values of suspension travel than the leaf spring ever can, so from an off-roading point of view, the leaf spring is clearly inferior in this regard.

  Ideally a modern damper should be soft on compression to allow the spring to compress and thus absorb the shock, and firm on decompression where the natural oscillating motions of the spring is rapidly halted, thus eliminating floatiness. These characteristics provide what is perceived to be a smooth ride.

  A leaf spring has roughly the same amount of friction on both compression and decompression. Friction in any suspension system inhibits the compressing of a spring, which leads to more road shock being transmitted into the cabin. This characteristic leads to what is perceived to be a rough ride.

  Leaf springs are completely out of date today but they can still be found in the rear ends of many highly priced trucks and SUVs. It is quite rare nowadays to find a truck with leaf springs on all 4 corners but they still exist. Many imported SUVs now have the frictionless coil springs all round but live axles (see below) are still the rule rather than the exception. American SUVs are starting to use torsion bar springing or coil springs for the front suspension. With just one or two exceptions, rear ends tend to almost be universally sprung with leaf springs.

• High unsprung weight and live axles

  A live axle is possibly the simplest and crudest way possible to connect two wheels to a body. On a driven axle it looks like a dumbell with the weight in the middle. Some nickname it the "pumpkin". You see it on most trucks and SUVs in the rear and sometimes in the front. The live axle is almost extinct in cars today, even four wheel drive ones. Why is the live axle evil? The more obvious reason is that shock that is encountered on one side is faithfully transmitted to the other. Live axled vehicles tend to be more easily upset if they hit a bump while turning.

  The less well known reason, however, is that a live axle carries with it a large amount of unsprung weight. Unsprung weight is the portion of the vehicle that is not sitting on top of the suspension. Examples of unsprung components are the wheels, tires and brakes. Unsprung weight is a highly undesirable attribute in suspension design, because the more inertia the unsprung components have, the less inertia the sprung masses have to remain unperturbed over a bump. This means that with less unsprung weight the suspension is able to react faster to bumps rather than simply transmitting the shock into the cabin.

  The ratio of sprung to unsprung weight is a key determining factor in how smoothly a car rides. Remember the old wisdom of how a heavier car rides better than a lighter car? The truth is that the heavier car most likely has a higher sprung to unsprung weight ratio than the lighter car. This also means that lighter cars can be made to ride better than heavier cars if they keep their unsprung weight down. And it also explains why these heavy SUVs still ride so roughly while handling so poorly compared to cars much lighter than them. They have too much unsprung weight, much of it comes from using antiquated live axles.

  Compared to the modern independent suspension, a live axle has the entire differential assembly, protective casing and all final driveshafts as unsprung masses while the main driveshaft is partially sprung. All the variations of modern independent suspension only have the final driveshafts partially unsprung. The main driveshaft(s) and differential assemblies are all part of the sprung masses attached to the body. Thus one can see how much of an unsprung weight penalty there is by using a live axle.

  A high performance car with measures taken to reduce unsprung weight can trade the gains in ride quality for stiffer suspension. Designers of high performance cars go to extreme lengths to reduce unsprung weight. In the Porsche 911 turbo for example, not only are the wheels made of lightweight alloy, the spokes are also hollowed out. All this in a fanatical attempt to reduce unsprung weight. Some higher priced cars have also introduced suspension components that are made of aluminum instead of steel. The aforementioned Porsche, the Audi A4 and the BMW 5 series are some examples.

  With low unsprung weight one has a car capable of sizzling handling while riding reasonably. Conversely, a SUV with its high unsprung weight has to have what little handling sacrificed for a tolerable ride. The use of live axles and leaf springs can be traced back to at least 50 years ago, while the use of coil springs can be found in railroad cars dating back to the 19th century. Paying substantial money today for such inferior and crude technology is analogous to paying $3,000 for a PC powered by a 80286 CPU today.

• Crude specification

  Given the high ground clearance and high loading capacity of a SUV, spring rates are specified to be very stiff, both to counter roll and squat. This of course is the simplest, cheapest and crudest possible solution to the problem. Modern suspension has developed methods to counter roll and squat without necessitating extremely high spring rates and a resulting bone jarring ride.

  For off roading needs, it is indeed necessary to specify stronger suspension components to handle the extra stress. The added weight of these components would increase the unsprung weight and thus worsen the ride. For the high asking prices of SUVs it would not be unreasonable to expect some effort made at reducing unsprung weight by using a modern specification of suspension components and using lighter and stronger materials.

  Cynically conceived SUVs have the worst of both worlds. By using leaf springs and live axles they handle and ride poorly. And because they are not expected to do serious off roading, the suspension bits are also not specified to be very tough. Thus one has a vehicle that is bad both on and off road!

Q2: Why do SUVs have such poor fuel economy?

A: Mediocre engineering standards

• Barn door aerodynamics

  SUVs have a marketing need to look tough. Curvy aerodynamic shapes are not considered tough. Also, a streamlined shape requires a lot of wind tunnel testing to optimize. Recall that research spending on SUVs is quite modest. Hence one ends up with a tough looking, commercially attractive shape that cost little to no windtunnel development. A win-win from the maker's point of view!

  The larger size of SUVs also lead to an increased frontal area. It is easy to see how an increased frontal area leads to more drag.

  The high ground clearance needed for off roading also means that there is a great potential of the air travelling under the vehicle to become turbulent. Turbulent air leads to increased drag. Since most SUVs spend time on paved freeways at speed, all these combined aerodynamic debits add up to significantly increased energy required to push air out of the way. A particular variant of the Range Rover offers air springs whereby the ride height of the vehicle could be adjusted depending on its current mission. Using slightly more advanced technology like this would mean that a SUV could potentially excel both on and off road, but the market has shown very little appetite for excellence of this sort.

• High weight due to inefficient design

  Pickup trucks are invariably constructed out of the "body on frame" design. (Also called "twin rail" frame) i.e. A stiff steel "ladder" frame is laid out horizontally whereby the engine, transmission and suspension are installed onto it. A cab and a bed is then installed on top of this frame. A biological analogy to this design is a vertebrate such as a human being, where the skeleton is concealed inside the exterior surfaces.

  Modern cars use unit bodies, also called unibodies or monocoques. Monocoque design came directly from the aircraft industry where low weight is considered paramount. Quite simply, there is no ladder frame. The entire skin of the car is a stress bearing structure. A biological analogy is the invertebrate such as a crab where the outer shell forms the structure. The advantage of a monocoque in car design is that sheet metal used to form the car's shape is also used as its structure. Hence there is more efficient use of the raw materials and lower weight.

  A monocoque design does not make sense in a pickup truck because a monocoque needs to form a completely closed box to be optimally stiff. In a pickup truck, the shape of the bed provides very little structural stiffness if it were to be part of a monocoque. A convertible is a good illustration of this limitation of the monocoque.

  SUVs that are evolved from pickup trucks simply attach a complete passenger cabin over the ladder frame of the pickup. This is wasteful and inefficient because the inherent strength of a closed box formed by sheet metal is not used as a part of the vehicle's load bearing structure. A lot of unnecessary extra weight is the result with no gains in structural strength.

  Furthermore, the ladder frame is very weak in torsion. Torsional forces are those that come into play when the vehicle is turning or if the suspension is unevenly loaded. If a vehicle structure is weak in torsion, one end of the vehicle will be deformed by twisting forces more than the other. This makes suspension design difficult as it introduces an additional undesirable variable into the equation. Monocoque design, in contrast are being continually refined by increasingly powerful supercomputers, so it is not surprising to see that each new generation of a car has increased torsional stiffness over its predecessor with no gain in weight.

  A number of SUV makers have addressed this. The car based Honda CRV and the Toyota RAV4 for example, use monocoque construction, so does the Jeep Grand Cherokee (though it still uses live axles and still rides pretty poorly). The Mercedes M class SUV uses a hybrid of both monocoque and ladder frame as a compromise for light weight and longitudinal strength; the Range Rover and the Hummer use aluminum for body panels since they are not load bearing. SUVs that evolve from cars have superior dynamics than those that evolve from trucks because cars are engineered to have much higher dynamic standards than trucks.

• Poor handling

  As a consequence of using outdated suspension components and construction techniques, SUVs tend to be ill handling. Antiquated four wheel drive systems also compound this significantly. This subject is discussed in detail in my page on Introduction to all wheel drive systems. A ill handling vehicle will negotiate turns at lower speeds, which mean that they will need to shed more speed when entering a turn and consequently burn more fuel reaccelerating back to its original speed. The high weight of an SUV will further compound this.

Q3: Why aren't they making better SUVs?

A: Consumer ignorance and a uncritical media

The average suburban couple with three kids and a dog are unlikely to consult technical journals in evaluating their transportation needs. Apathy towards demanding higher standards of engineering is perpetuated by the automotive media who generally are reluctant to seriously criticise any popular product. Consumers in turn are too feeble minded to realize that there are no technical reasons why SUVs need to be so bad and simply excuse them for "being trucks". It is also my opinion that while Consumer Reports' writing is untainted by advertising money, they do not come across as very competent in evaluating vehicles, even from a consumer's point of view.

SUV defenders will even claim that all this ancient technology is somehow necessary because "they are tougher". This is complete nonsense. A close examination of the Hummer shows that this toughest of all off roaders has a thoroughly modern specification of independent suspension located by upper and lower A arms, ("double wishbones" in colorful marketing terms), coil springs and full time four wheel drive. When the maker of an off road vehicle needs to sell to the US military instead of the ignorant masses, one can see how much more modern and advanced designs can be. If these outdated components are indeed tougher they are not in evidence on the toughest truck on the market, chosen by the military for the toughest possible missions.

Further proof can be found in highly modified pickup trucks that participate in high speed off roading competition. These trucks have components that are not typically found in regular trucks, such as long travel coil springs and independent suspension all round. Notice too that coil springs and independent suspension can be found in the most modest of passenger cars, thus demolishing another fallacious argument that these components cost too much.

The profit margins on SUVs are large because R&D spending is minimal while markups are huge. Very few people realize that Ford spent over US$7 billion in designing the Contour/Mondeo, more money than even what Boeing spent conceiving the 777! Yet this car is sold for less than trucks and SUVs, vehicles that cost between one tenth and one hundredth as much to design. For makers of SUVs this is a heavenly situation, while the real losers are the trend happy consumers who buy them.

This bubble is about to burst, however, because foreign makers can smell the scent of huge profits and are about to offer technologically superior products to break into the market. This is not terribly difficult to do if one considers the fact that they can simply use a lot of the innovations in construction and suspension pioneered in passenger cars. The Mercedes M class SUV is technically respectable regardless of what its social statements may be or how it is going to be marketed. It is heartening to know that a upscale brand of car maker is not stooping down to the levels of irresponsible greed. This vehicle has an important role to play, namely to hopefully put an end to the madness of woefully inferior products and their unbridled commercial success.

The latest SUV, the Dodge Durango needs to be mentioned for being particularly cynical in exploiting the current SUV madness with a vehicle of minimal substance and maximum mark up and profit. Competing SUVs from other makers have shown progress (though only the Mercedes and Hummer can truly be considered modern) with one or more of the following modern components: Usage of coil springs or torsion bars all round, independent front and/or rear suspension, full time four wheel drive and monocoque construction. The Durango offers full time 4WD as an option but in every other regard it fails to advance SUV technology one iota, which is to say it remains firmly embedded in 1950s technology. Fans of the Durango are undoubtedly seduced by its macho styling and Chrysler knows this only too well. Slap on a high enough price tag ($30,000) to show you are a serious player and just reel in the dollars from customers fighting to pay it. It makes one wonder how many Durango customers buy highly priced 286 or 386 PCs dressed in stylish and colorful looking glass cases.

Q4: Aren't SUVs safer?

A: It depends

This ladder frame design is lethal when it comes to accidents. While SUV owners can take comfort that the rigid frame would quite possibly pierce through weaker vehicles with little deformation, they are probably less aware of the fact that collision with something much larger than them would lead to little shock absorption by the frame, thus transmitting the bulk of the impact forces onto their bodies. High g forces of internal organs colliding with the skeleton is a major cause of car accident deaths. If a SUV crashes into a smaller, softer object it comes out ahead. However if it crashes into something infinitely stiff such as a concrete wall it loses out compared to a modern car. This explains the apparent contradiction between real world experience and government crash testing reports.

Another area of safety involves the ability of a vehicle to avoid accidents. This is where superior braking and handling come in to play. While there has been no mention of SUVs' braking ability thus far in this article, it should come as no surprise that given their high weight and modest technical specifications in general that braking performance leaves something to be desired for most SUVs. The result is that while SUVs is probably safer in an impact with a car, they are also more likely to get into an accident in the first place. A very recent development in this issue involves insurance companies attempts at raising rates for SUV owners because of their disproportionate destructiveness in accidents.

Comments?

Credits: This article was inspired by Jay Karolyi's very funny web page on poseur SUVs.

This has historically been a volatile topic, possibly because the purchase of a SUV is usually indefensible on logical grounds!

Notice is given that I intend to publish on my web page all hate mail that I receive in response to this article.

Follow this link to sample the hate mail that I've gotten.

To be fair I am now publishing well argued retorts.