Some answers to questions we forgot to askBy Randolph Beckman Illustrations by Bill Dobson
Crazy John drives a 3-wheel car. He likes it because it is practical, not because of all the attention it gets -- which is almost more than he can stand. On freeways he finds himself boxed in on all sides by gawkers. In busy parking lots it draws a crowd of curious touchers. And at every traffic light, the driver of the car stopped next to him will roll down his window, look down, and ask one of just two questions: "What is it?" and "Why does it have only three wheels?" Only the driver of another exotic car can comprehend the inevitability of these questions. Crazy John could answer them from the depths of his sleep. Unless the asker happens to be a very sexy person of the opposite gender, the conversation goes precisely like this: Next Car: "What is it?" John (casually checking out the asker): "I built it myself." Next Car: "Why three wheels?" John (long pause, thoughtful expression): "Why do you need four?" The word "four" is always carefully timed to coincide with the green light, and away drives John, leaving the counter-question echoing in the other drivers' ears. Why indeed? Three points determine a plane. A fourth support is not only redundant, but it creates a "statically indeterminate structure." In other words, it is difficult to predict the actual load distribution among four suspensions. If you have four passengers, you arrange them in a rectangle, put a box around them, and put a wheel at each corner. But what if you only have one or two seats? Why add the air drag of the fourth corner and why add the cost and weight of the redundant wheel, tire, brake and suspension? Even ignoring the physics of the situation, state and federal regulations seem to encourage 3-wheel vehicles. They label them "motorcycles," allow reduced license fees and taxes, and require fewer safety and emission-control features that add cost and weight. Before we take a good look at the disadvantages of 3-wheel cars, let's test our prejudices and preconceptions. Before you go any further, make a mental note of your gut responses to these common beliefs:
As R&T noted in a story called "Are Three Wheels Enough?" in March 1955: "The question that most people want answered is whether the elimination of one wheel is dangerous." Surprisingly, the answers to all these questions finally came not from the world auto industry, but from a federal research program. When the U.S. Department of Energy (DOE) was formed, conservation-minded innovators inundated it with far-out ideas, requesting research funds and loan guarantees for their projects. Among these ideas were many versions of the electric 3-wheel commuter car. DOE, not wanting to risk a Golden Fleece Award, said in effect that there must be something wrong with the idea because not only did Detroit not build 3-wheel automobiles, Japan didn't either. Walt Dippold at DOE called the Department of Transportation and asked the National Highway Traffic Safety Administration to look into it. Dr. Joe Kanianthra at NHTSA soon discovered that while it was possible that 3-wheel cars were less stable, there wasn't any hard evidence to prove it. So NHTSA let out a small contract for someone to run down the facts. Fortunately, the research was done by an experienced car enthusiast. Paul Van Valkenburgh is a former auto writer, race car engineer, builder and driver. When he did the project at Systems Technology, Inc., he tested eight 3-wheelers, for with a single front wheel and four with a single rear wheel, and he compared them to four roughly equivalent 4-wheelers. With 20/20 hindsight, R&T probably could have come to the same conclusions using its own testing procedures. But given federal funding, a wider range of 3-wheel examples to select from and far more extensive tests, the results are not only more comprehensive, but essentially bulletproof. The final technical research report runs to more than 100 pages of explanation, graphs and details, but I can translate most of it from sophisticated tech-talk and bureaucratic dullness into plain English. The first concern of those who insure, if not those who drive, 3-wheelers is the possibility of turning over. A theoretical dissection of the problem turns out to be less important than simple observation that one test 3-wheeler had better overturn resistance than the best 4-wheeler, a Fiat X1/9. Sound ridiculous? It isn't if you look at the numbers. If you get the center of gravity (cg) low enough and close enough to the 2-wheel end, and have a wide enough track, you can build a 3-wheeler that won't overturn in the most extreme maneuvers on flat pavement. Naturally, if you retain the identical cg location and track width of a given 4-wheeler, three wheels will provide a lower overturn safety margin. But these design parameters are never cast in stone and, in limited production, you can put the cg and track just about anywhere you choose. To be honest, the 3-wheeler with the best overturn resistance was sans, body, and therefore had an unrealistically low cg. But another 3-wheeler, fully equipped was still almost as overturn resistant as the Fiat. Also, I should note that one of the 3-wheelers could have been overturned in testing and in fact was by its owner. It had a high, rearward cg and narrow track, and at 0.6g it would take up on two wheels, precisely as predicted from static tests. To a skilled driver, this was no problem, and it could be balanced there like a motorcycle as long as you had enough time, space and presence of mind. But now I come to a philosophical question: Is a vehicle that will overturn during hard cornering "unsafe"? Perhaps it is even more of a legal question, as there are some current lawsuits trying to determine if Jeeps and other recreational vehicles are "unsafe" in overturn. If overturn in cornering is unacceptably dangerous, what about tall, narrow, commercial vehicles, such as loaded tractor-trailers, which can go into oversteer at about 0.3g and overturn at about 0.6g? Even if I assume that engineers of 3-wheel automobiles would optimize the overturn design limits, I still need to know how their stability and handling compare. I will ignore the myriad ways of analyzing stability (static, dynamic, transient, steady-state, oscillatory, divergent, covergent) and just consider oversteer/understeer. Most car enthusiasts have an intuitive feel for these terms and respect for the potential dangers of unexpected oversteer at the limit. As it turns out, there was a strong distinction between 4-wheel cars and single-front-wheel cars -- but not single-rear-wheel cars. Although the sample was admittedly small, the inescapable conclusion is that all single-front wheel 3-wheelers will oversteer at their limit of adhesion. Conversely, all single-rear-wheel cars had strong understeer at the limit. And in neither case could the opposite effect be created, in spite of all the chassis tuning. An accompanying graph shows how all three groups of cars compare in steer angle versus lateral acceleration on the skidpad. Conventional 4-wheelers have a constantly increasing steer angle as speed or g-forces increase. The same is try (to a potentially greater degree) with single-rear 3-wheelers. But the steering on single-front 3-wheelers levels off and then decreases with increasing g's, requiring counter-steering to avoid a spin.
If these results were difficult to predict, they are easy to explain after the fact. Oversteer/understeer is a result of many vehicle dynamics factors, such as tire size, type and pressure, suspension characteristics, steering compliance, weight distribution and roll resistance distribution. With all other factors being roughly equal, the end of the car with the greatest weight and greatest roll resistance (springs or anti-roll bar) will have the lower limit of adhesion. Put another way, a nose-heavy car or one with lots of its roll resistance up front will understeer, and vice versa for rear/oversteer. The implications are obvious. With a single front wheel, most of the weight is at the rear, not to mention all of the roll resistance. On some of these 3-wheelers, extremes of state-of-the-art chassis tuning tricks were attempted, with negligible effect. Regardless of large tire and pressure differences, camber changes and changes in weight distribution that were reasonable from an overturn standpoint, they still oversteered. But again, is oversteer unacceptable? There are a lot of naive folks driving around out there in oversteering production sedans (because of low tire pressures) who will never encounter that limit even in an emergency. The fact of oversteer is easy to obtain; the implications are more than a little speculative. So overturn can be avoided, and single-rear 3-wheelers have a comfortable degree of understeer. But what about handling -- how do they feel? Professional researchers resist being quoted on subjective impressions, but at least here they report a numerical value for handling response. These yaw response times represent the time required for a car to reach a steady cornering condition after a quick steering wheel input. Ordinary 4-wheelers range form perhaps 0.30 seconds for a large car with soft tires, to 0.15 sec for sports cars. All of the 3-wheelers were below 0.20 sec, to as low as 0.10 sec. and that is quick. The answer is not in the number of wheels, or their location, but in mass, tires and polar movement. The effect of polar movement has been considered for years, but this is the first report I have seen with actual figures. The 3-wheelers had, on the average, about 30 percent less polar movement (normalized for weight) than 4-wheelers, because of centralized masses and less overhang. And the ones with the lowest figures and best tires had the quickest response. Van Valkenburgh says that some of the 3-wheelers had yaw characteristics akin to those of formula cars. All of the rest of the tests showed no measurable difference between 3- and 4-wheelers. The testers subjected the cars to crosswinds, bumps in turns, braking in turns, free steering return, lane changes and off-camber turns, and although there were many vehicle-specific problems (as you would expect with one-off prototypes) the number of wheels was unimportant. However, all this research, original as it was, had a relatively narrow orientation. There is more to a practical automobile than handling and braking. Although most design factors, such as quality, esthetics, value, reliability, comfort, etc. are unrelated to the number of wheels, there are still a few other important considerations. It was assumed that a 3-wheeler could be inherently more aerodynamic than a 4-wheeler. As a part of this research, two of the more complete 3-whllers were coast-down tested for aerodynamic drag. In both cases, the product of the frontal area and drag coefficient (A x Cd) was about 8.0, as compared to 8.4 for a Plymouth Horizon or 8.8 for a Honda sedan or Ford Fiesta. Certainly this is an improvement, but not as good as the 6.5-7.0 for comparable low-drag prototype 4-wheelers. This peculiarity partly results from the necessary increase in frontal area on a wider-track 3-wheeler will not automatically be less. Another theoretical justification for a 3-wheel chassis is that it can be lighter because it requires no torsional resistance as one wheel rises and falls. In fact, however, one builder reported that he had a torsional stiffness problem on one of his 3-wheelers. While the single rear suspension was strong enough to resist the camber forces in hard cornering, the frame was not and it not only flexed in torsion, but allowed disturbing vibrations. Ride quality can be a problem as well, not in vertical ride rate which is independent of the number of wheels, but in 1-wheel rates. Because the 2-wheel end of the car must resist all roll, roll resistance requires more stiffness than necessary for a soft ride. And, as Van Valkenburgh discovered, roll damping suddenly becomes an important consideration when all of it has to be handled at one end. Finally, all this research ignores the question of impact protection, and rightly so, because it's hard to imagine the number of wheels being a factor. However, the intent of a 3-wheeler is to have reduced mass, and a side effect of three wheels is a "motorcycle" classification, which means that federal impact regulations do not apply. Again, it's a philosophical question. Is a lightweight 3-wheeler a safer motorcycle, or a less save automobile? NHTSA is currently struggling with the problem. Subjectively speaking, I can't see why a front-engine front-wheel-drive 3-wheeler is inherently any less safe in frontal impact than a comparable 4-wheeler, although crush space or diagonal impacts on a corner with no wheels could be a problem. Now that fwd subcompacts are flourishing, it would seem natural to adapt them to three wheels, either in original design or aftermarket kits. The technical problems involved in producing a practical 3-wheel car do not appear to be overwhelming. And the potential benefits of cost and fuel conservation would seem to make it worthwhile. As Van Valkenburgh succinctly put it, "a properly engineered 3-wheel car can be made as stable as a properly-engineered 4-wheel car." But recall your initial reaction to the questions of stability and handling. The big problem is psychological -- market acceptance of a radical change. Even if the 3-wheel layout were twice as good, I wouldn't speculate about its future. One of the most powerful forces on earth is the inertia of an existing idea. But if 3-wheelers ever have a chance to make it, their time is now. |
