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Motorcycle Cornering

Tilting balances vector of forces.
Figure T1




Rollover Stability of
Non-Tilting Vehicles
Three-Wheeler rollover threshold
Figure T2




Comparison to Four-Wheel Car
Comparison to conventional vehicle rollover threshold
Figure T3




GM's Lean Machine
GM's Lean machine
Figure T4



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Project 32 Slalom
Technical Overview

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(Text and graphics courtesy Larry Edwards, Transit Innovations.)

         Project 32 Slalomnegotiates moderate turns like a motorcycle. To appreciate the impact of this on cornering power and rollover stability, consider that a motorcycle has no side-by-side wheels, yet even in the hardest of turns it does not flip over. A motorcycle's cornering power and stability come from its ability to bank into curves with a lean-angle that precisely balances the vector of forces in the turn. 
         The vector of forces might be envisioned as an arrow extending from the vehicle's center of gravity (CG). Normally the arrow points straight down, in line with gravitational pull. But in a turn, there is also centrifugal force acting outward from the CG. The vector of forces then represents a combination of gravitational force acting downward and centrifugal force acting laterally toward the outside. Consequently, the vector assumes an angle or slope that is independent of vehicle weight. On a motorcycle, the rider steers to keep the tire-to-road contact patch aligned with this vector of forces. In other words, a lean (or bank, or tilt) is accomplished to match the angle of this vector of forces. As shown in Figure T-1, this is how the cyclist maintains balance.  

  Stability of Conventional Vehicles

         As explained above, the force vector slopes outward in a turn. But ordinary three- and four- wheel vehicles stay largely perpendicular to the pavement; they do not lean like a motorcycle. So in a turn, the point at which the force vector projects to the pavement shifts outward toward the "rollover threshold," which is an imaginary line extending between the two outermost wheels. (Those two wheels carry all of the weight just before tip-over, and stunt drivers will sometimes navigate a considerable distance on those wheels alone.) Whenever the force vector crosses over this line, overturn follows quickly.
         A four-wheel car has its rollover threshold well to the side, and the CG can be toward the front or toward the rear; it doesn't make a great difference. But a three-wheeler must get all its rollover resistance from the side-by-side wheels at one end of the platform because the single wheel at the other end offers no such resistance. If the CG were midway along the wheelbase, a conventional three-wheeler would need its side-by-side wheels twice as far apart, i.e. the track would have to be super-wide. In actuality most three-wheelers have their CG close to the side-by-side wheels, typically within 30% of the wheelbase, reducing the penalty in track width to the order of 30%. The three-wheeler's triangular footprint causes another penalty in rollover margin that is not present in conventional cars. It's not large, but designers must allow for it. Braking or strong acceleration causes the force vector to shift forward or backward, and this brings the vector a little closer to the rollover threshold. For typical geometry, this adds a few percent to the track width needed for safety comparable to a four-wheeler. Figure T-2 shows the kind of analysis that must be performed to assure a good margin against rollover.

  P32 versus Automobile in Turns

         P32 Slalom takes advantage of its tilt feature to cope with the foregoing challenges. By leaning, it keeps the force vector in line with the body during typical turns, as shown in Figure T-3. This allows to a saving of over 12 inches (305 mm) in width that would otherwise be needed, and it drastically reduces the side-forces experienced by occupants. It also reduces structural weight, aerodynamic drag, and the space needed for parking. 
         Only in aggressive turns beyond the limit of lean does the force vector begin to shift away from Slalom's centerline and toward the outside. But since the magnitude of lateral turn-forces is limited by the tires' ability to grip the road, a large margin of safety against rollover is still maintained. And the side forces on occupants are only a fraction of those experienced in a conventional automobile during an equivalent turn. 

  P32's Proprietary Suspension

         P32 is not the first vehicle with a tilting suspension system. General Motors experimented with a tilting three-wheeler in the '80s, the Lean Machine (Figure T4), and reported fuel economy of 100 mpg, lighting fast acceleration, and superb cornering capabilities. But the vehicle could not seat more than a single occupant, it had a high learning curve (balancing it with pedals took practice), and it was never developed for production. GM's internal market studies, however, indicated a high market potential, even with the vehicle's inherent limitations.
         More recently, Mercedes' experimental Life Jet F300 extended the performance envelope, and it can carry two occupants in tandem. A precursor to the Life Jet was the 3VG prototype developed, tested, and patented by staff of the environmentally-oriented Mother Earth News magazine some 15 years ago. Mechanical arrangement on the two is remarkably similar. Work on the 3VG was discontinued before automated leaning was perfected. 
         But no one has met the challenge of sociable seating for two in a road vehicle that tilts the way it should. P32 meets the challenge with a proprietary new suspension system that is elegantly simple and allows both free and electronically controlled tilt movements.

  Modes of Operation

         P32's suspension allows either of two modes of operation, called Mode A and Mode B. When operating in Mode A, tilt-angle is automatically controlled by an onboard computer. The computer senses lateral acceleration and adjusts the vehicle's tilt angle to keep it precisely in balance with the vector of forces. During Mode A operation, the driver is essentially unaware that anything unusual is taking place. The vehicle steers just like any four-wheel vehicle, and the computer takes care of the rest. In Mode B, the vehicle's tilt angle is controlled by the operator, just as it is with a motorcycle. The vehicle banks into curves to remain in balance with turn forces.  



Slalom is the result of a Transit Innovations development program codenamed Project 32.
Robert Q. Riley was a primary consultant on the project.
Construction plans and completed vehicles are not available.


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