Two front A configuration of two wheels in the front and one wheel at the back presents two advantages: it has improved
aerodynamics, and that it readily enables the use of a small lightweight motorcycle powerplant and rear wheel. This approach was used by the
Messerschmitt KR200 and BMW
Isetta. Alternatively, a more conventional front-engine, front wheel drive layout as is common in four-wheeled cars can be used, with subsequent advantages for transversal stability (the center of mass is further to the front) and traction (two driven wheels instead of one). Some vehicles have a front engine driving the single rear wheel, similar to the rear engine driving the rear wheel. The wheel must support acceleration loads as well as lateral forces when in a turn, and loss of traction can be a challenge. A new tadpole configuration has been proposed with a rear engine driving the front wheels. This concept (Dragonfly Three Wheeler) claims both stability and traction (two driven wheels), as well as a unique driving experience. With two wheels in the front (the "tadpole" form or "reverse trike") the vehicle is far more stable in braking turns, but remains more prone to overturning in normal turns compared to an equivalent four-wheeled vehicle, unless the center of mass is lower and/or further forward. Motorcycle-derived designs suffer from most of the weight being toward the rear of the vehicle. For lower
wind resistance (which increases
fuel efficiency), a teardrop shape is often used. A teardrop is wide and round at the front, tapering at the back. The three-wheel configuration allows the two front wheels to create the wide round surface of the vehicle. The single rear wheel allows the vehicle to taper at the back. Examples include the
Aptera (solar electric vehicle) and
Myers Motors NmG.
Two rear Having one wheel in front and two in the rear for power reduces the cost of the steering mechanism but greatly decreases lateral stability when cornering while braking. When the single wheel is in the front (the "delta" form, as in a child's pedal
tricycle), the vehicle is inherently unstable in a braking turn, as the combined tipping forces at the
center of mass from turning and braking can rapidly extend beyond the triangle formed by the
contact patches of the wheels. This type, if not tipped, also has a greater tendency to spin out ("swap ends") when handled roughly.
Lateral stability Source: The disadvantage of a three-wheel configuration is that lateral stability is lower than with a four-wheeled vehicle. With any vehicle, an imaginary line can be projected from the vehicles centre of mass to the ground, representing the force exerted on the vehicle by its mass. With the vehicle stationary, the line will be vertical. As the vehicle accelerates, that imaginary line tilts backward, remaining anchored to the centre of mass the point at which the line intersects the ground moves backward. As you brake it moves forward, with cornering it moves sideward. Should the point at which this line intersects the ground move outside of the boundary formed by connecting the tyre contact patches together (a rectangle for a four-wheeled car, or a triangle for a trike) then the vehicle will tip and eventually fall over. This is true for any vehicle. With all vehicles it is critical that the vehicle should be engineered to slide before this point of instability is reached. This can be achieved in several ways: • by placing the
center of mass closer to the ground • by placing the center of mass closer to the axle with two wheels (for three wheelers) • by increasing the
track width • by limiting the grip provided by the tyres, such that the vehicle loses adhesion before it starts to tip. • By tilting some or all of the vehicle as it corners. In the case of a three-wheeled ATV, tipping may be avoided by the rider leaning into turns. == Tilting option ==