Wheels incur mechanical and other disadvantages in certain environments and situations that would represent a decreased fitness when compared with
limbed locomotion. These disadvantages suggest that, even barring the biological constraints discussed above, the absence of wheels in multicellular life may not be the "missed opportunity" of biology that it first seems. In fact, given the mechanical disadvantages and restricted usefulness of wheels when compared with limbs, the central question can be reversed: not "Why does nature not produce wheels?", but rather, "Why do human vehicles not make more use of limbs?" The use of wheels rather than limbs in most
engineered vehicles can likely be attributed to the complexity of
design required to construct and
control limbs (see
robot locomotion), rather than to a consistent functional advantage of wheels over limbs.
Efficiency Rolling resistance N, with a component opposing the motion. (
W is the weight of the wheel plus the supported portion of the vehicle;
F is a propulsive force;
r is the wheel radius.) Although stiff wheels are more
energy-efficient than other means of locomotion when traveling over hard, level
terrain (such as
paved roads), wheels are not especially efficient on soft terrain such as
soil, because they are vulnerable to
rolling resistance. In rolling resistance, a vehicle loses energy to the
deformation of its wheels and the surface on which they are rolling. Smaller wheels are especially susceptible to this effect. Softer surfaces deform more and recover less than firm surfaces, resulting in greater resistance. Rolling resistance on medium to hard soil can be five to eight times greater than on concrete, and on sand it can be ten to fifteen times greater. While wheels deform the surface along their
entire path, limbs induce only a small, localized deformation around the region of foot contact. Rolling resistance led to wheels being abandoned throughout a large region at least once in history. During the time of the
Roman Empire, wheeled
chariots were common in the Middle East and North Africa. Yet when the Empire collapsed and its roads fell into disrepair, wheels fell out of favor with the local populations, who turned to
camels to transport goods in the sandy desert climate. In his book ''
Hen's Teeth and Horse's Toes'', Stephen Jay Gould explains this curiosity of history, asserting that, in the absence of maintained roads, camels required less manpower and water than a wheeled cart pulled by
oxen.
Efficiency of aquatic locomotion When moving through a fluid, rotating systems carry an efficiency advantage only at extremely low
Reynolds numbers (i.e. viscosity-dominated flows) such as those experienced by bacterial flagella, whereas
oscillating systems have the advantage at higher (
inertia-dominated) Reynolds numbers. Whereas ship propellers typically have
efficiencies around 60% and aircraft propellers up to around 80% (achieving 88% in the human-powered
Gossamer Condor), much higher efficiencies, in the range of 96–98%, can be achieved with an oscillating flexible
foil like a fish tail or bird wing.
Traction Wheels are prone to
slipping—an inability to generate
traction—on loose or slippery terrain. Slipping wastes energy and can potentially lead to a loss of control or becoming stuck, as with an automobile on mud or snow. This limitation of wheels can be seen in the realm of human technology: in an example of
biologically inspired engineering, the forest machinery company
Timberjack tested
tree harvesters with six legs from 1999 to 2011. These
walking machines allowed access to terrain too challenging for wheeled vehicles to navigate.
Tracked vehicles suffer less from slipping than wheeled vehicles, owing to their larger contact area with the ground—but they are less efficient and more mechanically complex.
Obstacle navigation Work by vehicle engineer
Mieczysław G. Bekker implies that the distribution of irregularities in natural terrains is
log-normal; that is, small obstacles are far more common than larger ones. Thus, obstacle navigation is a challenge for locomotion in natural terrains
at all size scales. The primary means of obstacle navigation are to go around obstacles and to go over them; each has its attendant challenges.
Going around Anatomist Michael LaBarbera of the
University of Chicago illustrates the poor maneuverability of wheels by comparing the turning radii of walking and
wheelchair-using humans. As
Jared Diamond points out, most biological examples of rolling are found in wide open, hard-packed terrain, including the use of rolling by
dung beetles and
tumbleweeds.
Going over Wheels are poor at dealing with vertical obstacles, especially obstacles on the same scale as the wheel itself, and may be unable to climb vertical obstacles taller than about 40% of the wheel height. Because of this limitation, wheels intended for rough terrain require a larger diameter. In addition, without
articulation, a wheeled vehicle can become stuck on top of an obstacle, with the obstacle between the wheels, preventing them from contacting the ground. Limbs, in contrast, are useful for climbing and are equipped to deal with uneven terrain. With unarticulated wheels, climbing obstacles will cause the body of a vehicle to tilt. If the vehicle's center of mass moves outside of the
wheelbase (side to side) or axle track (front to back), the vehicle will be statically unstable, and will tend to tip over. At speed, a vehicle can become dynamically unstable—that is, it can be tipped over by an obstacle smaller than its static stability limit, or by excessive acceleration or tight turning.
Suspension systems often mitigate the tendency of wheeled vehicles to overturn, but unlike fully articulated limbs, they do not provide any ability to recover from an overturned position.
Versatility Limbs used by animals for terrestrial locomotion are frequently also used for other purposes, such as
grasping,
manipulating,
climbing,
branch-swinging,
swimming,
digging,
jumping,
throwing,
striking, and
grooming. Without articulation, wheels cannot perform these functions. ==In fiction and legend==