Editing Torque (section)

=== Machine torque ===
[[File:Torque Curve.svg|thumb|Torque curve of a motorcycle ("BMW K 1200 R 2005"). The horizontal axis shows the [[rotational speed]] (in [[Revolutions per minute|rpm]]) that the [[crankshaft]] is turning, and the vertical axis is the torque (in [[newton-metre]]s) that the engine is capable of providing at that speed.]]

Torque forms part of the basic specification of an [[engine]]: the [[power (physics)|power]] output of an engine is expressed as its torque multiplied by the angular speed of the drive shaft. [[Internal combustion|Internal-combustion]] engines produce useful torque only over a limited range of [[rotational speed]]s (typically from around 1,000–6,000&nbsp;[[rpm]] for a small car). One can measure the varying torque output over that range with a [[dynamometer]], and show it as a torque curve. [[Steam engine]]s and [[electric motor]]s tend to produce maximum torque close to zero rpm, with the torque diminishing as rotational speed rises (due to increasing friction and other constraints). Reciprocating steam-engines and electric motors can start heavy loads from zero rpm without a [[clutch]].

In practice, the relationship between power and torque can be observed in [[bicycle]]s: Bicycles are typically composed of two road wheels, front and rear gears (referred to as [[sprockets]]) meshing with a [[bicycle chain|chain]], and a [[derailleur gears|derailleur mechanism]] if the bicycle's transmission system allows multiple gear ratios to be used (i.e. [[Single-speed bicycle#Advantages and disadvantages versus multi-speed bicycles|multi-speed bicycle]]), all of which attached to the [[bicycle frame|frame]]. A [[cyclist]], the person who rides the bicycle, provides the input power by turning pedals, thereby [[Crank (mechanism)|cranking]] the front sprocket (commonly referred to as [[Crankset#Chainring|chainring]]). The input power provided by the cyclist is equal to the product of angular speed (i.e. the number of pedal revolutions per minute times 2''π'') and the torque at the [[Axle|spindle]] of the bicycle's [[crankset]]. The bicycle's [[bicycle drivetrain systems|drivetrain]] transmits the input power to the road [[wheel]], which in turn conveys the received power to the road as the output power of the bicycle. Depending on the [[gear ratio]] of the bicycle, a (torque, angular speed)<sub>input</sub> pair is converted to a (torque, angular speed)<sub>output</sub> pair. By using a larger rear gear, or by switching to a lower gear in multi-speed bicycles, [[angular frequency|angular speed]] of the road wheels is decreased while the torque is increased, product of which (i.e. power) does not change.