View source for Traction (mechanics)

{{Short description|Force used to generate motion between a body and a tangential surface}}

'''Traction''', '''traction force''' or '''tractive force''' is a [[force]] used to generate [[Motion (physics)|motion]] between a body and a tangential surface, through the use of either [[dry friction]] or [[shear force]].<ref>{{citation|title=Evaluating Vehicle Mobility Using Bekker's Equations|publisher=U.S. Army TARDEC|date=2000|url=https://apps.dtic.mil/dtic/tr/fulltext/u2/a457941.pdf|archive-url=https://web.archive.org/web/20190705150218/https://apps.dtic.mil/dtic/tr/fulltext/u2/a457941.pdf|url-status=live|archive-date=July 5, 2019|first1=Sean|last1=Laughery|first2=Grant|last2=Gerhart|first3=Paul|last3=Muench.}}</ref><ref name="digger">{{cite book|title=Estimating Excavation|publisher=Craftsman Book Co|first=Deryl|last=Burch|date=1997|isbn=0-934041-96-2|page=215|chapter=Usable Power|chapter-url=https://books.google.com/books?id=BxSjyYKWn0wC}}</ref><ref>{{cite web|url=http://hyperphysics.phy-astr.gsu.edu/hbase/frict2.html|title=Friction|website=hyperphysics.phy-astr.gsu.edu|access-date=20 April 2018}}</ref><ref>{{cite web|url=http://metrotrainsimulation.com/definition.html|title=Metro Train Simulation|last=Abhishek|website=metrotrainsimulation.com|access-date=20 April 2018}}</ref>
It has important applications in [[vehicle]]s, as in ''[[tractive effort]]''. 

''Traction'' can also refer to the ''maximum'' tractive force between a body and a surface, as limited by available friction; when this is the case, traction is often expressed as the ratio of the maximum tractive force to the [[normal force]] and is termed the ''coefficient of traction'' (similar to [[coefficient of friction]]). It is the force which makes an object move over the surface by overcoming all the resisting forces like [[friction]], normal loads (load acting on the tiers in negative ''Z'' axis), [[air resistance]], [[rolling resistance]], etc.

==Definitions==
Traction can be defined as:
{{quote|a physical process in which a tangential force is transmitted across an interface between two bodies through dry friction or an intervening fluid film resulting in motion, stoppage or the transmission of power.|Mechanical Wear Fundamentals and Testing|Raymond George Bayer<ref>{{cite book|title=Mechanical Wear Fundamentals and Testing| first=Raymond George|last=Bayer|publisher=CRC Press|isbn=0-8247-4620-1|page=3|chapter=Terminology and Classifications| date=22 April 2004|chapter-url=https://books.google.com/books?id=Q64Kq2HlyucC}}</ref>}}

In vehicle dynamics, [[tractive force]] is closely related to the terms [[tractive effort]] and [[drawbar pull]], though all three terms have different definitions.

==Coefficient of traction==
{{see also|Coefficient of friction}}
[[Image:Aderenza asfalto (traction coefficient).svg|right|thumb|Diagram of the longitudinal coefficient of adhesion (fx) in function of the speed and the weather conditions for the asphalt:<br>
A) dry asphalt<br>
B) Asphalt drainage in wet conditions<br>
C) Asphalt in wet conditions<br>
D) Snow<br>
E) Ice]]
[[Image:Aderenza trasversale stagioni e manti stradali.svg|right|thumb|Change cross tack (Fy) average during the seasons (represented numerically from 1 to 12) and with different road surfaces.<br>
A) Hot Rolled Asphalt<br>
B) Gravel<br>
C) Quartzite<br>
D) Conglomerate cement<br>
E) mastic asphalt<br>
F) Gravel sedimentary (unbound)]]

The '''coefficient of traction''' is defined as the usable force for traction divided by the weight on the running gear (wheels, tracks etc.)<ref>{{cite book|title=Construction Management Fundamentals|first1=Clifford J.|last1=Schexnayder|first2=Richard|last2=Mayo|publisher=McGraw-Hill Professional|date=2003|isbn=0-07-292200-1|page=346|url=https://books.google.com/books?id=vxrvnfEwWMkC}}</ref><ref>{{cite book|title=Theory of ground vehicles|first=Jo Yung|last=Wong|isbn=0-471-35461-9|page=317|chapter=4.1.3 Coefficient of Traction|date=20 March 2001|publisher=John Wiley & Sons |chapter-url=https://books.google.com/books?id=LH8wd8im13AC}}</ref> i.e.:
{{indent|5}}''usable traction'' = ''coefficient of traction'' × ''normal force''.

===Factors affecting coefficient of traction===

Traction between two surfaces depends on several factors:
* Material composition of each surface.
* Macroscopic and microscopic shape ([[wikt:texture|texture]]; [[macrotexture]] and [[microtexture]])
* [[Normal force]] pressing contact surfaces together.
* Contaminants at the material boundary including lubricants and adhesives.
* Relative motion of tractive surfaces - a sliding object (one in kinetic friction) has less traction than a non-sliding object (one in static friction).
* Direction of traction relative to some coordinate system - e.g., the available traction of a [[tire]] often differs between cornering, accelerating, and braking.<ref>{{citation|title=J670 Vehicle Dynamics Terminology|url=https://www.sae.org/standards/content/j670_200801/|publisher=[[SAE International|SAE]]}}.</ref>
* For low-friction surfaces, such as off-road or ice, traction can be increased by using traction devices that partially penetrate the surface; these devices use the shear strength of the underlying surface rather than relying solely on dry friction (e.g., aggressive off-road [[Tire tread|tread]] or [[snow chains]])....

===Traction coefficient in engineering design===
{{Refimprove section|date=April 2009}}
In the design of wheeled or tracked vehicles, high traction between wheel and ground is more desirable than low traction, as it allows for higher acceleration (including cornering and braking) without wheel slippage. One notable exception is in the motorsport technique of [[Drifting (motorsport)|drifting]], in which rear-wheel traction is purposely lost during high speed cornering.

Other designs dramatically increase surface area to provide more traction than wheels can, for example in [[continuous track]] and [[half-track]] vehicles.{{Citation needed|reason= The total traction force is completely independent of surface contact area -- see equation above. Is this misunderstanding (increased surface area allegedly gives more traction) common enough that this article needs to specifically debunk it? |date=July 2017}} A tank or similar tracked vehicle uses tracks to reduce the pressure on the areas of contact. A 70-ton M1A2 would sink to the point of high centering if it used round tires. The tracks spread the 70 tons over a much larger area of contact than tires would and allow the tank to travel over much softer land.

In some applications, there is a complicated set of trade-offs in choosing materials.  For example, soft rubbers often provide better traction but also wear faster and have higher losses when flexed—thus reducing efficiency. Choices in material selection may have a dramatic effect. For example: tires used for track racing cars may have a life of 200&nbsp;km, while those used on heavy trucks may have a life approaching 100,000&nbsp;km.  The truck tires have less traction and also thicker rubber.

Traction also varies with contaminants.  A layer of water in the [[contact patch]] can cause a substantial loss of traction.  This is one reason for grooves and [[Siping (rubber)|siping]] of automotive tires.

The traction of trucks, agricultural tractors, wheeled military vehicles, etc. when driving on soft and/or slippery ground has been found to improve significantly by use of Tire Pressure Control Systems (TPCS). A TPCS makes it possible to reduce and later restore the tire pressure during continuous vehicle operation. Increasing traction by use of a TPCS also reduces [[tire wear]] and ride vibration.<ref>{{cite web|url=https://www.roadex.org/wp-content/uploads/2014/01/TPCS-Trial-RIII.pdf|title=Tyre Pressure Control on Timber Haulage Vehicles: Some observations on a trial in Highland, Scotland|first1=Ron|last1=Munro|first2=Frank|last2=MacCulloch|date=February 2008|publisher=ROADEX III Northern Periphery|access-date=20 April 2018}}</ref>

== See also ==
{{div col}}
* [[Anti-lock braking system]]
* [[Equilibrium tide]]
* [[Friction]]
* [[Force (physics)]]
* [[Karl A. Grosch]]
* [[Rail adhesion]]
* [[Road slipperiness]]
* [[Sandbox (locomotive)]]
* [[Tribology]]
* [[Weight transfer]]

{{div col end}}

== References ==
<references/>

[[Category:Force]]
[[Category:Vehicle technology]]
[[Category:Mechanics]]