Caster angle

Caster angle

Caster angle is the angular displacement from the vertical axis of the suspension of a steered wheel in a car, bicycle or other vehicle, measured in the longitudinal direction. It is the angle between the pivot line (in a car - an imaginary line that runs through the center of the upper ball joint to the center of the lower ball joint) and vertical. Car racers sometimes adjust caster angle to optimize their car's handling characteristics in particular driving situations.

The pivot points of the steering are angled such that a line drawn through them intersects the road surface slightly ahead of the contact point of the wheel. The purpose of this is to provide a degree of self-centering for the steering - the wheel casters around so as to trail behind the axis of steering. This makes a car easier to drive and improves its straight line stability (reducing its tendency to wander). Excessive caster angle will make the steering heavier and less responsive, although, in racing, large caster angles are used to improve camber gain in cornering. Caster angles over 10 degrees with radial tires are common. Power steering is usually necessary to overcome the jacking effect from the high caster angle.

The steering axis (the dotted line in the diagram above) does not have to pass through the center of the wheel, so the caster can be set independently of the mechanical trail, which is the distance between where the steering axis hits the ground, in side view, and the point directly below the axle. The interaction between caster angle and trail is complex, but roughly speaking they both aid steering, caster tends to add damping, while trail adds 'feel', and returnability. In the extreme case of the shopping trolley (shopping cart in the US) wheel, the system is undamped but stable, as the wheel oscillates around the 'correct' path. The shopping trolley/cart setup has a great deal of trail, but no caster. Complicating this still further is that the lateral forces at the tire do not act at the center of the contact patch, but at a distance behind the nominal contact patch. This distance is called the pneumatic trail and varies with speed, load, steer angle, surface, tire type, tire pressure and time. A good starting point for this is 30 mm behind the nominal contact patch.

History

Arthur Krebs applied the first positive caster angle to automotive front-train in his [http://en.wikipedia.org/wiki/
] English patent : "To ensure stability of direction by means of a special arrangement of fore-carriage, that is to say, to re-establish automatically the parallelism of the two axles of the vehicle when there is no tendency to keep them in any other direction, or after a temporary effort has caused them to diverge from said parallelism. [...] The axle of the fore-carriage is situated a suitable distance behind the projection of the axis of the pivot-pin in order to ensure the stability of direction above referred to."

See also

* Bicycle and motorcycle dynamics
* Camber angle
* Toe (automotive)

External links

* [http://www.240edge.com/performance/tuning-caster.html Caster and Race Car Suspension Tuning]
* [http://www.ozebiz.com.au/racetech/theory/align.html Camber, Caster, Toe - What does it all mean?]
* The 1896 Arthur Krebs automobile English patent: [http://rbmn08.waika9.com/Krebs_brevet_GB_07-09-1896-01.jpgpage 1] [http://rbmn08.waika9.com/Krebs_brevet_GB_07-09-1896-02.jpg2] [http://rbmn08.waika9.com/Krebs_brevet_GB_07-09-1896-03.jpg3] [http://rbmn08.waika9.com/Krebs_brevet_GB_07-09-1896-04.jpg4] [http://rbmn08.waika9.com/Krebs_brevet_GB_07-09-1896-05.jpg5] [http://rbmn08.waika9.com/Krebs_brevet_GB_07-09-1896-06.jpg6] [http://rbmn08.waika9.com/Krebs_brevet_GB_07-09-1896-07.jpg7] [http://rbmn08.waika9.com/Krebs_brevet_GB_07-09-1896-08.jpg8] [http://rbmn08.waika9.com/Krebs_brevet_GB_07-09-1896-09.jpg9] [http://rbmn08.waika9.com/Krebs_brevet_GB_07-09-1896-10.jpg10] [http://rbmn08.waika9.com/Krebs_brevet_GB_07-09-1896-11.jpg11] [http://rbmn08.waika9.com/Krebs_brevet_GB_07-09-1896-12.jpg12] [http://rbmn08.waika9.com/Krebs_brevet_GB_07-09-1896-13.jpg13] [http://rbmn08.waika9.com/Krebs_brevet_GB_07-09-1896-14.jpg14]
* [http://www.bikeforest.com/CAD/caster_angle.php Illustration of a dimension that is sometimes referred to as caster angle on bicycles.]


Wikimedia Foundation. 2010.

Игры ⚽ Поможем решить контрольную работу

Look at other dictionaries:

  • caster angle — the angle in side elevation between the steering axis and the vertical. It is considered positive when the steering axis is inclined rearward (in the upward direction) and negative when the steering axis is inclined forward …   Mechanics glossary

  • caster angle — The inclination or angle that a wheel makes when measuring the distance between the vertical post and the offset of the wheel placement …   Dictionary of automotive terms

  • Caster — A caster (or castor) is an undriven, single, double, or compound wheel mounted on an object to make movement easier. Found on shopping cart, rolling chairs, and material handling equipment, casters may be fixed to roll in one direction, or… …   Wikipedia

  • Caster (disambiguation) — A caster is a type of wheel. Caster can also mean:*Spellgun, a fictional weapon *Caster angle, one of the angles important to automotive suspension *Caster, a machine used in metal casting *A type of sugar *Caster (Fate Stay Night), an anime… …   Wikipedia

  • angle — See ackermann angle approach angle attack angle cam angle caster angle departure angle dwell dwell angle front triangle heading angle …   Dictionary of automotive terms

  • angle — Ackerman steer angle aerodynamic angle of attack ambient wind angle attitude angle bank angle camber angle caster angle course angle heading angle inclination angle phase angle sideslip angle …   Mechanics glossary

  • caster — casterless, adj. /kas teuhr, kah steuhr/, n. 1. a person or thing that casts. 2. a small wheel on a swivel, set under a piece of furniture, a machine, etc., to facilitate moving it. 3. a bottle or cruet for holding a condiment. 4. a stand… …   Universalium

  • caster — cast•er [[t]ˈkæs tər, ˈkɑ stər[/t]] n. 1) a person or thing that casts 2) a small wheel on a swivel, set under a piece of furniture, a machine, etc., to facilitate moving it 3) a bottle or cruet for holding a condiment 4) a stand for such bottles …   From formal English to slang

  • Camber angle — From the front of the car, a right wheel with a negative camber angle …   Wikipedia

  • rate of caster change — the change in caster angle per unit vertical displacement of the wheel center relative to the sprung mass …   Mechanics glossary

Share the article and excerpts

Direct link
… Do a right-click on the link above
and select “Copy Link”