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250px, θ is the caster angle, the red line is the pivot line, and the grey area is the tire. 250px, Front suspension of a race car, the caster angle is formed by the line between upper and lower ball joint. The caster angle or castor angle is the angular displacement of the steering axis from the vertical axis of a steered
wheel A wheel is a circular component that is intended to rotate on an axle bearing. The wheel is one of the key components of the wheel and axle which is one of the six simple machines. Wheels, in conjunction with axles, allow heavy objects to be ...
in a car,
motorcycle A motorcycle (motorbike, bike, or trike (if three-wheeled)) is a two or three-wheeled motor vehicle Steering, steered by a Motorcycle handlebar, handlebar. Motorcycle design varies greatly to suit a range of different purposes: Long-distance ...
,
bicycle A bicycle, also called a pedal cycle, bike or cycle, is a human-powered or motor-powered assisted, pedal-driven, single-track vehicle, having two wheels attached to a frame, one behind the other. A is called a cyclist, or bicyclist. B ...
, other
vehicle A vehicle (from la, vehiculum) is a machine that transports people or cargo. Vehicles include wagons, bicycles, motor vehicles (motorcycles, cars, trucks, buses, mobility scooters for disabled people), railed vehicles (trains, trams), ...
or a vessel, as seen from the side of the vehicle. The steering axis in a car with dual ball joint suspension is an imaginary line that runs through the center of the upper
ball joint In an automobile, ball joints are spherical bearings that connect the control arms to the steering knuckles, and are used on virtually every automobile made. They bionically resemble the ball-and-socket joints found in most tetrapod animals. ...
to the center of the lower ball joint, or through the center of the kingpin for vehicles having a kingpin. Caster causes a wheel to align with the direction of travel, and can be accomplished either by caster displacement or caster angle. Caster displacement moves the steering axis ahead of the axis of wheel rotation, as with the front wheels of a shopping cart. Caster angle moves the steering axis from vertical. In automobile racing, the caster angle may be adjusted to optimize handling characteristics for a particular venue. This is all connected to the front wheels.


History

Arthur Krebs Arthur Constantin Krebs (16 November 1850 in Vesoul, France – 22 March 1935 in Quimperlé, France) was a French officer and pioneer in automotive engineering. Life Collaborating with Charles Renard, he piloted Timeline of aviation - 19 ...
proposed placing the front axle of a car at a positive caster angle in his UK patent of 1896, entitled ''Improvements in mechanically propelled vehicles.'' In it he stated it was intended "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."


Positive caster angle

The steering axis is angled such that a line drawn through it intersects the road surface slightly ahead of the center of the contact patch of the tire on the pavement by a distance called
trail A trail, also known as a path or track, is an unpaved lane or small road usually passing through a natural area. In the United Kingdom and the Republic of Ireland, a path or footpath is the preferred term for a pedestrian or hiking trail. ...
. The purpose of this is to provide a degree of self-centering for the steering—the wheel casters around in order to ''trail'' behind the axis of steering. This makes a vehicle easier to control and improves its
directional stability Directional stability is stability of a moving body or vehicle about an axis which is perpendicular to its direction of motion. Stability of a vehicle concerns itself with the tendency of a vehicle to return to its original direction in relation ...
(reducing its tendency to wander). Excessive caster angle will make the steering heavier and less responsive, although in racing large caster angles are used for improving camber gain in cornering. Caster angles over 7 degrees with radial tires are common. Power steering is usually necessary to overcome the jacking effect from the high caster angle. Some front-end alignment calls for different right-side and left-side caster. This is called cross caster, and the difference is called the spread. Cross camber may also be specified, but not usually both.


Trail or trailing

The steering axis (the red 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 trail, which is the distance between where the steering axis intersects the ground, in side view, and the point directly below the axle. Caster angle and trail both influence the steering, albeit in different ways: caster tends to add damping, while trail adds "feel" and returnability. The caster wheel on
shopping cart A shopping cart (American English), trolley (British English, Australian English), or buggy (Southern American English, Appalachian English), also known by a variety of other names, is a wheeled cart supplied by a shop or store, especi ...
s are an extreme case the system is undamped but stable, as the wheel oscillates around the "correct" path. The construction has relatively high trail, but no caster, which allows changing of direction with minimal force. In this case the
lateral Lateral is a geometric term of location which may refer to: Healthcare *Lateral (anatomy), an anatomical direction * Lateral cricoarytenoid muscle * Lateral release (surgery), a surgical procedure on the side of a kneecap Phonetics *Lateral co ...
forces at the tire do not act at the center of the
contact patch Contact patch is the portion of a vehicle's tire that is in actual contact with the road surface. It is commonly used in the discussion of pneumatic (i.e. pressurized) tires, where the term is used strictly to describe the portion of the tire’s ...
, but at a point behind the center. This distance is called the
pneumatic trail Pneumatic trail or trail of the tire is a trail-like effect generated by compliant tires rolling on a hard surface and subject to side loads, as in a turn. More technically, it is the distance that the resultant force of side-slip occurs behind th ...
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 center of the contact patch.


Front-end alignment

When the front suspension of a vehicle is aligned, caster is adjusted to achieve a self-centering action in the steering, which affects the vehicle's straight-line stability. Improper caster settings will require the driver to move the steering wheel both into and out of each turn, making it difficult to maintain a straight line.


Two-wheeled vehicles

In the context of bicycles and motorcycles, caster is more commonly referred to as " head angle", "rake angle" or "rake and trail", especially in
American English American English, sometimes called United States English or U.S. English, is the set of varieties of the English language native to the United States. English is the most widely spoken language in the United States and in most circumstances ...
. The terms caster or castor angle are still predominantly used in
British English British English (BrE, en-GB, or BE) is, according to Oxford Dictionaries, "English as used in Great Britain, as distinct from that used elsewhere". More narrowly, it can refer specifically to the English language in England, or, more broadl ...
. Some bicycle constructors refer to the angle subtended by the mechanical trail at the wheel center as caster.


See also

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Bicycle and motorcycle dynamics Bicycle and motorcycle dynamics is the science of the motion of bicycles and motorcycles and their components, due to the forces acting on them. Dynamics falls under a branch of physics known as classical mechanics. Bike motions of intere ...
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Camber angle Camber angle is one of the angles made by the wheels of a vehicle; specifically, it is the angle between the vertical axis of a wheel and the vertical axis of the vehicle when viewed from the front or rear. It is used in the design of steering a ...
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Toe (automotive) In automotive engineering, toe, also known as tracking, is the symmetric angle that each wheel makes with the longitudinal axis of the vehicle, as a function of static geometry, and kinematic and compliant effects. This can be contrasted with ...
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Trail A trail, also known as a path or track, is an unpaved lane or small road usually passing through a natural area. In the United Kingdom and the Republic of Ireland, a path or footpath is the preferred term for a pedestrian or hiking trail. ...
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Vehicle dynamics For motorized vehicles, such as automobiles, aircraft, and watercraft, vehicle dynamics is the study of vehicle motion, e.g., how a vehicle's forward movement changes in response to driver inputs, propulsion system outputs, ambient conditions, air ...
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Caster A caster (or castor) is an undriven wheel that is designed to be attached to the bottom of a larger object (the "vehicle") to enable that object to be moved. Casters are used in numerous applications, including shopping carts, office chairs, ...


References

{{DEFAULTSORT:Caster Angle Automotive steering technologies