Toe (automotive)
Encyclopedia
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 steer, which is the antisymmetric angle, i.e. both wheels point to the left or right, in parallel (roughly). Positive toe, or toe in, is the front of the wheel pointing in towards the centreline of the vehicle. Negative toe, or toe out, is the front of the wheel pointing away from the centreline of the vehicle. Toe can be measured in linear units, at the front of the tire, or as an angular deflection.
In a rear wheel drive car, increased front toe in provides greater straight-line stability at the cost of some sluggishness of turning response, as well as a little more tire wear as they are now driving a bit sideways. On front wheel drive cars, the situation is more complex.
Toe is always adjustable in production automobiles, even though caster angle
and camber angle
are often not adjustable. Maintenance of front end alignment
, which used to involve all three adjustments, currently involves only setting the toe; in most cases, even for a car in which caster or camber are adjustable, only the toe will need adjustment.
One related concept is that the proper toe for straight line travel of a vehicle will not be correct while turning, since the inside wheel must travel around a smaller radius than the outside wheel; to compensate for this, the steering linkage typically conforms more or less to Ackermann steering geometry
, modified to suit the characteristics of the individual vehicle.
, the interaction between the tire and road surface causes the wheel to tend to want to roll in a curve, as if it were part of a conical surface (camber thrust
). This tendency to turn increases the rolling resistance
as well as increasing tire wear. A small degree of toe (toe-out for negative camber, toe-in for positive camber) will cancel this turning tendency, reducing wear and rolling resistance. On some competition vehicles such as go-kart
s, especially where power is extremely limited and is highly regulated by the rules of the sport, these effects can become very significant in terms of competitiveness and performance.
Automotive engineering
Modern automotive engineering, along with aerospace engineering and marine engineering, is a branch of vehicle engineering, incorporating elements of mechanical, electrical, electronic, software and safety engineering as applied to the design, manufacture and operation of motorcycles, automobiles,...
, toe, also known as tracking, is the symmetric angle
Angle
In geometry, an angle is the figure formed by two rays sharing a common endpoint, called the vertex of the angle.Angles are usually presumed to be in a Euclidean plane with the circle taken for standard with regard to direction. In fact, an angle is frequently viewed as a measure of an circular arc...
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 steer, which is the antisymmetric angle, i.e. both wheels point to the left or right, in parallel (roughly). Positive toe, or toe in, is the front of the wheel pointing in towards the centreline of the vehicle. Negative toe, or toe out, is the front of the wheel pointing away from the centreline of the vehicle. Toe can be measured in linear units, at the front of the tire, or as an angular deflection.
In a rear wheel drive car, increased front toe in provides greater straight-line stability at the cost of some sluggishness of turning response, as well as a little more tire wear as they are now driving a bit sideways. On front wheel drive cars, the situation is more complex.
Toe is always adjustable in production automobiles, even though caster angle
Caster angle
thumb|250px|θ is the caster angle, the red line is the pivot line, and the grey area is the tire.Caster angle or castor 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...
and camber angle
Camber angle
thumb|100px|From the front of the car, a right wheel with a negative camber angleCamber angle is the angle made by the wheels of a vehicle; specifically, it is the angle between the vertical axis of the wheels used for steering and the vertical axis of the vehicle when viewed from the front or...
are often not adjustable. Maintenance of front end alignment
Wheel alignment
Wheel alignment sometimes referred to as tracking, is part of standard automobile maintenance that consists of adjusting the angles of the wheels so that they are set to the car maker's specification. The purpose of these adjustments is to reduce tire wear, and to ensure that vehicle travel is...
, which used to involve all three adjustments, currently involves only setting the toe; in most cases, even for a car in which caster or camber are adjustable, only the toe will need adjustment.
One related concept is that the proper toe for straight line travel of a vehicle will not be correct while turning, since the inside wheel must travel around a smaller radius than the outside wheel; to compensate for this, the steering linkage typically conforms more or less to Ackermann steering geometry
Ackermann steering geometry
Ackermann steering geometry is a geometric arrangement of linkages in the steering of a car or other vehicle designed to solve the problem of wheels on the inside and outside of a turn needing to trace out circles of different radius...
, modified to suit the characteristics of the individual vehicle.
Interaction with Camber
When a wheel is set up to have some camber angleCamber angle
thumb|100px|From the front of the car, a right wheel with a negative camber angleCamber angle is the angle made by the wheels of a vehicle; specifically, it is the angle between the vertical axis of the wheels used for steering and the vertical axis of the vehicle when viewed from the front or...
, the interaction between the tire and road surface causes the wheel to tend to want to roll in a curve, as if it were part of a conical surface (camber thrust
Camber thrust
Camber thrust and camber force are terms used to describe the force generated perpendicular to the direction of travel of a rolling tire due to its camber angle and finite contact patch...
). This tendency to turn increases the rolling resistance
Rolling resistance
Rolling resistance, sometimes called rolling friction or rolling drag, is the resistance that occurs when a round object such as a ball or tire rolls on a flat surface, in steady velocity straight line motion. It is caused mainly by the deformation of the object, the deformation of the surface, or...
as well as increasing tire wear. A small degree of toe (toe-out for negative camber, toe-in for positive camber) will cancel this turning tendency, reducing wear and rolling resistance. On some competition vehicles such as go-kart
Go-kart
thumb|A [[Kart racing|racing kart]] at the [[Commission Internationale de Karting|CIK-FIA]] European Championship 2008A go-kart is a small four-wheeled vehicle...
s, especially where power is extremely limited and is highly regulated by the rules of the sport, these effects can become very significant in terms of competitiveness and performance.