Chapter 42 Front Axles and Vehicle Alignment Factors

scrub radius

the difference between the intersection of a point on the tire contact patch between true vertical and the KPI angle. Scrub radius forms the pivot point for the tire contact with the road.

skew

the difference in distance between any axle end and the perpendicular centerline. Also called axle setback or setback.

axle setback

the difference in distance between any axle end and the perpendicular centerline. Also called setback or skew

thrust line

the direction in which the rear wheels are pointing

axle perpendicularity

when the axles are square with the vehicle frame

axle parallelism

when the rear wheels of a vehicle follow the front wheels in a parallel manner

positive camber

when the tires are closer together at the bottom and farther apart at the top

negative camber

when the top of the tire is closer to the center of the vehicle than the bottom of the tire

geometric centerline alignment

an alignment method that establishes a vehicles centerline by placing a line from the midpoint of the front axle and midpoint of the rear most axle

frame centerline alignment

an alignment method that uses the vehicle frame and not its axles as the reference point for making alignment adjustments

steering arm

an arm that extends from the steering knuckle. The tie-rods connect to these arms in order to steer the wheels.

drop arm

an arm that transfers the steering box out-put shaft motion to the steering linkage by converting rotational movement into liner motion. Also called the pitman arm

steering axle

an axle that allows the vehicle to turn

live (drive) axle

an axle that does not supply power to the wheels

non-drive (dead) axle

an axle that does not supply power to the wheels

tie-rod end

articulating ball and socket joints attached to each end of the tie-rod

kingpin inclination angle (KPI)

The angle formed between true vertical and the angle of the kingpin. Also called steering axis inclination angle.

steering axis inclination angle (SAI)

The angle formed by an imaginary line running through the upper and lower steering pivots relative to vertical as viewed from the front. The angle formed between true vertical and the angle of the kingpin. Also called kingpin inclination angle (KPI).

steering stops

bolts used to limit the turning angle of the steering knuckle

negative scrub radius

a condition in which the point of center contact between the road surface and the tire and point where the steering axis centerline contacts the road surface intersect above the road surface

positive scrub radius

a condition in which the point of center contact between the road surface and the tire and the point where the steering axis centerline contacts the road surface intersect below the road surface

toe-in

a condition that exists when, as seen from above, the wheels are closer together at the front and farther apart at the rear

toe-out

a condition that exists when, as seen from above, the wheels are closer together in the rear and farther apart at the front

drag ling

a connecting linkage that transfers movement of the pitman arm to the upper steering arm

steering knuckle

a device that connects the front wheel to the suspension; it pivots on the top and bottom, thus allowin the front wheels to turn

steering geometry

a geometric arrangement of linkages in the steering of a vehicle designed to solve the problem of keeping the wheels properly oriented through various positions of the steering and suspension systems

toe-out on turns

a geometric steering concept that the inner wheel should have a smaller turning angle than the outside wheel. Also called Ackermann angle.

turning radius

a measure of how small a circle the vehicle can turn in when the steering wheel is turned to the limit

toe

a measurement of how much the front wheels are turned in or out from a straight-ahead position. The angle is referenced from a position directly above the tires and facing forward

lift axle

a non-drive (dead) axle that can be mechanically raised and lowered to meet requirements regulated for maximum axle weight loads

trailer axle

a non-drive axle used by trailers and which generally has no steering linkage unless it is a self steering axle

kingpin

a pin that connects each steering knuckle to the solid I-beam axle

pitman arm

a pitman arm converts the steering gear sector shaft movement to a sweeping arc resembling linear movement. Also called a drop arm.

tag axle

a rear non-drive axle mounted behind the drive axle.

pusher axle

a rear, non-drive rear mounted axle, ahead of the drive axle

tie-rod

a steering component that transfers linear motion from the steering box to the steering arms at the front wheels

solid I-beam

a type of solid steering axle named for its forged I-beam design

Caster

the angle formed through the wheel pivot points when viewed from the side in comparison to a vertical line through the wheel

included angle

the angle of camber added or subtracted to the steering axis inclination angle. This is the angle of the pivot points in relation to the camber angle of the wheel

Ackermann angle

the angle the steering arms make with the steering axis, projected toward the center of the rear axle. Also called toe-out on turns

frame angle

the angle the vehicles frame makes with regard to horizontal measurement to the ground.

positive frame angle

the condition where the vehicles rear is higher than the front

negative frame angle

the condition where the vehicles rear is lower than the front

scrub angle

the distance between two imaginary points on the road surface--the point of center contact between the road surface and the tire, and the intersecting point where the steering axis centerline and tire centerline contact the road surface. The difference between the intersection of a point on the tire contact patch between true vertical and the KPI angle.

setback

the distance one wheel is set back from the wheel on the opposite side of the axle.

Ackermann principle

the geometric alignment of linkages in a vehicles steering such that the wheels on the inside of a turn are able to move in a different circle radius than the wheels on the outside

wheel alignment

the positioning of the tires relative to the vehicle. Also called tracking

tracking

the positioning of the tires relative to the vehicle. Also called wheel alignment

thrust angle

the relationship between the centerline of the vehicle and the angle of the rear tires

axle

the shaft of the suspension system to which the tires and wheels are attached; used to transmit driving torque to the wheels

camber

the side to side vertical tilt of the wheel. It is viewed from the front of the vehicle and measured in degrees.

caster shimmy

the small, rapid, side to side movement of the steering wheel resulting from excess positive caster