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The necessities of steering geometry

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Ever noticed the steering wheel is misaligned when driving in a straight line or the vehicle pulls to one side? If the answer is yes, this could well be the classic signs of steering geometry problems.
Steering geometry problems generally tend to occur with aging and wearing of components. Nevertheless, they can also be caused by impacting potholes, driving over kerbs and most certainly after the vehicle is involved in a major impact.

Steering geometry, also known as wheel alignment, is the procedure required to check and if necessary, adjust settings when they have deviated away from the manufacturers’ specifications. However, it is also important to bear in mind, if after a steering geometry inspection there is cause for re-alignment, not all geometry values are adjustable and whenever noticeable deviations from pre-defined settings are observed, the only remedial procedure available could well be component replacement.

Here we take a look at a few steering geometry essentials from the technical experts Autodata, to help you in your service, maintenance and diagnostic work.

  • Camber angle

1, Negative camber. 2, Wheel vertical centreline. 3, Positive camber.

Camber angle is the direction the front wheel is leaning relative to the wheel’s vertical centreline and depending on the lean, the camber angle is either positive or negative. To explain in more detail, when observed from the front of the vehicle if the top of the wheel is leaning towards the engine, this is considered as negative camber. Conversely, if the top of the wheel is leaning outwards, this denotes positive camber.

If during a steering geometry examination, the measurements are outside specified tolerances and the camber angle needs amending, look for evidence of elongated holes on the suspension strut tower, or eccentric bolts or washers securing the upper and lower control arms as a means of adjustment. In their absence, scrutiny of the suspension and steering components is essential to check for any potential damage.

  • Castor angle

1, Negative castor. 2, Wheel vertical centreline. 3, Positive castor.

Castor angle refers to the positioning of the steering axis centreline from the wheel vertical centreline when viewed from the side of the vehicle. If the steering axis centreline contacts the road surface ahead of the wheel’s vertical centreline, this is said to be positive castor. Whereas, negative castor implies the steering axis centreline contacts the road surface behind the wheel’s vertical centreline.

Most modern vehicles are designed with positive castor, this in conjunction with the other geometry angles, reduces steering effort and allows the front wheels to self-straighten after a bend is negotiated.

Yet, to avert the vehicle from wandering towards the kerbside because of the apex in present day roads, the average vehicle castor and camber angles, also called cross castor and cross camber, can sometimes be set at slightly opposed settings from left to right depending on which side of the road the vehicle is driven.
In most contemporary vehicles the castor angle is non-adjustable; nonetheless, aftermarket kits do exist that can be tailored to the suspension to permit castor angle alteration.

  • King pin inclination (KPI)

1, King pin inclination

King pin inclination (KPI), also called steering axle inclination (SAI), is achieved differently depending on the suspension arrangement. Typically, with the MacPherson strut type suspension, KPI is attained by leaning the strut. While with the control arm type suspension, the angle of the upper and lower swivel joint pivots is offset.

KPI being non-adjustable can often be left unchecked or overlooked in collision situations. Incorrect KPI caused by worn or damaged suspension components usually results in accelerated tyre wear along with poor directional stability and increased steering effort – particularly when the vehicle is implementing a parking manoeuvre.

  • Toe-in and Toe-out

1, Toe-in. 2, Vehicle centreline. 3, Toe-out.

Toe-in and toe-out, commonly referred to in automotive terms as tracking, is the most frequent steering geometry adjustment undertaken. This is the degree to which the leading edge of the front wheels steer out or in from the vehicle centreline when observed from the front. Wheels pointing in towards the vehicle centreline is called toe-in whereas, wheels pointing away from the vehicle centreline is identified as toe-out.

Making sure the vehicle’s toe-in or toe-out measurement is correct offers many advantages including, improved straight-line stability, better road handling characteristics and more effective steering response.
Should it be necessary, this adjustment will also allow minor tweaks to correct suspension bush disparities caused in production or by accepted wear levels. If adjustment is needed, it is worthwhile remembering to adjust the track rods equally. Although, there is one notable exception to the rule which requires the repositioning of a misaligned steering wheel by minutely adjusting one track rod more than the other.

Guaranteeing precise steering geometry alignment is vital in prolonging the life of tyres and ensuring vehicle stability. Regular steering geometry checks are advisable, and not only when changing worn tyres, steering or suspension components. Checks should also be carried out if subframe removal is required to facilitate gearbox or clutch repair work.

Finally, it must be pointed out that rear wheel geometry can influence steering stability as well. It’s possible to have the front steering geometry angles correctly aligned and still have a vehicle that pulls to one side or displays abnormal tyre wear patterns. In such circumstances, it is imperative rear wheel geometry is also considered when confronted with a vehicle experiencing unusual tyre wear or stability issues.

Autodata has a dedicated Wheel Alignment module to further assist technicians with wheel alignment procedures and help provide workshops with an additional revenue stream. Within the module there is a comprehensive guide which includes information on subjects such as camber angle, ride height, tyres and adjustment procedures together with manufacturer-specific data.

For more information visit www.autodata-group.com

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