Steering Geometry and Four-Wheel Alignment

Steering Geometry and Four-Wheel Alignment

Safety Statement

Information and Administration: The course will start each day at 9.00am and finish at 5:00pm each day. Lunch and refreshments will be provided at the times advised by the training instructor. Workshop Safety • It will be expected that all necessary workshop health and safety procedures are followed e.g. the wearing of suitable work wear, safety footwear, eye and ear protection when in the practical workshops is mandatory. We recommend that barrier cream and workshop gloves are used at all times. • Ensure that you are fully aware of the emergency stop procedure for any of the rotating equipment used during the course. While working on rotary test equipment or engines, please ensure that all loose clothing is secure and can not get caught in the equipment or engine parts.

Objectives

• Understanding wheel alignment terminology • Understanding what influences wheel alignment • Understanding reset and calibration of wheel alignment

Introduction

Fire Escape Smoking Areas Toilets Mobile Phone, please switch off or on silent

Group introductions…..

Overview of Steering Geometry

Can you name 9 factors that effect wheel alignment?

1. Tie-rod ends 2. Steering linkage 3. Wheel bearings 4. Suspension components 5. Tyre pressures 6. Vehicle ground clearance 7. Tyre run out 8. Difference left and right wheelbase 9. Wheel alignment being carried out on uneven ground

Overview of Steering Geometry

A driver complains of excessive free play in the steering. Which components would you check?

1. Worn steering linkage 2. Worn steering rack, or incorrect adjustment of steering gear 3. Worn wheel bearings 4. Steering wheel not fitted properly

Overview of Steering Geometry Doing the actual corrections of wheel alignment is a fairly simple task and, in many instances, it is easily accomplished by some mechanical adjustments. However, Wheel Alignment diagnosis is not so straightforward, and you will need to understand the interaction between the wheel alignment angles as well as the influence the various angles have on each other.

In addition, there are also external factors one will need to consider.

Overview of Steering Geometry Before we look at steering geometry, we need to consider the other checks we need to carry our before successful wheel alignment can happen. • Road test, before & after

• All linkages/bushes for wear or damage • Brake bind • Check for accident damage • Ride height, broken springs etc

• Tyre pressures • Tyre condition • Tyre tread depths • Wheel bearing condition

Tyres The main functions of a vehicle’s tyres include supporting the vehicle load, transmitting traction and braking forces to the road surface, absorbing road shocks, and changing and maintaining the direction of travel. One of the main purposes of wheel alignment is to correctly contact the tyre with the road surface in such a way that tyre life is prolonged. As a wheel alignment specialist the tyres are usually the first item checked, they present us with key information and guidance before setting up alignment. Wheel alignment adjustments shouldn’t be made with defective or damaged tyres fitted to the vehicle.

Tyres

Car tyre pressure is a measurement of how much air is in the vehicles tyre, and ensures the tyres wear evenly and maintain the correct level of grip on the road surface. This is commonly expressed as PSI, or pounds-per- square-inch. Bar and more recently measured in K.P.A (Kilopascals) This needs to be checked before carrying out alignment because camber, toe and ride height will be affected.

Tyres

Direct tyre pressure monitoring

Car tyre pressure monitoring in most current designs of direct TPMS, a small electronic assembly which is rugged enough to be mounted inside a tyre, measures the pressure using a microelectromechanical system pressure sensor and then transmits this and other information to the vehicle. A direct TPM sensor fitted to the back of the valve stem on each wheel

Tyres

In-direct pressure monitoring

In-direct systems don't rely on sensors to do the work, or at least not pressure sensors. The systems rely on wheel speed sensor data to interpret the size of a tire based on how fast it rotates, a small tire would rotate faster than a larger tire, and an underinflated tire is smaller than one with proper inflation. Draw backs include the wheel needs to be moving for the system to work.

Tyres

Tread-depth Sufficient tyre tread depth is crucial in allowing tyres to maintain a firm grip on the road surface, particularly on wet roads as the deep grooves channel water away. Currently the UK tread depth legal limit is 1.6mm across the central three quarters of the usable tread. A simple way to find this is to divide the width by 8 and take this measurement from the edges of the tyre tread. Just remember a tyre at 2mm only has 0.4mm of usable tread left.

Tyres

Central tyre wear Over-inflating tyres causes the sidewalls and tread of the tyre to become harder than normal. This can reduce the traction and performance of the tire, as well as cause uneven tyre wear. When the pressure is increased beyond the recommended pressure, the contact patch of the tire shrinks. The centre of the tread wears out faster than the edges of the tread.

Tyres

Edge(both) tyre wear Under inflated tyres are one of the leading causes of tyre failure. Low pressure means that too much of the tyre touches the road, increasing friction and heat greatly reducing the life of the tyres. A tyre doesn’t need to look a little flat to be under inflated. The edges of the tread wear out faster than the centre of the tread

Tyres

Edge tyre wear The primary cause of the vehicle's tyres wearing on the inside, includes the inside of the tread on the front tyres wearing faster than the centre or outside edge. It is usually directly related to a Camber problem.

Tyres

Edge tyre wear The wear on the outside of the tyre has various contributing factors, worn ball joints, worn bushings, bent suspension components all cause a change to the castor set up, and of course could be due to excessive positive camber. This could also be due to over-enthusiastic cornering by the driver.

Ride Height Most wheel alignment data supplied by the manufacturer is given with the chassis to ground clearance at a specified height. The height is usually with the vehicle unloaded. The caster, camber, toe angles and steering axis inclination are all impacted by vehicle load

Wheel Alignment Specifications

Wheel Alignment Specifications are normally given in angular values of degrees and minutes A circle consists of 360 segments called Degrees , symbolized by the indicator ° Each Degree again has 60 segments called Minutes symbolized by the indicator ‘.

Overview of Steering Geometry Angular values are popular in that they are not reliant on wheel sizes (rims), nevertheless in certain instances the values of TOE are given in Millimetres. When this format is used then it is imperative that the wheel sizes (rim) are known, simply for the fact that as one moves away from a given point on the wheel (normally the centre) then the millimetre value will increase the further one moves off the centre. If the wheel (rim) size is used, then the point of measurement becomes fixed.

For example, a TOE value of 1mm on a wheel size of 10” = 10’ (minutes of angle) yet on a 15” wheel it represents 8’ (minutes of angle)

Overview of Steering Geometry Vehicle manufacturers normally quote the Wheel Alignment specifications as an “Actual value with a plus or minus tolerance”. For example, Toe-in , Target data is +2mm +/- 1mm This means that the ideal setting is +2mm, but a tolerance of 1mm is allowed to the negative and 1mm to the positive, So the Spread of Toe is from +1mm to +3mm on the Positive side. Tyre wear should not be excessive if the Toe is set between these two extremes. However, the ideal setting is at +2mm. The only prerequisite is that both wheels should be set at the same value. How did we get to the extremes of +1mm to +3mm? – Simple +2mm taking away -1mm = +1mm (the one extreme) and +2mm added to +1mm = +3mm (the other extreme).

Overview of Steering Geometry This rule also applies for degrees and minutes. However, one must be cautious in that when doing the calculation one needs to bear in mind that there are 60’ (minutes) to 1 ° (degree) So let’s look at an example being Camber = Target data is +30’ +/- 45’ The two extremes would be –15 (being +30’ taken away from - 45’) and +75’ (being +30’ added to +45’), however there are 60’ to 1 ° so +75’ would be +1 ° 15’. Thus the two extremes would be –15’ to +1 ° 15’ so the spread of Camber is from –15’ to +1 ° 15’ and if kept in this area it should not cause excessive tyre wear. However, the ideal setting is at +30’

Why Do We Need Wheel Alignment?

A vehicle’s wheels need to be aligned for two basic reasons. 1. To provide good road handling 2. To ensure good even tyre wear

Why Do We Need Wheel Alignment? Good road handling means that the vehicle moves in a straight line without the continual correction of the steering wheel. The vehicle must behave in a predictable manner when the brakes are applied, or a corner is taken. Good even tyre wear means all four tyres will last as long as possible. This can be achieved through “motion balance” which occurs when the wheels roll without scuffing, dragging or slipping under all road operating conditions. This gives greater safety in driving, easier steering, longer tyre life and less wear on the parts that make up the front and rear suspension of the vehicle. The ideal is for all tyres to always be in contact with the road, always rolling, never skidding forward or sideways. Modern suspensions, properly aligned, can come remarkably close in achieving this.

Overview of Steering Geometry

Toe In the automotive world, toe 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). Negative toe, or toe out, is the front of the wheel pointing away from the centreline of the vehicle. Positive toe, or toe in, is the front of the wheel pointing towards the centreline of the vehicle. Toe can be measured in linear units, at the front or rear of the tire, or as an angular deflection.

Overview of Steering Geometry

Toe What are three major areas of performance, toe settings can effect?

1, Tyre wear 2, Straight line stability 3, Corner handling characteristics

Overview of Steering Geometry The toe angle can take two forms, either toe-in or toe-out. Toe-in was introduced on early vehicles, to cancel out camber thrust generated through the use of positive camber. On modern vehicles the use of positive camber has been removed, and either negative or zero camber has taken its place.

Negative camber helps improve vehicle handling and reduces camber thrust; so on modern vehicles toe angle is used for straight-line stability.

Toe angle can be expressed as the difference in distance between the front of the front tyres and the rear of the front tyres. Toe-in is when the distance between the front of the tyres is less than the distance between the rear of the tyres. Toe-out is the opposite of toe-in.

Toe Angle

Toe In (front of the wheels point inwards)

Toe Out (front of the wheels point outwards)

Toe-In – Toe-Out

The vehicle toe can be measured as either mm or as an angle.

As there are three methods of measuring toe in mm what would be a better solution? How would this solution affect vehicles fitted with aftermarket wheels? Measuring Toe angle It wouldn’t as the angle wont change which ever wheels are fitted

European measured from wheel rim

US measured from the edge of the tyre Japan measured from the centre of the tyre

Toe-In – Toe-Out

a + b = total toe (degrees)

a b

B – A = toe (mm)

A

B

Toe-In – Toe-Out

a + b = total toe (degrees)

a b

B – A = toe (mm)

A

Note for every mm of incorrect toe this equals 1 metre of sideways scrub per mile driven and will decrease tyre life by at least 25%

B

Front Wheel Set Back

Positive setback indicates that the right front wheel is setback further than the left. Negative setback refers to the left front wheel being further back than the right.

Set back can be measured in mm or as an angle.

What usually causes setback?

Accident damage Front subframe incorrectly fitted

Thrust Axis

Thrust angle is the direction that the rear wheels are pointing in relation to the centre line of the vehicle. If the thrust angle is not zero, the vehicle will “crab” and steer to the opposite side to the thrust line. The steering wheel will not be centred. The best solution is to first adjust the rear toe. This is normally done during four-wheel alignment as long as the rear toe is adjustable. If the rear is not adjustable, then the front toe must be set to compensate for the thrust angle, allowing the steering to be centred.

Thrust Axis

What would you have to do, to the steering wheel to make this car drive straight? Turn it to the Left

Thrust Line

Ackerman Principle The intention of Ackermann geometry is to avoid the need for tyres to slip sideways when following the path around a curve. The geometrical solution to this is for all wheels to have their axles arranged as radii of circles with a common centre point. When a vehicle turns to the left or right the inside wheels will travel less distance than the outer wheel, therefore the inner wheel is required to turn a greater angle than the outer wheel.

18°

20°

Camber

Camber 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 rear. It is used in the design of steering and suspension. If the top of the wheel is farther out than the bottom (that is, away from the axle), it is called positive camber; if the bottom of the wheel is farther out than the top, it is called negative camber.

Positive Camber Positive camber gives a reduction of steering effort and prevents negative camber due to load. The disadvantages are poor road handling, increased tyre wear, understeer and torque steer

+5°

Negative Camber Negative camber gives better road holding and a reduction of torque steer, the disadvantages are increased steering effort required, increased oversteer and tyre wear.

-5°

Caster The function of Castor is to aid the driver in keeping the vehicle in the straight-ahead position. Caster can be seen by looking at the front suspension of the vehicle from the side and find out whether the steering axis is tilted either forwards or backwards. Forward tilt is called negative caster and backward tilt is called positive caster. Although caster is beneficial to aid straight line stability, too much caster will cause the steering to become heavy when steering away from the straight-ahead position.

Caster

Positive castor

Pivot Point

Tyre contact patch

Caster Advantages of caster are straight-line stability is improved due to the castor angle and wheel recovery (self centring), the disadvantages are the steering can be heavy and tyre wear. Vehicle will pull to the side with lower castor value.

On modern vehicles with power steering caster angle has been reduced to allow crisper turn in. Castor angle has also been further reduced to enhance crash safety – the suspension is more likely to fold under the vehicle than penetrate the passenger compartment.

Roll Steer

Roll steer is a compensation factor which some manufacturers build into the suspension geometry to assist in the correction of under steer or over steer. As the vehicle corners hard , the transfer of weight to the outside causes the suspension to compress on that side. The suspension geometry is arranged in such a way that this creates a small amount of toe out which will correct the tendency of the vehicle to continue turning into the corner harder push the tail out (over steer)

From above

Steering Axis Inclination (S.A.I) Steering axis inclination (S.A.I), is the angle between a line drawn at 90˚ to the road surface to a theoretical line that is drew from the top of the shock absorber mounting to the lower suspension ball joint.

G

If a solid king pin is used, then the angle of the king pin now forms the steering axis inclination. The distance between the two points that are generated when the lines contact the road is called offset.

Scrub Axis (Offset) The smaller the offset the less effort the driver needs to turn the steering wheel , and this small offset reduces the amount of road shocks felt by the driver. If a large offset is used, the rolling resistance or friction generated between the tyre and the road when steering wheel is being turned will cause the steering to become hard. Increasing positive camber can reduce this offset, but positive camber isn’t always an advantage as described earlier.

Scrub Axis (Offset)

Large positive offset Small positive offset Small negative offset

+

-

+

+

-

-

Large toe out

Small toe out

Small toe in

Included Angle

Included angle is the angle formed between the SAI and the camber. Included angle is not directly measurable. To determine the included angle, you add the SAI (a) to the camber (b). If the camber is negative, then the included angle will be less than the SAI, if the camber is positive, it will be greater. The included angle should be the same from side to side even if the camber is different but should never be more than 1.5˚ difference left to right. If it is not the same, then something is bent, most likely the steering knuckle.

b

a

G

Angle

Function

Pulling

Tyre wear

Determines steering load or improves cornering performance

The greater the difference the greater the pull

Large effect if the angle is increased a small amount

Camber

Provides self centring force & directional stability.

The greater the difference the greater the pull.

Increased angle will increase tyre scrub when cornering

Castor

Provides directional stability

Small effect unless the left/right difference is large

Increased angle will increase tyre scrub when cornering

SAI

Determines the direction of travel for the front of the vehicle.

If the steering wheel is straight ahead large effect

Increased tyre scrub

Front Toe

Determines the direction of travel from the rear of the vehicle.

Large effect if alignment with body is incorrect

Increased tyre scrub

Rear Toe

Thank you for your time, Any questions?