Front Wheel Angles Student Booklet En

Front wheel anglesStudent booklet

Front wheel angles - INDEX - 2007-01-24 - 16:59

- INDEX -

Front wheel angles The wheels on a truck are aligned in certain ways and that is no coincidence.In this module we are going to go thru how the wheels are aligned and why the wheel angles look like they do.The wheel angles influence on tyre were is also brought up.There are a number of measurement systems for measuring the wheel angles on the market but in this module non of them will be described or referred to.It only covers the basic principles behind the measurement methods.To get specific information about a certain system a special course for that system is needed.

Introduction General The 5 angles Caster 1 Caster 2 If wrong caster Camber 1 Faulty camber KPI 1 Incorrect KPI Toe-in 1 Incorrect Toe-in Curve angles 1 Incorrect Curve angles Curve angles 2 Tyre pressure 1 Axial clearence Centre position Ball joints Camber 2 Caster 3 KPI 2 Curve angles 3 Toe-in 2 Tyre pressure 2 Volvo tyre chart Axle Alignment Effect of tyre pressure Sum up Assessment

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Front wheel angles - Steer_wh_ang_01.swf - 2007-01-24 - 16:55

This course is dealing with front wheel angles. Here well go through the front wheel angles, why we have them and we´ll also analyze some fault symptoms.

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Front wheel angles - Steer_wh_ang_02.jpg - 2007-01-24 - 16:56

We all know that if a truck is to perform as it should on a road with minimum steering effort from the driver, the front wheels must have certain angles calculated and set by the factory. These angles have a certain interrelationship which cannot be altered without affecting the steering stability.

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Now what are the various wheel angles?Well, we have

1, Caster2, Camber3, King-pin inclination, which you´ve probably seen abbreviated to KPI4, Toe-in5, Curve angles.

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First let´s have a look at the caster. If you pull a mechanic´s creeper,the front wheels will always point in the way shown on the slide due to the fact that pivot centre A of the wheels is in front of the contact point B against the ground. The same thing applies to a bicycle. If you let go of the handle bars, the bicycle will try to go straight forwards, providing of course you don t try to make it lean to the one side.

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As far as a truck is concerned, the front axle is angled in relation to the vertical line V, as shown on the slide here. When the truck comes out of a bend, the front wheels strive to straighten up. The amount of steering effort involved in doing this will depend on the size of the angle. The caster also makes for less stress on the steering mechanism. Volvo trucks have a front wheel caster of 1° to 2°.

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But, if the caster is wrong, e.g., negative, the driver will think the steering is unstable. The truck no longer has the same tendency to straighten out after a bend. You can also see this on the tyres, which would have worn spots on them because of this. You get the same wear pattern in the case of imbalance, loose front wheel bearings or defective shock absorbers.

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Then we ve got camber A. A positive camber means that the upper part of the wheel inclines outwards in relation to the vertical line V. The camber distributes the forces in the wheel bearing so that the large bearing takes up most of them. The camber is 0.5 , but varies slightly according to vehicle type.

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Faulty camber can be recognized by the fact that the vehicle pulls to the side. Faulty camber is also noticed on the wheels. If the camber is excessive, you see that one side of the wear tread all round is worn. If the camber is too positive on the outside and negative on the inside.

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If you re riding a bike without holding the handle bars and if the bike leans to the one side, it will swing. The same thing applies to a front wheel which inclines, that is if we ve got a camber. To counteract this the king pin is inclined in the opposite direction to the camber. Because of the king pin inclination, the front end of the vehicle rises slightly when the front wheels are turned, which makes it easier to straighten out after a bend. The king pin inclination is from 5.5° to 6.0° .

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Incorrect king-pin inclination will also give the same type of tyre wear as we get with incorrect camber. The fault we see here can only be due to front axle member warp, so the front axle must be removed for checking.

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When you re driving straight ahead, wheel friction against the road surface causes the front edge of the wheels to strive outwards. To counteract this we "toe-in" the wheels, in other words, we adjust them so that, in the rest position, they slightly incline inwards at the front edge. This is what we call the toe-in. Usually in the order of 0 to 3 mm.

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Here we can see the effect of incorrect toe-in. The tyres have a saw-toothed pattern. The toe- in can be adjusted by altering the length of the tie rod.

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When a truck goes into a curve, the front wheel on the outside covers a larger arc than the inside one. But the wheel on the inside must roll more than the outside. This is achieved by pointing the steering arms (1 and 2) obliquely rearwards/inwards when the tie rod is behind the front axle. This gives all the wheels a common centre A for the rolling radii.

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Incorrect curve angles gives us the same tyre wear pattern as we get with incorrect toe-in. In other words a saw-toothed pattern. This fault may be due to a bent steering arm, which must be replaced.

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To prevent unnecessary tyre wear it is important that both front axles have the correct angles of deflection (V). This means that the inner wheels have to be turned more than the outer wheels when driving through a curve. All the wheels should follow a circular path around the same hub (A). Since the rear front axle is closer to the rear axle's centre line the angles of deflection will be less on this axle. The angles of deflection for the two front axles are calculated extremely precisely on the basis of length and anchorage points of the pitman arms and intermediate pitman arm.

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Some checks must be carried out before measuring the wheel angles. Among other things make sure the front tyres are of the same size and that they aren t unevenly worn. Also what the wear looks like. Tyre pressure, too, must be checked to ensure that it is correct.

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You also must check the axial clearance A on the king pin journalling and the radial clearance B, with the brakes applied. Check B again without applying the brakes. The difference between the two measurements is your wheel bearing clearance. Look up the specifications for the vehicle in question to see what the correct clearances should be.

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And now you check to make sure the wheels point straight ahead with the steering wheel correctly positioned and the steering gear in the centre position.

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Let someone turn the steering wheel back and forth while you check for any clearance in the ball joints. Check also to make sure the spring leaves are in good condition and that none of the leaves has lost resilience so that the vehicle shows a slight tilt.

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We ll only talk here about the principle for measuring wheel angles. This varies according to the different makes of measuring instruments. The camber is measured with the wheels pointing straight ahead. The angle can be read off from the instrument. First on the one side, then on the other.

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The caster can be measured straight off. It is the angle difference when the wheels are turned 20° inwards and 20° outwards. In other words, you turn the wheel 20°, 0-set the instrument, turn the wheels 20° in the other direction and read off the instrument. Follow the instruc- tions of the instrument maker to see how this is done in practice.

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The king pin inclination is also measured in the same way. But since this inclination is at right-angles to the caster, you must also measure the caster right-angle. The measurement you get on the instrument is corrected by the camber measurement.

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The curve angles are measured by turning one of the front wheels, say, 33° inwards, if this is the angle given in the specifications. Then you read off the angle for the other wheel. In this particular case it should be 50°. Hut note that the angles differ according to tyre size. Maximum wheel lock is adjusted with the stop bolts. If the angle difference between the two wheels is greater than that given in the specifications, you must change the steering arm.

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You should use Volvo s own equipment for measuring the toe-in. Here you measure the difference between measurements A and B. If this measurement is incorrect, you adjust by turning the tie rod accordingly.

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Up to now we have talked about the various reasons for abnormal tyre wear. Another thing which can give you abnormal tyre wear is wrong tyre pressure. If the pressure is excessive, you ll get wear according to fig l and if it is too low you ll get wear according to fig 2.To low pressure will also make the tyre get very warm and that could lead to a flat tyre or a tyre explosion.

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But it s worthwhile remembering that abnormal tyre wear on the front wheels does not necessarily indicate something wrong at the front end. There may be other reasons. For example the tow hook may be incorrectly mounted, likewise the 5th wheel, the frame could be warped, the bogie shafts out of parallel, and so on. The Volvo tyre chart you see here on this slide gives us a fairly good picture of the different types of tyre wear and the possible reasons for them.

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Axle Alignment

Given that the average fuel consumption of the vehicle above is 36 L/100Km with the axles correctly aligned.

The effect of misalignment will be an increase in fuel consumption of:+3.4% (37.2 L/100Klm) if two of the trailer axles are misaligned by 13mm in the same direction.

+23.4% (44.4 L/100Km) if two of the trailer axles are misaligned by 13mm in opposite directions.

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Effect of tyre pressureTyre pressure effects rolling resistance because it determines the amount of tyre that contacts the road surface.

A general rule of thumb is:For every 0,7 bar (10 psi) of under-inflation there will be a 1% increase of the fuel consumption.

The picture shows three tyres and the corresponding “foot print” that each has on the road surface. The tyre on the left is over-inflated, has a small foot print, and therefore less rolling resistance and lower fuel consumption. However the tyre life will be reduced as the centre of the tyre will tend to wear out.

The centre tyre is under-inflated producing a larger foot print, higher rolling resistance and therefore higher fuel consumption. The tyre life is also reduced due to the increased flexing of the tyre wall which also causes over heating.

The tyre on the right is at the correct pressure and as a result gives the best compromise between tyre life and fuel consumption.

The effects of tyre pressure on rolling resistance are also shown on the graph on the left. The graph on the right shows the effect of tyre pressure on tyre life.

Tyre pressure is the most important factor in terms of rolling resistance and, therefore, of fuel consumption and economy.

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Front wheel angles - Steer_wh_ang_sumup_33.swf - 2007-01-24 - 16:59

1. Which are the five different front wheel angles?

2. Why do the vehicle have a positive camber?

3. Match the different abnormal tyre wear pattern with the corresponding incorrect wheel angle.

4. How does wrong tyre pressure effect the vehicle?

5. What more could cause abnormal tyre wear?

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Front wheel angles - Steer_wh_ang_assess_34.swf - 2007-01-24 - 16:59

Assessment questions

1. What is the name of the wheel angle that will make the vehicle straighten up?

2. There is a rule of thumb regarding fuel consumption increase depending of under inflated tyres.According to that rule of thumb, how much will the fuel consumption increase for every 0,7bar (10 psi)?

3. Why does the front end of the vehicle rise slightly when the front wheels are turned?

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Front Wheel Angles Student Booklet En

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