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After Sales Services10:01-01

Issue 1 en

Facts about brakes

1 585 273 Scania CV AB 1995-10

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Contents

Contents

Wheel brakes ................................................4

Wheel brake operation ...................................4Maintained brakes are safe brakes .................5

Brake linings ................................................5Frictional stability ..........................................7Linings widths and their effect onservice life .....................................................9Short/long brake linings ..............................10

Brake drums ...............................................12

General ........................................................12Thermal cracking .........................................12Tension in brake drum whenapplying brake .............................................13Tension in brake drum after cooling ..........13Rough areas anddeep cracks ..................................................14

Factors that impair brake force ...............16Fading ..........................................................16

Comfort reducing phenomena vibration

.....................................................20Vibration, general ........................................20Brake squeal ................................................21Brake judder ................................................22Low frequency vibration,high speed ....................................................23Vibration caused by heat spots ...................23Low frequency vibration,low speed .....................................................24

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Wheel brakes

Wheel brakes

The most usual type of brake for heavy vehiclesis the drum brake.

There are different methods for actuating andcontrolling brake shoes but the difference ismarginal as regards function and performance.

Scania wheel brakes are of S-cam type, whereby the brake shoes are pressed against the drumby turning the brake camshaft.

Compressed air is used to actuate the wheelbrakes. The air is compressed by the compressorand stored in pressure tanks. From there,

the pressure is distributed to each brake chamberthrough the service brake valve, to obtainnecessary force for braking.

Wheel brake operation

In the brake chamber, the diaphragm (area pressure) pushes out the pushrod. Slackadjuster 1 is connected to the pushrod and turnsbrake camshaft 2 with its S-cam, pressing outthe brake shoes against the drum.

Thus, the pressure between the lining and the

brake drum is determined by the interactionbetween: the compressed air - brake chamber slackadjuster - brake camshaft S-cam.

The shape of the brake camshaft S-cam is suchthat the lifting height when braking (i.e. thevertical distance the brake shoes move towardsthe brake drum) is the same, irrespective ofwhere on the cam the pressure rollers are resting.

In the diagram to the right you can see the liftingheight for the brake shoes as a function ofwear and application angle. Irrespective ofwear, you can see that any specific turning ofthe eccentric shaft always gives the same liftingheight.

1Slack adjuster (with manual or automaticadjustment)

2Brake camshaft (S-cam)

3Pressure roller

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4Return spring

5Anchor bolt

6Brake linings

7Brake shoe

a gives the same backlift across the whole of r

1 = shoe liftingr = wear anglea = application angle

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Brake linings

Maintained brakes are safe brakes Substance number two is the material that

In order to maintain safe and optimum braking,all components must be continually maintained

and if necessary, adjusted according to our recommendations.

Well maintained brake reduce the risk of malfunctionsuch as pulling to one side, unevenlining wear or overload on any one of thewheels. Otherwise, malfunction can lead toknock-on damage and impaired brake functionor, at worst, brake failure (see heading Factorsthat impair brake force, - mechanical heat fading).

Brake linings

For competitive reasons, brake lining manufacturerswill not reveal the exact composition oftheir linings.

Normally, a brake lining consists of a numberof substances, each having its own specifictask.

We normally say that there are three maincomponents in a lining.

Substance number one is the friction material,

i.e. the substance in the lining that does thebraking and which is essential to the life of thelining (50-70 % is made of friction materialand the structure of the material contributes tostrength and greater wear properties).Here, the choice of material varies from onelining manufacturer to another. Examples ofmaterials that are used are iron, glass fibre etc.

It can generally be said that when selecting afriction material, the requirement is that it isstrong, gentle on the brake drum and heat-insulating,that is to say it does not conduct heattowards the wheel bearing.

bonds the friction material and makes the liningkeep its shape. Consequently, it is alsoknown as bonding agent (5-15 % of the lining).

The linings must keep their shape at extremelyhigh temperatures and under extremely highpressure.

Examples of bonding agents are rubber andresin.

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The third substance is the material that makesthe lining frictionally stable, especially at hightemperatures. This relieves a heavily loadedwheel brake by lubricating the lining and inthis way reducing the work of the wheel brake.This substance is called a modifier and canconsist of graphite, organic substances, sand

etc.

It does not seem complicated to make brakelinings for trucks.

In reality it is completely different. A longperiod of testing is required before the manufacturerhas a lining that might be of interest.


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Brake linings

What does a lining need to be approved byScania?

The requirements for a good lining are a compromise

between our own stringent requirements,the requirements of authorities (seeNational Legal Requirements) and the requirementsof our customers.

The following is a list of requirements andproperties that must be considered:

Frictional stability

The friction coefficient for the operating rangeof the lining (temperature variations) must be

nearly constant, that is to say not lead toimpaired braking ability (see heading Factorsthat impair brake force).

Stable shape

The lining must be able to cope with mechanicalstress without splitting or causing sticking.

Squealing/vibration tendencies

The tendency of a lining to vibrate may havean impact on comfort.

Service life of the brake lining

Long service life. The introduction of asbestosfree linings has reduced the choice of materials.

Service life of the brake drum

The effect of the lining on the drum should beas little as possible, i.e. it should minimize heatspots and cracking (see section Brakedrums).

Environmentally friendly

Legal requirements, asbestos free materials.

Manageable

Light, not harmful to the hands, dust free etc.


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Brake linings

Frictional stability

When selecting a brake lining, a great deal ofcare has to be taken, as materials of different

types can have completely different properties.Before approving a lining for fitting in production,it must go through a number of tests in ourlaboratory (performance, fading, strength,wear, vibration tests etc.). Only a small numberof brake linings pass these tests. These liningsgo through further tests in trucks in operation.

Investigations/comparisons between differentlining types reveal distinct differences in properties,depending on whichs properties aretested, and how the tests are carried out.

The friction of the linings varies with slippingspeed and temperature. Here, all linings havetheir own friction curve.

In a comparison of linings A and B at 3 km/h(roller brake tester) and at 60 km/h (legalrequirement), it can be seen that the liningsperform similarly at low speeds, and that liningA has higher friction than B at higher speeds.

Friction coefficient m changes as speedincreases and you can see how important it is

that the characteristics of the linings are consideredthroughout the whole speed range. Thisproperty is known as speed fade.

Friction coefficient m as a function of speed

The diagram to the right shows how the frictioncoefficient varies as a function of temperature.

Friction coefficient should be stable, as a sharpdrop leads to impaired brake function.

Friction coefficient m as a function of temperature


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Brake linings

A small drop in friction is a positive thing, inorder to avoid one wheel brake carrying out tomuch of the braking work. The overloadedbrake transfers some of the work and distributes

it to the other wheel brakes. The effect oftemperature is important to consider in order toreduce the risk of surprises in the form of suddenlypoor braking effect or impaired brakematching.

In a brake lining test where 24 different makeswhere examined as regards frictional stability,wide variation was observed. This test demonstratesthat some of the linings have such poorproperties that brake function under toughoperational conditions is impaired. See diagram

to right.

Heat fading and speed fading must be the samefor all linings on the vehicle combination, otherwisethere is an undesirable redistribution ofbraking work.

If the linings have a tendency to glaze, thisresults in lower braking capacity. Glazing is aphenomenon that causes a gradual loss of friction,and can arise during prolonged and lightbraking.

What happens when a lining glazes is that themodifier with the lubricating property gathersat the surface of the brake lining. This substanceis normally burnt away during braking,but remains at the surface due to the low temperatureduring light braking.(See the heading Factors that impair brakeforce.)

This experience underlines how important it isfor road safety that only linings tested for thevehicle are used. This is not a problem whenthe vehicle leaves the factory, but arises whenlinings are to be changed. For this reason, genuinelinings must always be used.


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Brake linings

Lining widths and their effecton service life

Scania currently use four different lining

widths:

A 127 mm (5) tag axle

B 178 mm (7) front and tag axle

C 203 mm (8) front axle, halfshaft and tag

axleD 254 mm (10) halfshaft

Ideally, the width of the lining should be

adapted to the work the brake is intended for.

The result of this is a difference in wear speed,where narrower linings wear more quickly.

The most important factor in lining wear istemperature, as wear speed increases with temperature.

It is therefore desirable to keep lining temperaturedown.

Wear speed for a normal Scania lining is

shown in the diagram to the right.

Heat is generated in the contact surfacebetween brake lining and brake drum and islead by the drum out to the air. Heat conductionis therefore extremely important to liningwear.

S = wear


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Brake linings

The heat conductivity of a wheel brake can belikened to buckets of water of different sizes.The size of the bucket (5, 7, 8, 10) correspondsto the size of the brake/drum and the

holes in the bottom of these to the ability toconduct away heat. The taps and the water flowinto the bucket correspond to brake work to becarried out.

With the same amount of braking energy/heatwe can see that the level varies in the differentbuckets. This corresponds to the temperature inthe drum, where high level corresponds to hightemperature.

If this symbolism is put into practice, it means

that a large drum is better able to take upenergy in the form of heat and also better ableto dissipate it.

The critical point is when the temperaturebecomes so high (the bucket becomes full),that lining wear increases significantly andthere is a danger of mechanical heat fading.

The conclusion from our example is that narrowlinings/small drums are worn more thanwide linings/large drums for the same amountof braking work.

Short/long brake linings

Since 1989, Scania has gone over to usingshort linings instead of the long liningspreviously used.

The amount of material in short linings is about65 % of that in long linings. This may seempeculiar as lining area has often been used as ameasure of quality.

In our tests we have been able to conclude thatthe total service life is not changed with shorterlinings. However, it is our experience that heatspots arise much less often in brake drums.

In addition, low frequency brake vibration disappearsat high speed almost completely. Seeheading Comfort reducing phenomena -vibration.


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Brake linings

Service life with short linings is not diminished(although we said earlier in connection withlining widths that a small area gives highertemperature for the same work and therefore

greater wear) as the contact between the liningand the drum is more efficient, and the drum iscooled better as the stretches between the liningswhere the drum is free are longer.

The difference between the contact of a shortand long lining against the drum is demonstratedby the figure by the right.

Here you can see two linings in cross section.You can see that the contact of the liningagainst the brake drum consists of a number of

contact points.

If the short and long linings are applied withthe same force, more contact points areobtained with the short lining. This results inlower temperature for the contact points, which

A = Short lining

more than compensates for the lining length.

B = Long lining


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Brake drums

Brake drums

General

A lot is demanded from a brake drum. A brakedrum must withstand high temperature, be ableto dissipate heat well, keep its shape, resistwear etc.

To create a brake drum with these properties,detailed and thorough testing is required, bothin the laboratory and in the field.

The choice of material for a brake drum isalways a compromise in order to optimize therequired properties.

The material in the brake drums is fine alloycast iron with as high a carbon content as possible.The fine alloy is for strength and stabilityof shape and the carbon is for thermal dissipation.

Despite the high quality of the brake drum, ithas certain limitations as regards withstandinglocal temperature increases. See heading Heatcracks.

Even but high temperature also leads to

changes, and if these are allowed to go too far,they can cause problems. See heading Factorsthat impair brake force mechanical heat fading.

During heavy braking, large amounts of kineticenergy are converted to heat energy. The heatwhich is generated by the contact between thelining and the brake drum is useless energy andmust mainly be dissipated by the brake drumout into the air. See heading Brake linings.

Despite the ability of the brake drum to dissipateheat, negative effects, in addition to normalwear, can arise such as thermal crackingand heat spots.

Thermal cracking

The diagram shows temperature distribution inthe brake drum 1, 2, 3, 4, 5, 6 and 10 secondsafter braking is started. The temperature in avery thin surface layer rises quickly and afterabout two seconds reaches its highest value.After this, the temperature drops as vehicle

speed has already had time to decrease andheat generation has dropped. The heat wavetravels through the iron and temperature is

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evened out more and more.


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Brake drums

Rough areas and deep cracks

Deep cracks and black heat spots occur whenthe brake is overheated to a limit where there

are changes in the materials of the linings anddrum.

Some parts of the lining will then be in contactwith the drum, which is not round. The contactsurface becomes less at the same time as a lotof energy is developed. Heat generation in thecontact surfaces is very great.

When the temperature becomes high, chemicalchanges take place in the lining, which meansthat friction increases with temperature, see

fig. A spot which is warmer develops moreheat and becomes even hotter than its surroundings.In this way unevenness in the drumis amplified.

The lining has now reached the point (temperature),where wear speed is abnormally high,causing the lining to be worn down locally.

The drum has a permanent heat spot with acrack.

If a number of these heat spots are close to

each other, the cracks can grow into each otherand weaken the drum.

A sufficiently long and deep crack can lead tothe drum breaking.

A = NormalB = Unstable

1 Brake drum with heat spots

2 Same drum as 1 plus 700 brake applications


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Brake drums

Heat spots may cause steering wheel vibrationor other vibration phenomena, even before anysignificant cracking has taken place.

Action to ease/avoid problem:

In the case of problems with cracked brakedrums, check and adjust the brake systemaccording to our instructions. These checksand adjustments prevent knock-on damage.For trailer units, brake matching should alsobe carried out. From a heat dissipation point of view, it isbetter to have wide linings. Short liningsalso give a better distribution of heatbecause of better and more even contact

with the brake drum. See sectionBrake linings

. Larger and heavier drums are better. These

are stronger and are more able to resist concentrationsof heat and a build-up of waves.In addition, heat dissipation propertiesincrease with increased weight/volume. Auxillary brakes such as exhaust brakes andretarders can be widely used. These reducethe stress on the wheel brakes.3 Brake drum with deep cracking

4 The same drum as 3 plus 700 brakeapplications

5 Limit for scrapping brake drum

6 Wear pattern that should lead to scrapping


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Factors that impair brake force

Factors that impairbrake force

There can be many causes of the partial or

entire loss of braking. The most commoncauses are incomplete draining of air tanks(freezing problems) and poor maintenance ofboth pneumatic components (such as valves,hoses and pipes) and mechanical componentsin the wheel brakes (such as brake chambers,slack adjusters, adjustment brake shoes andbrake camshafts).

If continuous maintenance is conducted asrecommended, the danger of a fault in thebrake system is reduced.

However, even if the entire brake system isworking, braking in a vehicle combination canvary depending on the type of lining and howthe linings work (working temperature).

Looking more closely at some of these phenomena.

Fading

1Heat fading - desired change in the properties

of the brake linings

As a rule, the friction coefficient of a lining isreduced with increased temperature. See figure.

Moderate fading is a desirable and stabilizingfactor, as a heavily loaded brake transferssome of the work to the other wheel brakes.This reduces the danger of heat damage.

Desired fading is obtained by mixing solidmodifiers in the linings, which start to workwhen heated.

The change in friction as a function of temperaturevaries a great deal depending on the typeof lining. Scania linings have always had well-controlled and desirable change of friction.

a) Fading = impaired m

b) Bonding agent collects at the surface(normal working range).

c) Bonding agent evaporates. Fric

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tion material remains.

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2Heat fading - undesirable changes in theproperties of brake linings

A lining with friction reduction, where m dropssignificantly as the temperature rises, is a poorand unreliable lining (see figure).

3Mechanical heat fading

This type of fading is often ignored, despite thefact that it can have serious consequences onroad safety.

What happens is as follows: during braking,large amounts of energy are developed in thewheel brakes. This energy is converted to heat.

As the temperature rises, the diameter of thedrums increases (thermal expansion). In additionto temperature, the change in diameter isdependent on the material and dimension.

In order to maintain the same brake torquewhen warm as when cold (drum expands),greater turning of the bake camshaft isrequired. Increased turning of the brake camshaft

requires a longer stroke in the brakechamber. The brake chamber is designed sothat the force for any stroke (max/min stroke,see Workshop manual) is more or less constant.If however the stroke becomes longer,

i.e. moves out of its operating range, force isreduced with increased stroke.If brakes are badly adjusted, the risk of poorbraking because of excessively long strokeincreases.

S = brake cylinder stroke lengthF = pressure force


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Extreme case

Even piston force effective slack adjusterlength decreases with extremes strokes, i.e. the

torque on the brake camshaft is reduced. (M1 =F L1 is greater than M2 = F L2. Force F is alsoreduced as stroke length increases, making M2even smaller.)

If the slack adjuster is not correctly set, thebrake chamber stroke may not be sufficient.

To prevent the negative consequences of thesefactors, the wheel brakes must always be maintainedas recommended (see maintenanceinstructions under group 0).

4 Glazing of linings

Glazing is a gradual reduction of friction as aresult of lengthy and light braking.

The lubricating substances collect on the wearsurface of the lining. These substances areessential to the service life of the lining andgive the lining a certain fading property (seeheading Heat fading - desirable changes inbrake lining properties).

During glazing, these substances collect on thesurface of the lining. These substances cangenerally be removed by braking heavily severaltimes. In this way, a new surface isworn/burnt into the lining.

Some linings have a greater tendency to glazingthan others.

The intention is that the friction coefficientshould not be affected, but in principal shouldremain constant.

However, the stability of the friction coefficientvaries greatly depending on the quality ofthe linings. It is therefore important to alwaysuse genuine linings.

Note: A glazed lining does not always have ashiny surface. It can also be covered in dust.

Normal working range (A)


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5 Speed fading

Speed fading is a reduction of friction with anincrease in speed. It is related to heat fading as

braking at high speeds quickly causes heatingin the contact surface between the lining andthe drum. As mentioned earlier, friction dropsas temperature increases. See heading Fading.

6 Ageing of lining

High temperature for a long period of time canage a lining prematurely, i.e. the substancesin the lining can change or disappear beforethey have started working. The properties of

the lining and friction can be impaired.

7 Other

Oils, water, dust etc. have a negative effect onfriction.


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Comfort reducing phenomena - vibration

Comfort reducingphenomena - vibration

General about vibration

Vibration generated by the wheel brakescauses disturbance across a wide frequencyrange, resulting in various problems dependingon frequency.

Normally we speak about five different vibration/oscillation phenomena:

Squeal

-high frequency vibration which disturbs hearing.

Judder

-vibration at 50 Hz generated when reversing.

Low frequency vibration

-Twice/wheel turn at high speed, above 70km/h

-Twice/wheel turn at low speed, approx. 40km/h

-caused by hardened spots. The frequencyrelates to the number of spots on the drum.

Vibration in the brake system most oftenoccurs when a rotating part such as the brakedrum is out of round. This causes the brakedrum to run out radially.

There is always disturbance in the form ofvibration. However, the brake components of atruck generally have integrated damping properties.If disturbance is minor, damping preventsoscillation being generated. It is whendamping is reduced, and/or disturbanceincreases, that oscillation can be generated andresonance may occur.

Resonance means that a part of the vehiclevibrates at its own natural frequency, ratherlike when a string is stroked by a bow.

Resonance can occur, for instance, by the frontaxle or the rear axle oscillating around itsrotating shaft or by the front wheels being

turned to and fro around the king pin bearings.This can be experienced as vibration in thesteering wheel.

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Brake squeal

Brake squeal is vibration at a frequency whereit causes a tone which is uncomfortable to the

ear.

High friction linings are generally more proneto squeal than low friction linings. Imagine theresin on the violin bow that is used to increasethe friction of the bow.

Soap on the bow = no sound

Soap on the brake = no squeal, but no brakingeither (low friction).

Another cause of brake squeal might be lightbraking from not excessively high speed or aworn bearing in the brake camshaft/brake shoeanchorage. Another cause may be faulty rivetingof the brake lining, causing the lining contactsurface with the brake shoe to be incorrect.This may result in the lining rivets becomingloose or the lining loosening around the rivets.

Riveting with to much force:

1 The lining material around the rivets swells,

causing the lining between the rivets to

loose contact with the brake shoe.

2 Cracks may appear around the rivet holes.

Riveting with insufficient force

1 Because of the movement between lining

and shoe, vibration is generated in the form

of brake squeal.

2 Movement between lining and shoe can

cause the rivets to become fatigued and

wear away at the rivet holes. The result is

that the lining becomes detached.

For correct fitting, the riveting force of the rivetingmachine should be between 16 and 18kN. This applies to all genuine Scania brake

linings.

When riveting brake linings, the contact surface

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for the brake shoe must always be properlycleaned.

Remove all loose rust and dirt from the shoes.

Grind down any unevenness on the brakeshoes, such as burr around the rivet holes. Do

not grind so much that hollows or flat surfacesare formed, see figure.

A No machining necessaryB Burr by rivet holesC, D Incorrect grinding

Brake shoe surface by rivet holes


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Brake judder

When braking, the brake shoes are pressedagainst the brake drum. The leaf spring and the

shaft member are twisted by the transmittedbrake torque. As the axle turns, the movementof the slack adjuster and pushrod is delayedbecause of the inertia in the system. The pressureof the brake shoes against the brake drumincreases and this leads to the leaf spring andthe shaft member turning even more. See figure.

Maximum brake force is soon achievedbetween brake shoe and brake drum. The brakeshoe then snaps back under spring force. The

slack adjuster is once more delayed and easesthe brake pressure more than necessary. Oscillationbetween braking and releasing soonstarts up. Oscillation then continues by itself atthe natural frequency of the shaft system.

This oscillation is more likely to arise if damping(= friction in system) is low, because oflow friction in the brake camshaft bushings.

As previously shown, oscillation arises whenthe brake drum and eccentric shaft move inopposite directions. If these move in the same

direction, the leading brake shoe is actuated bya smaller force, reducing self-locking properties.

10_210311 = Leading brake shoe


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Comfort reducing phenomena- vibration

Low frequency vibration,high speed

This type of vibration can arise during persistent

braking, such as on long down-hill slopes.

The temperature distribution in the brake drumis not entirely uniform. At position A in thepicture, the brake drum has a slightly highertemperature.

The material at this point expands more thanthe rest of the brake drum. The result is that thesurface of the brake drum buckles inward. Liningcontact is concentrated at this point, whichbecomes even hotter. The unevenness is amplified

and the drum becomes ovoid.

A brake drum which is out of round causes variationsin torque. When the uneven part of thebrake drum meets the toe of the brake lining,there is heavier self-tightening of the brakeshoe. The consequence of this is that the torquevariation around the anchor pin bearingincreases.

The further in on the shoe that the lining andbrake drum meet, the less the variation in

torque. Force is constant, leverage becomesshorter.

With short linings, there is less variation intorque under the same circumstances. In addition,the time of the disturbance is also shorter.

Vibration caused by heat spots

Heat spots occur when the brake is heatedabove the limit where material changes takeplace in the drum, see heading Brake drums.

Heavy braking from high speed and light brakingfor long periods can cause heat spots (unevennessin the form of hard patches).

Heat spots can cause vibration.

If, in addition, brake distribution between theaxles in incorrect, the problem is amplified.


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Low frequency vibration,low speed

Low frequency vibration, for example steering

wheel judder, can be disturbing when brakinglightly, when speed is lower than 40 km/h.

The variations in torque in the brake are transmittedvia the king pin bearing in the steeringlinkage and up into the steering wheel.

The cause is generally incorrect shape or dirt inthe joint between wheel, drum and hub.

A faulty wheel, which is drawn towards thebrake drum and hub, can cause deformation of

the brake drum.

Incorrect tightening of the wheel joint can alsocause deformation of the drum.

Tightening must therefore be conducted withthe recommended torque and using the recommendedworking methods. See the operatorsmanual or workshop manual group 9.


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