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CAUSES AND CONSEQUENCES OF DISC DETERIORATION: PRACTICAL EXAMPLES
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DETERIORATION
DUE TO FITTING THE DISC Description
of defect: Formation of cracks on the carrier surface in contact with
the hub. This is due to incorrect tightening during fitting. The tightening
sequence has not been followed and torque is insufficient. Failure to
follow the recommended sequence and tightening torque can cause distortion
of the carrier mounting surface even when there are no visible cracks. Consequences:
Distortion of the carrier mounting surface causes vibrations that can
be felt immediately after fitting when the brake pedal is pressed. Advice:
Fit another disc, following the recommended sequence and tightening torque. |
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Description
of the defect: The carrier mounting surface breaks away from the rest
of the disc. There are clear signs of excessive tightening, above all
in the area of the two fixing holes. This is evidence of a failure to
observe the recommended torque and sequence during tightening. Consequences:
Pronounced rasping noises and absence of braking torque. Advice:
Fit another disc, following the recommended sequence and tightening torque. |
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Description
of the defect: The carrier mounting surface can easily distort when
the disc positioning bolt is tightened too much. This excessive tightening
can cause the mounting surface to break as can be seen in the photo. Consequences:
Run outing cannot be kept within an acceptable tolerance. Heavy vibrations
occur during the very first kilometres following installation. Advice:
Tightening bolts are only intended to ensure that discs are positioned
correctly. Do not tighten them excessively. |
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4.
FITTING A DISC THAT DOES NOT CORRESPOND Description
of the defect: Formation of cracks on the carrier mounting surface.
Signs of a poor match between the diameter of the disc centring and that
of the hub. Consequences:
Disc contact with the wheel hub is incorrect. This defective installation
will immediately cause vibrations due to excessive run outing. Advice:
Check catalogues to determine the correct disc reference: model, year
of manufacture. Never force discs when fitting. |
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5. INCORRECT ASSEMBLY OF THE CALIPER BODY Description
of defect: The braking surfaces have broken away from the hub. Asymmetrical
braking surface wear can be noted: the central area of the external surface
in respect of the vehicle and the outer edge of the internal surface.
In this case mechanical stress has caused the disc to break and the braking
surfaces have come away. Consequences:
The impact of this breakage can be felt when braking. Very loud rasping
noises. Safety is compromised. Advice:
Before fitting new discs check the alignment and assembly of the caliper
body on the axle. |
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Description
of defect: An exaggerated tightening torque knocks the bottom out
of the bearing seat. Consequences:
Braking system functioning is compromised because of disc instability
relative to the bearings. Strong vibrations are felt right from the first
braking action. Advice:
Replace the hub, bearings and disc. Apply correct tightening torque when
installing. |
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Description
of defect: The hub surface was not cleaned when the disc was fitted:
presence of rust or dirt. Tightening against this unsuitable surface causes
the new disc to run out excessively. Consequences: This error during installation causes the onset
of vibrations after a few hundred or thousand braking actions, vibrations
that increase with the distance travelled. Uneven disc wear as a result
of the pads causes DTV to increase - this is the origin of the vibrations
- and noise (see section 3.3.3). Advice:
Clean contact surfaces very carefully. Check disc run out after installation. |
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Description
of defect: Excessive hub run out causes the braking surfaces to wear
as they are not parallel to the plane of the disc. Evidence of localised
overheating is indicated by the darker colour of the worn areas. This
is due to an alternating rubbing action of the disc and pads as a result
of pronounced wheel hub run out. Consequences:
Vibrations can be felt right from the start and progressively increase.
Onset of loud noises. Advice:
Check the wheel hub run out and ensure that this falls within the tolerance
specified in the disc installation instructions. |
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1.
NO RUNNING IN Description
of defect: Disc shows colouring of varying intensity and shades (blue,
violet, golden), mainly visible in the cooling areas (groove and carrier
interior). Consequences:
Initial slight vibrations that progressively increase. Overheating of
this type alters the mechanical characteristics of the cast iron in as
much as there is a change in structure due to the formation of cementite
(Fe3C). Advice:
Always advise users to observe a running in period. Brake only moderately
and briefly during the first 200-300 kilometres over a mixed itinerary.
Avoid prolonged pad to disc contact. |
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Description
of defect: Disc thickness is considerably less than the recommended
minimum thickness (4 mm less in total). Cracks are evident. Heat spots
are visible in areas corresponding to the ventilation blades. This is
advance warning that other cracks will form. Consequences:
Noises, vibrations. Advice:
This situation is typical of that found on certain sports vehicles which
are subjected to an intensive and excessive use on the road. |
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1.
EXCEEDING THE LIMIT Description
of defect: The thickness of the braking surfaces measured using a
micrometer is less than the recommended minimum thickness etched on the
outer disc rim or on the carrier. Consequences:
Performance and comfort decreases. Advice:
Check state of disc wear periodically. The disc should be replaced after
every two or three pad changes. Pads must be changed every time discs
are replaced. |
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Description
of defect: a)
disc The
minimum thickness indicated on the outer rim of the disc has been exceeded
by more than 1 mm. Furthermore, the disc has been subjected to excessive
operating temperatures as a result of the reduced braking surface thickness.
This causes cracks of a thermal origin. b)
pads More
marked wear can be noted in the central area of the disc whereas the area
in correspondence with the pads is less worn. This can be attributed to
the presence of hard points in the friction material, an indication that
the production mixture was not homogeneous. It can also indicate poor
functioning of the caliper. Consequences:
Distortion caused by vibrations during braking. In the long run cracks
may form that can lead to disc breakage. Advice:
Periodically check disc wear. The disc must be replaced every two or three
pad changes. |
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Description
of defect: The disc has been subjected to excessive operating temperatures:
in the outer area of the braking surface there are evident signs of overheating.
High operating temperatures lead to the appearance of cracks. Consequences:
Distortion and pad "bouncing" when hot that causes vibrations
during braking. With time, cracks form and the disc may break. Advice:
The worn disc must be replaced. Check the state of wear periodically. |
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Description
of defect: The disc is extremely worn and has been damaged by the
metal pad support, the friction material of which has completely disappeared.
The thickness of this disc when new was 7 mm, measured now it is 3.5 mm
compared to a recommended minimum thickness of 5 mm. Consequences:
Very loud noises, very long braking distances, caliper functioning is
critical. Advice:
Replace pads when they reach wear limits. Check out the pad wear warning
indicator circuit. |
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Description
of defect: Braking surfaces show evident signs of overheating due
to the considerably reduced mass of the worn disc. This situation has
caused the braking surfaces to break away from the hub. Consequences:
Violent impact during braking. A very strong rasping noise when braking.
Possible safety problem. Advice:
Always check the disc thickness every time the pads are replaced. Replace
the disc before it reaches its wear limit. |
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Description
of defect: Disc thickness less than the limit value. The disc has
been worn by the pad support as friction material is completely absent.
The metal support plate has come out of its seat in the caliper and has
almost entirely cut the disc brake surface from the carrier. The braking
surfaces are near to breaking away from the carrier. Consequences:
Rasping noises, loss of efficiency (considerable brake pedal slack), possible
rubbing between the disc and caliper, thrust of the caliper is not symmetrical. Advice:
Check and, if necessary, repair the caliper. Replace the discs and pads. |
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1.
UNEVEN WEAR OF VARIOUS PARTS Description
of defect: A braking surface of one of the discs is in contact with
the metal pad support. Examining the pads it is noted that wear is uneven
due to blocking of the caliper (the pads on the other caliper are in perfect
condition). Consequences:
The caliper's two pads are completely worn down with consequent onset
of noises and vibrations, the brake pedal has to be depressed completely.
Braking distance is very long. Advice:
Check, repair or replace the caliper. Replace all discs and pads. |
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Description
of defect: Presence of a very fine deposit of friction material on
the disc surface which now has a shiny finish (corrosion has then caused
the detachment of part of this layer). Consequences:
Loss of braking efficiency, braking distances too long. The pedal is very
stiff but with no braking action. Advice:
Replace discs and pads. Use high, original, quality pads. |
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Description
of defect: The two pads were not positioned in the same manner with
respect to the disc. Wear has not been the same for both surfaces. Heat
spots can be noted at the centre of the braking surface due to high thermal
exposure. The caliper and/or the pads were not fitted properly. Consequences:
Gradual onset of vibrations due to the heat spots. Possible appearance
of cracks. Advice:
Check and, if necessary, repair the calipers. Check the pad model and
friction material quality. |
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Description
of defect: Deep scores caused by penetration of foreign bodies between
pads and disc can be noted. This can be due to unsuitable pad material
or the presence of badly distributed abrasives in the mixture. Consequences:
Very unpleasant noise both during braking and when travelling. Diminished
efficiency as a result of the reduced contact surface between disc and
pads. Advice:
Replace discs and pads. |
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Description
of defect: The braking surfaces are completely covered by very dark
spots. These spots are due to deposits of pad friction material. This
causes overheating which in turn leads to a transformation of the cast
iron with the formation of very hard cementite. Consequences:
Onset of vibrations that become progressively worse. Advice:
Only install friction material that is suited to the brake and vehicle. |
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Description
of defect: The braking surface only shows wear in the outer area.
The inner area never comes into contact with the pads as indicated by
the presence of corrosion (rust). This situation could be caused by: • incorrect installation of the caliper and
hence the pad which does not make full contact with the disc • loss of part of the friction material • fitting of wrong pads Consequences:
Reduced braking torque. Increase in operating temperature as the surface
on which the pad works is reduced (by about 50% in this example) compared
to its normal working surface. There is a risk of localised overheating
and therefore the onset of hot judder-type vibrations. Moreover, reduced
system efficiency leads to high stress levels and therefore rapid and/or
uneven wear (cold judder). Advice:
check the installation and correct functioning of the caliper. Check the
fitting and condition of the pads. Check that the pad model is suitable
for the particular vehicle. |
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7. PARTIALLY WORN AND VITRIFIED Description
of defect: This disc shows a deterioration that is the sum of defects
described in 2 and 6 above. In this case the pad only acts on the inner
section of the braking surface. High stress has led to overheating, vitrification
(deposit) and detachment of friction material. Consequences:
Progressive reduction in efficiency of the system and, when friction material
detaches, total inefficiency of the system. Advice:
As for 2 and 6. |
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Description
of defect: The pads only act on the central section of the braking
surface. They may be curved. Consequences:
Reduced braking action with the consequences described in points 6 and
7. Advice:
Check pad planarity and the functioning of the calipers. |
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CHANGES
IN DISC SIZE When
initial measurements of the fitted disc and successive observation do
not give clear answers, it may be useful to analyse disc dimensions further. Placing
the disc on a bench as seen in the illustration, it is possible to measure
its dimensions without these being affected by other brake components. -
After cleaning the surface coming into contact with the hub using sandpaper,
the disc must be able to rest on the bench bearing without run out. This
is done by holding the disc at the extremities of a diameter with the
fingers and checking its vertical movement. If the disc is unstable this
indicates distortion due to incorrect or excessive tightening. -
With the aid of a DTI Gauge mounted on a fixed base it is possible to
measure the disc's production run out, provided that it has not been rectified
by the mechanic at the time of fitting. This measurement is carried out
by resting the DTI Gauge against the external or internal edge of the
disc where the pads have not worn the braking surfaces. If the run out
value measured is greater than 0.05 mm it means that the disc originally
had a marked run out, a certain cause of vibration. If the value is less,
then the acquired run out is measured by placing the point of the DTI
Gauge at the centre of the braking surface: if it is beyond the tolerance
this indicates incorrect installation that causes vibrations after a few
thousand kilometres (cold judder due to a DTV increase). -
Lastly, when travelling, it is possible to determine if vibrations are
caused by the front discs or the rear drums. Proceeding at low speed,
pull lightly on the handbrake: if there are vibrations the problem concerns
the rear drums (check and, if necessary, replace them). |
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BREMBO
ADVICE TO USERS 01
- Braking style - just like driving style - must
always be adapted to climatic conditions and the state of road and traffic. 02
- Braking distance not only depends on braking
system efficiency but also on the conditions of tyres and suspension. 03
- Optimal braking within a limited distance is
which provides maximum deceleration without locking the wheels, the consequences
of which would be the loss of vehicle stability and an increase in braking
distance. 04
- In the case of long downhill stretches it is
advisable to use the engine brake and above all not to switch off the
engine. After a brief stop, check pedal efficiency before starting off
again. 05
- When the situation requires a prolonged and continuous
use of the braking system, release pressure on the brake pedal from time
to time for a brief period. 06
- After stopping for a long period the first braking
actions will be affected by the stop, and also by climatic conditions.
A certain number of braking actions should be performed in order to re-establish
the full efficiency of the disc-pad unit. 07
- Ensure that all parts of the braking system function
correctly: fluid level, pad and disc wear, rear brake lights, parking
brake indicator, etc. Follow the manufacturers instructions as regards
replacement of the brake fluid (frequency, quality). 08
- Periodically check the efficiency of the parking
brake by trying to move the vehicle when this brake is on. 09
- Only use spare parts produced by recognised manufacturers. 10
- If possible, use a vehicle equipped with a braking
control system (ABS, etc.). |
