Analisis Comparativo Pruebas de Frenado
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Transcript of Analisis Comparativo Pruebas de Frenado
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International Journal of Automotive Technology, Vol. 13, No. 5, pp. 735742 (2012)
DOI 10.1007/s122390120072x
Copyright 2012 KSAE/ 06605
pISSN 12299138/ eISSN 1976-3832
735
COMPARATIVE ANALYSIS OF VEHICLE BRAKE DATA IN THE
MINISTRY OF TRANSPORT TEST ON THE ROLLER BRAKE TESTER
AND ON FLAT GROUND
C. SENABRE*, E. VELASCO and S. VALERO
Miguel Hernndez University, Elche 03202, Spain
(Received 27 May 2010; Revised 5 January 2011; Accepted 23 December 2011)
ABSTRACTThis study performs a comparison between what occurs when braking on a Ministry of Transport (MOT) brake
tester and on flat ground. The tire pressure is changed, but the other parameters remain constant. The results from this research
and from the in-depth comparative study conducted by the mechanical engineering staff in the mechanical laboratory at the
Miguel Hernndez University in Elche have led to the following main conclusions: By varying the tire pressure, false results
can be obtained with the MOT brake tester, which means that, if the tires are inflated at a low pressure but the brakes are in
good condition, the vehicle will not pass the MOT. Conversely, if the brakes are in poor condition but the tire pressure is higher
than what is recommended by the manufacturer, a false pass is produced. This article shows that the MOT brake testing
equipment is often wrong and inexact, and the data and graphs presented prove that the tire pressure is a determining factor
when assessing the condition of brakes.
KEY WORDS : Braking, Ministry of transport (MOT), Roller brake tester, Roller bed, Tire pressure
1. INTRODUCTION
Currently, the International Motor Vehicle Inspection
Committee (CITA) states that the condition of brakes must
be checked at the Ministry of Transport (MOT) Test
Stations. The European Community Law (2002) in Law
no. 29 237, Article 14, states that the driver of the vehicle
must present the vehicle to the MOT Ministry of Transport
Test Station, with the right tire pressure. However, the
MOT procedure manual from Ministry of Industry,
Tourism and Trade of Spain (2006) does not mention the
tire inspection procedure at the stations. This paper
analyzes how the tire pressure and extra weight on the
vehicle can affect the longitudinal braking action and
sliding on a MOT brake tester, and this braking action is
compared with brake sliding on a flat road.
When a vehicle is taken to a MOT testing facility, the
brakes are tested on a roller bed to check the brake circuit.
Several questions on the efficiency of the MOT testing
facilities need to be answered:
Does the braking on a roller brake tester accurately
reproduce braking on flat ground?
To what extent does the tire pressure affect the
measurements taken on the rollers?
Is this test safe enough to assess the condition of the
brakes?
Is this brake test 100% effective?
The aim of the study is to calculate a vehicle's braking
capacity by measuring the slippage on a roller brake tester
at a MOT center, compare this slippage with similar
measurements taken on flat ground, and use these results to
assess the machines reliability to test brake systems.
This is the first time that a comparative analysis of the
longitudinal braking on a MOT roller brake tester and on
flat ground has been conducted.
The contributions from many authors on longitudinal
braking have been analyzed.
1.1. Background
The oldest model, Fiala (1954), was the first to ever
produce an iteration of the tire on the ground, although the
tire was not considered with dimensions. Bergman (1965)
conducted the first theoretical analysis of the drift during
braking. Meyer and Kummer (1962) published a study on
the transmission mechanisms of the forces between the
wheel and the ground. Nordeen and Cortese (1963a)
studied the relationship between the F side and the F drive
and braking. Nordeen and Cortese (1963b) published
studies on the characteristic forces and moments that are
involved in tire rolling. Frank and Hofferberth (1967)
studied the mechanical properties of tire technology and
the qualities of rubber. Bekker (1969) published the first
study on the interactions in terrain-vehicle systems.
Livingston and Brown (1969) compared the effects of
different types of brake pressure. Dugoff et al. (1971)*Corresponding author. e-mail: [email protected]
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736 C. SENABRE, E. VELASCO and S. VALERO
developed the friction coefficient slip-speed expression of
the tread. Hays and Browne (1974) made a list of the
theory and data from the friction of rubber tires. They also
studied the physical properties of the drive wheel. Moore
(1975) conducted an extensive study on the relevant slip
and friction properties of the vehicle. Bernard et al. (1977)
published a study on the semi-empirical tire model used for
continuous slipping. Potts et al. (1977) studied the
vibrations that tires. Shearer (1977) analyzed the rolling of
the wheel and established methods to improve tires of the
wheels. Pacejka (2005) came up with mathematical models
that outlined the behavior of the steady drift and the
relationship between the rim and other parameters. Topkins
(1981) provided a massive collection of tire theory and
data. Sakai (1981) conducted in-depth theoretical and
experimental studies of advanced tire models and their
dynamic properties. Milliken and Rice (1983) published a
study about non-nimensionalizing tire data for vehicle
simulation. Sharp and El-Nashar (1986) conducted a
computer application-based mathematical model to
simulate rupture forces on the tires of wheels. Bakker et al.
(1989) devised a pseudo-experimental model, which is
called the Magic Formula model. Gim and Nikravesh
(1990), from the University of Arizona, conducted studies on
tire forces. Senabre et al. (2004) developed mathematical
models to simulate tire movement and interaction with the
surface (Mechanical Conference). Garca-Pozuelo et al.
(2009) studied improvements in the inspection procedure of
brake disc warping (Securitas Vialis).
2. TESTING METHODS
The vehicle used in this test was a Renault 21 model
Nevada that seats 7 and has a diesel engine. It has front disc
brakes with sliding clamps, uses DOT 4 brake fluid, and
uses a tandem brake pump. The rear wheels have drum
brakes.
The measurements were taken from 2 encoders fitted to
the roller brake tester, and the wheels of the vehicle and a
pressure sensor were attached to the vehicle's brake circuit.
These instruments were used to measure the pressure in
the hydraulic circuit after pressing the brake pedals and to
correlate the data to the speed of the wheels
The two tests conducted are described below:
2.1. Test 1
The braking is measured on the roller brake tester at a
MOT center by placing the vehicle on rollers that rotate at
5 km/h (see figure 2). All roller banks on the MOT brake
testers rotate at 5 km/h in all MOT stations in Spain. This
velocity is enough to obtain 100% wheel slip.
The vehicle tries to stop the wheel through braking. A
sensor in the hydraulic pipe placed on the front right wheel
is used to obtain data on the brake. The slip value was
measured using the angular velocity of the rollers and the
vehicle wheel data.
The emergency brake should not be on. The brake pedal
has to be pressed down until 100% slippage is obtained.
After 100% slippage and with a fully locked wheel, the tire
cannot transmit additional brake torque to the terrain or the
roller tester sensor, which means that the pressure in the
brake circuit after this point is ineffective.
The torque on the rotation axis of the rollers is measured
using a strain gauge. The pressure in the hydraulic brake
circuit on the right front wheel of the vehicle is also
measured using a sensor in the hydraulic pipe of the right
front wheel, as shown in figure 1. There is a pipe for the
brake of each wheel, and the pipe in the right front wheel is
no longer than 2 m; therefore, the position of the sensor on
the pipe does not affect the measurements because there are
no flow losses.
Measurements were taken from the 2 encoders attached
to the roller brake tester and the right front wheel of the
vehicle, and a pressure sensor is attached to the vehicle's
brake circuit. These instruments were used to measure the
pressure in the hydraulic circuit after pressing the brake
pedals and to correlate the data to the speed of the wheels.
The test data were recorded using an LMS Pimento
portable, multi-channel analyzer. This analyzer provides for
real-time monitoring of the process and data recording.
To measure the revolutions of both the rollers and the
vehicle wheels, 2 OMRON encoders were used. One
encoder was the E6B2-CWZ6C model, and the other
encoder was the E6B2-CWZ1X model. One encoder was
attached to the right-hand roller of the roller brake tester (see
Figure 1. Position of the hydraulic sensor.
Figure 2. Hydraulic sensor in pipe to right front wheel.
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COMPARATIVE ANALYSIS OF VEHICLE BRAKE DATA IN THE MINISTRY OF TRANSPORT TEST 737
figures 3 and 4), and the other encoder came in contact with
the front right-hand wheel of the vehicle (see figure 4). To
ensure that the rotation of the encoders was synchronized
with the rollers and the wheels, connectors were built as
shown in figures 3 and 4 such that a spring guarantees that
good contact is achieved.
The slip in test 1 was calculated from the data recorded
during the test using expression (Dixon, 1996):
(1)
where rvehicle wheel is the brake torque on the wheel / the brake
force on the roller.
The torque at the roller axis is measured using the MOT
sensor, which is a gauge that determines the torque on the
shaft.
The brake torque was calculated in the following
manner. The driver released the brake pedal until the
torque generated at the wheel was higher than the
pressure applied to the brake circuit such that the torque
produced a rotation of the wheel (see figure 5). A triggers
is used, it measure 0 volts when the wheel rotate and a
circuit with a 5 volts battery is open, in that point the
measurement of the pressure in the brake circuit is
measured to calibrate the pressure sensor.
2.2. Test 2
In this test, the vehicle runs on flat ground until it attains a
velocity of 40 km/h, which is required to obtain 100%
slippage. In the test 2, It is braked the vehicle until the car
slides by 100%. The same signals as in test 1 are recorded
for the brake test. Both sensors are used in the two tests to
make sure that the measurement has been acquired in the
same manner.
A fifth wheel, driven by the vehicle, has been attached to
measure the slippage (see figure 6). This is used as a
reference. The track is assumed to be completely flat, with
no changes in the adhesion coefficients of the route. A
spring was attached to the fifth wheel to ensure good
contact. The velocity of the car is 40 km/h, as this velocity
is required to obtain 100% wheel slippage.
The values of the coefficient of adherence () (Dixon,
1996), which are affected by several internal factors
associated with the wheel and the vehicle, by external
factors, and even by environmental conditions where there
is movement, appear to be constant.
The adhesion coefficient is 1 for test 1 and 1 for test 2
(Dixon, 1996). This coefficient from the tester value is the
factory-supplied value for the MOT testers.
Therefore, the only difference in the measurements is the
change in tire pressure in both tests.
In test 1 and in test 2, once the test data were recorded,
any data points that were not characteristic were filtered and
deleted so that better results and a suitable interpretation
could be obtained.
Slip 1speed of vehicle wheel
roller speed-------------------------------------------------------=
1rvehicle wheel* Wr of vehicle wheel
rroller* Wr of roller-----------------------------------------------------------=
rroller 0.1 m=
Figure 3. Test 1 roller brake tester at the MOT center.
Figure 4. Encoder attached to the roller brake tester.
Figure 5. Pressure relation in the brake circuit and brake
torque on the wheel.
Figure 6. Fifth wheel or the drive wheel.
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738 C. SENABRE, E. VELASCO and S. VALERO
Sick Stegmann DKS-40 encoders were used in this
test, and the encoder was attached to the fifth wheel of the
vehicle, the drive wheel (see figure 4), and the other
encoder was attached to the front right-side wheel of the
vehicle, which is the same as that in the previous test.
The slip was calculated from the data recorded during
tests by means of expression (Aparicio, 2001):
(2)
The slip can be calculated if the angular velocity is
calculated:
All of the slip data are obtained when the wheel is
locked, hence: wr of vehicle wheel = 0 and wr of fifth wheel >0, where re
fifth wheel is the fifth effective rolling radius and can be written
as re fifth wheel = Vvehicle / wr of fifth wheel, r e vehicle wheel is the wheel
effective rolling radius, and Vvehicle is the vehicle velocity.
The value of wvehicle wheel/wfifth wheel are multiplied by the
fraction re vehicle wheel / re fifth wheel , which is considered to be
constant to simplify the calculation.
The vehicle velocity is determined with a Wilcoxon
784A accelerometer that is placed inside the car, as shown
in figure 7.
The velocity is obtained by integrating the acceleration
values acquired from the accelerometers, located on the
floor by the rear seats of the vehicle, which is the center of
gravity of the vehicle.
Additionally, 2 more accelerometers were installed to
make sure there is no vertical (z axis) or horizontal (y
axis) acceleration.
3. RESULTS
A tire with pressure that is too low overheats, the surface
does not wear uniformly so more fuel is consumed. The tire
is less durable, becomes more sensitive to impact and has a
lower resistance to stress and strain, which means it might
become irreparably damaged. At a pressure lower than 1
bar, the tire may even come loose from the rim. This did
not happen in this study because a pressure of 1 bar or
greater was used. The tires have been inflated to have
pressures of 1, 1.5 and 2 bar. These three values clearly
show how events occur, although the range was increased in
later studies.
The measurements of the brake pressure on the vehicle
and the slippage in the braking wheel were obtained for
both tests.
A comparative analysis was conducted on the braking
and slip measurements for the same test performed with
different tire pressures to examine how the test evolved.
The braking and slippage data were also compared for both
tests at the same tire pressure to investigate if there are any
differences in the braking measurements between the roller
tester and flat ground.
3.1. Comparison of the Braking-slip Ratio Data at Various
Tire Pressures for Both Tests
3.1.1. Results and discussion from test 1
When the brake pedal is pressed down, the pressure values
in the circuit and the corresponding slippage that is obtained
in test 1 for different tire pressures are shown below.
The measurements produced from the pressure sensors
versus time are as shown in figure. 8:
From the data obtained, only the measurements taken up
to the maximum slippage are considered, which corresponds
to 100% wheel slip.
Once the measurements have been processed, which
Slip 1speed of vehicle wheel
seep of fifth wheel-------------------------------------------------------=
Slip 1speed of vehicle wheel
speed of fifth wheel--------------------------------------------------------- ==
1re vehicle wheel* Wr of vehicle wheel
re fifth wheel* Wr of fifth wheel--------------------------------------------------------------=
Figure 7. Accelerometer installed inside the vehicle.
Figure 8. Pressure measurements in the brake circuit of the
vehicle versus time on the MOT roller brake tester and on
flat ground with 1-1.5-2 bar of wheel pressure.
Figure 9. Start and end points of the measurements.
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COMPARATIVE ANALYSIS OF VEHICLE BRAKE DATA IN THE MINISTRY OF TRANSPORT TEST 739
means removing anomalies, such as sensor or wire
disconnection, the useful part of the measurement is
obtained. All data files have a clear start and end point (see
figure 9). The starting point has to be the last point with 0
pressure in the brake circuit, and the last point is the
maximum slip value. If the measurements begin with a
torque value greater than 0 the file is removed, and if the
measurement finishes without a maximum and constant
slip value, the file is deleted.
A summary for test 1 was obtained for each tire pressure
value and is shown in figure 10.
As seen in the graph in figure 10, when the tire pressure
increases, the braking pressure required for the braking
system to be able to stop the vehicle on the MOT roller
tester increases as well, and there is some delay in the slip
increase. This means that when the tire pressure is too low,
the maximum slip value can be obtained with less pressure
on the vehicles brake circuit.
3.1.2. Results and discussion from test 2
This test shows that there are smaller differences in the
brake curves in relation to the slippage at various tire
pressures compared with the MOT test 1, as shown in
figure. 11.
The maximum pressure applied in the brake circuit to
stop a vehicle moving on flat ground is observed to decrease
by 6.09% from 128.62 to 121.24 bar as the tire pressure
increases from 1 to 2 bar, as shown in the maximum values
in figure 11. This, however, is not as significant as the
48.1% increase in the maximum value of the pressure in the
brake circuit in test 1, which increased from 45.07 to 86.83
bar and was enough to obtain 100% slippage as the tire
pressure increased from 1 to 2 bar (see figure 9).
3.2. Comparison of the Results from Both Tests
When comparing the pressure curves in the brake circuit
for both tests and the slippage for various tire pressures, the
graphs in figs. 11, 12 and 13 show a clear and distinct
difference in the pressure required in the brake circuit to
stop the vehicle.
As shown in figure 12, the % difference of the maximum
value of the pressure in the brake circuit between both tests
for a tire pressure of 1 bar is 65% if 100% slippage is to be
obtained. This difference decreases to 40.64% when the
tire pressure is 1.5 bar (see figure 11), and it drops to
28.38% when the tire pressure is 2 bar (see figure 14).
Figure 10. Pressure in the brake circuit for the slippage in
test 1 at various tire pressures.
Figure 11. Pressure in the brake circuit for slippage in test 2
at various tire pressures.
Figure 12. Comparison of the pressure in the brake circuit
versus slippage when the vehicle is on the roller tester and
on flat ground at a tire pressure of 1 bar.
Figure 13. Comparison of the pressure in the brake circuit
versus the slippage of the vehicle on the roller tester and on
flat ground at a tire pressure of 1.5 bar.
-
740 C. SENABRE, E. VELASCO and S. VALERO
Although figure 14 shows a reduction in the difference
in braking pressure between both tests, a roller brake tester
does not reproduce braking on flat ground.
3.3. Comparison of the Braking Data for Both Tests
According to the tests conducted, the pressure in the brake
circuit required to stop the wheel for the test on flat ground
decreases as the tire pressure increases. Therefore, at tire
pressures lower than 2 bar, a higher pressure is required in
the brake circuit to stop the vehicle. This should be avoided
so that less energy is consumed and the vehicle can be
stopped in time.
However, this result does not correspond to what occurs
on the roller brake tester. As observed in fig. 15, the
maximum pressure applied to the brake circuit to obtain
maximum slippage (and the vehicle leaves the tester, as a
result) increased as the tire pressure increased, and
therefore, a greater hydraulic pressure in the brake circuit
has to be applied in the case of higher tire pressure for the
vehicle to leave the tester.
One of the possible reasons why this tendency is
observed at the peak braking values in the MOT is that the
tire deforms on the rollers, and at higher tire pressures, it
deforms less, resulting in less contact area between the tire
and roller. This means that with a smaller frictional area, a
greater force is required with the rollers before the vehicle
can leave the tester.
In contrast, the braking force required to stop the vehicle
on flat ground, regardless of the tire pressure, is always
greater than the force required on the roller tester. This is
due to the fact that, when a moving vehicle is stopped, the
forward inertial motion must be counteracted, and this does
not occur on the roller brake tester.
Quantitatively, if the tire pressure is 2 bar, the pressure
sensor in the roller brake tester sensor captures a 28%
difference in the measurements from the test on flat
ground, and this increases to 65% if the tire pressure drops
to 1 bar.
The minimum brake efficiency required to pass the
MOT test is 48-50% for vehicles without ABS. Our vehicle
does not have ABS. This is pursuant to Directive 96/96
CEE (EC) and the MOT vehicle inspection manual.
The efficiency is:
where E is the efficiency, F is the sum of the braking forces
from all the wheels, M.M.A is the maximum permissible
vehicle mass in kg (1350 kg), and g is the acceleration due
to gravity.
If there is a pressure of 1 bar in the brake circuit, 13.87 N
of force is available to brake each wheel (see figure 5).
Therefore, 121.24 bar (maximum brake value) in the
brake circuit is equal to 1681.5 Nm, and this value over 4
wheels is 6726 N.
If the pressure of the brakes is equal in the 4 wheels, the
efficiency is:
E= 6726 N/(1140 kg*10 m/s2)= 59%
Therefore, this vehicle should pass the MOT test
because its brakes are in good condition, and this vehicle
should pass the test with pressure of 2 bar or less in the
tires.
However for 2 bar of wheel pressure, the maximum
value in the brake circuit is 86.83 bar for test 1, and for 4
wheels, the brake torque is 4817.3 N.
E = 4817.3 N/1140 kg*10 m/s2= 42.2%
Therefore, this vehicle will not pass the test 1 with 2 bar
of tire pressure or less.
If the tendency of the maximum bar in the brake circuit
for test 1 (figure 12) is captured by the following:
EF
M.M.A.g---------------------100=
EF
M.M.A.g---------------------100=
Figure 14. Comparison of the pressure in the brake circuit
versus slippage of the vehicle on the roller tester and on
flat ground at a tire pressure of 2 bar.Figure 15. Relation of the maximum pressure in the brake
circuit for different tire pressures when the vehicle is
running on the MOT roller tester and on flat ground.
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COMPARATIVE ANALYSIS OF VEHICLE BRAKE DATA IN THE MINISTRY OF TRANSPORT TEST 741
y = 41.78x+8.81,
where x is wheel pressure, the maximum brake value for
2.5 bar of wheel pressure can be predicted.
Hence, the efficiency for 2.5 bar of tire pressure will be:
E = 107*13.87*4 N/1140 kg*10 m/s2 = 52.4% > 48%
The car will pass test 1 with 2.5 bar of wheel pressure
because the efficiency is greater than 48%.
Our conclusion is that the car will pass test 1 only with a
wheel pressure 2.5 bar or more, and it will fail the MOT
test with a wheel pressure lower than 2.5 bar even if the
brake system is in good condition.
4. DISCUSSION AND CONCLUSIONS
This study was conducted after ensuring that the measurement
control was the same in both tests. Indeed, large differences in
the pressures in the vehicle brake circuit were required to
stop the vehicle before obtaining 100% wheel slip. The
maximum pressure applied in the brake circuit to stop the
vehicle movement on flat ground was observed to decrease
by 6.09% from 128.62 to 121.24 bar as the tire pressure
increased from 1 to 2 bar (see figure 7). However, this is
not as significant as the 48.1% increase in the maximum
value of pressure in the brake circuit, from 45.07 to 86.83
bar, in test 1 to obtain 100% wheel slippage, as the tire
pressure increased from 1 to 2 bar (see figure 6). Therefore,
there is an adverse tendency in tracking the pressure
measurements in the brake circuit related to the wheel slip
for the 1 and 2 bar tire pressures in both tests.
On flat ground, less braking force was required to stop the
vehicle as the tire pressure increased, and the opposite was
observed with regard to the roller test when tire pressure
increased. The braking force was required to increase as
well, i.e., the pressure in the brake circuit has to be increased
to obtain 100% slippage. This means that, as the tire pressure
increases, the roller brake tester captures the measurements
that are closer to those of the flat ground test.
In summary, a low tire pressure on the MOT roller brake
tester produces values in the brake pressure circuit that may
vary up to 65% from the real braking force required on flat
ground.
The difference in the measurements between the two
tests may be due to the following:
(1) There is a visible difference between the processes
compared: in test 2 (on flat ground), the brake circuit
pressure required to stop the vehicle was measured,
and (on the roller tester) in test 1, the pressure required
to eject the vehicle from the roller tester was also
obtained.
(2) On flat ground (test 2), the forward motion inertia has to
be counteracted, which is not observed on the roller
tester.
(3) The contact zone between the tire and the MOT roller
test track decreases as the tire pressure increases;
therefore, a larger area of friction with the rollers will
involve a greater braking force.
(4) The contact zone between the tire and the flat ground is
230 mm2 and the contact zone between the tire and the
roller is 48.04 mm2 for a wheel pressure of 2 bar. This
difference affects the maximum value of the pressure in
the brake circuit because less contact means that 100%
slippage with a lower pressure in the brake circuit can
be obtained (see figures 16 and 17).
(5) According to the measurements taken when the vehicle
is stationary, the contact surface between the tire and
the roller, at any tire pressure, is always less than the
contact area of the wheel on flat ground.
(6) The velocity could be a reason why there is a difference
between the brake measurements in the two experiments.
Therefore, the brake efficiency has been demonstrated to
depend on the condition of the brakes and on the tire
pressure during inspection on a roller brake tester. As
mentioned before, the car would pass test 1 only with a tire
pressure of 2.5 bar or more, and the car would fail the
MOT test with a tire pressure lower than 2.5 bar, even if the
brake system was in good condition.
Consequently, the suitability of a roller brake tester to
determine whether a vehicles brakes are in good condition
Figure 16. Print on flat ground.
Figure 17. Print on the MOT brake tester.
-
742 C. SENABRE, E. VELASCO and S. VALERO
is debatable.
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