Rolling resistance measurement on the road:A dream?
Dr. Jürgen Bredenbeck
Tire Technology Expo,
14.-16. February 2012 Cologne

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Motivation
Introduction of the used Measurement Equipment
Introduction of the theoretical approach
Description of the Test procedure
Results
Summary / Conclusions

Motivation
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The demand to higher efficiency concerns each component of future
vehicles
ehiclesTire resistance is identified as one of the areas for efficiency
improvements independent of vehicle drive concepts
Understanding the behavior in real road conditions will becomemore important
Standard testing methods (drum based) do not deliver roadcondition related information
Real road conditions measurement was suffering from:– Accurate measurement equipment for the forces asTire resistance value is relative low– Low repeatability– Ability to separate different influence sources

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Motivation
Introduction of the used Measurement Equipment
Introduction of the theoretical approach
Description of the Test procedure
Results
Summary / Conclusions

Measurement Equipment on Road
• Vehicle measurement System (VMS)– Wheel Force Sensor(WFS)– Wheel Position Sensor (WPS)– Other sensors such as GPS– Vehicle ECUInformation

Rig Measurement Equipment• Flat belt tire testing rig (steel belt)
 Best simulation of the road• Test is performed with the same sensor used for the vehicle testing

Rig Specification
Velocity

0~200km/h

Slip Angle±20deg（0~3Hz）

Camber Angleg±15deg（0~1Hz）g

Up & Down

0~50mm（0~25Hz）

Load

Fx: ±10 kN
Fy: ± 10 kN
Fz: 12 kN

Flatness of the steel belt
(under load condition)Less than 10 μm

Bearing under the belt

Air bearing

Wheel Force Sensor （WFS)

6 component iin wheel
h l fforce sensor maini propertiesi• 3 axis of force and 3 axis of moment• Total error 0.1%
• Capacity:– Fx = 24KN, Fy = 15KN, Fz = 24KN
– Mx= 4.5 KNm, My =4 KNm,
Mz = 4.5KNm• Resolution 1/4000– 6N or 1.8Nm• Data acquisition up to 1kHz
• Lightweight 3.2 Kg

Unique Force Detection Method•

Model Based Sensor concept– Shared force detection method• Eight bridges are applied to the spring element• NoN didirectt ddetectiont ti off eachh componentt• Components are re-composed by model based calculation using realplatform
time calculation DSP p• Digital conversion of all signals
and electronically re-composing
overcomes disadvantages ofanalogue approach• Cross talk error can be canceledout

Di stributed
force bridge

High speed
A/DHigh speed
DSP

Minimized Temperature effects• Vehicle measurement is a challenge for the temperature influence– Temperature gradient e.g. break side outside– Quick change of temperature depending on driving maneuver• Need for robust design against Temperature effects– Share Force method allows to place the strain gauges very close toeach other
T: High– Total gradient on each gauge is very small– Small temperature effect on themeasurement
4 elements– At the same time robustness againstgauge
dynamic temperature changes

T: Low

Mechanical and Electrical sensitivity•
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Application needs stiff sensor and high accuracy
Sensor sensitivity:– Mechanical sensitivity x electrical sensitivity
Stiff Spring element design results in:– Increase of robustness– Increase of eigenfrequency– Reduction of mechanical sensitivity
Increase electrical sensitivity by utilizing:– High precision A/D/ converting off nV order– Low noise design from less analog circuit– Optimized temperature compensation from gauge layout
The combination of all technology results in a high accurate sensor
with 1/4000 resolution

Wheel Force Sensor Configuration

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Motivation
Introduction of the used Measurement Equipment
Introduction of the theoretical approach
Description of the Test procedure
ResultsSummary / Conclusions

Tire Loss Theory• Tire loss can be calculated from measuredparameters
ton theth wheelh l• Measurement parameters• Tire rolling inertia

Jt in kg・mkg m2• Tire effective radius

rt in mDriven Wheel
Tire• Wheel torque My in Nm• Tire longitudinal force Fx in N
ω in rad/s• Tire Angular speed• Tire Angular acceleration

ώ rad/s2• Calculated parameter• Tire loss (rolling resistance) Rx in NRx My  J t  rt Fx

MyFxRx

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Motivation
Introduction of the used Measurement Equipment
Introduction of the theoretical approach
Description of the Test procedure
Results
Summary / Conclusions

Testing procedure on the test track• Target: Determine “Tire Loss” from real driving condition• Test car: BMW Mini Cooper SStopSt tStart• Test Track:- East straight line: 550m
- West straight line: 554m

• Driving Maneuver:– Cost down at East straight line
– TestT t laps:l
10 llaps•

100Hz data acquisition

Coast doCown– Acceleration at west straight line

Acceleeration- Total length: 1,792m

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Motivation
Introduction of the used Measurement Equipment
Introduction of the theoretical approach
Description of the Test procedureResults
Summary / Conclusions

Test Track Measurement Results• Example plot of one round• Fx shows mainly difference
ffbetween front and rear wheel• Fz shows change
g between
left and right

Coast Down

Acceleration
Stop

70s

60s

0s
10s

50s

Bank20s40s
30s

StartBump

Parameter Determination

Direct Measures from the sensor:• Wheel torqueMy in Nm

• Tire longitudinal force

Fx in NRx 

Indirect Measures:• Tire rolling inertia

Jt in kg・m2• Tire effective radius

rt in m• Tire Angular acceleration ώ rad/s2My  J t  rt Fx

Wheel inertia• Tire rolling inertia is premeasured
using free load rotating wheel inacceleration and deceleration
condition• Measurement
Mt ititems– Tire angular speed ω [rad/s]– Angular acceleration ώ [rad/s2]– Wheel torque Myfree [Nm]• Rolling inertia formula:

My freeJt 

TTorqueVs.
V AngularAl accelerationl tiAngular acceleration determination
70s• Tire angular speed is measured from sensor
angle encoder.
• Tire angular acceleration is calculated fromangularl speed
d signalilbby titime dderivativei ti• Measurement item:– Tire angular speed
ω [rad/s]– Tire angular acceleration dω
dt

60s

0s
１0s

50s

20s
40s
30s

Tire radius determination• Tire mean radius is calculated from vehicle velocity and tire angularspeed.
d• Vehicle velocity is measured from optical Doppler sensor• Instant tire mean radius is measured.
• Measurement items– Vehicle velocity against
road Vph [m/s]– TireTi angularl speeddω [rad/s]• Tire radius formula(Not considering tire slip)

rt Vph［m］Measurement parameter: Wheel torqueand longitudinal force• WheelWh l torque M
My anddlongitudinal force Fx aremeasured from 6 componentsof the Wheel Force Sensor(WFS)

70s

60s

0s
10s

50s

20s
40s
30s

Rolling Resistance Results• To avoid tire slip error, driven
wheel data is evaluated• 10 laps of data
• To avoid some high frequency
noise a low pass filter (4 Hz) isapplied to the measurement data
• Very good repeatability for 10laps
70s

60s

0s
10s

50s

20s
40s
30s

Rear Left Wheel results• Average Rx: Rx = -76.1N (Acceleration), Rx = -72.8N (Cost down)• 10 laps data variation 3σ : 2.8N
2 8N (Acceleration)(Acceleration), 33.6N
6N (Cost down)• Rx for Acceleration and Rx for Cost down data are very close to each other: 3.3NAcceleration

Bump
Coast Down

Acceleration

Coast Down

Rear Right Wheel results• Average Rx: Rx = -87.6N (Acceleration). Rx = -82.6N (Cost down)• 10 laps data variation 3σ: 2.5N (Acceleration)., 6.6N (Cost down)• Rx for Acceleration and Rx for Cost down data are very close to each other: 5.0N

Acceleration

Bump

Acceleration

Coast Down

Coast Down

Measurement result :Test rig
Test condition:• Slip angle :0 [deg]• Camber angle: 0 [deg]• Wheel driven by steel belt
• Vertical load Fz: 1kN,, 2kN,, 5kN• Static velocity: 5km/h, 10km/h,20km/h, 60km/h, 80km/h, 120km/h• Rolling resistance is directly measured
from Fx using same sensor as on theroadResults:• Rolling resistance is proportional to thevertical load and is not a function ofvelocity• Rolling resistance at 2.7kN is 42N

Comparison: Real road vs Test rig
Real road rolling resistance :• Rx(Left)= 74 N• Rx(Right)= 82 N
Test rig:• Rx = 42 N

Reasons for the difference:• Tire alignment on Road and rig isdifferent• Road surface condition• Environment conditions– Wind force to tire
– Temperature•

Measurement errors– Tire effective radius measurement

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Motivation
Introduction of the used Measurement Equipment
Introduction of the theoretical approach
Description of the Test procedureResults
Summary / Conclusions

Summary and Conclusion
Summary:• A&D Sensor
Sdeliversd lihighhi h quality
lit ddatat• Repeatability of 10 lab data did show good match• It was possible to measure the tire loss (rolling resistance) during real driving
condition• Great match on the results though 10 laps of data
• Rolling resistance measurement result is depending on driving conditions• We did observer difference between acceleration and coast down
conditions• There are differences between road and test rig results
CConclusion:l i• WFS is a useful tool for analyzing energy loss at real driving condition• We are very close to the dream and will continue this investigation

Thank you for your attention!

You can find us on booth no. 8387 of the Expo