# Presentation: 262121

## Rolling Resistance of Tires for Light Vehicles, II:  Comparison of Rolling Resistance Values Obtained from Different Test Methods and in Different Laboratories 

_**Larry Evans**_** & John Harris, Transportation Research Center, Inc.**
**James D. MacIsaac Jr., USDOT - NHTSA**
**Presented at ITEC 2008**
**Akron, OH**
**September 17, 2008**

## Objective

**Select a Method to Measure Tire Rolling Resistance in Order to Compare Tires**
- SAE J2452 values at various speed, load, and inflations used to relate tire properties to vehicle
- SMERF (Standard Mean Equivalent Rolling Force)
- SAE J1269 and ISO 18164 describe the tire response at various load and inflation conditions
- SRC (Standard Reference Condition)
- SAE J1269 and ISO 28580 test a tire at  standard conditions of speed, load, and inflation

## Measuring Rolling Resistance

***Measuring Rolling Resistance***

**Evaluate Sources of Variability in Testing**
-  Tire
- Different Tire Types
- Different Tires of Same Type
- Same Tire – Repeat Testing
-  Lab
- Test

## Test Matrix

**Five Test Protocols**
**25 Tire Types**
**2 Labs**
**Capped or Regulated Pressure (J1269)**
**First, Second and Third Tests on Individual Tires**
**Different Inflation Gas**
**641 Tests**
**Only one tire was an outlier from other tires of the same model**

## ANOVA

**Analysis of Variance was conducted on results from each test**
- SAE J1269 Single-Point
- SAE J1269 Multi-Point
- Calculated value at SRC using regression specified
- ISO DIS28580 Single-Point
- ISO 18164 Multi-Point
- Calculated at SRC using J1269 regression method
- SAE J2452
-  SMERF
- Calculated value at SRC using J2452 regression

## SAE J1269 Single-Point

**Dependent Variable: Rolling Resistance**
**                                               ****	****Sum of****	****     Mean**
**       ****Source                      ****	****DF         ****	****Squares     Square    ****	****F Value    Pr > F**
**       ****Model                       ****	****29     ****	****49358.7      1702.0    ****	****15122.2****   ****<.0001**
**       ****Error                      ****	****191              21.5            0.11**
**       ****Uncorrected Total    ****	****220     ****	****49380.2**
**                       ****R-Square     Coeff Var      Root MSE       RR Mean**
**                       ****0.995985****      2.371565      0.335487        14.14623**
**       ****Source                      ****	****   DF     Type III SS     Mean Square    F Value    Pr > F**
**       ****Lab Where Tested            1           9.45        ****	****   9.45          83.99    <.0001**
**       ****Procedure for Inflation    1   ****	****     2.99        ****	****   2.99          26.62    <.0001**
**       ***Test Order                         2           0.07        **	**   0.036          0.32    0.7265*
**       ****Type (Tire Model)           24     4871.64      ****	****202.98     1803.49    <.0001**

## ANOVA Comparison

| Test | J1269 Single-Point | J1269 Multi-Point | ISO DIS28580 | ISO 18164
(10 Tires) | J2452 SMERF |
| --- | --- | --- | --- | --- | --- |
| F Value | 15,122.2 | 15,929.5 | 8,320.9 | 2,687.6 | 23,535 |
| R2 | 0.9960 | 0.9960 | 0.9967 | 0.9891 | 0.9953 |
| F Value -  Lab | 83.99 | 11.24 | 2.14 | 4.30* | 544.34 |
| F Value -  Capped / Regulated | 26.62 | NA | NA | NA | NA |
| F Value – Test Order | 0.32 | 0.23 | 0.68 | 4.30* | 26.57 |
| F Value – Tire Model | 1,803.49 | 4,574.38 | 853.61 | 89.94 | 1,699.70 |

** Variables were confounded*

## ANOVA Conclusions

**F and R2 values indicate that the sources of variation are accounted for in model**
- Tire type is the most significant variable
- The two labs studied produced significantly different values
- Capped vs. regulated inflation pressure during the test was significant
- First, second, or third test on the same tire was not a significant variable

## Lab Variation

**Test order (1****st****, 2****nd****, 3****rd****) ignored**
**Coefficient of Variation within a lab for a tire type on a test was ****≤ ****2.5%**
- Values were normally distributed within each lab
**There was a significant variation between labs for all tests**

## Lab Offset J1269 Single-Point Test

***Rolling Resistance @ Lab B***** =  0.257 + 1.024*(Lab A)**

## Lab Offset

| Test | Correlation of Lab B to Lab A | R2 |
| --- | --- | --- |
| J1269 
Single-Point | B = 0.257+1.024*A | 0.9975 |
| J1269 Multi-Point | B = -1.746 + 1.012*A | 0.9659 |
| ISO 28580 Single-Point | B = -0.099 + 1.012*A | 0.9623 |
| ISO 18164 | B = 0.714 + 0.908*A | 0.9623 |
| J2452
SMERF | B = -0.143 + 1.077*A | 0.9915 |

## Lab to Lab Correlation

**Statistically significant offset between labs.**
- Lab B had higher values than Lab A for 3 of the tests
- Lab B had lower values than Lab A for 1 test
- Equal on average for 1 test
**A slightly better prediction of lab offset could be obtained with a non-linear equation, especially for multi-point tests**

## Lab-to-Lab Offset

**Correlation of Lab B to Lab A for all conditions of the J1269 Multi-Point test. ****Test Point 1 ***[Capped Inflation]*** is significantly offset**

## Variability of Test

| Test | Coefficient of Variation |
| --- | --- |
| J1269 Single-Point | 2.4% |
| J1269 Multi-Point | 2.4% |
| ISO DIS28580 | 2.2% |
| ISO 18164 (10 Tires) | 5.2% |
| J2452 SMERF | 1.9% |

## Measuring Rolling Resistance

***Measuring Rolling Resistance***

**Sources of Variability in Testing**
- Different Tires of Same Type
- Only 1 Tire significantly different
- Same Tire – Repeat Testing
- No significant effect
- Lab
- Significant differences
- Varies by test and conditions
- Test
- All tests have low Coefficient of Variation (C.V.)

## Ranking Tire Rolling Resistance

**Data for values from Lab A corrected to Lab B values using correlation equations**
**Do tests rank-order tires differently?**
**Rolling resistance of tires was grouped using least significant differences at 95% confidence level for data from each test**

## Correlation of Test Data

| 1 =  ISO 28580 single-point value |
| --- |
| 2 = SAE J1269 multi-point value @ SRC |
| 3 = ISO 18164 value @ SRC |
| 4P = SAE J2452 value @ SRC, Passenger Tires |
| 4T = SAE J2452 value @ SRC, Light Truck Tires |
| 5P = SAE J2452 SMERF value, Passenger Tires |
| 5T = SAE J2452 SMERF value, Light Truck Tires |

## Passenger Tire Groups

| Group
 | J1269 Single-Point | J1269 multi-point@ SRC | ISO 28580 | ISO 18164 | J2452 @ SRC | J2452, SMERF |
| --- | --- | --- | --- | --- | --- | --- |
| 1 
(Lowest) | B11
G8
G11 | G11
B11
G8 | G8
B11
G11 | G11
G8
B11 | G11
B11
G8 | G11
G8
B11 |
| 2 | G9
G10
M13
M14
B10 | G9
G10
M14
M13
B10 | G9
M13
M14
G10
B10 | G9
M14
G10 | G9
M13
G10
M14
B10 | G9
M13
G10
M14
B10 |
| 3 | D10
U3
P5
B14
B15 | U3
D10
P5
B14
B15 | D10
B14
U3
B15
P5 | U3
B14 | D10
U3
B14
P5
B15 | D10
U3
B14
P5
B15 |
| 4
(Highest) | R4
B13
B12 | B12
R4
B13 | R4
B13
B12 | B13
B12 | R4
B12
B13 | R4
B12
B13 |

## Light Truck Tire Groups

| Group
 | J1269 Single-Point | J1269 multi-point@ SRC | ISO 28580 | J2452 @ SRC | J2452, SMERF |
| --- | --- | --- | --- | --- | --- |
| 4 
(Lowest) | M10
M12
M11
D8
K4
D7
P4 | M10
M12
K4
M11
D8
P4
D7 | M10 
M12 
M11
K4
P4
D8
D7 | M12
M10
M11
K4
P4
D8
D7 | M12
M10
M11
K4
P4
D8
D7 |
| 5 | D9 | D9 | D9 | D9 | D9 |
| 6
(Highest) | C9 | C9 | C9 | C9 | C9 |

## Selection of Method

**All methods produce data with low variation**
**All methods rank tires into the same groups**
**Data from any method can be correlated to data from any other method**
**Single-point method is most efficient**
**Any method selected will need a procedure to account for lab-to-lab differences**

## Ranking Tires Using RRC

**Rolling Resistance Coefficient (RRC) is often used to report the rolling resistance of tires**
- RRC is Rolling Resistance Force / Normal Force
- Using same units removes the dimension
**When comparing tires tested at the same load, there is no change in comparisons**
**Can RRC be used to compare tires of different load ratings?**

## Correlation of Test Data

| 1 =  ISO 28580 single-point value |
| --- |
| 2 = SAE J1269 multi-point value @ SRC |
| 3 = ISO 18164 value @ SRC |
| 4P = SAE J2452 value @ SRC, Passenger Tires |
| 4T = SAE J2452 value @ SRC, Light Truck Tires |
| 5P = SAE J2452 SMERF value, Passenger Tires |
| 5T = SAE J2452 SMERF value, Light Truck Tires |

## RRC Calculation

**While RRC is dimensionless, it is not independent of conditions of testing**
**For passenger tires using J1269 RR:**
*RR**_Force **= Load*(A**1** + A**2***Load + A**3**÷**Pressure)*
*Dividing both sides by load*
*RRC = A**1** + A**2***Load + A**3**÷**Pressure*
*At constant pressure*
*RRC = A**2***Load + A**1 **+ A**3**÷k*
***RRC is still a function of load***
*J2452 and LT tires are even more complex*

## RRC Vs. Load – Tire Type D7

**RRC is between 0.0093 and 0.0161, depending on test condition.**
** ****It is not an intrinsic property of the tire**

## Conclusions (I)

**Up to 3 repeat tests of the same tire has no significant effect on rolling resistance values**
**As expected, testing with capped inflation pressure produces a lower value, due to increased pressure**
**Tires of the same model and size produce equivalent rolling resistance values, ± ~6%**
- Data is normally distributed
- 1 outlier tire discovered

## Conclusions (II)

**Lab-to-Lab variation is significant **
- Dependent on test conditions and protocol
**All tests produce reliable data with low variation**
**All tests rank order tires into the same groupings**
**Values for all tests are approximately linear functions of the values of any other test**
**Rolling Resistance or RR Coefficient describe a tire’s response at the conditions of test only**