Pull: The Science of a Nuisance

1. Pull: The Science of a Nuisance By Marion Pottinger M’gineering, LLC M'gineering, LLC

2. OUTLINE • Automobile and Light Truck Response to Pull • Pull Lateral Force and Aligning Torque Sources • Measuring Pull Force and Torque • Test Equipment Problems • Sorting Pull Problems in the Field • Summary and Conclusions M'gineering, LLC

3. In Trim... • Traveling down a straight road in a straight line. • @ Sideslip b • With Steer d • Driver happy and unconcerned if: • No effort required to maintain trim. • Steering wheel straight. • Driver unhappy and a pull problem exists if: • Driver effort required to maintain trim. • Steering wheel appreciably crooked. M'gineering, LLC

4. FY & MZ Near a = 0° • NOTICE • FY & MZ approximately linear near a = 0° • FY & MZ usually not zero at the same a. M'gineering, LLC

5. How Tire Induced Pull Occurs • Treat front and rear axles separately. • Add contributions of both tires on an axle. • Rear easy, no steer. • Car sideslips until FYR = 0. • MZR unimportant. • Front steers. The story is here. M'gineering, LLC

6. Pull in Fixed Control FY = FYL + FYR MZ = MZL + MZR • Staying in trim requires driver supplied torque. • Find FY = 0 . • Drop to MZ . • Project to vertical axis. Read MZF . MZ M'gineering, LLC

7. Pull in Free Control FY = FYL + FYR MZ = MZL + MZR • Driver releases steering wheel. Car turns. • Find MZ = 0 . • Drop to FY . • Project to vertical axis. Read FYF . M'gineering, LLC

8. Non-tire Contributors to Pull • Road Camber • Incorrect Suspension Alignment • Correct Alignment and Tire Properties differ for right hand and left hand driving countries. ROAD M'gineering, LLC

9. Types of Tire Pull FY & MZ • Camber Like – Conicity • Steer Like – Described in two terms • PRAT - Plysteer Residual Aligning Torque or • Plysteer Residual Lateral Force M'gineering, LLC

10. Conicity • Due to off-center belt, 3 mm significant. • Distribution centered at FY@ 0. • Belt slightly conical • Rolls toward small side. On a car - • If right in RR, then left in LR. • OEMs solve by limiting conicity FY. • Basically, if good on one auto brand, good on all. # OF TIRES M'gineering, LLC

11. Plysteer (PRAT or PRLF) • Due structural anisotropy • Design effect including tread pattern. • Narrow distribution. • Same in RR or LR on a car. • OEMs control by only accepting tires within specified limits. • Limits vary by OEM. • Different limits for driving on left or right. LATERAL FORCE M'gineering, LLC

12. Measuring Pull FY & MZ • On a car: • Conicity in RR = - Conicity in LR. • Plysteer in RR = Plysteer in LR. • Suggests determination by measuring in RR then in LR. • There are two ways to do this. • Use two mounting senses rolling in one direction. • Use one mounting sense and roll in two directions. M'gineering, LLC

13. Two Mounts + One Rolling Direction, Flipped FYM1, MZM1 FYM2, MZM2 M'gineering, LLC

14. Two Mounts + One Rolling Direction, Flipped • Describes Pull in a Tire Coordinate System. M'gineering, LLC

15. One Mount + Two Rolling Senses, Backward/Forward FYM3, MZM3 FYM1, MZM1 M'gineering, LLC

16. One Mount + Two Rolling Senses, Backward/Forward Describes Pull in Test Machine Coordinate System. M'gineering, LLC

17. Solutions as Algebra Flipped FYPLY = 0.5 (FYM1 + FYM2) FYCON = 0.5 (FYM1 – FYM2) MZPLY = 0.5 (MZM1 + MZM2) MZCON = 0.5 (MZM1 – MZM2) Backward / Forward FYPLY = 0.5 (FYM1 – FYM3) FYCON = 0.5 (FYM1 + FYM3) MZPLY = 0.5 (MZM1 + MZM3) MZCON = 0.5 (MZM1 – MZM3) • FY equation different signs. Are FYPLY & FYCON the same? • MZ equations no concerns. M'gineering, LLC

18. FYPLY & FYCON are the same. FYM3 = FYCON - FYPLY FYM2 = - FYCON + FYPLY Since FYM3 = - FYM2, substitution in the FYCON & FYPLY equations for both test methods gives the same answer. M'gineering, LLC

19. Problems with Test Equipment • Machine Plysteer and Conicity • Can’t find machine plysteer from tire tests. • Can find machine conicity from tire tests. • For cantilevered spindles it may be load dependent. • Differential Belt Tracking Error • Causes errors in Backward / Forward tests. • Test Surface Curvature M'gineering, LLC

20. Test Surface Curvature Effect • PRAT & PRLF must be on flat. • Round / Flat relationship differs for FY and MZ. • Round / Flat relationship depends on: • Tire / Drum curvatures. • + • Tire structure. CONICITY - Linear Regression Flat Surface vs. Factory Drum M'gineering, LLC

21. Mounting to Sort Pull in the Field Must mount front tires two ways (a) and (b) for test. Vehicle Sides WSW is the side of the tire intended to mount outward. M'gineering, LLC

22. Test Surface for Sorting Pull in the Field • Must allow safe testing. • Preselected straight surface without side slope. • Uncrowned road. • Up or down parking lot slope. • Lined parallel to motion for easy judgment of lateral position. • Allow constant speeds above 40 km/hr (25 mph). M'gineering, LLC

23. Tests for Sorting Pull in the Field • Mount tires in sense (a). • Drive in trim at a constant speed on test surface. • Release the steering wheel. • Measure lateral movement over either: • Fixed time. or • Fixed distance. • Steer back to intended path & return to shop. • Mount tires in sense (b) and repeat steps a–d. M'gineering, LLC

24. The Answer in the Field • Conicity if mountings (a) and (b) yield opposite and essentially equal lateral drift. • PRAT if mountings (a) and (b) yield essentially equal lateral drift with same sense. • Foolers • Both problems are present. • An OEM has used a particular PRAT as part of the package to give Trim on a particular road crown. M'gineering, LLC

25. Summary and Conclusions • Discussed pull in both free and fixed control. • Explained FY and MZ causing pull along with OEM philosophies for limiting conicity and PRAT associated problems. • Compared Backward / Forward and Flipped testing modes to eliminate confusion. • Noted items causing FY and MZ measurement problems. • Outlined a way to field sort pull problems. M'gineering, LLC

26. ? ITEC QUESTIONS M'gineering, LLC