Pull: The Science of a Nuisance By Marion Pottinger M’ gineering, LLC 9/14/20111 M' gineering,...

Pull: The Science of a Nuisance

ByMarion PottingerM’gineering, LLC

9/14/2011 1M'gineering, LLC

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

9/14/2011 M'gineering, LLC 2

In Trim...


•Traveling down a straight road in a straight line.

@ Sideslip With Steer

•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.

FY & MZ Near = 0°


•NOTICE

FY & MZ approximately linear near = 0°

FY & MZ usually not zero at the same .

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.


Pull in Fixed Control


M Z

•Staying in trim requires driver supplied torque.

Find FY = 0 .

Drop to MZ .

Project to vertical axis. Read MZF .

FY = FYL + FYR

MZ = MZL + MZR

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 .

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

Types of Tire Pull FY & MZ

• Camber Like – Conicity

• Steer Like – Described in two terms

PRAT - Plysteer Residual Aligning TorqueorPlysteer Residual Lateral Force


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.

9/14/2011 M'gineering, LLC 10#

OF

TIRE

S

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

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.


Two Mounts + One Rolling Direction, Flipped


FYM1, MZM1

FYM2, MZM2

Two Mounts + One Rolling Direction, Flipped


•Describes Pull in a Tire Coordinate System.

One Mount + Two Rolling Senses, Backward/Forward


FYM1, MZM1

FYM3, MZM3

One Mount + Two Rolling Senses, Backward/Forward


Describes Pull in Test Machine Coordinate System.

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.

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.

Problems with Test Equipment

• Machine Plysteer and ConicityCan’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 ErrorCauses errors in Backward / Forward tests.

• Test Surface Curvature


Test Surface Curvature Effect


MANUFACTURER SIZE Slope = Flat/Factory Intercept (N) R2

A P205/75R15 1.623 -11.83 0.9834B P205/75R14 1.445 45.35 0.9576B P205/75R15 1.722 -3.95 0.9626C P205/75R14 1.518 26.60 0.9883

CONICITY - Linear Regression Flat Surface vs. Factory Drum

•PRAT & PRLF must be on flat.Round / Flat relationship

differs for FY and MZ.•Round / Flat relationship depends on:

Tire / Drum curvatures.+Tire structure.

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.

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).


Tests for Sorting Pull in the Field

1. Mount tires in sense (a).a. Drive in trim at a constant speed on test surface.b. Release the steering wheel.c. Measure lateral movement over either:

i. Fixed time.ori. Fixed distance.

d. Steer back to intended path & return to shop.

2. Mount tires in sense (b) and repeat steps a–d.


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.

• FoolersBoth problems are present.An OEM has used a particular PRAT as part of the

package to give Trim on a particular road crown.


Summary and Conclusions

1. Discussed pull in both free and fixed control.2. Explained FY and MZ causing pull along with

OEM philosophies for limiting conicity and PRAT associated problems.

3. Compared Backward / Forward and Flipped testing modes to eliminate confusion.

4. Noted items causing FY and MZ measurement problems.

5. Outlined a way to field sort pull problems. 9/14/2011 M'gineering, LLC 25


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Pull: The Science of a Nuisance By Marion Pottinger M’ gineering, LLC 9/14/20111 M' gineering,...

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Transcript of Pull: The Science of a Nuisance By Marion Pottinger M’ gineering, LLC 9/14/20111 M' gineering,...