Steering System

• Ackerman

• Linkage geometry

• Road wheel geometry

• Caster

• Kingpin inclination

• Compliance effects

Ackerman Steer

o i

RL

t/2 Inboard off-tracking

L2 R

2

tan-1R+t/2

Lo

tan-1R-t/2

Li

TurnCenter

R+t/2

R-t/2

L

L

For large radii, R >> t/2

AckLR

Ackerman – Truck Steering System

Straight ahead

Right turn

Left turn

Ackerman Steer

-60

-40

-20

0

20

40

-50 -30 -10 10 30 50

Right Wheel Steer Angle (deg)

Lef

t W

hee

l S

teer

An

gle

(d

eg)

AckermanParallel

50%

Car Steering Systems

Rack and pinion

GearboxRack

Tire rod

Steering arm

Gearbox

Steering arm

Tire rod

Relay linkIdler armPitman arm

Truck Steering Systems

Tire rod

Steering arm

Drag link

Pitman arm

Gearbox

Compliance in Steering System

Kss Kss

To steering wheel

Geometry at the Wheel

Center of Tire Contact

Caster AngleKingpin Inclination Angle

Kinpin Offset at the Ground

Kingpin Axis

Lateral Inclination Angle

( ) sin sinzl zrM F F d

Fzr

d

Fzr

d sin

Torques from Lateral Inclination

4530150-15-30-45-200

-100

0

100

200

Total

Steering Angle (deg)

Ste

eri

ng

To

rqu

e (

in-l

b)

STEERING TORQUE FROMLATERAL INCLINATION ANGLE

Right Wheel (600 lb)

1" Offset

10 Inclination Angle

Left Wheel (800 lb)

M = - (F + F ) d sin sin V zl zr( ) sin sinzl zrM F F d

Caster Angle

( ) sin coszl zrM F F d

Fzr

d

Fzr

d cos

Torques from Caster Angle

4530150-15-30-45

-200

-100

0

100

200

Steering Angle (deg)

Ste

eri

ng

To

rqu

e (

in-l

b)

STEERING TORQUE FROM CASTER ANGLE

Total

Left Wheel (800 lb)

Right Wheel (600 lb)

1" Offset

5 Caster Angle

M = (F - F ) d sin cos V zl zr cossin)( dFFM xrxl

Lateral Force

tan)( rFFM yrylL

Fyr r tan

Tractive Force

dFFM xrxlT )(

Fxr d

4 Wheel Steer - Low Speed

R =frf

L

RL

f

r

Turn Center

4 Wheel Steer - High Speed

f

r

ax

• Four-wheel in-phase steering• Only at high speed (typically

above 35 mph)• Rear steer angles less than

front• Rear steer angles limited to a

few degrees

Steering System Applications• Effect of steering geometry on performance

– Understeer (linear range)– Limit cornering (non-linear range)

• Steering torques and feel– On-center feel– Torque gradients– Linearity– Power assist characteristics– Friction and damping

• Evaluate effects of asymmetry– Manufacturing tolerance

Steering Ratio

Steering Ratio = Steering wheel angle (deg) / Road wheel angle (deg)

Steering ratio for cars = 15 to 20

Steering ratio for trucks = 20 to 40

Steering Ratio

Assignment

• Design linkage geometry to meet requirement of the minimum turn radius and provide close to Ackerman geometry (X-Y plane is ok)

• Design steering ratio with measurements on your car