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
Top Related