steering system ackerman linkage geometry road wheel geometry caster kingpin inclination compliance...
TRANSCRIPT
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Steering System
• Ackerman
• Linkage geometry
• Road wheel geometry
• Caster
• Kingpin inclination
• Compliance effects
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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
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Ackerman – Truck Steering System
Straight ahead
Right turn
Left turn
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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%
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Car Steering Systems
Rack and pinion
GearboxRack
Tire rod
Steering arm
Gearbox
Steering arm
Tire rod
Relay linkIdler armPitman arm
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Truck Steering Systems
Tire rod
Steering arm
Drag link
Pitman arm
Gearbox
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Compliance in Steering System
Kss Kss
To steering wheel
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Geometry at the Wheel
Center of Tire Contact
Caster AngleKingpin Inclination Angle
Kinpin Offset at the Ground
Kingpin Axis
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Lateral Inclination Angle
( ) sin sinzl zrM F F d
Fzr
d
Fzr
d sin
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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
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Caster Angle
( ) sin coszl zrM F F d
Fzr
d
Fzr
d cos
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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
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Lateral Force
tan)( rFFM yrylL
Fyr r tan
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Tractive Force
dFFM xrxlT )(
Fxr d
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4 Wheel Steer - Low Speed
R =frf
L
RL
f
r
Turn Center
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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
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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
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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
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Steering Ratio
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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