Steering System Ackerman Linkage geometry Road wheel geometry Caster Kingpin inclination Compliance effects

Ackerman Steer d = tan-1 ----- = ----- o d d o i L L d = tan-1 ----- = ----- o d R+t/2 o d i R+t/2 L L d = tan-1 ----- = ----- i R-t/2 R-t/2 For large radii, R >> t/2 L d = -- Ack R L R Turn Center t/2

Ackerman – Truck Steering System Straight ahead Right turn Left turn

Ackerman Steer 50% Parallel Ackerman

Car Steering Systems Rack and pinion Gearbox Steering arm Rack Tire rod Tire rod Gearbox Steering arm Pitman arm Idler arm Relay link

Truck Steering Systems Tire rod Steering arm Gearbox Pitman arm Drag link

Compliance in Steering System

Geometry at the Wheel

Lateral Inclination Angle

Torques from Lateral Inclination

Caster Angle

Torques from Caster Angle

Lateral Force

Tractive Force

4 Wheel Steer - Low Speed R = L d (1 + d /d ) f r

4 Wheel Steer - High Speed 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