Transcript Steering
Steering System • • • • • • Ackerman Linkage geometry Road wheel geometry Caster Kingpin inclination Compliance effects Ackerman Steer do L =----L -1 d= o tan ----R+t/2 R+t/2 di L =----L -1 d= i tan ----R-t/2 R-t/2 For large radii, R >> t/2 L d=-Ack R L R t/2 Inboard off-tracking 2 L 2R Turn Center Ackerman – Truck Steering System Straight ahead Right turn Left turn Ackerman Steer 50% Left Wheel Steer Angle (deg) 40 20 0 -50 -30 -10 10 30 -20 -40 -60 Right Wheel Steer Angle (deg) 50 Car Steering Systems Rack and pinion Steering arm Gearbox Rack Tire rod Tire rod Gearbox Idler arm Relay link Steering arm Pitman arm Truck Steering Systems Tire rod Steering arm Gearbox Pitman arm Drag link Compliance in Steering System Geometry at the Wheel Kingpin Axis Center of Tire Contact Caster Angle Kingpin Inclination Angle Kinpin Of fset at the Ground Lateral Inclination Angle Fzr F s zr in Fzr F s zr in d d sin d d M = -( Fzl Fzr ) d sin sin d Torques from Lateral Inclination M M =VFzlzl+F F )d sin dsin d sin d sin ) zr - (F =( zr Steering Torque (in-lb) 200 STEERING TORQUE FROM LATERA L INCLINATION ANGLE 1" Off set 10 Inclination Angle 100 0 -100 -200 -45 Right Wheel (600 lb) Left Wheel (800 lb) Total -30 -15 0 15 Steering Angle (deg) 30 45 Caster Angle Fzr sin F zr Fzr d d in F zrs M = ( Fzl - Fzr ) d sin cos d d cosd Torques from Caster Angle MV == (F(zlF - F zr ) d sin cos d M xl - Fxr ) d sin cosd Steering Torque (in-lb) 200 100 STEERING TORQUE FROM CASTER ANGLE 1" Off set 5 Caster Angle Left Wheel (800 lb) Total 0 -100 Right Wheel (600 lb) -200 -45 -30 -15 0 15 Steering Angle (deg) 30 45 Lateral Force Fyr r tan M L = -(Fyl Fyr ) r tan Tractive Force Fxr d M T = ( Fxl - Fxr ) d 4 Wheel Steer - Low Speed d f L R d Turn Center r R= L d(1d/d) r f f 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 d f ax d r 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