Steering System Report 3g55a

STEERING SYSTEM OBJECTIVE The main objectives of the steering system is to handle stress for safely directing the vehicle through any type of terrain and to provide the driver with an accurate, predictable, and reliable method of driving along with good ergonomics.

TARGETED DESIGN SPECIFICATIONS In order to meet the objective we decided to attain  100% Ackermann geometry for directional control and easy cornering of the wheels.  Minimum turning radius to increase the chance of winning the maneuverability and endurance race.  Minimum steering ratio to decrease driver’s effort for maneuvering the vehicle.  Over steer gradient (K<0)

DESIGN SPECIFICATIONS ACHIEVED: In order to meet the targeted design specifications 14” centralized rack and pinion was chosen from desert karts. This rack and pinion setup has a rack travel of 4.25” (107.95mm) and steering wheel turn equal to 1.5 turns. This setup is used to achieve the Ackermann geometry using iterative methods resulting in achievement of 85% Ackermann geometry which was verified using LOTUS software.

Outer Steering angle (δ o) = 24.261 deg = 0.4234 rad Inner Steering angle (δ i) = 34.177 deg = 0.5965 rad δ = (δ o + δ i) / 2 = (24.261 + 34.177) / 2 = 29.219 deg = 0.5099 rad δ = 29.219 deg δ=L/R Radius of turn (R) = L / δ = 1625.6 / 0.5099 = 3187.668 mm R = 3.187 m. Thus targeted minimum turning radius was attained. In order to provide a better ergonomics to drive and minimize driver’s effort for maneuvering, the steering turn was reduced to 1.2 turn and this was achieved by providing external steering lock-to-lock turn. Thus rack travel of 107.95mm was restricted to 86mm. 1.5 turns (540 deg) of steering wheel = 4.25 inch (107.95 mm) of rack travel. Hence for 86 mm of rack travel = 1.2 turns (432 deg) of steering wheel. Steering Ratio = degrees of steering wheel angle : Front wheel angle = 432 : 58.438 Steering Ratio = 7.39 : 1 In order to reduce the slip of the rear wheels of a rear wheel drive vehicle which posses C.G nearer to the rear of the vehicle, it should possess over steer gradient (K <0) We know that during steady-state cornering of the car it over steers due to the reason that load in rear axle is more than load in front axle (Wr >Wf) and possess only over steer gradient (K) which has negative value. Hence the front slip angle (αf) and rear slip angle (αr) are given by

Mass of the Vehicle (M) = 330 Kg Lateral Force (Fy) = 0.49*g*M = 1586.27 N = 355.825 lb Vertical Load (Fz) = 0.4*g*M = 3237.3 N = 290.471 lb Weight on front axle (Wf) = 0.4*M*g = 1294.92 N = 290.47 lb Weight on rear axle (Wr) = 0.6*M*g = 1942.38 N = 435.71 lb αf = 2α / 3 = (2*4.5) / 3 = 3 deg αr = 4α / 3 = (4*4.5) / 3 = 6 deg Fyf = 250 lb ; Fyr = 500 lb Fy = Cα*α Cαf = Fyf / αf = 83.33 lb/deg Cαr = Fyr / αr = 83.33 lb/deg K = {(Wf / Cαf) – (Wr / Cαr)} = {(290.47 / 83.33) – (435.71 / 83.33)} = - 1.74 deg/g K = - 1.74 deg/g. To prove the over steer gradient theoretically, the condition to be satisfied is δf > δ δf = (57.3 L / R) + {((Wr / Cαr) – (Wf / Cαf))*V2 / (g*R)} = 29.2271 + 1.7209 = 30.948 deg δf (30.948 deg) > δ(29.219 deg)