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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 drivers effort for
maneuvering the vehicle.
Over steer gradient (K
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Outer Steering angle ( o) = 24.261 deg = 0.4234 rad
Inner Steering angle ( i) = 34.177 deg = 0.5965 rad = ( o + i) /
2 = (24.261 + 34.177) / 2 = 29.219 deg = 0.5099 rad = 29.219 deg =
L / R Radius of turn (R) = L / = 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
drivers 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 Wf) and possess only over steer gradient (K)
which
has negative value. Hence the front slip angle (f) and rear slip
angle (r) are given by
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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
f = 2 / 3 = (2*4.5) / 3 = 3 deg r = 4 / 3 = (4*4.5) / 3 = 6 deg
Fyf = 250 lb ; Fyr = 500 lb
Fy = C* Cf = Fyf / f = 83.33 lb/deg Cr = Fyr / r = 83.33 lb/deg
K = {(Wf / Cf) (Wr / Cr)} = {(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 f > f = (57.3 L / R) + {((Wr / Cr) (Wf /
Cf))*V
2 / (g*R)} = 29.2271 + 1.7209 = 30.948 deg
f (30.948 deg) > (29.219 deg)
