STEERING REPORT
STEERING OBJECTIVE
To provide directional stability to the vehicle
To facilitate straight ahead recovery after completing a turn
To obtain a turning circle radius (T.C.R.) as minimum as possible
To obtain perfect rolling of all the wheels about a single point
PERFECT STEERING CONDITION
cot φ – cot Θ = c/b
Θ = angle turned by inner wheel
φ = angle turned by outer wheel
b = wheel base
c = distance between pivot points
STEERING MECHANISMS
We analysed Davis and Ackermann steering mechanisms.
Davis steering mechanism obtains the required steering angle using sliding
pairs. Due to the presence of such sliding pairs, mechanical wear and tear q
increases. This increases the possibility of failure. Also due to the increase in
the number of links, it increases the weight making it bulky and inefficient.
Ackermann steering mechanism is basically a 4 bar linkage mechanism. It
consists of turning pairs and no sliding pairs. This helps to decrease the wear
and tear of steering mechanism.
Thus, we prefer Ackermann steering mechanism over Davis steering
mechanism
Ackermann steering mechanism is basically of 3 types -
Ackermann steering mechanism-Angle turned by inner wheel is greater
than that of outer wheel.
Anti-Ackermann steering mechanism-Angle turned by inner wheel is less
than that of outer wheel.
Pro-Ackermann steering mechanism-Angle turned by outer wheel is
equal to that of inner wheel.
In case of Anti-Ackermann, there was also a problem of interfering of tie-rod
with the suspension strut. In Pro-Ackermann, wear of tires is more. Instead,
Ackermann steering mechanism is used.
RACK AND PINION STEERING MECHANISM
Considering the following aspects this
mechanism was opted
It minimizes the steering effort.
It occupies less space.
It does not involve bulky linkages.
Due to less linkages possibility of wearing
out of joints is minimum.
Rotary motion of the steering wheel is transmitted to the pinion through
universal joints. The circular motion of the pinion is converted into linear
motion of the rack, which is further relayed through ball joints and tie-rods to
the wheels to be steered.
WHEEL ALIGNMENT
Camber angle- It is the angle made by the wheels
from the true vertical when viewed from the front of
the car. When camber is set chances of slipping
increases. In case of positive camber outer edge of
tyres wears out faster and vice versa. Hence it was
decided to set 0 camber.
Caster angle- It is the angle between steering axis
and the vertical, in the plane of the wheel. When the
Caster Angle line hits the ground in front of where the tyres contact with the
ground, this is Positive Caster. It determines the amount of self-centring the
steering will have, influence the straight-line running. Also the steering axis
inclination will influence the camber change when cornering as a function of
the steering input. Large Caster Angles mean greater camber changes can be
created and that means better negative camber when cornering and smaller
camber on the straight, ideal for both performance and wear of the tyres.
Unfortunately too large a caster angle can lead to poor turn-in. In our vehicle it
is 3 degrees.
Toe angle- Deviation of the wheels from
straight ahead position. In case the wheels
are pointing inward it is called toe in and if
outward then toe out.
Scrub Radius- The scrub radius is the distance in front view between the
steering axis and the center of the contact patch of the wheel, where both
would theoretically touch the road. Effective scrub radius is calculated later.
RACK SELECTION
Manuverability is a desired trait of ideal BAJA vehicle. Consequently a BAJA
vehicle is expected to have a low turning circle radius. Hence we tried to
survey racks of cars with relatively small wheelbase and TCR.
Most suitable racks for our use found were those of REVA and Maruti 800.
However REVA’S rack was unavailable in the market and that of Maruti was
still bigger than our requirement.
Other than this many efforts were made to manufacture a rack of our own.
Following are the briefs of the rack and pinion assembly designed
RACK
Dimension Value
Module (m) 1.5 Length = 14”
No of teeth = 28
Addendum (1m) 1.5 Cross section(Circular) = DIA 22
Dedendum (1.25m) 1.875 PINION
Pitch(π m) 4.712
PCD = 30 mm
Width at pitch 2.362 No of teeth = 20
Width of teeth = 17 mm
Tooth thickness (1.5708m) 2.3562 Length = 100 mm
The material selected was EN8 (Mild
steel grade). It is also the material used for rack in Maruti 800. The new rack
had a larger pinion reducing the lock angle from mid position of the steering
wheel by 70⁰. It showed positive results in FE analysis giving factor of safety of
6 under dynamic load, clearly being safe even in accidental conditions.
The weight of the rack and pinion assembly along with the casing was about
3.3 kg. This design was too large and even slightly heavier as per our
requirement.
Hence, it was decided to use the rack from the previous vehicle as it was light
in weight, provided the appropriate lock to lock angle and feasible steering
effort (discussed later). Thus the majority of the steering components and
setup from the previous vehicle remains the same. However the goal of the
steering modifications this time is to make the steering compact to achieve
strict ergonomic targets set forth by the team and more importantly, to better
the Ackerman geometry.
GEOMETRY
Factors fixed before performing iterations:
Wheel track = 55”
Wheel base = 66”
Rack length = 14”
Rack travel = 2.125”
Steering Wheel lock to center = 270 degrees
Iterations were performed by varying length of tie-rod and steering arm and
rack offset from axle in AutoCAD.
Based on these iterations the following values were obtained:
Steering arm = 110 mm
Tie rod = 353 mm
Rack offset = 112 mm
Outer turning circle radius = 3.277 m
Initial Angle of Steering Arm = 20.9449 degrees
Ackermann Angles
Inner Wheel = 43.3291 degrees
Outer Wheel = 28.6705 degrees
BUMP STEER
The steer angle generated by the vertical motion of the wheel with respect to
the vehicle body is called bump steering. Bump steering is usually an
undesirable phenomenon and is a function of the suspension and steering
mechanisms. If the vehicle has a bump steering character, then the wheel
steers when it runs over a bump or when the vehicle rolls in a turn. As a result,
the vehicle will travel in a path not selected by the driver. Bump steering
occurs when the end of the tie rod is not at the instant centre of the
suspension mechanism. Hence, in a suspension deflection, the suspension and
steering mechanisms will rotate about different centres.
The above figure shows the graph of toe angle vs. wheel travel, which indicates
bump steer. As seen in above figure, bump steer is minimal.
STEERING RATIO
The steering ratio is the amount of degrees you have to turn the steering
wheel, for the wheels to turn an amount of degrees.
Steering ratio = (Lock to lock steering wheel angle)/(sum of inner and outer
wheel angle)
Hence for our vehicle it is equal to 7.5:1
Calculation of scrub
radius
X=96.7925 mm
Y=13.6442 mm
Effective Scrub Radius, Z=
√ X 2 +Y 2
Z=97.7 mm
STEERING EFFORT
Steering effort in static condition
Maximum Normal Reaction at Front Wheels = 1030.05 N
Torque at steering arm = μ x Normal reaction x Scrub
= 0.6 x (1030.05 / 2) x 0.0977 = 30.191 Nm
Force perpendicular to steering arm = 274.464 N
Force along the rack = 2 x (274.464/cos (20.9449)) = 587.764 N
Torque at pinion = Force along rack x Radius of pinion = 4.702 Nm
Radius of steering wheel = 150 mm
Effort at steering wheel = Torque at pinion / Radius of steering wheel= 31.347
N
Steering effort considering dynamic weight distribution at full braking
Maximum Normal Reaction at Front Wheels = 2403.45 N
Torque at steering arm = μ x Normal reaction x Scrub
= 0.6 x (2403.45 / 2) x 0.0977 = 70.446 Nm
Force perpendicular to steering arm = 640.416 N
Force along the rack = 2 x (640.416 / cos (20.9449)) = 1371.449 N
Torque at pinion = Force along rack x Radius of pinion = 10.971 Nm
Effort at steering wheel = Torque at pinion / Radius of steering wheel = 73.143
N
