UWE Bristol Engineering showcase 2015

Matthew Edwards
MEng Mechanical Engineering
A Study on four-wheel-steering in road vehicles.
Project summary
This study is to investigate how a four wheel steering (4WS) system affects a vehicles performance. Two different types of 4WS were investigated during this
study, in phase and out of phase 4WS. In phase steering is when both the rear and front wheels steer in the same direction once a stimulus is given. Whereas
out of phase 4WS is when the rea wheels turn in the opposite direction to the front wheels for a given steering input. Both types of 4WS systems where used
over a range of ratio between the front and rear wheels. These different ratios where used to investigate the effect that both types of 4WS had on different
vehicle characteristics when the vehicle is travelling around a corner.
The chosen vehicle:
For the vehicle modelling an Audi A5 Coupe 2.0 TDI 177hp quattro
from 2012 – 2015 was selected. The information regarding the
specification of the vehicle was found at (cars-data.com, 2015). The
information taken from this website was the mass of the vehicle, at
1510Kg, and the wheelbase, at 2.751m. Another piece of
information needed was the corner stiffness coefficient, which was
found in Wong J.Y. stated as (2008, p. 381) “The corner stiffness of
each of the front tyres is 38.92kN/rad (8750lb/rad) and that of the
rear tyres is 38.25kN/rad (8600lb/rad).”
Steady State cornering:
The first area which was investigated during this project was for
steady state cornering, which occurs once a vehicle has settled into
a constant radius corner. Once a vehicle is travelling around a steady
state corner it is easy to compare the different characteristics of in
phase, out of phase and two wheel steering. During this stage of the
investigation many different variables where looked at, such as the
off-tracking and the yaw velocity of the vehicle. Many of the
different variables enabled other more complex variables to be
calculated, such as the slip angles of both the front and rear tyres.
Transient cornering:
The study then moved on to start looking at transient cornering,
which is what happens immediately after a steering input is given.
During this period key characteristics of the vehicle can be
observed, such as the change in the yaw velocity and lateral
acceleration. These variables will give a good idea on how the
vehicle will be travelling around a corner, as well as what size corner
radius the vehicle can travel around.
Steering angle (rad)
yaw velocity (rad/s)
2.75
2.5
2.25
2
1.75
1.5
1.25
1
0.75
0.5
0.25
3.5
3
2.5
2
1.5
1
0.5
0
3
0
A chart showing how the steering angle changes with the corner radius over a range of steering
ratios, at a velocity of 5mph.
0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150
Corner radius (m)
A graph to show how the yaw velocity changes against the vehicle velocity over a range of steering
ratios, when the steering angle was 15 degrees.
0 10 20 30 40 50 60 70 80 90 100
vehcile velocity (mph)
sr = -0.8
sr = -0.6
sr = -0.4
sr = -0.2
sr = 0
sr = 0.2
sr = 0.4
sr = 0.6
sr = 0.8
sr = 1
sr = -0.8
sr = -0.6
sr = -0.4
sr = -0.2
sr = 0
sr = 0.2
sr = 0.4
sr = 0.6
sr = 0.8
sr = 1
Project Supervisor
Benjamin Drew
Project Objectives
The main aim of this thesis is to investigate the
effectiveness of four-wheel-steering in road cars, by
evaluating the vehicle's handling, agility and stability
when cornering. The effectiveness will be determined
by the extent of any improvements to the handling,
stability or agility, such as higher cornering speeds or
a smaller turning circle. This will be achieved with the
use of mathematical models. These models will take
into account various different parameters, allowing a
comparison between four-wheel-steering and twowheel-steering.
The handling of the vehicle concerns
the ability to negotiate a corner, which is regarding
the vehicle speed, corner radius and turning angle. A
vehicle's agility is its ability to negotiate a corner at
speed, so the quicker it can corner the more agile the
vehicle is. Also the vehicle stability is to do with the
slip angle and the side-slip angle, as the lower these
values are the more stable the vehicle is.
Project Conclusion
In conclusion of the results you can see that out of
phase steering is beneficial for a vehicle at lower
speeds. This is because the vehicle is able to turn
quicker and manoeuvre around tighter corners. If a
vehicle has these characteristics than it is more suited
for city driving, as the majority of the obstacles the
vehicle will have to overcome are based around the
turning circle of the vehicle. Therefore, as out of
phase steering has the effect of shortening the
wheelbase of a vehicle it will become more practical
for larger vehicle to use within a city. This means that
vehicles such as vans will have better
manoeuvrability, therefor delivering and transporting
goods will become easier as the van will be able to
drive through tighter roads. However out of phase
4WS is less beneficial for travelling along motorways
at high speeds. For a vehicle travelling along
motorways and at high speeds in phase steering
would be more beneficial. This is because the vehicle
will be less sensitive to a steering input, meaning the
vehicle is less likely to spin out of control because of a
sudden increase in the steering angle. This means
that a vehicle will benefit from both in phase and out
of phase four wheel steering if it is involved in both
motorway and city driving.