Conference PaperPDF Available

Analysis of an off road 4WD vehicle’s suspension system modification – Case study of aftermarket suspension lift and modification of wheel track size

Authors:

Abstract and Figures

In this research, a four wheel drive (4WD) suspension of a vehicle has been modified by increasing the ride height to investigate stability and cornering potential of the vehicle through load transfer and variation of roll angle. Further investigation has been conducted to observe the characteristics which are deemed desirable for off road application but detrimental to the on road application. The Constant Radius Cornering Test (CRCT) was chosen as a base method for experimental investigation to observe the effect of the suspension modifications. The test was carried out by undertaking a known radius and cambered corner at a constant speed. For this test, the acceleration and gyroscopic data were measured to check and compare the accuracy of the analysis performed by OptimumDynamics model. The tests were conducted by means of negotiating the curve at the speed of 80 km/h and it was gradually achieved to allow a good consensus of the amount of body roll the vehicle experienced. Using a surveyor’s wheel, the radius of the corner was estimated as 160 m and using the gyroscopic sensor, the corner camber was measured at 4 degrees. While comparing the experimental results with the simulation results, the experimental constraints led to higher values than those of the analytical results. The total load transfer reduced by 2.9% with the increased track size. It has been observed that the dynamic load transfer component is lesser than the standard suspension with the aftermarket suspension lift and the upgraded anti-roll bar (ARB). With the simulation of the fitment of the other modifications aimed to improve the characteristics of the raised vehicle, the vehicle showed a reduced tendency towards roll angle due to the stiffened anti-roll bar and the maximum increased wheel track demonstrated reduced lateral load transfer and body roll. Even with these modifications however, the decrease in load transfer is minimal in comparison to what was expected.
Content may be subject to copyright.
Analysis of an off Road 4WD Vehicle’s Suspension System
Modification – Case Study of Aftermarket Suspension Lift
and Modification of Wheel Track Size
J. Ross1, M. A. Hazrat1, a), and M. G. Rasul1
1School of Engineering and Technology, Central Queensland University, Queensland 4702, Australia
a)Corresponding author: h.ali@cqu.edu.au
Abstract. In this research, a four wheel drive (4WD) suspension of a vehicle has been modified by increasing the ride
height to investigate stability and cornering potential of the vehicle through load transfer and variation of roll angle.
Further investigation has been conducted to observe the characteristics which are deemed desirable for off road
application but detrimental to the on road application. The Constant Radius Cornering Test (CRCT) was chosen as a base
method for experimental investigation to observe the effect of the suspension modifications. The test was carried out by
undertaking a known radius and cambered corner at a constant speed. For this test, the acceleration and gyroscopic data
were measured to check and compare the accuracy of the analysis performed by OptimumDynamics model. The tests
were conducted by means of negotiating the curve at the speed of 80 km/h and it was gradually achieved to allow a good
consensus of the amount of body roll the vehicle experienced. Using a surveyor’s wheel, the radius of the corner was
estimated as 160 m and using the gyroscopic sensor, the corner camber was measured at 4 degrees. While comparing the
experimental results with the simulation results, the experimental constraints led to higher values than those of the
analytical results. The total load transfer reduced by 2.9% with the increased track size. It has been observed that the
dynamic load transfer component is lesser than the standard suspension with the aftermarket suspension lift and the
upgraded anti-roll bar (ARB). With the simulation of the fitment of the other modifications aimed to improve the
characteristics of the raised vehicle, the vehicle showed a reduced tendency towards roll angle due to the stiffened anti-
roll bar and the maximum increased wheel track demonstrated reduced lateral load transfer and body roll. Even with
these modifications however, the decrease in load transfer is minimal in comparison to what was expected.
INTRODUCTION
Standard passenger vehicles generally perform poorly under Australian off road conditions due to insufficient
underbody clearance, suspension travel and articulation, and fitting of open differentials. Towing a heavy load, such
as a large caravan or boat on factory suspension can cause the suspension to sag, reducing the ride quality, safety
and travel of the suspension. Even the top of the line four wheel driver touring vehicles from popular manufacturers
such as Toyota, Nissan, or Land Rover which have a reputation for being the toughest on the market will suffer
under these conditions, where it is often seen that the suspension at the rear end is nearly bottomed out. This may not
be too much of an issue when driving on a good quality road, but this can cause major issues when trying to get to
your desired location if it involves any off road travel or the driver were to hit an imperfection in the road surface at
high speed such as a pothole. For those who wish to travel around Australia, the dreaded corrugations convince most
to upgrade their suspension systems, and for those who do not will generally suffer a terribly uncomfortable ride,
and risk potentially damaging their vehicle.
The passive suspension systems installed in the majority of today’s vehicles are unfortunately often a
compromise with driving conditions1. This compromise is almost always governed by the major contributor to all
vehicle components quality and performance, cost. A suspension system from the factory also needs to be designed
to perform acceptably under the likely driving conditions for that vehicle type; this means that it won’t always be
ideal for an individual’s driving conditions. One of the most prominent examples of this would be for four wheel
A
IP Conference Proceedings 1754, 030016 (2016); https://doi.org/10.1063/1.4958360
https://aip.scitation.org/doi/pdf/10.1063/1.4958360?class=pdf
drive (4WD) vehicles. Due to the majority of these vehicles being bought primarily for use in urban conditions with
the occasional off road or towing encounter, they aren’t designed exactly up to the standard some would hope for.
Those who choose to upgrade their vehicle to be capable in off road terrain, will almost always choose to upgrade to
a suspension kit with longer springs and shock absorbers in order to gain greater ground clearance, further
suspension travel and install larger tyres, which will give extra ground clearance under the differential, axles and
also further clearance for the under body. The majority of 4WD owners who decide to upgrade their suspension
system generally install a system that will give more under body clearance and more suspension travel; however it is
not unknown for 4WD vehicles to be upgraded in the opposite direction if it is known the vehicle will only be used
for on road use. As the vehicle is designed from the factory to be able to perform acceptably in the most likely
conditions, swapping out the suspension system for a system suited to your needs will almost always give a very
noticeable improvement. Suspension modification is required either for towing a heavy caravan around the country
or off road adventures.
There is strong prejudice towards suspension modifications of four wheel drive vehicles, with a common view
that any deviation from the original setup dramatically hinders the handling capability of a vehicle. Crash statistics
research conducted by Keall & Newstead2 found that sports cars, which have superior braking and handling systems
in comparison to 4WD vehicles show a considerably higher risk of crash involvement. Many suspension
manufacturers have come up with different methods to provide some level of diversity, however these advanced
systems are generally only available on higher end vehicles. The most common solution being employed currently is
adjustable damping, which allows damping to be controlled electronically, providing some level of improvement
between surface conditions. This solution is limited however, as it can only affect the damping properties of the
suspension system whereas a suspension system is made up primarily of two components, a spring and a damper. A
mechanical spring cannot be varied between surface conditions so the current adjustable suspension systems are
once again, a compromise.
In this article, both the experimental and simulation analyses are presented by modifying a 4WD vehicle. The
objective was to improve the vehicle’s suspension performance in off-road conditions. Due to the nature of the
modification being an increase in ride height, the main investigation is to observe the effects on the stability and
cornering potential of the vehicle due to this modification by means of the load transfer and roll angle of the vehicle.
The investigation aims to observe how the characteristics which are deemed desirable for off road handling will be
detrimental to the on road handling and the methods that could be employed to counteract these effects.
CONSIDERATIONS ON MODIFICATION OF 4WD VEHICLE
Few of the considerations undertaken for this experiment are presented as in the following subsection.
Lift Modification
There are various types of lift modifications that are aimed to increase the off road capability of a vehicle. All
these methods have their own benefits and drawbacks. The most common types of four wheel drive vehicle
modification that are aimed to increase vehicle clearance have been simplified to three types for this experiment.
This includes the fitment of oversized tyre, suspension lifts and body lifts. Suspension lift could be achieved either
by fitting a longer spring and shock absorber or by the fitment of spacer kits between the suspension and chassis
mount and an extended shackle for a leaf spring suspension system. When a suspension kit is installed it causes the
vehicle to sit higher, the height gained in static ride will be lost in the available down travel of the suspension.
Besides, the centre of gravity of the vehicle is affected due to lift modification. In this research, the vehicle was
fitted with an Independent Front Suspension (IFS) setup using the double wishbone configuration.
Body lift is one of the least common types of lift modifications carried out on a 4WD vehicle, albeit arguably
one of the safest for on road conditions if installed correctly. A body lift is achieved by placing spacer blocks
between the mounts of the body of the vehicle (i.e. the cab) and the chassis. The advantage of a body lift over other
types of lift modifications is that the body of the vehicle is almost a negligible amount of mass in comparison to the
remainder of the vehicle, hence producing a minimal effect on the vehicles centre of gravity. Fitment of oversized
tyres3 (large diameter tyre combined with aggressive tread pattern) is deemed beneficial for off-road vehicles but
detrimental to on road application. Oversized tyre could be effective when installed with other two types of
modifications. This type of lift raises the vehicle centre of gravity equal to the increased tyre diameter as both the
sprung and unsprung masses of the vehicle are raised. The most common drawbacks are change of gearing and
requirement of higher ratio differentials, which are to be improved to maintain secured drivability of the vehicle.
Handling Characteristics
Good handling characteristics for an on road vehicle is defined as high cornering potential with minimal body
movement while good off road handling characteristics are defined as good vehicle articulation and stability
throughout the full range of suspension travel. The largest contributor of the modifications for off road capability is
the centre of gravity of the vehicle. When the centre of gravity is raised, the static and transient lateral acceleration
potential of the vehicle is altered. The camber angle is effective for on road application, but insignificant for off road
uses. Increasing the track width of the wheels on a vehicle is the simplest way to counteract the stability issues
caused due to a raised centre of gravity however it also has negative consequences. The Static Stability Factor (SSF)
is a static determination of a vehicle’s resistance to rollover using the most influential geometric characteristics of
the vehicle4, 5. It is defined as the track width of the vehicle divided by twice the height of the centre of gravity of the
vehicle. It is important to note that the SSF is only derived from the geometric relationships of the vehicle
characteristics and the effects of the suspension are not taken into account. It is a simple way of determining the
intensity of vehicle instability due to lift modifications. As the load is transferred to the outer wheel, the available
traction for this tyre increases, however the traction available to the inner wheel is reduced then. Under extreme
circumstances, when the load transfer is equal to over half of the vehicles total weight, it would start to roll over.
Australian Codes and Regulations
In order to be confident that the vehicle is still legal to be driven on Queensland public roads without the
requirement of a certificate of compliance to be issued to the vehicle which would require third party testing and
inspection, scrutiny of these codes and regulations have been undertaken prior to any modification being undertaken.
The modifications were performed as minor category modifications according to both the Queensland Code of
Practice (QCOP) and the National Code of Practice (NCOP)6, 7.
METHODOLOGY
The first and most important specification to be determined for the vehicle is the mass distribution of the vehicle.
Through the use of load cells provided by Dobinsons Suspension8, the mass distribution was determined by
measurement of the mass at each wheel as seen in Fig. 1. Then, with the known spring rate from aftermarket
suspension kit manufacturer, the unsprung mass was determined. The Hooke’s law was applied to experimentally
determine the sprung mass. The anti-roll bar used in this experiment was of similar material as the original anti-roll
bar, so the calculation was performed with Puhn’s formula9 as in Fig. 2.
FIGURE 1. Determination of mass distribution of the sample
vehicle (Mitsubishi Triton)
FIGURE 2. Sway Bar Stiffness Estimation by Puhn’s
formula9
To conduct the Constant Radius Cornering Test (CRCT), some adjustments were performed to suit with the
experimental objectives. The acceleration and gyroscopic data was measured as a means to test and compare the
accuracy of the OptimumDynamics model. The test place was a national highway including a turn-off to another
road (Tee-junction), where maximum speed limit was 100 km/h. The other test speeds were of 60 km/h, 70 km/h
and 80 km/h. By using a surveyor’s wheel, the radius of the corner was estimated as 160 m and using the gyroscopic
sensor, the corner camber was measured at 4 degrees. The centre of gravity of the vehicle could be determined by
the following formula (1)10:
ܥܩ=×(ି௠)
×௧௔௡ ఏ (1)
Here, the parameters, CGh= height of the centre of gravity (m), Wb=wheelbase length (m), m1=front vehicle mass
at ground level (kg), m2= front vehicle mass at raised condition (kg), mT WRWDOYHKLFOHPDVVNJș VLQ-1(total raise-
wheelbase length).
Using the characteristics of the vehicle found, the simulation model using OptimumDynamics could be created
to illustrate the behaviour of the vehicle under various simulations. When the measured data of accelerometer was
exported the OptimumDynamics model for simulation, noise recorded by the sensors were filtered bypassing the
data through the Butterworth11 filter in Matlab. During simulation, the rear suspension was made as the rear
suspension was modelled with coil springs when in reality the vehicle uses a leaf sprung suspension system. The
springs were defined with the correct characteristics however, so the only variation in the model is visual.
RESULTS AND DISCUSSION
Both the experimental investigation and computational analysis are discussed as follows:
Experimental Determination
The vehicle was weighed without occupants in the vehicle, which when taken into account would cause the
symmetry to shift, dependent on the mass and layout of occupants. According to the experiment, the total corner
mass (kg), sprung mass (kg) and unsprung mass (kg) respectively of the driver side (front) were 527, 472.52, 54.48;
passenger side (front) 562, 507.20, 54.80; driver side (rear) 453, 353, 100; and passenger side (rear) 450, 350, 100.
The front of the vehicle was only separated by 35kg which can be attributed to be factors such as the offset of the
engine and the transfer case mass being influenced towards the driver side to allow the prop-shaft to navigate to the
front differential, which is also offset towards the driver’s side of the vehicle. In order to establish the unsprung
mass of the vehicle, the mass distribution was used in conjunction with the displacement of the coil spring. The front
spring preload was determined by using the Hooke’s law (F=kx; where, F is load in kg-wt, k is stiffness const. and x
is the displacement of the sprint at the defined load condition), which is 219.385 kg (with aftermarket assembly). As
the measured corner mass at the front driver side corner was 527 kg, it is known that of the sprung segment of this
measured mass will first need to exceed this spring preload of 219 kg prior to any further displacement of the spring
occurs. The sprung mass on this side was calculated with the total displacement of spring for preloaded mass, which
was 28 mm. Then the constant of Table 1 was used to determine the desired sprung mass from the Hooke’s law. The
unsprung mass corner mass was determined as the difference between total corner mass and sprung corner mass.
When measured from the ground plane with standard tyres fitted, the centre of gravity height was found as 716.8
mm (with standard suspension) and 757.9 mm (aftermarket suspension). As only the sprung mass is being raised
with the suspension lift, the increase in height of the centre of gravity is not equal to the suspension lift. It can be
seen here that the aftermarket suspension fitment relates to an increase in centre of gravity height of 41.1 mm
compared to the 50 mm lifted. The stiffness constants were found as follows in Table 1. The aftermarket front spring
is considerably stiffer as a medium duty spring was chosen to allow the future addition of accessories such as a bull
bar without causing suspension sag. The rear spring chosen was a ‘comfort’ spring as the vehicle does not haul large
loads, and as such, the high stiffness rating of the original springs was not required. It is expected that the softer rear
spring will allow the suspension to more effectively absorb road imperfections, and an expected side effect is that
the oversteer characteristics of the vehicle to be slightly reduced as the vehicle is able to more efficiently transfer
load to the rear of the vehicle.
TABLE 1. Stiffness constants of various springs of suspension system
Description of constants With original springs (N/mm) With aftermarket springs
(N/mm)
Front Sprin
g
Constant 71.31 165.495
Rear Sprin
g
Constant 104.045 40.74
T
y
re Vertical Stiffness 296.262 296.262
Anti-Roll Bar Stiffness 21.89 52.45
From the measured spring and wheel travel, the motion ratio (ratio between wheel travel and spring travel) for
the front and rear suspension setups were found as 2.08 and 1.5 for front and rear suspensions respectively. Using
the CAD tool, the roll centres for both the standard and aftermarket suspension setups were found. The difference in
height between the roll centre height and the centre of gravity height is known as the roll moment, which is the
primary contributing factor to how much chassis roll the vehicle will experience. Standard suspension front roll
centre height was -21.35 mm, and that of aftermarket suspension was -41.29 mm.
Simulation Results
In order to test the model under various scenarios, experimental data was gathered from the vehicle. In case of
input data collection from experiment, this experimental data was limited to what was able to be gathered legally on
public roads due to project constraints. The lateral acceleration and body roll were measured. Lateral acceleration is
useful to contribute towards load transfer over the suspensions and it’s a key handling characteristic of the vehicle.
The measured body roll was used to validate the simulation result, i.e. comparing simulation result of body roll with
experimental result instead of using as input. Various other characteristics, e.g. suspension geometry, centre of
gravity height and roll centre height, etc. can be explained with the validation of body roll. Fig. 3(left side) is the
filtered accelerometer data that was used for the simulation testing at 80 km/h. It is important to note that this is
simply set up as a magnitude, not taking into account direction.
Validation of Simulated Results with Experimental Determinations
FIGURE 3. Filtration of acceleration data (left), and comparison between measured and simulated roll angle (right)
FIGURE 4. Lateral load transfer (left) and chassis roll angle (right) variation for experimental and simulated investigations
Figure 3 shows the comparison of the simulated and measured data of the constant radius corner negotiated at a
constant 80 km/h. It is observed in this figure that the degree of roll angle achieved by the actual vehicle is
significantly higher than that was shown by the simulation software. The simulation peaks the body roll at 2.24
degrees in comparison to the measured 6.08 degrees, a variation of 271 percent. As discussed, the cause of this
variation is likely due to the experimental nature used to evaluate the specifications of the vehicle. Another factor to
be taken into consideration is that the software uses a steady state simulation method, indicating that the dampening
characteristics of the shock absorbers are not taken into account. This could have a considerable effect on the
accuracy of the results.
Moreover, Fig. 4 shows the results of experimental and simulated investigation for total load transfer with and
without modification. It shows that the total load transfer is reduced from 244.95 kg to 238.18 kg (i.e. ~2.9%
reduction) with the track increase from the simulated results. As the lateral load transfer component is only
dependent on the centre of gravity height, wheel track width and the lateral acceleration experienced, the upgraded
anti-roll bar will not have any effect on this major contributor towards the total load transfer. The upgraded anti-roll
bar acted to stiffen up the vehicle, reducing the body roll experienced, i.e. effect on the dynamic load transfer caused
by the motion of the centre of gravity of the sprung mass. Unfortunately, at this small displacement of chassis roll,
the dynamic load transfer is so small that the effect of the upgraded anti-roll bar is negligible and cannot be seen in
the reduction of the total load transfer. When observing the chassis roll angle in Fig. 4 (right side), it can be seen that
when the upgraded anti-roll bar is fitted, the roll angle achieved by the vehicle is actually reduced to a level below
that of when the standard suspension is fitted. It could be observed from this figure that the dynamic load transfer
component is actually lesser than the standard suspension with the aftermarket suspension lift and the upgraded anti-
roll bar. It has been observed here how the stiffer sway bar could be more effective at higher lateral accelerations as
the potential dynamic load transfer components increased from the higher potential roll angles achieved by the
vehicle. The comparative analysis of load distribution on track and anti-roll bar showed that the load was
transferring from front to rear, which was also the similar case with standard suspension. Besides, the widened
wheel track and upgraded anti-roll bar have no effect on the load transfer towards the rear of the vehicle.
CONCLUSION
The vehicle model which was considered in this experiment was Mitsubishi Triton 4WD. The effect of
modifying the vehicle by fitments of suspension lift, widened wheel track and upgraded front anti-roll bar were
analysed both experimentally and computationally with some limitations in each sections. Without taking into
consideration the effects of the damping effects of the standard and aftermarket shock absorbers, the simulation
results showed the predicted results of enhanced tendency towards roll angle due to the increase in roll moment with
just the fitment of the suspension lift. The softer rear springs also demonstrated a slightly improved ability for the
load to transfer towards the rear of the vehicle, illustrating a slightly reduced tendency to demonstrate oversteering
characteristics. With the simulation of the fitment of the other modifications aimed to improve the characteristics of
the raised vehicle, the vehicle showed a reduced tendency towards roll angle due to the stiffened anti-roll bar and the
maximum increased wheel track demonstrated reduced lateral load transfer and body roll. Even with these
modifications however, the decrease in load transfer is minimal in comparison to what was expected. The
aftermarket suspension actually showed less roll angle than that was found in the simulated results due to no
damping effect consideration in the software. As a result, the simulated displayed higher roll moment and greater
centre of gravity height. From this it can be assumed that the standard damping used in the vehicle is considerably
‘softer’, designed for comfort, whereas the aftermarket is ‘harder’, designed to reduce the dynamic load transfer
component and other handling effects of excessive body roll such as undesirable camber gains. To observe real
effect the modifications, the damping effect of the suspensions, which was not considered as the aftermarket
suspension dampers should be considered. A better and expensive computational tool like MSC Adams could be
more effective in providing details analysis. Moreover, the facility of more sophisticated but safe experimental
investigation and data management could be beneficial to provide industry scale results of modifying the 4WD
vehicles for off-road conditions.
ACKNOWLEDGEMENTS
The authors would like to acknowledge the significant contribution by Central Queensland University.
REFERENCES
1. 4wdonline, in 4WD Suspension (http://www.4wdonline.com/A.hints/Suspension.html, 2013).
2. M. Keall and S. Newstead, Report No. 262, 2007.
3. AAMVA, in Study Findings (American Association of Motor Vehicle Administrators, 2003).
4. R. Elvik, A. Hoye, T. Vaa and M. Sorensen, The Handbook of Road Safety Measures, 2nd ed. (Emerald
Group Publishing Limited, UK, 2009).
5. E. A. Harwin and H. K. Brewer, Journal of Traffic Medicine, Vol. 18, No. 3, 1990, p. 109-122 (1990).
6. TMR, in Vehicle Modifications (The State of Queensland (Department of Transport and Main Roads),
Australia, 2015).
7. NCOP, in Section LS Tyres, Rims, Suspension and Steering (Version 2) (Australian Government-
Department of Infrastructure and Transport in Canberra, Australia, 2011).
8. Dobinsons, (Dobinsons Spring & SuspensionTM), Vol. 2015.
9. F. Puhn, How to Make Your Car Handle. (H. P. Books, 1976).
10. Longacre, in http://www.longacreracing.com/technical-articles.aspx?item=42586 (Longacre Racing
Products, 2002), Vol. 2015.
11. Butter, in MATLAB butter (MathWorks, 2015).
ResearchGate has not been able to resolve any citations for this publication.
The Handbook of Road Safety Measures
  • R Elvik
  • A Hoye
  • T Vaa
  • M Sorensen
R. Elvik, A. Hoye, T. Vaa and M. Sorensen, The Handbook of Road Safety Measures, 2nd ed. (Emerald Group Publishing Limited, UK, 2009).
How to Make Your Car Handle
  • F Puhn
F. Puhn, How to Make Your Car Handle. (H. P. Books, 1976).
longacreracing.com/technical-articles.aspx?item=42586 (Longacre Racing Products Google Scholar
  • In Longacre
  • Http
  • E A Harwin
  • H K Brewer
E. A. Harwin and H. K. Brewer, Journal of Traffic Medicine, Vol. 18, No. 3, 1990, p. 109-122 (1990).
Suspension and Steering (Version 2) (Australian Government-Department of Infrastructure and
NCOP, in Section LS Tyres, Rims, Suspension and Steering (Version 2) (Australian Government-Department of Infrastructure and Transport in Canberra, Australia, 2011).