Page 1 of 10

14AC-0245/2014-01-0095

Reconciliation of ATV/UTV Handling Characteristics and the Operator 

Author, co-author (Do NOT enter this information. It will be pulled from participant tab in 

MyTechZone)

Affiliation (Do NOT enter this information. It will be pulled from participant tab in MyTechZone)

Copyright © 2014 SAE International

Abstract

This paper examines the directional handling characteristics of 

several vehicles in their original condition, then examines 

modifications to a few of these vehicles to determine if the 

handling characteristics can be made more forgiving of normal 

operators without sacrificing utility and without substantial 

increases in cost. These analyses of vehicles are made in the 

context of what normal operators are capable of performing 

with regards to steering response.

Introduction

Humans have a limited ability to control any machine. The 

operator perceives what is wanted and applies a control input 

to have the machine perform the desired operation. In the 

case of vehicles that is a steering, throttle, or brake input. In 

this paper the concentration is on the steering input with the 

desired path of the vehicle as the machine output. Control of a 

vehicle begins with a decision regarding what direction the 

operator desires the machine to go. A steering input is 

applied, the vehicle responds, the operator observes the 

response and adjusts the inputs for any error that occurred 

between the steering input and the desired vehicle response. 

Through the last century, vehicle dynamic control has been the 

subject of serious study and applied to all vehicles on land, 

sea, and air. The principles of dynamic stability apply in all 

three of the realms and are all limited by what a person can do 

with regards to perception, reaction, and response. 

Maintenance of dynamic control of a vehicle is paramount in 

the safe operation of the vehicle. This paper reports on an 

examination of human control characteristics and applies these 

to all-terrain vehicle (ATV) and utility-terrain vehicle (UTV)

control. 

In this paper the handling characteristics necessary for human 

control of various ATVs and UTVs are presented. Most of 

these vehicles had handling characteristics that traditionally 

have been found to not be conducive to vehicle control or 

safety. Several vehicles were modified to alter the 

oversteer/understeer characteristics to be consistent with 

accepted control parameters of automobiles. This was done to 

both determine if that can be done economically and without 

negatively affecting utilitarian functions and improve safety 

performance. This paper reports the modified vehicles’

measured handling characteristics and the rider heuristic 

evaluation of handling. This is then compared to the traditional 

vehicle handling design to determine if there is a potential for 

improved safety through vehicle design.

First a short discussion of the possible connection between 

control and crash frequency will be given. This will be followed 

by a description of how humans control vehicles and the 

vehicle characteristics that are most important for human 

control. Then the results of testing various vehicles from EI 

Consultants independent tests will be presented. The modified 

vehicles will be described and their handling characteristics will 

be given. Conclusions concerning the possible effects of 

traditional understeering on drivability and safety are 

presented.

Crashes and Control

ATV fatalities have been a major concern in the USA for many 

years, with the CPSC reporting 11,688 fatalities in the three 

decades 1982 to 2011 [1]. Of these approximately 25% were 

younger than 16 years of age. Similarly in Australia, ATV 

fatalities are a major issue, particularly on farms, with 124

fatalities in the period 2001 and 2010 [2]. Improvements in 

Quad Bike and SSV handling has been highlighted by authors 

such as Roberts [3] and others as being practical means to 

reduce crash and rollover risk. (Note: Australian for ATV is 

quad bikes and UTV is side by side )

In the CPSC study of 1989 they found that the cause of 80% of 

ATV crashes was affected by lack of “rider judgment” [4]. 
Page 2 of 10

Figure 1. Percent of crashes and potential fixes based on Scheers, et 

al (1990).

From an engineering standpoint, it seems that rather than 

relying solely on “improved judgment” by ATV operators to 

reduce ATV crashes, ways to improve vehicle safety through 

design should be examined. Certainly part of the fix to 

overturn crashes could be attributed to the rider’s judgment, 

but vehicle design also affects the ability of the rider to 

maintain vehicle control and possibly prevent crahes. Since 

1990, efforts have been made to assist riders to improve their 

judgment through training and education. Frustration after 

attempting to improve judgment by training and education 

alone was expressed in a recent meeting in Australia1

. In the 

FAQs associated with a discussion paper released by 

Australia's Federal Minister for Employment and Workplace 

Relations in August 2012, the following statement was made: 

" For over 20 years the focus on quad bike safety in Australia 

has been on other approaches like training and education, 

awareness raising and helmet use to address the high number 

of fatalities and serious injuries sustained by quad bike 

riders. The time has come to focus on design and engineering 

controls for improving quad bike safety.” Looking at handling 

characteristics of the existing ATVs and UTVs in light of 

operator handling capabilities could be a way of approaching 

the problem from another direction.

Controllability of the vehicle, in conjunction with riding ability, is

linked to vehicle design characteristics that will assist in 

allowing the vehicle to be controlled by inexperienced riders or 

riders who are not intent on the act of riding. Farmers and 

ranchers, and those who are confronted with emergency 

situations and must react quickly and properly to avoid a crash 

are the individuals being considered. The first portion of this 

paper is about determining what vehicle characteristics 

contribute to human control of vehicles and how can these 

characteristics be quantified by testing. Understanding these 

dynamic properties of a vehicle for predictable human control 

will allow the selection of a set of tests to measure the vehicles 

for human control compatibility. Due to space restrictions, only 

 

1 http://www.safeworkaustralia.gov.au/sites/swa/whsinformation/agriculture/quad-watch/pages/quad-bike-safety-forum

one of these characteristics will be discussed here. The others 

will follow in future publications.

Characteristics Affecting Human Control of 

Vehicles

Analysis of the control of any system by a human requires two 

things. Knowledge of the human ability to perceive, process, 

and react to events encountered by the vehicle as it follows the 

intended path is one. The system dynamic characteristics to 

receive a control input and respond in a stable manner and 

provide feedback to the human controller is the other. A block 

diagram describing such an operator/vehicle system is shown 

below [5].

Figure 2. Block Diagram of Driver/Vehicle System for Lateral Steering 

Control and Regulation [5]

The McRuer study was to determine “how the response of a 

vehicle to a driver’s steering input affects the driver’s ability to 

maintain precise control over vehicle path.” He went on to 

state in his opening paragraph of his report, “With this 

knowledge we can identify those vehicle steering response 

characteristics which may lead to imprecise vehicle path 

control and which may, in turn, lead to accidents.” Can the 

ATV/UTV be studied to yield similar results of vehicle 

characteristics? Would the handling characteristics of 

automobiles, which must respond to inputs and give feedback 

to the same humans that operate ATVs/UTVs, be relevant to 

the study of ATVs/UTVs? Use of research regarding dissimilar 

vehicles as a starting point for improving and understanding 

vehicle control is not a new or novel endeavor. A 1956 study 

drew on the work concerning the stability of aircraft and ships 

to understand and improve the handling of road vehicles [6]. 

Milliken and Ollie also relied on aircraft stability studies to 

improve the handling and longitudinal stability of the modern 

automobile [7]. Similarly, the results of automobile/operator 

interface have been used in the study of the handling of ATVs

[8]. Indeed, in the SAE Technical Paper "An Introduction to the 

Operational Characteristics of All-Terrain Vehicles" [9] the 

authors argue: "...handling qualities include, in general, 

controllability which in turn involves ease of control, the ability 

to follow a desired path or make the desired maneuver, and 

the ability to suppress disturbance inputs, whether they arise 

from the environment or from within the system itself."

Control of any machine by a human requires that it responds 

quickly to a control input. The human controller then perceives 

the effect of the control input through visual or tactile 

perceptions. He or she then analyzes the response, and 

determines any error with respect to the desired effect. Then

he/she adjusts the control input to correct for any error in the 

response. Control implies that the machine must respond the 
Page 3 of 10

same way to a given control input, respond in a timely manner, 

and not change the response over time with constant control 

inputs. For dynamic systems such as vehicles, the response 

time must be short enough to allow the controller to adjust the 

control inputs to safely manage a high speed vehicle along a 

desired path. When the vehicle or machine ceases to respond 

or is random in its response then that vehicle is “out of control.”

There are main control characteristics that determine if a 

vehicle is controllable by a human operator. These are

understeer/oversteer [6], the steering response time [5], and 

steering gain [5]. For off-road vehicles, the vertical and yaw 

response to striking a series of bumps at the suspension 

harmonic frequency is also an element of concern [10]. For 

the scope of this paper only understeer/oversteer will be 

addressed. Pitch and bounce response, step steer response 

and steering gain will be addressed in later papers.

Understeer and Oversteer

The first order of business is to differentiate between the 

common usages of the terms understeer and oversteer which 

are results of specific driver inputs and the engineering use of 

the terms that describes a characteristic of the vehicle. A manoff-the-street, discussing oversteer, understands that word to 

be how he can cause oversteer. He causes oversteer by 

pulling on the parking brake or applying throttle to cause the 

rear of the vehicle to break away and swing out in a turn. 

These are maneuvers used in racing and recreational riding. 

The vehicles being driven in oversteer actually have an 

inherent understeering design characteristic. This driver 

induced “oversteer” is not what is being discussed here. 

Understeer is a measure of a handling characteristic of the 

vehicle independent of the driver. It is measured using a circle 

test as described in SAE J266 (ISO 4138:2012) [11]. When 

the numerical value of the measured understeer of the vehicle 

is negative, it is then an oversteer vehicle, meaning that the 

yaw rate responses are diverging from the steering angle 

magnitude. This is a classic instability. To measure this 

characteristic a vehicle is driven in a circle laid out on a flat 

surface of consistent coefficient of friction. While gradually 

increasing speed, the steering angle and lateral acceleration 

are measured. The plot in Figure 3 below shows the steering 

angle versus the lateral acceleration for a typical, well 

behaving, understeer vehicle.

Figure 3. Understeer characteristic for a stable handling vehicle.

The vehicle described in this graph will require a continually 

increasing steering angle as the lateral acceleration increases. 

The yaw rate response is proportional and gradually 

decreasing as the steering increases. Understeer is quantified

by the slope of the graph between 0.1 and about 0.4 g’s. For 

the vehicle illustrated in Figure 3, that slope would be about 2 

degrees per g. This vehicle would respond to a lateral force 

from a gust of wind or a change in road camber by drifting 

away from the applied force. This would establish a new 

stable path without any effort by the driver. However if that 

slope of the above graph is negative, the vehicle is called an 

oversteer vehicle. If a road camber angle change occurred, 

the vehicle would tend to move toward the applied force or 

uphill to the slope of the road. That would then decrease the 

turning radius of the vehicle. That decrease in turning radius 

would in turn cause an increase in the lateral force on the 

vehicle. It would respond by turning in an even smaller turning 

radius. This cycle would continue increasing the lateral forces

which decrease the turning radius in an unstable manner. In 

an Advance Notice of Proposed Rulemaking (ANPR) for 

recreational off-highway vehicles, the CPSC states,

"If a vehicle oversteers in a turn, ..., the rear wheels 

lose traction and the steering wheel needs to be 

turned less to stay on the turn. This condition is 

directionally unstable because it can result in spin out 

or rollover of the vehicle. Controlling oversteer 

requires driver skill and knowledge in using 

acceleration and steering that is beyond the average 

driver" [12]

Conducting the same SAE J266 circle test and graphing the 

results as was discussed by the CPSC in 2009 [12] and by 

Heydinger et.al [13] for a typical UTV or ATV will often yield a 

graph as shown in Figure 4 below.
Page 4 of 10

Figure 4. Understeer characteristic for typical UTV

By contrast to the first vehicle’s understeer graph in Figure 3, 

this vehicle demonstrates too much understeer at low lateral 

accelerations. This can be as much as 9 degrees per g. 

Reasonable handling vehicles have an understeer gradient 

between 2 and 6 degrees per g. At a lateral acceleration of 0.2 

g, the vehicle transitions to oversteer. At this point what was 

once a severe understeering vehicle becomes a severely

oversteering vehicle. The slope shown above is about -8 

degrees per g. Due to the steep understeer gradient at the 

very low lateral accelerations the vehicle instability will be 

masked while driving straight at low lateral accelerations. 

However, any steer maneuver that would cause the vehicle to 

exceed a lateral acceleration of 0.2 g will move the vehicle 

response into the oversteer realm causing the rider to exert a 

significant effort to maintain control. These are all results 

consistent with what was found by Allen, et al [8, 9, 14], “Four 

wheeled ATVs with mechanical suspensions tend to oversteer 

under hard cornering conditions due to rear axle side force 

saturation...” In addition, Heydinger et al. [13] concluded, 

“Several of the vehicles tested exhibit understeer at low levels 

of lateral acceleration and then transition to oversteer at higher 

levels of lateral acceleration.”

A recent paper suggested that an oversteering vehicle for offroad or UTV handling characteristic was preferable to a slightly 

understeering vehicle [15]. The data from their testing actually 

suggests that a heavy understeering vehicle of 9 degrees per g

is undesirable. That is understandable since 9 degrees per g 

is far higher than anything that has been found to be 

controllable or desirable. That vehicle was sluggish and 

difficult to control. The most desirable vehicle in the study was 

a stock Polaris RZR that illustrated a good understeer 

characteristic and was preferred over the other vehicle 

configurations in the study. These results support what has 

been found in this study.

Dynamic Testing for Understeer and Oversteer

Dynamic testing and analysis of various ATVs and UTVs has 

demonstrated that several stock, unmodified vehicles exhibit 

terminal oversteer often preceded by understeer at low lateral 

acceleration levels. It was found that the oversteer vehicle was

difficult for riders to control. The fact that the vehicle exhibits 

both understeer and oversteer depending on the lateral 

demands placed on the vehicle increases the unpredictable 

handling characteristics of these vehicles and the demand on 

the rider. 

ATVs

ATVs are rider active vehicles. This means that the rider is 

free to move around on the seat and can affect the handling of 

the vehicle by shifting his or her position. During the ATV 

testing, the rider remained as still as possible in a neutral 

seating position.

The testing summarized in this paper involved stock ATVs, 

modified ATVs for which suspension layouts and components 

were changed, and tuned ATVs where existing geometries 

were used and the shocks and springs were tuned. General 

characteristics of the vehicles are shown below.

Table 1. Summary of ATVs tested by EI Consultants

ATV A and ATV B are typical small ATVs with a solid axle rear 

suspension. The solid axle design creates a near infinite rear 

roll stiffness with the only rear compliance being flex in the 

mountings/bushings and from deflection of the tires. This 

massive roll stiffness differential between the rear and the front 

of the vehicle results in an oversteer vehicle.

Testing of the vehicles both subjectively and quantitatively 

confirmed this prediction. The driver felt an initial pushing from 

an understeer tendency at very low speeds then was forced to 

apply constant steering corrections by taking out steering as 

the test speed increased. This is mirrored in the test data. As 

seen with ATV A (Figure 5), the initial positive slope of the data 

indicating understeer is seen at very low lateral accelerations. 

Before even 0.1 g is developed, the vehicle is a neutral steer 

vehicle with a transition to oversteer just beyond 0.1 g. The 

scatter of the data seen above 0.2 g is indicative of the driver 

having to input and remove steering to keep the vehicle on 

course as the rear of the vehicle is losing lateral traction. 

Vehicle 

Designation

Stock/Modified

/Tuned

Year 

Model

Engine 

Size

Unladen 

Weight 

(lbs)

Front 

Track 

Width 

(in)

Rear 

Track 

Width 

(in)

Wheelbase 

(in) Rear Suspension

ATV A Stock 2001 250 462 30.75 30.75 44 Solid Axle

ATV B Stock 2006 250 421 31.625 31.875 44 Solid Axle

ATV C Modified 2006 250 468 31.625 35.125 44 IRS with Spool

ATV D Modified 2006 250 468 31.625 35.125 44 IRS with Differential

ATV E Tuned 2012 700 745 35.625 36 53.625 IRS with Spool

ATV F Tuned 2012 800 854 36.25 38.75 57.625 IRS with Spool
Page 5 of 10

Figure 5. ATV A understeer/oversteer plot

ATV B showed similar characteristics (Figure 6). A difference 

between ATV A and ATV B is that there is more scatter in the 

ATV B data. This indicates a more unpredictable vehicle 

requiring more rider corrections to stay on course. The general 

trend for both vehicles is predictably similar. 

Figure 6. ATV B understeer/oversteer plot

ATV C (Figure 7) and ATV D (Figure 8) were modifications of 

ATV B. A matching chassis (same year, make, and model) 

was modified with an independent dual control arm rear 

suspension with a wider track. ATV C had a spool rear axle 

with an anti-sway bar, whereas, ATV D had a differential and 

no sway bar.

The figure below shows the testing with ATV C with spool 

drive. Driver confidence was improved, and there was a 

decrease in the data scatter. Transition to oversteer occurred

at a higher lateral acceleration than the stock vehicle.

Figure 7: Understeer/oversteer plot for ATV C

The addition of an open differential allows the roll stiffness 

distribution to be tuned without the need of a rear sway bar to 

unweight the inside tire. The open differential installed on the 

ATV gave much greater freedom when setting the roll stiffness 

distribution between the front and rear of the vehicle. 

The driver’s confidence was much improved, and the vehicle 

response was much more predictable. The data shows a 

defined lack of scatter throughout the test. In addition the data 

shows that the vehicle is neutral to slightly understeer up to the 

limits of the test. 

Figure 8: Understeer/Oversteer plot for ATV D

ATV B and ATV D were also compared in an off-road 

environment. A small, oval course with a gravel surface was 

used for this comparison. This course consisted of two short 

straights and two sweeping curves of near constant radius 

connecting the straights. The course was driven 

counterclockwise meaning the rider was inputting turns that 

would cause the vehicle to enter and hold left-hand turns. The 

track is shown below. 
Page 6 of 10

Figure 9. Gravel test course.

The handling differences between ATV B and ATV D were 

drastic on this course. The driver was much more confident 

with ATV D. ATV B was very unpredictable and constant 

sawing of the handlebars was required by the driver. 

Corrective steers required were so extreme that the rider was 

often inputting right steers to stay on the left curve. 

The following figures demonstrate the improved handling from 

the modifications. The colored boxes in the figures correspond 

to the colored boxes in Figure 9. These boxes show what part 

of the course the ATV is traversing on the data plot. Note that 

turning the handlebars to the left is indicated by a negative 

wheel angle and turning right is a positive angle. It is shown 

from the first figure that the rider was alternately steering right 

(positive) and left (negative) to stay on the left curve. Figure

11 shows that the steering required to stay on course is less 

erratic and did not require the corrections necessary with the 

stock vehicle. Considering that the speeds are the same for 

both tests, it is clearly demonstrated that improved on-road 

handling stability translates to an off-road environment.

Figure 10: Plot of ATV B steering on gravel course

Figure 11: Plot of ATV D steering on gravel course

ATVs E and F have the original stock suspension geometry 

only the shocks and springs were replaced. Spring stiffnesses 

and damping rates were selected with the aid of computer 

modeling and tuning for flat ride over cyclical rolling bumps 

commonly known as “whoop-di-doos”. 

In addition to tuning the springs and shocks, the tires were also 

modified. The force versus deflection properties of the tires 

were measured at various pressures. Based upon this 

research, it was found that 10 psi was the optimum pressure 

for the tires to transmit terrain forces to the suspension. This 

will be discussed in a subsequent paper about the terrain 

effects. 

A constraint was placed on the tuning by the fact that the spool 

drive was retained. As mentioned above, this made it difficult

to obtain the desired front and rear roll couple and overcome 

the effects of the solid axle on understeer. Still, the final tuning 

of the vehicles resulted in predictable handling as felt in the 

constant radius testing. The constant radius tuning was also 

evaluated in an off-road environment to ensure that the 

understeer did not compromise the off-highway mobility. The 

representative plot below shows the understeer/oversteer plot 

for the tuned ATV F. Notice from the plot that the vehicle is 

basically neutral steer with a slight turn to understeer at higher 

lateral accelerations. 

Figure 12: ATV F understeer/oversteer plot
Page 7 of 10

The testing of these ATVs demonstrated that tuning of the 

suspensions and components can result in predictable 

handling during testing on pavement. This improved handling 

on pavement was also reflected in the off-road environment, 

both on the gravel test track, where quantitative measurements 

demonstrated the improvement, and through subjective 

evaluations at the EI Consultants test track.

The suspension tuning not only improved the handling during 

the on-road testing, but also improved the handling in an offroad environment. Travel and clearance was not reduced, so 

no off-road mobility was lost due to these adjustments. In 

addition, it is feasible to offer limited slip and lockable rear 

differentials yet provide improved understeer performance that 

increases rider confidence and reduces rider work load. 

UTVs

UTVs are distinguished from ATVs in that the operator is 

seated on a more conventional bench or bucket seat and they 

are typically equipped with seatbelts and a roll cage. The 

driver operates the vehicle via a steering wheel and foot 

pedals. In addition, UTVs are not rider active as are ATVs. 

However, the suspension, tires, and drivetrain of UTVs are 

virtually identical to ATVs. As such, UTVs demonstrate similar 

handling behavior to an ATV.

For the purposes of this paper, research and analysis of six

UTVs are presented. Certain characteristics of the vehicles

used in this study are summarized in Table 2 which is seen 

below.

Table 2: Summary of UTVs tested By EI Consultants

All UTVs tested had dual control arm independent suspension 

systems on the front and rear. All modified UTVs had 

suspension modifications as well as spring and shock absorber 

tuning.

UTV A, a stock UTV, showed both understeer at low lateral 

accelerations and a transition to pronounced oversteer at 

higher ones. This placed a large burden on the driver to keep 

the vehicle under control. The test driver lost control in a 

slalom maneuver due to not catching the oversteer response. 

This total loss of control resulted in a tip onto the outriggers. 

The understeer and oversteer are clearly seen in the data plot 

for the constant radius circle testing (Figure 13). In addition, 

scatter is also seen as the driver is constantly correcting the 

steering so the vehicle remains on-path. 

Figure 13. Understeer/oversteer plot for UTV A

UTV B was also tested in stock configuration. This vehicle 

also exhibited a transition to oversteer. Test termination was 

often a tip-up event. The vehicle starts understeer, quickly 

transitions to neutral steer, then becomes oversteer around 

0.25 g. The oversteer range is also characterized by erratic 

steering inputs in order to keep the vehicle on course as 

evident by data scatter as seen in Figure 14.

Figure 14. Understeer/oversteer for UTV B

UTV C and UTV D were direct modifications of UTV A and 

UTV B respectively. Modifications included suspension tuning 

and increased track width. UTV C and D were modified to 

achieve a static stability factor of 1.0 with vehicle plus driver. 

The modification to meet these goals was a track width 

increase achieved by fabricating longer suspension arms for 

UTV C and adding wheel spacers for UTV D. In addition, UTV 

D was fitted with a rear differential. 

UTV C’s suspension tuning consisted of replacing the OEM 

shocks and springs. Adjustable dampers were installed to tune 

the damping characteristics. Spring stiffness were altered

based on mathematical calculations concerning ride rates and 

pitch characteristics discussed in reference [10]. In addition, 

new sway bar mounts and mounting points were fabricated 

allowing tuning of the roll stiffness on the front and rear 

suspension. 

Vehicle 

Designation

Stock/Modified

/Tuned

Year 

Model

Engine 

Size

Unladen 

Weight 

(lbs)

Front 

Track 

Width 

(in)

Rear 

Track 

Width 

(in)

Wheelbase 

(in) Rear Suspension

UTV A Stock 2011 700 1320 44.25 46.625 74.75 IRS with Spool

UTV B Stock 2007 450 1107 43.5 42.5 75 IRS with Spool

UTV C Modified 2011 700 1321 51 55 75 IRS with Spool

UTV D Modified 2007 450 1113 51.5 50.5 75 IRS with Differential

UTV E Modified 2011 400 1178 52.5 49.25 74.75 IRS with Spool

UTV F Modified 2011 800 1232 51.125 46.625 74.75 IRS with Limited Slip
Page 8 of 10

The dynamic handling characteristics of the modified vehicles

were predictable and the driver demands were greatly 

reduced. The trend seen is increasing understeer up to test 

termination with very little scatter as evident in Figure 15 and 

16. 

Figure 15. Understeer/oversteer plot for UTV C

Figure 16. Understeer/oversteer plot for UTV D

The modifications to UTV E and F were based on static 

analysis and computer modeling. The shock and spring 

specifications were based on mathematical and computer 

analysis again discussed in reference [10]. Slight tuning of the 

roll stiffness at the test track was achieved via adjustable rate 

sway bars.

Track width was increased on both vehicles by fabricating 

longer control arms. The track width values were based on 

static analysis and the design goal that the static stability 

coefficient, Kst, value be 1.0 or greater with a driver.

Note the lack of scatter in both plots below (Figures 17 and 

18). Understeer is seen throughout each test. Also note that 

the limited slip differential on UTV F allowed slightly more limit 

understeer to be tuned into the vehicle. 

Figure 17. UTV E understeer/oversteer plot

Figure 18. UTV F understeer/oversteer plot

As with the ATVs, the UTV testing demonstrated an inherent 

directional instability in the stock vehicles. The modified 

vehicles demonstrated more controllability as reported by the 

driver and less steering demand as seen in the data.

Summary/Conclusions

Human control of a vehicle requires that after a disturbance or 

control input, it establish a new stable trim. The handling 

characteristic that is easily measured and translates across to

off road vehicles is the understeer gradient. Tests reported 

here illustrated that the predominate characteristic of ATVs and 

UTVs is a high understeer gradient at low lateral accelerations 

that transitions to oversteer at relatively low lateral 

accelerations. 

It was shown that when the vehicles were adjusted to have

nominal amounts of understeer throughout the driving range 

the driver demand is reduced and steering variations required 

to maintain a circular path at elevated lateral accelerations 

were reduced. This was illustrated by the driver reports during 

the tests and the lack of scatter seen in the data as higher 

lateral accelerations were reached.

As can be seen from this paper, design constraints from drive 

trains and suspensions affect the ability to tune the suspension 
Page 9 of 10

for an ideal configuration. However, as the 

oversteer/understeer curve approached the form shown in 

Figure 3, controllability increased and driver demand 

decreased.

It is understood that the particular demands for control of offroad vehicles is more challenging than on-road vehicles. 

Friction coefficients are changing, the wheels of the vehicle are 

often not on the ground, and the terrain is undulating and 

rough. However, these vehicles are often driven at high 

speeds on flat and sometimes paved surfaces. Providing 

designs that can manage in “All” terrains is truly a challenging 

goal. This research has indicated that directional control

instabilities do exist with many ATV and UTV designs. By 

adjusting these designs to follow well established principles of 

longitudinal stability, the driver demand is reduced and 

increased rider confidence is gained while maneuvering the 

vehicle.

Improvements in Quad Bike and SSV handling has been 

previously highlighted by one of the authors and others [3] as 

being a practical means to reduce crash and rollover risk. The 

methodology presented in this paper has now been used by an 

Australian/US research team (that includes four of the authors) 

at Australia’s New South Wales government Crashlab facility to 

assess the handling characteristics of eight typical ATVs and 

five UTV designs used in the workplace, and three sports/ 

recreational type ATVs, as part of a safety rating program 

outlined in reference [16]. The results of these tests will be 

presented in a later paper.

This work should be the beginning of marrying on-road and offroad design principles to improve the rider’s experience. As 

McRuer said about automobiles, “With this knowledge we can 

identify those vehicle steering response characteristics which 

may lead to imprecise vehicle path control and which may, in 

turn, lead to accidents. [5]” The research presented here 

illustrates that learning from the past vehicle design and 

development may assist ATV and UTV designers in “...

focus[ing] on design and engineering controls for improving 

quad bike safety.” 

References

1. Garland, S., 2011 Annual Report of ATV-Related Deaths 

and Injuries. CPSC, 2013. 

2. Lower T, Pollock K & Herde E (2013). Australian Quad 

Bike Fatalities: What is the Economic Cost?. Aust N Z J 

Public Health. 2013 April; 37(2):173-8

3. Roberts A, “Dynamic Analysis of Side-by-Side Utility and 

Recreational Vehicles”, Paper No.09-0260-O, 21st 

International Safety Conference on the Enhanced Safety 

of Vehicles, 2009 

4. Scheers, N.J., Fulcher, D., and Polen, C., Four-Wheel 

ATVs and Lateral Stability, Emergency Room Treated 

Injuries (1989), U.S. Consumer Product Safety 

Commission, 1990.

5. McRuer, D., Klein, R., “Automobile Controllability –

Driver/Vehicle Response for Steering Control,” DOT-HS801-407, Volume 1, 1975.

6. Segel, Leonard, “Research In the Fundamentals of 

Automobile Control and Stability,” Transactions of the 

SAE, 1956.

7. Milliken, Milliken, Notes of Vehicle Dynamics – Maurice 

Olley, SAE publications, 2004.

8. Allen, R., Szostak, H., Rosenthal, T., Klyde, D. “Steady 

State and Dynamic Properties of All Terrain Vehicles 

Related to Lateral Directional Handling and Stability”, SAE 

891105, 1989.

9. Weir D.H. and Zellner J.W. An Introduction to the 

Operational Characteristics of All-Terrain Vehicles"; SAE 

860225. Dynamic Research Inc, Torrance CA, 1986.

10. Renfroe, D., Fleniken, G., “Designing for Pitch and Bounce 

Motions of Single-Passenger Off-Road Vehicles, SAE 

940273, 1994.

11. SAE International Surface Vehicle Recommended 

Practice, “Steady-State Directional Control Test 

Procedures for Passenger Cars and Light Trucks,” SAE 

Standard J266, Issued January, 1996.

12. US Consumer Product Safety Commission (CPSC), 

“ANPR for Standard for Recreational Off-Highway 

Vehicles,” Federal Register/Vol. 74, No. 207, October 28, 

2009.

13. Heydinger, G.J., Bixel, R., Jebode, H., Peterman, E., 

Sidhu, A., “Vehicle Characteristics Measurements of 

Recreational Off-Highway Vehicles,” CPSC, April, 2011.

14. Allen, R.W., T.J. Rosenthal, and H.T. Szostak, “Steady 

State Transient Analysis of Ground Vehicle Handling,” 

SAE 870485, Nov. 1986.

15. Brown, Jeffrey C., Larson, R., Fowler, G., Kuhn, R., 

“Recreational Off-Highway Vehicle (ROV) Handling and 

Control,” SAE 2012-01-0239, April 2012.

16. Rechnitzer, G., Grzebieta R.H., McIntosh A.S., and 

Simmons K. (2013). “Reducing All Terrain Vehicle Injuries 

(ATVs) And Deaths - A Way Ahead”, Proc. 23rd

International Safety Conference on the Enhanced Safety 

of Vehicles, Soeul, Korea, May, Paper No. 13-0213. 

Contact Information

David A. Renfroe; EI Consultants, LLC; dar@enginst.org

Alex Roberts; EI Consultants, LLC; alex@enginst.org

Raphael Grzebieta, University of New South Wales, 

r.grzebieta@unsw.edu.au

George Rechnitzer, University of New South Wales, 

g.rechnitzer@unsw.edu.au

Keith Simmons, KND Consulting Pty Ltd, 

keith_simmons@bigpond.com

Definitions/Abbreviations

ATV all-terrain vehicle

UTV utility-terrain vehicle

SSF static stability factor
Page 10 of 10

critical speed on an oversteer vehicle, critical 

speed is that speed at which yaw 

rate tends to infinity

Kst static stability constant

ROV recreational off-highway vehicle

ROHVA Recreational Off-Highway Vehicles 

Association