NEW FOLDABLE URBAN CAR CONCEPT

Abstract

A growing demand for individual mobility, increasing number of cars and rising car dimensions require actions to improve the parking situation in urban areas. One way to scope with this challenge includes the implementation of new vehicle and transportation concepts, which focus on the substitution of conventional passenger cars. Alternative vehicle packaging and new chassis, propulsion and suspension technologies are capable to fulfill specific parking and important customer requirements at the same time. In this paper, current urban car concepts with variable wheelbase technologies are investigated and a car concept including novel variable wheelbase mechanism and a new packaging solution for three passengers (EvoCare), is presented. The development of such vehicle and suspension concepts challenges common processes and requires multidisciplinary interaction of different disciplines, e.g. kinematics simulation, vehicle packaging, ergonomics & comfort, propulsion system layout, as well as driving dynamics and driving stability investigations. In this context, the novel suspension mechanism for the folding process is introduced and discussed. Furthermore, the required parking space and resulting costumer benefits of the concept are analyzed and illustrated by a comparison of different vehicle types in the target class.

Full text

F2016-AVCA-002
NEW FOLDABLE URBAN CAR CONCEPT
1Walzel, Bernhard*; 1Hirz, Mario; 1Brunner, Helmut
1Graz University of Technology, Institute of Automotive Engineering, Graz, Austria
KEYWORDS – urban car concept, individual urban transport, efficient parking, variable
wheelbase, 3-wheeler
ABSTRACT
A growing demand for individual mobility, increasing number of cars and rising car
dimensions require actions to improve the parking situation in urban areas. One way to
scope with this challenge includes the implementation of new vehicle and transportation
concepts, which focus on the substitution of conventional passenger cars. Alternative
vehicle packaging and new chassis, propulsion and suspension technologies are capable to
fulfill specific parking and important customer requirements at the same time.
In this paper, current urban car concepts with variable wheelbase technologies are
investigated and a car concept including novel variable wheelbase mechanism and a new
packaging solution for three passengers (EvoCare), is presented. The development of such
vehicle and suspension concepts challenges common processes and requires
multidisciplinary interaction of different disciplines, e.g. kinematics simulation, vehicle
packaging, ergonomics & comfort, propulsion system layout, as well as driving dynamics
and driving stability investigations. In this context, the novel suspension mechanism for the
folding process is introduced and discussed. Furthermore, the required parking space and
resulting costumer benefits of the concept are analyzed and illustrated by a comparison of
different vehicle types in the target class.
1. INTRODUCTION
Parking space in urban areas is a scarce resource. In addition to aspects like urban
densification and increasing numbers of vehicles, steadily rising vehicle dimensions lead to
parking space shortages and complicated parking processes. This time, the vehicle fleet
character is changing: besides small and compact cars, high volume Vans and Sport Utility
Vehicles (SUV) are increasingly present [1]. The average length of passenger vehicles has
grown by 4 % from 2000 to 2010, and the vehicle height has increased by 16.5 %.
Supporting the big success of large car types, increasing safety requirements and legal
provisions as well as comfort demands are responsible for those growing space
requirements. Searching free parking lots is very exhausting. In a study of stressfully
situations during car driving, more than 35 % of German car drivers define parking lot
search as most stressful and driving pleasure reducing [2].
Small city cars are becoming increasingly popular, but customer benefits of these vehicle
types are limited, because of small interior space and comparatively poor driving comfort.
The three-seater urban car concept “EvoCare” from the Institute of Automotive
Engineering, Graz University of Technology improves parking in urban areas without
sacrificing the driving behaviour and costumer comfort (Figure 1). The vehicle wheelbase
represents a significant variable for the external and internal dimensions of a vehicle, as
well as for the driving behaviour on various road conditions. With variable wheelbase, the
EvoCare concept combines both benefits, a short wheelbase for parking as well as
increased wheelbase supporting driving stability at high velocities and under poor road
conditions.

The conceptual development process of EvoCare is based on a comprehensive analysis of
users’ mobility and dynamic driving performance behaviour, including transport and
driving distance demands of various user types. In case of those studies, several
advantageous characteristics of three seater cars were identified. In a share of 44 % of all
car-driving activities, the majority transport demand requires storage space for 4 pieces of
goods and a maximum number of three seats. In this context, one challenge in the
development process of the EvoCare concept was the fulfilment of sufficient space for 3
persons and 4 luggage pieces within a length of only 2.2 m in “parking mode” [3]. The
variable wheelbase is realized by a specific folding system. During the folding operation
from “driving mode” to “parking mode“, the cabin is lifted, which decreases the vehicle
length. The resulting low space demand allows small parking and maneuvering areas and
provides the possibility of parking perpendicular to the road.
Figure 1: EvoCare. Left: Initial design proposal including folding mechanism. Right: 3D-model of the concept
vehicle with passenger packaging solution [4].
1.1. Development targets
Initial research work included the definition of customer target groups, the elaboration of
technical characteristics and the development of vehicle packaging and ergonomics
concepts [5]. One aim of the subsequent concept development process included the
realization of a suitable folding mechanism. Target of development was a simple, cost
efficient and robust solution, which provides sufficient vehicle body stiffness and safety
characteristics. To enable a low cost solution, it was a target to perform the folding process
without additional actuators. This has required the development of a smart kinematics
system, which is able to be actuated by the electric propulsion motor of the car.
1.2. Methology
In a first step, a technology and patent research of state of the art vehicles with variable
body and wheelbase, focussing on chassis and variable suspensions technologies was
carried out.
Wheel suspensions are safety relevant components; so the mechanical load situation had to
been taken into account. Another important concern was the development of a cost-
efficient solution for the variable wheelbase mechanism. Based on the vehicle-packaging
concept of EvoCare, a new kinematic mechanism was created and tested in a
comprehensive kinematics and kinetics simulation. The simulation was used for evaluation
of research results, further optimization and the development of concept proposals.
Furthermore, the space saving potentials of the novel urban car concept were discussed and
investigated.
2. CONCEPT DEVELOPMENT
2.1. State of the art
Figure 2 demonstrates a selection of urban car concepts with variable wheelbases. State of
the art are mechanisms, which are propelled by an actuator system, e.g. electric motors,

hydraulic or pneumatic linear actuators. Kinematics and kinetics transmission is mainly
performed with one link including a central bearing, or a linkage system. Such systems
have an impact on stiffness and safety characteristics of the vehicles. For instance, a link
with an integrated rear wheel and a central bearing has to carry high forces, leading to very
stable and complex (heavy) constructions. With these solutions, sufficient ratios of vehicle
length in driving and parking mode can be reached. However, complex wheel suspension
systems, including wheel hub motors or even 360°-rotatable wheels, result in high costs.
Figure 2: Selection of urban car concepts with variable wheelbase. Upper row: Hiriko folding mechanism [6],
Ultralight Road Vehicle [7], Renault Zoom [8]. Lower row: Vehicle with adjustable wheelbase [9], car with
small parking area demand [10], EO smart connecting car [11].
2.2 Concept proposal
Automobiles are usually carried, steered and driven by wheels. Number and layout of
wheels as well as the type of propulsion and suspension systems have essential impact on
the body concept, the interior and external dimensions, as well as on the driving behaviour.
Figure 3 includes vehicle specifications and a kinematics model of a novel concept
proposal for a variable wheelbase vehicle. The EvoCare concept was designed to fulfill the
requirements on future urban cars, as described in Section 1.1. The simple vehicle
architecture includes two main modules: A cabin containing passenger and luggage space
as well as a steerable front axle, module (1), and the propulsion module including battery,
inverter, electric motor and the driven rear axle (2). The mentioned folding mechanism is
capable to move front- and rear axle relatively to each other. As a speciality, the folding
process is significantly supported by a spring system (3) and the relative movement of
cabin module (1) and propulsion module (2). In this way, the vehicle length can be reduced
by one-third for parking and low-speed routing, without sacrificing driving comfort and
safety at higher speed on highways trips.
Figure 3: Left: EvoCare vehicle specifications [3]. Right: Novel kinematics proposal for
vehicles with variable wheelbase.

Figure 4 shows folding mechanism and component positioning of the concept in the three
operation modes “driving“, “folding“ and "parking“. The folding mechanism consists of
the propulsion modul (2), linkages (3 and 4), main carrier (5), spring and damper (6), spots
for fixation (7) and the corresponding fixation mechanism, which is integrated in the
propulsion modul. In „driving mode“, the propulsion modul (2) is fixed connected with the
cabin (1) by fixation spots (7). This enables a stiff body and a comfortable driving
behaviour at high velocities, due to the long wheelbase. Both sides of the linkages (3 and 4)
are mounted on the main carrier (5) and the propulsion modul (2). Via the linkages, the
main carrier (2) with the integrated rear axle is moved towards the front axle.
For “folding” the released fixation device decouples the propulsion modul (2) from the
cabin (1). The folding process starts by braking front wheels and subsequently driving the
rear wheel by the main engine (9). Two springs and dampers (6) are rotateable mounted on
the propulsion modul (2). The rear linkages (3), which are placed symetrically to the
longitudinal axis of the vehicle, support the folding process until the parking position is
reached. For “parking”, the fixation mechanism (7) connects the propulsion modul (2) with
the cabin (1). The propulsion modul (2) houses heavy components like battery (8), electric
traction motor (9), gear box (10), suspension with drive belt (12), rear wheel and fixation
mechanism. A systematic positioning of the components leads to a deep centre of gravity
that supports the folding process, due to a predefined movement of the heavy components
during folding. Folding is solely operated and controlled by the traction motor - expensive
actuators are not necessary. Vehicle dimensions and exemplary parking space demand of
the “EvoCare” are shown in Figure 5 and Figure 6. The provision of a variable wheelbase
enables a comparatively high variability of the vehicle length. In “parking mode”, the
length is just 2.1 m, which technically enables parking perpendicular to the road and small
parking and manoeuvring spaces in general. At a longitudinally parking lot with a length of
6 m length and a width of 2.3 m, 3 EvoCare vehicles can be parked side by side. The
achieved space benefit is displayed in Figure 6 by a size-comparison with a Smart ForTwo
and a BMW E63 Coupe.
Figure 4: EvoCare driving modes and main (heavy) component packaging.
Figure 5: External dimensions (in mm) and size comparison with Smart ForTwo 2007 and
BMW E63 Coupe 2010.

Figure 6: Exemplary EvoCare parking space demand.
3. KINEMATICS AND KINETICS SIMULATION AND TESTING
For realization of the new vehicle concept, the development of a suitable folding
mechanism represented a main challenge. Target was the creation of a smart low cost
mechanism, which was integrated into the vehicle architecture under consideration of
vehicle packaging, ergonomics requirements and the electric propulsion system.
The folding mechanism was investigated by use of comprehensive kinematics and kinetics
multi-body simulation. Besides geometrical and power-related aspects, friction behaviour
between wheels and the road surface plays an important role for proper operation of the
system. The coefficient of friction is defined by relationship of the wheel longitudinal force
Fx and the wheel force normal to the surface Fy. Generally, the coefficients of friction µ
and µmax are defined as [12]:
  



  


 (1), (2)
Road Conditions [13]
Bachmann [14]
Mundl [15]
Gustafsson [16]
Mitschke [17]
PC Crash [18]
Barace [19]
Asphalt, dry
Concrete, dry
Cobblestone, dry
1.05 – 1.2
1.05
-
-
1
-
-
0.88-1.15
-
0.7 - 0.9
0.9 – 1.2
Gravel, dry
-
0.5a
0.5
-
-
0.5
Asphalt, wet
Concrete, wet
0.73 – 0.81
0.66 – 0.8
0.85
-
0.7
-
0.7 – 0.95
0.5 – 0.7 (wet)
0.4 – 0.5 (very wet)
0.7 – 0.9
0.65 – 0.8
Asphalt, sandy
-
-
-
-
-
0.5b
Asphalt, snow
Asphalt, ice
(scattered)
Asphalt, ice
-
0.18 – 0.38c
-
0.2
-
0.1
0.3
-
0.1
0.07 – 0.02
0.1 – 0.5
0.05 – 0.25
0.2 – 0.3
0.1 – 0.2
a Mean value for loose gravel
b Assumed to be in the range of gravel
c Harsh ice
Table 1: Ranges of friction potentials of different road conditions, based on a literature review [19].
An overview of coefficients of friction potential of different road surfaces is shown in
Table 1, which provided the basis for the folding mechanism characteristics simulation
under different road surface conditions. Focus of the simulation was the determination of
required folding forces and the needed coefficients of friction for rear and front wheels.
Without enough “grip”, the rear wheels would spin and the front wheels would slip during
the folding process. Because of this behaviour, the weight balance of front and rear axle is
important. During folding, the centre of gravity moves and changes the axle loads, which
requires a consideration of the loading situation in the simulation process. Furthermore,
different load situations are caused by different car occupancy (1, 2 or 3 persons), which
also changes the force situation and influences the kinetics characteristics during folding
process. The multi body simulation provides longitudinal and vertical tire forces, under
consideration of the rear wheel movement and the required coefficients of friction. In the
simulation, the vehicle is divided into wireframe models: front carriage (cabin), rear
carriage (propulsion module), linkages and wheels. The wireframes are linked with joints
and represent the kinematic characteristics during folding. The tire forces are calculated by
different occupancy situations from 1 to 3 persons (cabin: 340 kg, propulsion module: 350

kg, weight per person: 80 kg). The centres of gravity of the components are supplied by
CAD-model data.
3.1 Simulation result
Figure 7 show the force curve of Fx at the rear wheel (longitudinal force at the 2 front
wheels act equivalent contrariwise) and the vertical tire forces Fz for a folding length of
1100 mm without support of the springs ((6) in Figure 4). In “driving mode”, the weight
distribution of front and rear axle for all loading situations is between 43 to 57 % and
increases to the rear direction during folding process. In parking position, the weight
distribution is 36 % to 64 %. In this way, the (braked) front axle limits the maximum
longitudinal forces. The simulation result for the coefficient of friction at the front wheels
at launch of folding for one-person load is above a value of 1, and for 3 persons
approximately 1.4, Figure 8. Under consideration of Table 1, folding will be possible for
one person and restricted possible for 3 persons on dry asphalt surfaces. To solve this
kinetics problem, additional springs have been applied (see Figure 3) to decrease the
required longitudinal force significantly, Figure 8. The resulting coefficient of friction
(Figure 9) suggests sufficient adhesion and a successful folding process by using these
supporting springs under all possible conditions, even on wet road surface. With a
maximum occupancy of 3 persons, the maximum required coefficient of friction is 0.46 at
rear wheel and 0.57 at front wheels.
Figure 7: Left: Longitudinal force Fx for folding. Right: Normal forces Fz acting on the 2 front
wheels and the 1 rear wheel.
Figure 8: Left: Required minimum coefficient of friction at front wheels. Right: Reduced force Fx because of
spring support in the kinematics mechanism.
Figure 9: Required minimum coefficients of friction for successful folding. Left: Front wheels. Right: Rear
wheel.

3.2 Prototype
For representation of the novel urban car concept and for verification of the simulation
results, a vehicle model has been developed in scale 1:5 (Figure 10). Vehicle chassis,
propulsion system and the folding mechanism with mechanical and electrical parts were
built by low-cost components; many of them using radio-controlled model car elements.
For comparable test conditions, model weight and axle load distribution under different
loading situations have been considered. On dry and wet asphalt, the folding mechanism
was successfully tested and confirmed the simulation results.
Figure 10: EvoCare Prototype in scale 1:5 including folding mechanism
4. CONCLUSION
Driving comfort, small parking spaces, low production costs, as well as satisfying different
user requirements are essential for a successful implementation of future urban car
concepts. Based on these challenging issues, a newly developed space- and cost-saving
urban car concept has been presented. The novel concept integrates a new vehicle
packaging design and an innovative folding mechanism for a variable wheelbase. As a
difference to previous foldable car concepts, the presented folding mechanism does not
need any additional propulsion system, like electric motors, hydraulic or pneumatic
actuators. In this way, the EvoCare car represents a simple and economically producible
approach. In combination with a smart packaging solution of all car components, the
folding system is able to decrease the vehicle parking space demand by 33 %. In “parking
mode” the length of the vehicle concept is only 2.1 meters, which allows parking
perpendicular to the road and provides the basis for small parking and maneuvering spaces.
Furthermore, the concept offers space for 3 passengers and luggage. It is based on a simple
chassis design and smart packaging solutions of heavy electric drive train and energy
storage system components. The folding mechanism is based on a newly developed
kinematics, which has been investigated by use of computational kinematics and kinetics
simulation and by a vehicle model. The simulation and model test results confirm a proper
operation of the mechanism under different dry and wet road conditions. An optimization
of the vehicle components weight distribution and the application of specific spring
systems improve the longitudinal force situation during folding. In this way, the presented
concept represents itself as a very cost-efficient solution to address one of the main
challenges in urban traffic – the parking situation without sacrificing driving safety and
comfort at higher speed operation.
CONTACT INFORMATION
Bernhard Walzel, Dipl.-Ing.
Scientific Project Researcher
Institute of Automotive Engineering, Graz University of Technology
Inffeldgasse 11, A - 8010 Graz
Phone: +43 316/873-35278
Email: bernhard.walzel@tugraz.at

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