Advanced Testing Devices for Suspension Components and Tyres
ATZ 100 (1998), Nr. 9
Philip Köhn, Peter Holdmann
Kinematics and Compliance Test Rig
Dynamic Tyre Test Rig
Summary
Suspension components and tyres have a tremendous influence on ride
and handling qualities of motorcars. Therefore the testing technology
for these components is of major interest for both theoretical and
practical vehicle dynamics investigations. Consequently, two new
testing facilities for chassis components investigations have recently
been taken into service at the Institute of Automotive Engineering
(ika) at the Aachen University of Technology (RWTH Aachen). These test
rigs are equipped with highly sophisticated measurement data
aquisition systems and they provide a new quality of testing at a
university institute. Both the rigs and first testing results will be
introduced in this article.
Kinematics and Compliance Test Rig
Kinematics and compliance properties of vehicle suspensions have a
major impact on ride and handling properties. The ika K & C test rig
is designed to investigate both full vehicles and stand-alone
suspension systems [fig. 1 and fig. 2].
Fig. 1: Vehicle on the new ika K&C test rig
Fig. 2: Twistbeam Axle on the ika K&C test rig in operation
Design of the K & C Test Rig
The rig mainly consists of four posts equipped with hydraulic
cylinders that allow any desired vertical deflection of the car?s
wheels. Additionally, there are two more small cylinders in every post
to simulate lateral and longitudinal force application. A car can
either be tested with its own wheels mounted or with special kinematic
devices mounted that simulate the kinematics of a rolling wheel. These
devices can be adjusted to match the tyre semidiameter and pneumatic
trail value. With the devices being mounted, lateral and longitudinal
force application is no longer limited to the maximum friction force
in the tyre contact patch; this increases the rig's field of operation
and it makes testing less complicated.
In both cases there are air cushions mounted between suspension and
rig to reduce friction to a minimum value of about 20 N even at
maximum wheel load. This allows the suspension to deflect in lateral
and longitudinal direction as it does in reality if horizontal forces
are applied.
The maximum force range in lateral and longitudinal direction is 10
kN. The rig is adjustable to any wheel base between 2000 mm and 3250
mm; the track width front and rear is independently adjustable between
1180 mm and 1650 mm.
Generally, very large forces will be applied to the car body during a
kinematics and compliance investigation. Consequently, a variable but
nethertheless very stiff system of fixing elements and beam elements
has been built up to connect the car body as stiff as possible to the
test rig. There are fixing elements available to be mounted to the
car's door sills, bumper fixings and strut top mounts. Mostly the top
mounts prove to give the best fixing of the body. However, in most
cases a combination of different fixings will be used.
Sometimes there are no complete vehicles but stand-alone suspension
systems to be tested. In such cases there is a number of rigs
available to test these suspensions. Fig. 2 shows a rear suspension
system assembly ready for measurement on the rig.
Test Rig Operation System
The rig consists of in total 12 electro-hydraulic control loops that
may be operated either in displacement control or in force control
mode. The basic functions of the rig such as run-up, run-down or
emergency routines are provided by an extremely reliable
microcontroller unit. Every control loops is continuously monitoring
itself. If, for instance, one cylinder exceeds its previously adjusted
force limit, all cylinders will be locked up automatically. Thereafter
the rig will slowly run down to the neutral position. These safety
routines ensure that the suspension cannot be be damaged by the rig
due to overload.
Additionally, there a two operation modes available for the whole rig.
Forces and displacements can either be adjusted manually or
automatically. In the automatic operation mode a PC system generates
the values of forces and displacements in the course of time.
Consequently, full driving manoeuvres such as a steady-state circular
run can be simulated on the rig.
Apart from that the standard
investigations can be automatically performed as well:
· investigation of roll stiffness
· longitudinal and lateral compliance investigation
· identification of roll axis position
Fig. 3: Test rig operation system
The identification of a suspension's roll axis position gives a good
example of the rig's capabilities. The roll axis position is being
identified with one post operating in displacement control and the
other post in force control mode. While one post is being lifted, the
other one is being lowered in a way that the overall axle load stays
constant.
Moreover, the roll axis position is not measured geometrically because
this does not provide a sufficient precision. The roll axis can be
identified more precisely by applying a constant lateral force to both
wheels. This lateral force will provoke a change in the measured wheel
load due to the jack-up effect (in most cases the applied lateral
force will increase the measured wheel load). The relation between
applied lateral force and resulting wheel load change gives a good
value for the roll axis position.
Measurement Data Aquisition
All relevant data such as lateral, longitudinal and vertical
displacements, wheel loads, lateral and longitudinal forces, toe and
camber angles are constantly measured with reliable sensors and they
are continuously converted into digital data by several A/D
converters.
Investigation of the K&C Properties of a Rear Axle as an Example
Ergebnisse am Beispiel einer einzelnen Hinterachse
Typical investigations of suspension mostly cover both kinematics and
compliance properties. Consequently, the changes in wheel alignment
and wheel loads as a function of wheel travel are being measured. The
results of such a measurement are displayed in Fig. 4. In this case
the changes in wheel alignment are caused both by kinematic influences
(the camber angle in design position turns into toe angle changes when
the axle starts rotating during wheel travel) and compliance effects
(twisting and bending of the axle tube). These effects cannot be
separated from each other.
Fig. 4: Change of wheel alignment as a function of wheel travel
measured on a twist-beam rear axle
The additional investigation of lateral and longitudinal force
compliance steer gives more information on the axle's properties.
Genarally, the forces applied should not exceed the overall axle load
as this is a natural limit in driving operation (unless there is an
accident or offroad overload situation). For this investigation the
wheels have to be replaced by the devices mentioned above because the
normal friction behaviour in the tyre contact patch does not allow the
application of such forces unless the wheel load is significantly
increased. Moreover, the simulation of the tyre's pneumatic trail is
not possible with a non-rotating tyre. Fig. 5 shows the typical
behaviour of a twist-beam rear axle. The lateral force applied causes
the axle tube to bend almost only in one direction which directly
leads to large camber angle changes and relatively small toe angle
changes. However, the wheel alignment changes towards the wrong
direction (toe-out and positive camber) which causes a reduction of
the rear axle's overall cornering stiffness.
Fig. 5: Wheel alignment changes as a function of lateral force applied measured on a twist-beam rear axle
Dynamic Tyre Test Rig
The dynamic tyre test rig is one of two tyre testing facilities at the
institute of automotive engineering Aachen (ika). It is designed to
investigate stationary and dynamic properties of both motorcar and
motorbike tyres at a maximum wheel load of 10 kN. Tyres can be tested
on virtually every rim size between 13" and 19".
Fig. 6: The ika dynamic tyre test rig in operation
Design of the Test Rig
The rig is mounted on a large drum that is operated by an electric DC
motor at a maximum speed of 180 km/h. During testing wheel load, tyre
inflation pressure, camber and slip angle can be adjusted manually or
in an automatic operation mode. Adjustment of slip and camber angle is
realized independently from each other and there is no lateral
movement of the tyre contact patch versus the drum if camber or slip
angle are adjusted. This is of major importance particularly under
dynamic operation conditions. Camber and slip angle are adjusted
hydraulically for highly dynamic operation, whereas wheel load is
applied by means of an airspring to ensure a smooth and constant load.
The rig's characteristic data are:
- drum speed: 180 km/h
- diameter of drum: 1700 mm
- maximum wheel load: 10 kN
- adjustment range: camber angle: +50° to -25°
slip angle: + 12°
- dynamic properties:
slip angle: 2° amplitude at 10 Hz sinusoidal excitation
camber angle: 5° amplitude at 5 Hz sinusoidal excitation
Operation System
The test rig is controlled by digital position controllers and a
reliable microcontroller unit that provides the test rig's basic
functions. Additionally, there is a PC system installed that serves as
man-machine-interface, provides the signal generation and includes the
data aquisition system. Due to the fact that test rig operator is not
directly in touch with the rig but only via PC and microcontroller
unit, a maximum reliability and repeatability of the measurements can
be ensured.
Fig. 7: Overall view of the dynamic tyre test rig
The test procedures can either be performed with synthetic standard
signals (e. g. sinusoidal or triangular input) or with custom-made
test signal that may also result from driving tests. The only
requirement is that the generated signals stay within the rig's
adjustment range and that they do not contain harmonics with
frequencies greater than the limits mentioned above. Signals may be
generated with any editor; they only have to be converted into an
ASCII file shape afterwards.
Measurement Data Aquisition
There is a 5 components measurement hub installed to measure all force
and moments reactions from the tyre. The hub is of a strain-gage type
(see Fig. 8); its signals are processed by an amplifier and low-pass
filters. Moreover, there are additional sensors to measure tyre
temperature, slip and camber angle, longitudinal slip, tyre
semidiameter and speed. All signals are constantly converted into
digital information and displayed to monitor the measurement.
Fig. 8: Measuring hub of the dynamic tyre test rig
Measurements on a Motorbike Tyre as an Example
The flexible layout of the control system makes design of testing
routines easier, especially if complex routines have to be realized.
As an example, the tyre behaviour of a motorbike in a weave mode
oscillation with camber and slip angle rapidly changing with a fixed
phase relation to each other can be simulated to obtain a deeper
understanding of tyre behaviour under these operation conditions. To
give an easier example of the rig's capabilities, the investigation of
a motorbike's lateral force performance under slip and camber angle is
demonstrated. The necessary time histories of the relevant parameters
are displayed in Fig. 9.
Fig. 9: Time courses for a testing programme to measure a motorbike
tyre in a wide range of slip and camber angles (wheel load not shown)
It is obvious that the tyre behaviour resulting from pure slip and
pure camber angle is investigated at first. Thereafter camber will be
increased in steps of 10° each and at each camber value the resulting
lateral force is measured during a slip angle sweep. This test cycle
has a length of in about 85 s. Thereafter the wheel load will be
increased automatically (not shown in this graph) and the whole
procedure will begin again. After finishing the measuring procedure
the tyre is characterized in a wide slip and camber angle range under
various wheel loads. The advantage of such automatic testing routines
even for relatively simple tests is obvious: Testing takes less time,
it is extremely reproducable and reliable. Having finished the test
procedure the data need some postprocessing (e. g. low-pass filtering,
elimination of crosstalk from the measuring hub). The following Fig.
10 displays the results of this test procedure for a motorbike tyre
measured at a wheel load of 1.900 N. The graphs clearly show that
maximum lateral force is only available at large camber angles. This
is due to the fact that the contact patch area of most motorbike tyres
is significantly increased at large camber angles.
Fig. 10: Lateral force as a function of slip and camber angle measured
on a motorbike tyre at a wheel load of 1.900 N
Summary
Having taken into service these two new test rigs, the number of
testing facilities for suspension components avaliable at the
Institute of Automotive Engineering Aachen has been significantly
increased. Consequently, a wide range of research work on vehicle
dynamics will be realized from first concepts and simulations up to
prototype manufacturing and testing. This gives the students of ika
the unique chance to work on almost all fields of automotive
engineering during their studies and they will leave university with
an education that is as close as possible to reality in the automotive
industry.
