Graduation Thesis: Suspension Design for an Electric Cargo Vehicle
For my graduation thesis, I chose a subject which would
involve work with the design of suspensions. I found that at my university with
HAN Automotive Research. The project was done with a Dynteq. My project with
them involved the design of a rear suspension for an electric cargo bike. This
project is protected by confidentiality, therefore not all the details and
images can be shared on this post. However, I would like to share my experience
of designing a suspension.
Cargo bikes are becoming quite popular in Europe as the
streets of old cities are very small and narrow, having cargo bikes allows
delivery companies to use them for use within city deliveries, as they don’t
take up much space on the roads and in some countries and cities they can also
make use of cycle lanes, thus reducing the van traffic congestion. Having
electric bikes also allows for less pollution within the city.
A typical delivery van consuming space on both the road and pavement in a narrow street
Dynteq was making the Electric bike for one of its
customers, which is a delivery company. The cargo vehicle had one wheel in the
front and two on the rear axle. The frame and cargo box were already designed;
therefore, the challenge was to design a suspension for the rear axle within
the constraints of the chassis present. This was especially difficult since the
vehicle was very compact, as it had to meet the rules to use bicycle lanes and
meet legislation.
This project also suffered a setback at the 3/4 stage of the project. This happened because the customer changed the chassis and cargobox design, this change came about because the vehicle was in the prototype stage and it was being improved. This however caused the entire suspension to be changed, the entire suspension type and geometry was changed due to this change. Since this change came about at such a late stage of the project it also put a lot of time pressure to meet the goal within the set amount of time. However, by prioritising the important processes and products, the goal was achieved within time.
In this project, to design the suspension the first thing that was done was
to get the customer requirements, the most important things for the customer
were to have a safe suspension which can provide stability and handling and in
loaded and unloaded conditions. The customer made it clear that they do not
want active suspension. That made it tricky to make a suspension that worked in
both loaded and unloaded conditions as the load difference was quite a lot.
Once the customer requirements were known a competitor
analysis was done using the Tripl (vehicle used by DPD), the Groupil (vehicle used
by Picnic Netherlands) and the Armadillo (vehicle used by DHL). This competitor
analysis was done to see the strengths and weaknesses of the competitors, this
was also done to see that kind of suspensions systems were being used by the
competitors.
Following the competitor analysis, a choice table was made between different suspension types. The suspension types chosen for the choice table were suspensions that would fit within the construction of the electric cargo vehicle. The suspension chosen for the vehicle was a modified version of the control blade suspension, (like the one used by the 2005 Ford Focus) The details of the suspension cannot be shared due to the confidentiality agreements. The suspension geometry was optimised to provide maximum stability as well as predictable handling. This was done as the riders of the vehicle should not require any training to drive the vehicle, therefore the vehicle should be easy to drive and have predictable handling.
Control blade suspension in the Ford Focus
For the design of the suspension, the spring rates and damping
rates need to optimised too, as the motion ratio of the suspension has an
effect on those factors. Due to the limited space in the vehicle, the wheel and
suspension travel allowance was not too much, therefore using helical springs was
not possible. Therefore, the only
options were torsion bars or compression mounts, however, finding torsion bars
that offered the spring rates, compactness and wheel travel required was very
difficult and indeed there were no suppliers that offered the torsion spring
required.
The solution was to use a compression mount, these mounts
are usually used for vibration damping and used as bump stops. However, these
mounts were ideal for use in the vehicle, to achieve the progressive spring
characteristics a parabolic compression mount was used, using a spring with progressive
spring allows for favourable spring rates in both loaded and unloaded
conditions. Since the spring is a polymer spring, the material friction also
provides the damping required for the suspension. This removes the need for a
damper on the suspension.
Progressive Spring characteristic, Force on Y axis and Displacement on X Axis
Along with choice of the spring and damper, the design of the suspension arms was to be done. To do this several load cases were analysed, such as impulse bump, steady state cornering with maximum load transfer as well as acceleration and braking. Once the load cases were analysed, the forces on all the suspension members were known, using those forces the geometry of the suspension arms was designed and optimised used FEA and techniques I learnt from my minor.
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