Dr Andrei Lozzi
Machine Design & CAD
MECH4460 & 5416
School of Aerospace Mechanical
& Mechatronic Engineering
The central torsion tunnel, to suit a kit car
Handed out: 4pm Thursday 28h March 2013
To be collected: 4pm Thursday 25th April 2013
This assignment should take an average student 16 hours to achieve a pass
All students are required to submit a signed statement of compliance with all work submitted to the
University for assessment, presentation or publication.
You are to design, analyse and build a model of a frame with a tunnel that contributes most of
the torsional stiffness to the whole frame for an open two seater kit sport car. The construction
is to be made entirely using bamboo skewers of about 3mm dia. glued at the nodes using 5
minute Araldite adhesive. All this material is readily available at most supermarkets. The scale is
to be 1/8th, correct to within 5 mm. It is suggested that you make plane jigs on cardboard to
assure yourself of reasonable dimensional accuracy.
The rectangular prism shown on figure 8 page 4, represents the envelope within which the frame
has to be contained. Your model is to be a simple version of a realistic frame, you are to just
allow space for two occupants within a structure that is capable of torsional and bending loads
between the two axles. No allowance is to be made for suspension, engine, transmission, doors,
body shell, seats and other lesser items. Your model is to have, among other elements, two
parallel skewers at the locations of the axles shown on figures 8. The torsion tunnel essentially
enhances the stiffness of the overall frame by carrying some of the load from one axle to the
other.
The space required by the occupants is indicated on figure 10 & 11. The necessary opening for
the occupant access is shown on Fig 10. They occupants are to face forward and to have
reasonable space for their limbs within the frame. The envelope available for the central torsional
tunnel is specified on Figure 9. All dimensions are in mm. A commercial kit car frame may be
viewed in Bob Halliday’s lab, by arrangement with the workshop manager
The frames will be judged based on torsional and bending stiffness, simplicity (mass) and
strength. A sample torque is to be applied to one axle and resisted at the other. A bending
moment is to be generated by applying vertical forces at the two front lower nodes of the torsion
tunnel and resisted at the axles. An analysis of your design may be done SW Simulation or other
FEA package.
You must submit a report showing how you arrived at your final solution. You may work in pairs
but each member’s contribution to the report is to be on separate A4 pages and initialled by the
its author.
1
Figs 1 left, 2 below.
The brilliant but poorly manufactured 1957-63 Lotus Elite.
The monocoque chassis for this car is arrived at by gluing
together the outer body shell (top left), to the inner shell (centre)
and to the floor pan (bottom), all made as thin fibreglass panels.
The engine mounts, seats, door hinges etc. were all bonded
directly to the fiberglass monocoque.
The concept was brilliant, the car is particularly elegant, simple
and extremely light, its roadholding is legendary, but the built
quality was very poor. As a collector’s car, it is a nightmare.
Figs 3 below, 4 lower right.
The car that followed the Lotus Elite was the Elan,
shown below.
In order to provide adequate strong points for the
mounting of engine, transmission and suspension a
backbone sheet steel sub-frame was made. This
frame can be judged to be stiff and strong in bending
but not so in torsion.
Torsional rigidity is significantly enhanced once the
fibreglass body shell is attached to the sub-frame.
Some American texts refer to this sub-frame as
“immensely strong”. Somehow American authors and
possibly some engineers are not aware that torsional
stiffness is important and how it may be achieved.
2
Figs 5 above, 6 far right.
The STP Indianapolis turbine car, shown here, reflects
ignorance on behalf of the designers. It does not utilise
the outer boundary of the car to provide a stiff chassis in
torsion and in bending. Placing the driver, structure and
engine side by side, maximises cross-section to the air
flow and consequently drag. It also unfortunately
minimised the polar moment of area of the chassis.
American books and journals still laud this as a
breakthrough design (yet I believe it was never
repeated). Lotus at about the same period had the turbine
directly behind the driver.
Fig 7 below.
This kit car’s frame exhibits a somewhat
discontinuous and incomplete space frame. The
torsion tunnel in particular is not well braced to
contribute to the torsional stiffness of the frame.
Compared to the Lotus
F1 frames it lacks
ingenuity in its attempt
to provide rigidity and
lightness. See Costin &
Phipps
3
Axles
The frame must be
contained within
this envelope
Space available for
tunnel: 1400 long,
550 high, 400 wide.
Front
550
2000
1300
Rear
Fig 9
Fig 8
Lower front nodes
Opening for
occupant access
800x450
800
Fig 10
450
1400
550
Fig 11
1400
4