What you see on the following page is the concept diagram for Godiva’s chassis. You can see that the diagram has ‘layers’ to help make sense of the overall image (note all red/green/black lines are steel tube, coloured sections are honeycomb panel). However, before going into the details as such we should consider the overall design philosophy behind the construction method proposed.
For those of you used to looking at space frame chassis design, you will notice the lack of triangulation, or cross bracing of the chassis, particularly in three dimensions. Additionally it will seem that it has very large unsupported sections. This is because the chassis is (at this stage) intended to be a hybrid chassis, part monocoque and part space frame. The large open sections in the chassis are actually filled with aluminium honeycomb panels, which form a structural part of the chassis and are not just appliqué. The body will be mostly bonded to this structure with some simple mechanical fixtures and the side sections of the body will be foam filled to provide crush resistance and added protection for both the driver and the chassis.
For those of you not familiar with honeycomb panels, they are constructed from two aluminium sheets of varying thicknesses separated by and bonded to an aluminium honeycomb foil. This honeycomb core material can be of various thicknesses, but the general rule is that the thicker the panel core the stiffer the panel. This aluminium honeycomb panel is the same type of material that some of the first Formula One composite tubs were made of (using a cut and fold method of construction) and to our joy there are a couple of manufacturers of such honeycomb panel materials within Australia. The panels are very stiff/rigid for their weight, have excellent intrusion/deformation performance, are relatively easy to maintain and can be repaired. The local manufacturers produce the panels primarily for marine use, which means that they have good corrosion resistance relative to other types of sheet aluminium.
Of course the downside to all of this panel goodness is that they are more expensive relative to the use of added steel triangulation and aluminium sheet cladding. But as is always the case the comparison between the use of panels and steel is not that simple as we need to consider the resources available, the acceptable level of complication/ease of construction and the desire to use of more advanced technology/techniques.
The first two aspects noted above are intertwined. When we make a space frame, we need to make it as accurately as possible. However, the more complicated the steel structure, the more difficult it can be to make straight, particularly for a novice such as myself. This is partly due to my level of inexperience and skill with steel space frame construction; space frame construction is definitely a skilled endeavour. It is also partly due to some of the details of the construction method. When we weld steel together, the steel expands or contracts due to the heat imparted by the welding process. (This is more of a problem with inexperienced welders.) Thus, unless we are skilled in what we are doing when constructing the chassis, the carefully laid out structure will distort and we will no longer have an accurate or straight and useful chassis. This distortion is no doubt even more likely when we attempt to make a steel structure in three dimensions without the appropriate safeguards and processes.
If a manufacturer of such space frame chassis is going to make a number of the same chassis then they usually make a number of very rigid ‘jigs’, which hold the individual tubes in place and thus limit the distortion to manageable levels. They also will have worked out the sequence of construction required to minimise the distortion in the chassis.
I do not have these options. I do not want to have to construct a substantial jig to hold the chassis I construct and I am sure that others would like to avoid it should they wish to build a Godiva of their own. In addition the chassis jig would in itself be a challenge to construct accurately and it would absorb considerable financial; space and time resources, so perhaps best if we can avoid it. With this in mind the simpler the chassis the better, within reason of course.
Approaching the chassis issues from the honeycomb side, we face similar issues regarding skills and resources used in construction. It is true that the composite monocoque chassis, such as planned in Godiva, was largely discontinued by most race car manufacturers in the late 1980s. However, it was revived by Lola for a series of their Le Mans prototype sports cars in 2000, albeit with an aluminium space frame in contrast with our planned steel one (Lola’s B2k/40b series of cars). Most manufacturers moved on from this hybrid construction for a good reason: it is not as ‘good’ (i.e. as stiff, light and compact) as a full monocoque (although the current Daytona Prototype rules mandate this hybrid construction in safety conscious USA).
However I also have to be realistic; I definitely do not have the skills and resources to attempt a full monocoque car; the engineering involved is well beyond what I can achieve at this point in time. The other aspect is that for Godiva’s intended purpose, using a full monocoque in either aluminium honeycomb panel or composites would be asking for trouble, Godiva needs to be rugged enough to survive years of road and competition use without ongoing maintenance in the chassis department. If Godiva is damaged in some way, it should be easy for the builder to repair. I feel that the hybrid chassis also has great advantages in this area. The planned bonded bodywork can be easily removed with minimal tooling, as it is anticipated that the bodywork would be adhered with polyurethane or structural acrylic adhesive. This use of adhesive would eliminate the need for the vast majority of structural fixing points on the chassis and also nearly eliminate the need for localised reinforcement in the composite bodywork where a structural fixing (bolt/screw) would be used.
The bodywork could also be used as additional structural components in some circumstances, such as the foam filled side panels.
It is worth noting that the honeycomb panels that we intend to select and use may have much thicker skins than would otherwise be used on a competition car. This will undoubtedly add weight, but it is felt that it will simplify the techniques and materials needed to attach the panels to the steel chassis (but more on this aspect in a future issue). Thus this may assist all future Godiva builders as well as increasing the overall durability.
The honeycomb panels themselves can be repaired in place if it is a simple intrusion dent in one skin over a small area, or they can be completely removed from the space frame if badly damaged and another panel inserted in its place. The steel space frame itself would be repaired in the usual manner, by straightening or replacing the damaged tubes.
As far as safety is concerned, the hybrid arrangement seems to have advantages. Honeycomb panel has a good reputation for coping with impacts and the steel space frame incorporates a roll cage and remains a good point to attach critical components to (i.e. the driver and harness for a start).
The downside: yes there is a downside; the secure attachment of the honeycomb panels to the space frame is absolutely critical. Any failure at the bond point will lead to a great reduction in chassis performance. Also due to the hybrid construction, the bonding of the honeycomb panels is more challenging from the point of bonding two dissimilar metals. However I have been assured that the effectiveness of structural adhesives has improved over the past ten years and that such adhesive construction is within my skill level.
The chassis design itself is relatively straightforward. The chassis is a three-box arrangement (my terminology), with the front sub-frame for mounting the front suspension, the middle cockpit and the rear sub-frame for the power train and rear suspension. All black lines represent 450Mpa square section steel tube, nominally 50mm X 50mm with a 1.6mm wall thickness, though the diagonals in the front and rear ‘boxes’ are envisaged to be 32mm X 32mm with the same wall thickness.
The front sub-frame is totally conventional, with the dampers running across the top of the sub- frame to a central mount, which is supported at the points of the tetrahedron cross-bracing.
The cockpit section is slightly more complicated as it has to fulfil a variety of functions. It has to provide sufficient interior room, ease of access to the interior via the door, sufficient structural integrity for impact, torsional and bending loads.
If you look at the side view of the chassis, you will see that the front subframe is supported by two tubes that continue rearwards – one at base level to the bottom of the vertical tube and one at mid-level, angled upwards to the base of where the windscreen looks like it would go. This section is bisected by the diagonal from the base of the vertical tube to the top of the front subframe. As the front subframe is not square to the vertical tube a panel could not be used here without another tube inserted. This tube is required to give two flat planes for the panel to brace. Looking at the plan view, you can see that the tube cross bar at the base of the windscreen (mid-level) to the front subframe is supported by honeycomb panel. This cross tube is also, naturally enough, the start of the dash, which is a full width aluminium honeycomb panel, partially ‘boxing’ the front of the cockpit section and possibly linking down to the longitudinal tunnel. Looking at the base and mid-levels you will see a slim rectangular longitudinal box running the full length of the car. This is the tunnel to house coolant lines and gear change mechanism, it is constructed out of honeycomb panel and locks into the front and rear bulkhead panels, as well as the dash panel (all not shown in this diagram).
You can see in the base level plan view, that the complete floor is honeycomb panel with some steel structure. There is a cross tube at which the diagonals terminate which is the forward point for the transverse box section, that you can see as a triangle in the side view. This is a triangular, full width box immediately behind the driver and co-driver’s seats. This is made from honeycomb panel and houses the fuel tank. It is placed there as it will put a major variable mass (i.e. the fuel load increases and decreases depending on the amount in the tank) as close to the Centre Of Gravity (COG) as possible. It also means we can use the transverse box structure to add as much strength or stiffness to the chassis as possible, as well as placing such structure as close to the driver as possible to reduce the potential intrusion in event of a side impact. It does mean that we also place the engine/transaxle further from the driver, which is good for weight distribution if we are considering a transverse engine/transaxle as per a FWD car as we effectively will getting a more even weight distribution.
There are unequal length wishbones front and rear with additional upper links at the rear. The top wishbones are pyramidal in design front and rear and are then used as a rocking arm to actuate the top mounted damper. This top mounted damper arrangement is not really very good for the centre of gravity, as it raises mass higher than it could otherwise be placed. However the plan is to use very good quality mono-tube coil-over dampers to assist overall suspension performance. Placing the dampers higher puts them in a less harsh environment, possibly making them less likely to be damaged if I should knock a corner off in an accident and most importantly making the dampers (and their adjustment knobs) very easy to get to at any event. I have definitely had enough of grubbing around in the wet, muddy wheel wells of my cars trying to adjust a damper! At this point, swaybars are conventional in type and application front and rear.
This is the concept sketch and design outline only and it is likely to change, but not to a great degree unless we get adverse feedback or a radical change in packaging. The reason for this is simple: we have not finalised the mechanical package and in particular the suspension package. It may seem an unusual statement, but Godiva’s final layout will be heavily influenced by the front upright choice!
The front upright chosen will provide the outboard suspension points which cannot be changed easily or without significant expense. The desired suspension design will thus be based on these point, which will then define the inboard (i.e. on the chassis) suspension mounts and hence the structure at the front of the car, as the chassis layout at the front will naturally be defined by these points. I know that an upright can be fabricated to suit the suspension design. However such fabrication would be expensive as each component would have to be properly engineered, constructed and undergo examination and testing by the certifying engineer. The other aspect is that everyone who would like to replicate Godiva would also have to go through the same process. So I will try to avoid this if at all possible by using a production component. To ease Godiva’s way through ADR compliance it will also assist to select an upright source from a current production car.
The suspension uprights will also define the rear chassis, but here we need to what comes with the total power train (i.e. engine, transaxle, drive-shafts and uprights) and modify the rear uprights to suit the suspension geometry required. This will be potentially simpler as the rear end will be adapting a strut arrangement from a production car and it is easy enough to modify a strut to become an upright.
So, you can see that yet again we face many compromises even at this very early stage in the process.
As always I welcome any informed comment on what you have read.