Secondary Structure

In Issue five, we had a brief look at the proposed hybrid chassis that we intend to use as a basis for Godiva. The chassis proposed uses a simple steel SHS spaceframe stiffened with aluminium honeycomb panels to achieve the desired performance. The chassis described so far is not the complete structure of the chassis, as naturally enough, the body of Godiva needs to be attached. But we need to be mindful of our goals and perhaps equal first is the weight target for Godiva.

Weight is an important element to consider as we have a fairly challenging weight target. So, any chance we have to reduce the number and weight of components used, the more likely we are to achieve our overall weight target.

However a reduction in component weight does not always mean that we reduce overall complexity, as a smaller total number of components may mean an increased complexity in the individual components themselves – both in the manufacturing skills and techniques required to produce the component and in the use of the component to build the complete car. In addition to which, we should also remain aware of how complex these components will be to repair, as there is little point in saving 0.5kg if you have to remove 25% of the vehicle to repair the damaged component; after all less time required for repairs means more chance of completing an event and less time off the road between events should damage occur.

For a large scale manufacturing operation such as one of the car companies, the ease of assembly is critical for manufacturing speed and thus profits, especially when thousands of vehicles need to be produced each year. For us, the time/profit equation and priorities may be different. We have more time to do things how we want and to get them ‘right’ for us and our needs. We can also use techniques and methods that would simply be uneconomic for any manufacturer. However we also need to acknowledge that we do not have access to the machinery that OEMs have and we also do not have the skill pool to draw upon to get some things done. With this in mind we need to be sure that the component design is within our abilities and that the individual components do not cost more than required in terms of time and money.

So, design Rule 1: Always try to use one item for two or more purposes; or put another way, there may be the primary function and then secondary functions or benefits from a particular design or use of materials.

A potted history of body work attachment

The structural layout chosen for the chassis of any car is of course not the final product and does not represent the ‘look’ of the car. Simply put, a body has to be attached, though it may be somewhat limited in covering in the case of a clubman style car. This bodywork includes all fixed and opening panels, wings, diffuser etc. Such bodywork establishes the aerodynamic performance of the vehicle, as well as affording the driver and component whatever protection is available. In the distant past, such bodywork was constructed from carefully beaten and shaped aluminium which was then usually attached to a tubular structure which supported it and attached it to the structural chassis. This method was the basis for the great sportscars such as the original Cobra and Ferrari’s etc. As you can imagine, it is not a simple procedure to make the aluminium panels and supporting framework, nor is it simple to keep it looking good. The aluminium panels on such cars often had a reputation for fatiguing and cracking if they had a hard life. Given the tarmac event plans for Godiva, this would not be a wise choice even if I had the skill and finances to follow this option.

The other method of panel construction involves composites. Initially these panels were supported by a simplified steel structure as with the aluminium panels, but methods of composite body panel construction changed as composite knowledge increased and soon panels were formed so that they were self-supporting. These type of composite panels are what we are familiar with and they usually attach to the chassis/structure via mechanical fixtures (screws/nut and bolt) to where the steel panels usually attached. Or in the case of ‘specials’ and race cars, the bodywork was attached to ‘tabs/brackets’ welded onto the chassis tubes or even directly through the chassis tubes themselves. This use of composites was an improvement in some ways, that is, no need for additional tubular structure. However, it did have its own complications. The body panels needed to be suitably strong in the local areas where they were attached to the chassis, which meant that the composite needed to be significantly increased in thickness at this point. This also meant that considerable attention had to be paid to the overall panel design and construction; otherwise the composite panel would eventually distort and become ‘wavy’ as the varying amounts of resin in the panel (particularly those areas of added thickness at attachment points) continued to contract over time. Also this localised thickening of the panels did not mean that the panel was impervious to stress, as over tightening of the mechanical fixtures combined with the vibration of the vehicle and heating/cooling by the sun and mechanical components all added to the degradation of the panels. This often resulted in the dreaded spider-web style cracking of the gel coat. This could be repaired, but such repairs are often best left to the skilled, the brave or those with large amounts of time on their hands…or someone who is all three! Efforts to reduce the cracking lead to some constructors using rubber vibration absorbing mounts to partially alleviate the road/running stress. Vehicle heat could be controlled to some extent and different resin systems that could tolerate higher levels of heat and vibration were also used; for example, ‘Vynalester’ and epoxy resin. Perhaps the greatest benefit was the improved knowledge and experience regarding the construction of composite panels, such that those people who are dedicated to producing composite panels (such as Alfa FG) produce stable and rugged products.

However these advances are not enough to convince me to go this way with regards to Godiva’s bodywork. You see, producing a composite body attached loosely to the chassis means that the bodywork is only that: panels hung on the outside of the body to provide shape. With all of the additional material and mass it seemed a backward step if I was not to try to get the most benefit from the material used, either in a structural sense or benefit for some other purpose.

Godiva’s secondary structures

As alluded to in the previous issue, we plan to attach Godiva’s body using modern structural adhesives. This use of adhesives will greatly reduce and possibly eliminate the use of structural mechanical fixtures for a lot of the bodywork. It will also reduce the complexity of the chassis, as no precisely arranged mounting points will be required for mounting the majority of the bodywork. It will also allow us to manufacture panels that are thinner, lighter and more consistent in their thickness, though any weight advantage may be reduced by the fact that the panels will be larger in area and mass due to the need to adhere them to the chassis…

However, such a method will allow us to use some of the attached panels and sections for different purposes and in doing so we will in effect create secondary structures that will conform to design rule No. 1.

Sill Box

The first secondary structure will be the sill box, which will be the large box glued to each side of the chassis. It will extend 10mm below the chassis to the bottom of the doors and it will be approximately 100mm wide and a minimum of 300mm high. It will be constructed from tri-weave fibreglass cloth and vynalester resin and it will be foam filled. As it will be glued full length and at both ends it is thought that it will create a structural component.

The sill box will be foam-filled for two reasons: the first is that it will create a substantial crush box to reduce potential damage to the chassis if say you have an ‘off’ on a tarmac event and hit something like a 4”x4” white post. Naturally it will also assist in protecting the occupants of the vehicle.

The second reason for the foam filling is that it will stiffen the composite structure’s skin and thus assist in achieving a predictable structural performance. Naturally the downside to the foam filling is the added weight, but given the perceived benefits it seems worth trying. Of course we will test our theory.

Undertray

The floor of Godiva will comprise honeycomb panels, which will have relatively thin skins of 1.5mm. These will be easily damaged if I or any other owner/builder should encounter rocks, branches, garden gnomes and wayward feral cats on any given tarmac event! Damage to any honeycomb panel is to be avoided if possible so we need to devise some sort of protection. To do this I suggest a full length undertray. This will again be a composite structure which is bonded with structural urethane adhesives. This undertray is shown in image 1. You can see that the raised ‘buffers’ have a specific construction in that there is foam fill for impact resistance, a wide mounting section to spread to load of any localised impact and also the surface closest to the road surface uses kevlar tape in the composite for abrasion resistance. The secondary benefit from using an undertray is that we can shape it to assist any aerodynamic benefits generated by the diffuser at the rear. Of course there are downsides such as added complexity, added weight and quite a large component to manufacture and attach, but again it seems at this early stage to have a useful benefit/cost ratio.

Front box

The front box is the secondary structure forward of the front suspension. Its primary role is to support the radiator and also to fully duct the air from the intake at the front of the car to the radiator (there will be removable ducting post radiator), which lies virtually horizontal in the car. Although the shape is as yet undecided, it will invariably be a box section, albeit of unusual shape. As with the sill boxes, the front box will be foam-filled, which combined with the radiator itself, will make the front crush structure – its primary secondary function. However the other advantages are also worth noting. Firstly, the structure will be made to be easy to replace as a unit and it will be able to support a significant amount of the front bodywork. Secondly, the front box structure will be made stiff enough to support the loads from the front splitter/diffuser, which will be mounted below the front box and which can be made adjustable to assist aerodynamic tuning. Thirdly, the internal aerodynamics of the ducting can be varied to suit the radiator or even the body style used whilst maintaining the other secondary benefits. It is envisaged that this front box will be bonded to the front chassis tubes with urethane adhesives, thus making it relatively quick to remove and also quick to replace. Some mechanical fixtures may be required.

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Windscreen base

This is the box section that, as the name suggests, runs across the car at the base of the windscreen. This naturally enough supports the base of the bonded in windscreen and also the ‘A’ pillars which run down from the roof panel. In addition to this, the boxes’ secondary function will be the mounting point and housing of the wiper mechanism and it will possibly house the windscreen washer bottle. If possible we will use the wiper mechanism from the car that will donate its windscreen and the box will be designed to house that. This means that the wiper mechanism will have been well tested with the screen and thus should work well enough for our purposes.

Rear diffuser

The primary function of this panel is aerodynamic as the name suggests. The intention is to extend the diffuser back to the rear bumper and forward to connect well to the full body undertray. However its secondary function is to seal the underneath of the engine bay and thus allow us the opportunity to manage the internal airflow within the engine bay as best as possible. I have learned some of the vagaries of airflow management reasonably recently when a carefully planned vent on the top of a bonnet actually fed air into the engine bay at speed instead of drawing it out! This finding was confirmed with wool tufts and observation at different speeds on the highway. The conclusion was that there was a lower pressure in the engine bay than on top of the bonnet, as a result of the open bottom of the engine bay. As we will want to ‘flush’ hot air out of the engine bay of Godiva as much as possible by ducting cool air in, we may as well try to use the diffuser to seal the engine bay floor.

Summary

The less remarked on side benefit of using secondary structures as described is the additional flexibility they may allow. In building Godiva, we will not be constrained by carefully formed metal structures to hold body panels. Instead new structures will be able to be attached in relatively short timeframes. In addition to which, I would argue that the methods of construction may also enable us to make lighter and more rugged bodywork, resulting in a lighter car and naturally better performance and longer service life.

The likely benefits of crush structures should also be seriously considered, as in making an Individually Constructed Vehicle such as Godiva we essentially eschew the crash performance of well-developed car such as a Porsche. Mind you, such crash testing was never intended to replicate that of a motorsport event, particularly with a roll cage etc. installed! In any case, you have now seen the initial planned structure of Godiva in full. From here we will start to make planes for construction and the first step is to make a wooden mock-up of the chassis centre to ensure that I can get in and out(!) and that I have a better idea as to which components will fit and how they should be positioned.

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