In the previous issue we set out some of the background on the project. The stated goals were:
- Weight approx 800-850kg
- Power approx 200kW
- 4 wheels
- Must meet expected safety standards such as FAI/CAMS approved roll cage.
- Must be able to accept a tall driver (if I can fit almost anyone else will be able to fit easily)
- As advanced a suspension system as can be reliably built by an amateur.
- Cheapish to build using production car bits
- As fast as the expensive cars!
- Able to be road registered.
With these general goals, we now need to start discussing some of the design priorities. Where do we start? Basically we have to decide what ‘Godiva’ is to be designed for and what the priorities are. What I see is that Godiva should be able to take a driver and one passenger fast as possible and as safe as required around any racetrack in Australia or on a ‘Targa’ type road rally event. That’s about it; everything else is negotiable.
We may as well start with the passenger compartment. Basically I have to fit - simply I am paying for this exercise and the car should be capable of fitting a 203cm, 120kg, (balding) driver…and a passenger. If I can fit then, the remaining 98 percent of the population should have no problem getting in there! The passenger compartment must be as safe as possible, with ample roll over protection that meets CAMS approval in its design and construction. It is very unlikely that the car that I build will ever be raced by me(I can’t afford the time when running this magazine), but if others are to be built, then it makes sense to start with safety in mind. In addition to this if there should be any dispute with an engineer or registration authority then CAMS approval may sway an argument a little in our favour.
Meeting CAMS approval is one thing; making the passenger compartment as safe as possible is another.
Given the intention to road register Godiva and also mix with very speedy traffic on the track (I don’t know which is more of a risk!), side-impact/intrusion is an issue. The car will have a primary crash/roll cage structure with an energy-absorbing secondary structure lining the sides of the passenger compartment. These energy-absorbing structures should be demountable and easily replaced. They should also serve the secondary purpose of protecting the chassis from damage in a minor coming together with another solid object. This will add weight, but is not really a negotiable element of the car so far (this is compromise number one). Other elements of the ‘cabin’ area will have to comply with the relevant Australian Design Rules (ADR’s) and with advice from the engineer employed to support and approve the project. Obviously if the car is to be shared with other drivers, the seat, pedals and steering column will need to be adjustable in some sense and this will need to be reflected in the choice of items and in the basic structure/construction of the chassis.
So, we will have two people in the car on occasion, though it is unlikely that both will be my size! Then, how do we arrange them? McLaren broke the mould in road going sportscars with the F1, by seating the driver in the middle of the car. This has a number of performance advantages:
It sites the diver’s mass in the middle of the car and may make weight distribution easier to achieve and predict for a driver only situation.
Any change in driver mass may have (in theory) less affect on the handling of the car
Lateral Polar Mass Inertia is reduced as drivers mass is located centrally with wheelbase
It allows different structures to be explored for the chassis as the occupants are in a different orientation
The driver is sited centrally and has an equal perception of each side of the car and perhaps a better perception of the relationship of the rear of the car to the front.
There MAY be some weight advantages if less material mass is required for steering components etc.
The frontal cabin cross section may be minimised and optimised in plan view
One design car be used in any country
It allows central placement of the fuel tank, or a more forward placement of the drive-train to reduce Polar Mass Inertia, or reduce wheelbase.
It places the driver in the safest possible position in the car
Possibly a more complex structure with more mass when made from ‘traditional’ mild steel etc
Access to the driver’s seat entails a complex door arrangement that necessitates cut outs in the roof. This may interfere with the structure of a spaceframe and thus take away some of the perceived advantages of the ‘novel’ structure possible.
Not exactly easy to get in and out of by report in other magazines when the McLaren F1 was tested
Some of the advantages of siting the driver’s mass centrally is reduced if a driver and co-driver are in the car as the co-driver is offset
Yamaha followed the F1 with their experimental show car, the OX99-11 (see photo). It was even more radical in that it had a fighter style cockpit ala’ a F18, with the passenger behind the driver, instead of ‘side-saddle’ like the F1. This is ideal in a number of ways:
Both human masses are central in the same way a motorbike and passenger are.
In addition to this the frontal cabin cross section can be minimised and this would no doubt help aerodynamics, particularly the flow to the rear wing.
The passenger and driver would be central which is good for side impact safety for both and would also leave ample room for the cars structure to go either side of the cabin.
However there are disadvantages:
The passengers will not see much (“so what?”, some of you will say!…but it’s hard to impress a partner/girl-boyfriend if they are just stuck in the backseat looking at the back of your head!).
The passenger compartment will be long enough that it may compromise the desired wheelbase (particularly with taller drivers) and power train packaging.
It may be difficult to make the passenger compartment compliant to CAMS regulations (including belts) as the cage would be a very unusual shape.
It may pose even a greater challenge to any registration organisation as it has not been done by anyone on a production car before.
Frontal impact worthiness may be difficult to predict with the passenger close behind the driver.
At the moment, we might be more conservative than Yamaha and McLaren and opt for the side/side seating for now. This means that we have sited one of the most variable masses within the car, as drivers can vary in weight from a paltry 60kg to my more generous 120kg and occasionally more. A variance of this magnitude would be difficult to imagine with any other item excluding fuel.
From this point we go on to deciding where to put the engine. Current wisdom is reasonably fixed and that is that a mid-engine is best at reducing the Polar Mass Index (PMI – see sidebar) and thus thought to be best for handling. But what is a ‘mid-engined’ car? Where does the engine have to actually be in the car to be ‘mid-engine?’
From a couple of weeks asking questions of various car-design professionals, the general consensus was that ‘mid engine’ means that the entire engine is within the wheelbase of the car, this makes a mid-engine. Simple!
The engine can be either in front of, or behind the cabin (no one has yet figured how to put one under the cabin in a racecar). As such the latest TVR’s and the MR2’s are both ‘mid engine’, with the TVR being front-mid and the MR2 being rear-mid (as noted above there is no mid-mid in a production car, though some race cars do so!)
Both front/rear-mid positions eat into the available cabin space for the driver and passenger. So yet again there are advantages and disadvantages.
I will only cover one of these disadvantages this issue, and that is heat. 1kW is roughly equivalent to the heating element on my kitchen stove turned up to red hot to boil a pot of water or cook dinner. We are planning 200kW of power and this will, as you can imagine, create quite a bit of heat! If we go for a front-mid engine position we will put all of the heat (from engine/transmission and exhaust) in front of and possibly partially under the cabin via the transmission tunnel etc. We can reduce and control some of this heat by ducting airflow to the hot areas and accepting the compromises that this will bring (less overall aero efficiency etc) and also by judicious use of insulation materials (which adds weight). However both of these control measures also add complexity. In addition it may also mean the use of more heat resistant materials (as well as the other measures) instead of those that might be ‘ideal’ in a weight/strength view. For example, we cannot use structural composites and some adhesives if the local ambient temperature is above their recommended operating temperature.
If we look at the mid-rear engine, we can see immediately that we are not faced with the same issues as the front-mid. The heat is behind the cabin and thus can be ‘flushed’ away from the cabin by the prevailing and ducted airflow. The exhaust and the transmission heat do not have to go past or under the cabin either. If we did need to use insulation materials, say on the rear firewall, then we may have to use far less, thus saving weight. However not all is beer and skittles. If we want to place the radiator up in the front of the car to take advantage of the reliable airflow and balance some of the engine weight, then we will lose some of the weight advantage as we will have long coolant pipes that will go past/through the cabin. This will obviously potentially add some heat to the cabin, though far less that the front-mid as the coolant is unlikely (we hope) to get above 120C.
But these heat issues may not be the only deciding factor in the front vs. rear mid engine battle.
In the next issue we will mull over exhausts, transmission, gear linkages etc etc.