As I said originally I would pay attention to those readers who showed interest in building a ‘Godiva’ of their own. To my surprise the overwhelming request was 95% for a mid/rear engined car and I also admit that it is my current preference as well (note this may have biased some of the reasoning below, so feel free to send me your views if you disagree!).
Though no doubt we have not fully covered the details below in great depth, we can see from that there seem to be some advantages for the mid/rear engine position.
In the previous issue of Race we looked at a comparison of front/mid and rear/mid-engine placement – specifically the difference in heat management from the power train of the car. The conclusion of this was that the mid/rear engine position seemed to have significant advantages in this area with regards to heat management.
In discussing these points I am mindful of some readers’ views that this discussion should not take too long as it will take forever to get to build the car! So, the discussion below is a little abbreviated and the ‘full’ version may be made available later to those who are interested.
If our target weight of 800kg is going to be met, we need to remain very aware of the weight, not only of the engine/transmission, but also the associated structure and components that go with it.
So in this issue we will compare some aspects of the two layouts:
Recently a Sports-sedan driver reported that he had difficulty driving his car as the red-hot exhaust was passed too close to the pedal box and things were uncomfortably hot, even with race boots (until he found a solution, which was ceramic paper insulation). We need to be mindful that we do not have the budget or testing abilities of the major manufacturers, so we need simple, cheap, light and effective first solutions.
As in the previous issue of Race, we said that the exhaust will require careful forethought to ensure that it does not heat the passenger compartment excessively. In addition to which, the exhaust will most likely have to pass down the side of the vehicle (easier to manage the heat and also to keep a flat floor under the car for aerodynamic reasons) and thus ‘protective’ covers (to stop people getting burnt) will be required to meet ADRs. These can be incorporated as part of the body work, but they will potentially compromise the bodywork design somewhat. They should also be made from a durable material such as stainless steel, which is not easily formable. If the exhaust does run down the sides ala’ Cobra then the chassis construction in this area must also be able to withstand high temperatures without loss of acceptable performance, which may mean a more expensive construction method or a lower tech approach with greater weight (e.g. steel shear bracing welded to the spaceframe). The other option is to adequately insulate the area that is affected by the exhaust heat, adding more weight, particularly where the catalytic converter is located).
The other aspect of this Front/mid engine position is that the exhaust will be longer (from the engine to the rear of the car as per ADRs) and most probably heavier.
This position has one similar issue with heat management, in that the exhaust may pass close to structural components such as the bulkhead at the rear of the passenger compartment. However this is most likely to occur over a much smaller area of the car’s structure that will closest to the engine’s exhaust. From this bulkhead back, the majority of the structure would most likely be a spaceframe and thus less likely to be affected by the exhaust’s heat.
The exhaust would require shielding from any components that might be heat affected e.g. rear dampers, wiring, fuel/oil lines etc. It is also likely that the exhaust in the mid/rear layout will be much shorter and need less supporting structure, thus end up lighter than the front/mid layout.
The rear muffler could also have a secondary use, as per other cars, as a crush zone. However this is not considered a major benefit as this crush zone can be accomplished a number of ways and it is best to keep the centre of gravity as low as possible.
It has often been said that the east/west engine layout consumes less torque than the longitudinal layout. I do not feel that we need to consider this aspect too closely as we may need to make our engine/transmission choice on other grounds. What we do need to do is compare component and structure weight of the two layouts:
Front/mid vs. Mid/rear
With both layouts there are similarities in that we have and engine, transmission and differential. The mid-rear may have a slight advantage in weight just looking at the components as the differential is integral with the transmission and thus no separate housing (often made of heavy cast iron in the front engined cars) is required (though there is some additional structure to the FWD transaxle casing). The greater advantage of the mid/rear engine and transaxle is that it has no driveshaft, which however you look at it, is a big heavy rotating mass that would be nice to do away with if possible.
The other aspect of this comparison is that the various torque loadings that need to be dealt with by the chassis. I believe that they are more complicated to deal with in the front/mid engine layout. For example we need to mount the engine and transmission plus the differential, all of which has a central layout requiring more structure to house and support it (e.g. a wide spaceframe tunnel) and thus more mass. Some of this structure can of course be well used as primary chassis structure, e.g. the ‘backbone’ chassis in most Lotus’ and Bolwell’s etc. However as we want to ensure good safety in event of a side impact (and to meet CAMS/FIA roll cage requirements) we still need considerable structure at the side of the car This combined with the tunnel may drive overall weight up.
The other aspect of having a tunnel is that it will force the placement of the driver (and thus one of our variable masses) away from the centreline of the car and closer to the side. This is not a good thing for the balance of the car and also for the driver’s/passenger’s desire to be as far away from any potential side impact! The other aspect of this is that the chassis of the car may need to be wider at this point and thus again more material may be required for the chassis. The differential will require adequate bracing to ensure it is well located and can react the torque forces into the chassis. However as the rear suspension loads also need to be reacted into the chassis in the same general area, the additional structure to support the differential may not be enormous, but it all adds up.
The Mid/rear engine design has fewer issues regarding chassis structure. The engine/transaxle being from a FWD car is most likely to be a very compact unit that has a lot of attention paid to the packaging and overall weight (too much weight is disastrous in the front of a FWD car).
The engine/transaxle does not require a wide tunnel and thus the occupants of the vehicle can be placed closer to the midline, aiding overall balance and permitting a narrower structure (less materials and thus less mass). In addition to which, any tunnel that is used to house coolant hoses/gear change etc, does not require the same level of strength as per the front/mid engine placement, as it may not be an important structural element of the chassis.
Mid/rear will however require more structure at the rear of the car to react the torque of the engine/transaxle into the chassis, which may not be good for front/rear weight distribution. This additional structure/weight may not be a great deal more than would be required for the front/mid layout, as both need to react the suspension forces into the chassis at a similar point.
From this it might be seen that the mid/rear has some significant advantages.
Torque Reactions (gyroscopic effects)
We are building a light car. In fact we are going to build as light a car as possible! Into this light car we want to put quite a large amount of torque/power. This torque needs to be ‘absorbed’ by the structure of the car and ultimately transmitted to the tyres via the suspension.
This torque has two components. The first is the ‘driving torque’ that turns the wheels and the second is the ‘reactive torque’ (note this is my terminology!) that is fed into the chassis of the car. In such a light car we need to be mindful of the effects of the ‘reactive’ torque. A driver of a 200kW V8 Westfield once remarked that you did not want to put the foot down hard in some corners as the torque from the engine would compress the suspension down even further and lift one of the front tyres, reducing grip. He said it was exciting, but not the fast way to corner!
Essentially the reactive torque of front/mid layout will attempt to twist the car, which may in some circumstances upset the lateral balance of the car.
The mid/rear layout also has to deal with ‘reactive’ torque, but this torque is reacted longitudinally. So it may be reasonable to expect that the suspension loads laterally will be less affected. However traction and braking may be affected. This is of course a simplification of the problem, but we may deal with it in more detail later.
Overall Packaging and Access
I hate having to reach some of the impossible nuts in the engine bay of my current car. For example, the engine mounts need to be done is a specific sequence!
Since we have the ability to do a car from scratch accessibility is very high on the list of priorities. This is desirable not only for general servicing, but also so that if a problem does arise at the track or on a stage we can access the area without a struggle. A quick, successful repair means that we can continue to enjoy our motorsport on the day and not waste money in lost entry fees etc.
The front/mid engine design will require a large bonnet opening which may not be that aesthetically pleasing, but can be incorporated into the panel design. It could also in effect be a large ‘clamshell’ (ala ‘E’ type Jaguar) whereby the whole front of the car tilts up (usually forward as it’s less likely to fly open if the catches fail!) as one panel.
This would make a complex and most probably heavier front body shell and lots of attention to the latches. As we especially want it to be durable we have to acknowledge the downside that any damage at a track or on a stage would likely mean the end of the day and/or a tricky repair.
A disadvantage the front/mid layout has is the packaging of the transmission/driveline. Any repair to these components is always a challenge and in my experience a lengthy process. Naturally enough the ease of service of these components can be built into the design, however not without compromises (e.g. the need for access plates/mounts or open tunnel and loss of aero efficiency).
Rear/mid has the same issues of access, however the transmission and engine are all in the one spot and no tunnel access would be required. If you ever have a look at the new BMW Mini you will see the excellent access possible for the engine/transaxle used. In the mid-rear position the access would not be as good; however it might be acceptable and certainly in the images of the Lancia 037 rally car the accessibility at the rear of the car was astoundingly good.
So the reasoning, desire and reader feedback is nearly all going in the same direction, thus Godiva will get a mid/rear engine and transaxle.
Now we need to decide on the engine/transaxles to be used. Note: for ADR compliance this will need to be from an Australian delivered car produced from 2003/2004 onwards.