To All,
Monocoque as applies to the GT40 from a torsional rigidity standpoint is superior to the tubular spaceframe. It is possible to build a rigid space frame but not with the limitations that the GT40 body design entails. Among other things the door opening, you can't run tubing from the top of the A pillar directly to the B pillar. It's a similar drawback as a convertible.
In racing torsional rigidity is paramount and the original GT40 monocoque is very rigid. Looking at photos of the GT40 tube frames offered by the present builders it appears some may not be significantly torsionally rigid.
I'm building a GT40 from scratch (except for the body) using a tube frame chassis. To design the frame I started by looking at an Integrity frame (originally KVA, now Sabre?) I measured the frame and built a 1/6th scale model and performed torsional rigidity testing on it. It's not much better than a ladder frame out of your typical AC Cobra.
I then added to the model to achieve more rigidity. It's very hard to get a lot more rigidity with tubing only due to the body design. You can but you end up with diagonals through the pass compartment or something that is very heavy.
The design ended up with sheet metal boxes similar to the original monocoque in the rocker panel area. The large boxes that run the length of the rocker panel from wheel to wheel in the original monocoque are key to the rigidity. I also added a larger central box tunnel to the design (larger than original monocoque). The 3 boxes, rockers and tunnel, more than tripled the torsional rigidity of the model.
At this point the design is completed and the chassis is almost to the point where torsional rigidity testing will take place. The model was tested at about 12 various stages of design and the actual chassis will also be tested as pieces are added on. The tubing is 1-1/2 square by .095 wall steel and the 3 torsion boxes are 2024 T3 .056 Alum. Alum panels are also going to be incorporated similar to a stressed skin chassis.
If anyone is interested full scale torsional rigidity testing is not that hard to do. The most difficult/costly part is the anchor you need to hold the chassis at one end. Hold downs are available as body shop equipment, and you have to drill your concrete to accept them. A very good description of the process is in the book "Chassis Engineering" by Herb Adams, ISBN 1-55788-055-7. 10,000 lbs-ft/degree is a good starting point. Indy and Formula cars are at 30,000. Another good book is "Race Car Chassis" by Forbes Aird, ISBN 0-7603-0283-9.
All considerations of torsional rigidity as applies to GT40 replicas can be a moot point depending on the desired application. If all you want is something affordable that looks cool and may be used for some historic type racing where participation and not competitiveness is the goal, then it really doesn't matter. Most people who spend this much time and effort are not going to want to risk "on the edge" type race driving anyway. And you have to consider cost and availability.
One might ask why I'm interested in torsional rigidity if I'm not going to race competitively. It's sort of a "just to see if I can do it" type of thing. It started with an earlier Cobra project. It was a similar project in which I scratch built the chassis. After the chassis was complete I performed torsional testing and it failed miserably. I didn't employ the model testing with the Cobra design so hopefully that will make the difference this time. I didn't do a full monocoque because I knew I probably couldn't improve on the original design and didn't want to make just a copy, and all my experience has been with tubing anyway. Another thing I will admit to is that my design might not be cost effective from a production point of view in that it is labor intensive similar to the monocoque design.
I'll post some pics on the web site.
thanks, regards, Kalun D, Seattle
[ November 18, 2001: Message edited by: Kalun D ]