Monocoque Talk

[Guest]

Both Roaring Forties and CAV appear to be heading down this road to offer us even more choices--and, more choices in any market are great for potential buyers. But, apeaking only for myself, I'd be most interested in the views of those of you who know a lot more than I with regard to chassis design and dynamics (in my book, that's all of you)--how much of a difference a monocoque chassis really makes. Both the CAV and Roaring Forties space frames look to be very good to me, and the reports from those that have driven either car speak very highly about handling. Will a monocoque car be significantly better? Or is it more of an authenticity issue? Other advantages? Thanks in advance for any thoughts on this stuff. I can't do it right now, but a GT40 is definitely in my future.

Best regards,

Tom T.

 

[Bill D]

Hi Tom

A full monocoque chassis is lighter than the semi-monocoque design. The advantage of the latter design is low cost, at a slight expense in weight.

The full monocoque chassis is much lighter, but much more expensive to make (tooling jigs, etc.)

From a purist's point of view, a monocoque chassis is preferred. If you need to save a few $$$ (which we all do), a semi-monocoque works just as well.

Hope this helps
Bill D
Los Angeles, CA

 

[Guest]

FYI

John Hester advises that he will also be offering a monocoque next year.
For those who are unaware, John purchased
the Mark I tooling from Sabre and will soon have his website up as Blue Oval Performance.

Amazing! Years of ERA having the market alone and multiple competitors now appearing
in a short period of time. Life is good.

MikeD

 

[Robert Logan]

Monocoque chasis are there for the pureist and for this reason I am building a FULL monocoque for myself. This will be the basis of our car if there is a demand !!!!!
I do not agree with the weight issue because it is possible to get a space frame down to under 900 kg ( 2000 lbs) a number rarely seen in originals. Even taking into consideration all the domestic pleasures of A/C , carpet, etc and weighing ( no pun intended ) them against dry sumps and the like all of our GT40 replica's are light. The major improvements are with torsional rigidity and beam strength. A space frame is far superior. The only improvements that can be made are with exotic materials and to follow the progress of F1 racing over the last decades shows what changes are best or not for PURE performance. I love our space frame car and how it handles, but as a GT40 nut I want a full detailed COPY and therefore a monocoque was my only option togrther with ZF gearboxes, Gurney Westlake heade, 48 webbers, " a burger with the LOT" as they say in Aus.
Best wishes, Robert

 

[Guest]

Robert

Of course you are quite right.
Story I heard was that Eric Broadley wanted the original GT40 to be an aluminum monocoque
but Ford would not allow it.

I don't know why Ford insisted on steel, but I understand that this was one of the major issues that lead to a parting between Ford and Broadley who went on to develop his
T-70 aluminum monocoque that was VERY light
and succesful as a race car.

Then of course Ford developed the Mark IV
aluminum tub car anyway. I don't think we'll
see a replica of these anytime soon! Do you?

MikeD

 

I also read that a monocoque chassis sits the engine a few inches lower than most space frames do, which would lower the centre of gravity I guess, and flatten cornering.
Is this true about the engine being mounted lower in a monocoque?

 

[Ron Earp]

I don't see why the frame type would affect engine location. Both designs are completely open underneth the engine, I could lower the engine in the RF until it is only 1" off the ground if I wished. As it is the car has 4-5" of ground clearance and I feel this is necessary for road use.

Ron

 

[Brian]

Ron, I agree, with some every simple frame modifications I can lower the engine in my GTD until the sump is lower than the floor pan. I think the larger differences between the the space frame and the monocoque have to do with anti-dive,squat and turn-in characteristics of the front suspension more han anything else. If one were to construct a monocoque which utilized the same suspension geometry as my space frame, I doubt that the average person could tell the difference when driving.

Brian

 

Ok thanks guys, I guess things have changed a lot since that book was written.
I still have yet to buy my GT40, and I am still in the research part, which I am sure you all know, is a big part.

 

[Guest]

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 ]

 

I am about to scrach build a GT40 chassis and am looking for the specifications. I was considering round tubing and or chrome molly does anyone have the specs that they will share with me?

I also have seen the buck that Ford is using for their part testing on the new GT40. Has anyone used this approach for chassis construction. I would be very interested in any feedback.

I can't wait to get started.

 

[Jim Rosenthal]

I suspect at the speeds that I will be competent to drive, the difference in handling between a monocoque chassis and a tubular-frame chassis are not very significant. The cost difference is considerable, as I am finding out. What the monocoque has going for it, in addition to better structural rigidity, is authenticity; if the original cars had been built up from tube sections like a 300SL or Birdcage none of this would be an issue. I decided to have a monocoque built because I really wanted to get as close as I could to the car I would have bought had I been able to in 1965 or 1966.
That said, I would much rather be riding around in a well-made tube frame car than in a monocoque that was indifferently welded together. And, as well as expense, the monocoque is worse in one area- collision repair of a monocoque is much more difficult.
Just got photos of the tub for my car, which is really looking like a GT40 now!!!

 

Ive read all the posts above, and im still not quite sure. My main concern is not performance of the 2 types of frames, but safety. In a crash which would offer the most protection.

 

[wpeter]

I doubt whether anyone can say with certainty which is the best in a crash. Monocoque performs poorly in a pinpoint or piercing type of impact, wereas impacting a tube in the same manner will outperform a flat panel. But , most auto crashes are into a larger surface which would allow monocoque to act as a large box taking impact and transfering it to the whole panel. Nascar or Champ car which is safer? What it really comes down to is whether or not you are willing drive a car with an overall height of 40 inches capable of traveling under almost any other car on the road. It is not the impact you have to worry about. The roof structure is there for rain and nothing else.The rigidity in the chassis is from the bottom 1/2 of the car, and, by the way, the sponsons are only there to hold gasoline. If you look at the design they really have little structural integrity except to support the inner panel.They will absorb impact but remember that an SUV bumper is about eyeball height.
This is a copy of a race car capable of killing the best race drivers. You just have to remember these cars were meant to be driven with other cars of their type and be very careful if you drive on city streets.
By the way, I am building another, MK4. If anyone is interested in an exact honeycomb copy of the real thing, let me know. It won't be cheap.

Bill

 

[Jim Glickenhaus]

I agree with JWHAT? None of these are going to be very safe to be in on the street if you're hit by a SUV. If you drive anything like this on the street you have to be very carefull.
Best
Jim

 

[Robert Logan]

Jim,

Great to finally meet with you and thanks for the dinner. Glad that things are progressing well !

Best wishes,

Robert

 

Hi Kalun D,

Interesting that you mentioned Integrity, KVA, Sabre and I wonder if you can help.

About 10 years ago an American by name of Max Tooker was involved with two South Africans building Cobra replica for export to the US.

I met the guys shortly after we had completed the build of our KVA type GT40 which laid the foundation about seven years later for the GTD/CAV deal. I had taken about 90 photographs of the various build stages of our car and when I told Max about this he asked if he could he could borrow the negs from which he would make prints and return them ‘within a day or so’.

The next thing I heard was that Max had returned to the US and was somehow involved with Jules Hoffmann (Sp?) of Integrity Coachwerks (Sp?) in Florida.
I tracked Jules down and he admitted that he had the negs. A specific issue made it clear that they were my negs. My partner on the project, Norman Lewis from the UK had a huge, beautiful Great Dane name Bryn. For one shot I parked my Ford Sierra next to the GT40 and asked Bryn (politely!) to sit between the two cars. Bryn obediently obliged and the pic was great with Bryn being taller than the GT40.

The first thing Jules said when I phoned him was, ‘Yeah, the pics with the dog’.

After all this time I would still like my negs back and if anyone can help I would be most grateful.

No hard feelings!
Andre 40