Monocoque on ebay

Thanks Mike.
I am an SGT space frame owner / builder. Just wondered from an investment point of view if it would be a better choice of chassis.
I have collected a fair selection of original / reproduced parts - engine, ZF box, pedal box, calipers etc. Rack and suspension parts not too much of a problem for me either. So building up into a runner wouldn't be too much of an issue. I probably wouldnt be looking for out and out 100% originality.
thanks
pab
 

Mike Pass

Supporter
Whether the chassis would be worth more than a space frame car like an SGT is a good question. If someone values originality most then they may be prepared to pay more for it. I guess it is down to the value that someone places on originality compared to ease of everyday use. As it is made from mild steel it would need good underneath and general protection from grit and salt etc. These original mono chassis are much more rigid than the usual panelled steel frame cars which lack triangulation and are road cars not race cars.
My personal view is that it would be a nice car to build up into a quick good handling road/track car with a strong element of original componentry. The correct mono chassis would give the car that extra element that most replicas don't have even though they have lots of original spec parts bolted on.
It's main value lies in it's being compliant with race spec. However the cost of building up this chassis to FIA/HTP needs to be compared to a Gelscoe car or converting a Superformance to the correct spec to get HTP papers. So from a cost point of view an original spec road car may be the way to go.
Cheers
Mike
 
These original mono chassis are much more rigid than the usual panelled steel frame cars which lack triangulation and are road cars not race cars.
I can't say I disagree with that statement in any way at all but I am curious as to opinions on what triangulation you would add to the spaceframe to bring it closer to the stiffness of one of these mono chassis.

Not being a chassis engineer I only have unqualified personal ideas on what additions / changes to make. It would be useful to have a more qualified opinion with some relatively simple reasons behind the methodology.
 
Easy to speculate on chassis stiffness. Only real way is to torsion test both back to back in a similar manner. If I take the plunge on a original style mono, I may well test both.
pab
 
Chassis torsional stiffness is something that has always interested me, and has been the subject of several discussions in previous threads in the chassis forum - and agree, the only real way is a torsion test, ideally to verify results from structural analysis or as suggested as a form of back-to-back comparison. Unfortunately, I don't have that capability but can offer a small anecdotal example.
A few years ago a friend and I were considering making a 906 replica (about the same time Chris Melia and Michel were starting out on their amazing 908 build). Sadly our dream didn't get very far but as an early step I made a wire frame model (1/5 scale) of the 906 chassis. Once completed, I was amazed at the rigidity of the structure and decided to do a simple torsional test clamping across the rear and applying an offset weight on a level arm across the front shock mounting points. After taking measurements, I continued to add weights to see if the angular deflection would remain linear with increasing torque. Ultimately one of the tack welds broke (circled in red in photo). Before re-welding it, I remeasured and noticed that the deflection readings had more than doubled. After rewelding, the deflection results were virtually the same as the originals. I was amazed that the single node where the roll cage connected to the rear roll hoop could effectively double the overall torsional stiffness on the chassis. After thinking about it for I while, I remember reading how a number of different car manufacturers have built convertibles that are heavier then the coupe versions in order to achieve similar chassis stiffness.
So to come back to your original question Mark, based on my little experiment, I would be tempted to say that a steel spyder (similar to the originals) would add a useful amount of stiffness when combined with a solid rear bulkhead. Most replicas (mine included) have fiberglass spyders so that may reduce stiffness a bit. I'm not a big fan of adding roll cages in a street car as unhelmeted heads and steel tubes aren't very compatible. If anyone was offering a separate steel spyder I would be tempted to retrofit it even if it took quite a bit of effort.
 

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^ any chance you have another pic that incorporates that node, but more from side angle? Having trouble figuring out where that element ties in - is it the top bar running from windshield "header" to top of rear bulkhead/roll hoop? If so, I am blown away by its stiffness contribution, especially since the top bars are parallel, and can shear relative to each other by bending the roll hoops. If they had a shear panel between them, and their mounting points at the hoops were triangulated down to the bulkheads I could see that being a huge contributor.
 
Hi Tom,
Here is a side view. Sorry it isn't a bit clearer. I too had the same reaction initially so checked to see if any of the tack welds directly involved (hoops and supports) were suspect and may be contributing accidentally to the difference, but they were all solid so don't think it was a source of error.
 

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Hi mates !!
Torsionnal stiffness have been always abig big concern for all car racing manufacturers
I did by the past alot of torsional stifness test during 40 years career and could say that to compare one chassis to another is very very dificult because
its depends mostly on how panels or tubes have been corectly welded and if the welding have been done "professionalmy" say with care of introducing stress or not ( depending of the welder hability to follow welding proces thru the chassis)
So it's a tuff work to compare chassis ( different or same )!! an other point is
I should agree with the fact that an original GT tub will always be stiffer tahn areplica tube chassis ; even with a rollbar stuff on it
Monocoques have always give stiffness on a chassis I don't think that a steel spider onto a replica base chassis will transform the stiffness ratio enought to beat a monocoque chassis
monocoques chassis have been done in variuos method ( while following the same panels drawings ) Thickness of the steel , material and treatmenton thhe panel before welding are more parameters influing onto the ratio torque measurement
 
Forgot to say that on 2 chassis of different manufacturers ( for example Mono vs tubular) you will never apply the load on the same longitudinal area
So figures obtained are subject to evaluations so means very hard to compare exactly .....
Another point is ; weight
a mono will for sure be much more heavier than a tube one so ration in beetween kgs /mega pascal is to be analysed
 
Hi Michel,
First of all I want to say I'm in awe of your 908 build. I wish I had your skills and abilities. But came to the realization that I personally was never going to do justice to a 906 by starting from scratch.

Like most people on this website, we share a love for the GT40. In my case going back over 50 years. I also realized many years ago that a real one was priced far out of my price range (and getting farther away!) So made the decision over 35 years ago to make one. This was before most of the kit car versions had come onto the market. I spent quite a lot of time looking at whatever photos existed of the GT40 chassis, trying to figure out in an intuitive way where the strength and stiffness of the chassis was coming from. After a couple of years of doodling different designs, I happened to see an ERA MkV at a car show north of LA and instantly fell in love! Although I was tempted to mortgage the house to buy one, my head over-ruled my heart and I decided against buying. However, I was really interested in the way ERA had maintained the 'spirit" of the monocoque but had simplified the construction to something closer to my capabilities. After, several more design iterations, I came to the conclusion that a steel monocoque centre section with front and rear sub-frames might provide a good compromise between lightness and stiffness. So now the project began.

As I've always been obsessed with keeping weight to a minimum, (hence my fascination with the 906), I was originally planning to make the rear subframe out of aluminum, however due to time constraints (long story) I decided to use square tubing with a geometry similar to GTDs. I've attached a few photos of the build. Although the workmanship may not look all that professional, I have attempted to add stiffness where I thought it was most needed. For example, in (1) I added additional shear panels that form the tip of a structural pyramid (2) adjacent to the front shock mounting points. I think the original GT40 used the half-circle of the tyre carrier as the equivalent structural member. I couldn't hope to match Eric Broadly and his team in attention to design but tried t at least capture some of the key elements. Similarly, I thought the U-shaped mounting of the rear sub-frame to the monocoque was a good way to distribute loads into the skin of the centre section.
With this as a basis, I have always considered that the geometrical shape and stiffness of the spyder to be a significant contributor to the stiffness of the centre section. I have always wanted to do a finite element simulation to see if my intuition was remotely in line with reality, but was introduced to NASTRAN many years ago when punch cards were still the method for computing and still bear the scars to this day!!
Apologies for this rambling trip down memory lane and thread hijack
 

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It's alway very very interesting to read each homebuilder story ; very nice one from you and how you achieve perfectly one of your dreams !
Congrats !!!!
Al your details on tecnics are right !!!
 

Neil

Supporter
Trevor, I smiled when I read:

"I have always wanted to do a finite element simulation to see if my intuition was remotely in line with reality, but was introduced to NASTRAN many years ago when punch cards were still the method for computing and still bear the scars to this day!!"

In the late Sixties, I designed a round tube space frame chassis for a Sabel Mk III sports racer using "STRUDL" running on an IBM 360/30. All my data inputs were on punch cards, too. What a nightmare! (but, to be fair, it was a great tool at that time. :(
 
Hi Neil,
Yes, in many ways I wish that I had started my engineering career in the era of MATLAB and incredibly powerful laptops instead of (similar to you) IBM 360s and pocket calculators. I have come to use both in the last few years before my retirement and realize how much more I could have achieved if they had been available when I was a young sproget just starting out.
 

Neil

Supporter
Yes, Trevor, I agree with you. One benefit of growing old is the ability to appreciate that things were not always the way they were now. Very few now know how much easier & accurate things were after the HP-35 displaced the slide rule, for example.
 

Neil

Supporter
Mark,

The HP-35 was RPN. I also had an RPN calculator made by National Semiconductor that would drain its 9V battery dry in 5 minutes. The logic family must have been TTL. Now the topic is truly in the weeds....
 

Mike Pass

Supporter
I am very much an amateur and not a structural engineer but I can read and look up stuff on the web. There is lots of stuff out there but to find the useable stuff that is headache free can be quite hard. Anyway here are my homespun ideas.

A couple of the more useful and understandable items.

https://www.buildyourownracecar.com Or go to “Car basics and build tips” and then drop down menu to “Chassis”

From 2min.25s

From the above you can see that the lightest stiffest chassis is the box monocoque construction. The best space frame design is the one made up from tubes in triangles with either round tubes or rectangular section. The rectangular tube is much easier to join and to fit body panels and brackets to.
The GT40 space frame is a tricky one. Seen from the side without a roll cage it is like a convertible and is a U or channel shape, which is not good. Fitting a roll cage will help as it closes the box. However due to the door tops being part of the roof there is only a narrow section joining the front rail to the bulkhead rail. In most roll cages this effectively a rectangle (oddly shaped), which is not a stiff structure. This should be panelled top and bottom to make it into a box. Nothing much can be done about the front arms of the roll cage, which must follow the periphery of the screen apart from making the tubes out of high grade steel. There is the issue that Frank Catt has pointed out that the part of the roll cage at the top of the screen is a brain damage hazard unless wearing a helmet. A possible way round this is to make this section of tube tightly fitted to the roof and formed into a wedge shape by deforming the round tube and cover this with closed cell foam so the head will skid off it.
The rest of the chassis can be triangulated in the normal way by fitting diagonal tubes across any rectangles.
The F – R stiffness of the GT40 mono chassis is provided in a large part by the sill structure which are rib -stiffened boxes. On a GT40 space frame these sill structures are not boxed in so much stiffness is lost. On cars like the Lotus Elan the F-R stiffness is provided by a central spine which is good for handling but very poor for a side impact! The pic shows a partly built upgraded Gilbern Invader chassis I built a long time ago which has a stiff triangulated central spine linking front to rear for F to R suspension loads and engine/box to fixed differential. For safety it also had side impact frames. The roll cage is not yet fitted. The tubes were quite thin gauge and the whole chassis weighed 235 pounds.
On the GT40 space frame there is no room for a big central spine and this tube often carries pipes for cooling water etc, The best area for stiffness enhancement after roll cage and general triangulation is to look at ways of turning the sills into box structures whilst keeping a reasonable fuel tank capacity possibly by replacing the side frames with boxes.
To test your theories build a balsa wood frame and test it for stiffness by clamping one end and resting the other end on a pivot and applying twist by hanging weights on a bar.
Over to you….

Cheers
Mike
 

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Neil

Supporter
MIke;

Using "torsion boxes" on each side of a tube chassis is a good way to increase its torsional stiffness as well as provide side impact protection. It provides a volume space for a fuel cell in each side as well. My chassis was built to SCTA requirements so it is far heavier than a street or road racing car would need to be. My whole chassis is 359 lbs, primarily due to the full roll cage and heavy- wall tubing that the SCTA requires.

Making the aluminum panels covering a tube chassis into stressed panels is also a good way of increasing chassis rigidity but that requires high strength aluminum alloy (6061-T6, 2024-T3, or 7075-T6) and good rivets (CherryMax, HuckBolts, etc) with proper spacing instead of a few hardware store pop rivets. A structural panel is no heavier than one intended to be decorative or to simply keep the cockpit sealed. It is somewhat more expensive, though. No criticism intended of anyone's build- just suggestions based on years of experience.
Your balsa wood model is an excellent way to analyze a chassis structure.
 

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On the GT40 space frame there is no room for a big central spine and this tube often carries pipes for cooling water etc, The best area for stiffness enhancement after roll cage and general triangulation is to look at ways of turning the sills into box structures whilst keeping a reasonable fuel tank capacity possibly by replacing the side frames with boxes.
And that is the area I've been looking into to evolve a rib system that could be grafted into the spaceframe.

Many CAD models later I might come up with something.
 
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