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  1. #1
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    Space frame chassis vs. Perimeter frame

    I've searched around and didn't find any real discussion on space frame chassis as a viable option for a pro touring build (especially uni body vehicle). It seems just about every build thread I see where there has been a significant amount of thought and effort put into fabricating a custom chassis is based on some sort of variation of a perimeter frame with a roll cage. I assume this is due to a number of reasons. Note, im not talking about installation of aftermarket solutions. Rather I am referring to the custom one off builds.

    To paint a picture, think about professional road race chassis and high end kit car chassis. A good example is anything from factory five racing. The overall design of these chassis are significantly different from what we see with just about all pro touring cars. Do you feel it would be a viable option to deviated from the rectangular tube parallel frame rail plus a roll cage configurations we see and fabricate a chassis much more like a tubular (square tube) space frame?

    I'm curious to hear your thoughts.

  2. #2
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    I think the benefit doesnt fit into many of the pro touring builds out there... a perimeter frame with good structure can be very strong, maintain original floor and tunnel structures and not cramp some already very tight interiors. The additional torsional strength may not be worth the added headaches and cost. Its much easier to design a car around a chassis, than to design a chassis around a car.
    -Chris
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  3. #3
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    I think the term "pro-touring" has just been over expanded, what was once improving old cars to be on par with modern performance cars has gone off on a tangent.
    I'm seeing more and more people asking why something is not being done this way or that way, and why "others " have not done it.
    In this case and most others it's a very simple answer, cost.
    To take a frame car and make it a unibody would require re-engineering the entire body of the car, load and stress analysis and design, engineering and testing, implementation, stamping and tooling, then volume.
    https://www.pro-touringf-body.com "doing what they say can't be done"

  4. #4
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    Quote Originally Posted by 79T/Aman View Post
    I think the term "pro-touring" has just been over expanded, what was once improving old cars to be on par with modern performance cars has gone off on a tangent.
    I'm seeing more and more people asking why something is not being done this way or that way, and why "others " have not done it.
    In this case and most others it's a very simple answer, cost.
    To take a frame car and make it a unibody would require re-engineering the entire body of the car, load and stress analysis and design, engineering and testing, implementation, stamping and tooling, then volume.

    I don’t think “pro touring” has boundaries. The concept has been around for a while long before we had the goodies we have today. As products become available and folks get more creative, we will see more and more unique builds.

    I have already started to see more full frame 1st and 2nd gen Camaros. In the years it will probably be the thing to do..who knows. I have already seen 2nd gen Camaros with tubular perimeter re-enforced structures. Tubes running across the rockers, tubes running across the interior that’s completely hidden by the OEM Interior. Neat stuff people are doing.
    1970 Camaro/DSE build


    Are you driver enough? Maybe....come on blue!
    https://www.pro-touring.com/threads/...71#post1147371

  5. #5
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    Square/rect tubing versus round tubing is not a huge structural difference in the big picture.

    The fact that most chassis are done with square on the bottom and round on the sides/top is mostly a matter of practicalities. Straight-sided tubes are easier to build perfectly squared (flat & symmetrical) to give the whole chassis a level starting point. Straight edges are also easier to fabricate suspensions & floors & firewalls onto. Whereas round tubing is easier to tuck up against interior walls/roofs and it doesn't present sharp edges to whack your head & body against.

    Furthermore, for the part of the car's structure around the driver's body, round tube cages are safer for collapsing. Welds are prone to cracking & breaking more than bends. It's better to have a cage made out of a few tubes + many bends, rather than many smaller straight sections of tube all butt-welded together. Advantage: round tube. The OEMs don't use round tube for the passenger "safety cell" but they make elaborately-shaped stampings that curve and bend and overlap at joints. It creates redundancy at the joints and "give" when it crumples, more like a round-tube cage than a bunch of square/rect sections.


    Most pro touring cars are perimeter frames + rollcages because most of them (at least on this forum) are GM-based. GM loved perimeter frames so much that they even put half-frames onto their compact cars in the muscle era. Ford & Mopar were much friendlier to unibodies back then. The PT builds on those Ford & Mopar unibody cars mostly retain the stock rockers & subframes. That's just for practicality. It's simply easier (and usually lighter weight) than custom-fabbing all that stuff. No need to reinvent the wheel.

  6. #6
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    I don't think the OP was asking about frame design, round VS square/rectangle, but about unibody/monocoque VS bolt on body and frame and the answer is cost and manufacturing, Factory Five and other "kit cars" are new bodies and can incorporate such designs from the onset and most kit cars of monocoque design are based on using such oem vehicles as bases for the kit, so it goes back to what a major automaker designed.
    Other "kit" cars use frames not unlike a tube chassis race car.
    https://www.pro-touringf-body.com "doing what they say can't be done"

  7. #7
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    I don't think the OP was asking about frame design, round VS square/rectangle, but about unibody/monocoque VS bolt on body and frame and the answer is cost and manufacturing, Factory Five and other "kit cars" are new bodies and can incorporate such designs from the onset and most kit cars of monocoque design are based on using such oem vehicles as bases for the kit, so it goes back to what a major automaker designed.
    Other "kit" cars use frames not unlike a tube chassis race car.
    Easy solution for getting a uni/mono body PT chassis: Use a unibody car to start with. None of the commandments on Moses's stone tablets specified that you had to use a Camaro or Chevelle.


    If you must use a GM car and you want a uni/mono: Then you'll have to design your own undercarriage. You won't be able to (safely) get it as lightweight as a factory unibody because you don't have access to a car company's massive R&D and real-world testing for a platform. You'll have to err on the thick/heavy side with the structural pieces . . .

    . . . and that means you will be almost copying a perimeter frame with raw tubing. (Hey, even unibody cars need subframe connectors in the mid-section to be fully stiff.)

    And by that point, you might as well just use the OEM perimeter frame for the bottom, and do a round tube rollcage for the sides & top . . . which is what everybody normally does with full-framed PT cars.

  8. #8
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    Quote Originally Posted by mikedc View Post
    Square/rect tubing versus round tubing is not a huge structural difference in the big picture.

    The fact that most chassis are done with square on the bottom and round on the sides/top is mostly a matter of practicalities. Straight-sided tubes are easier to build perfectly squared (flat & symmetrical) to give the whole chassis a level starting point. Straight edges are also easier to fabricate suspensions & floors & firewalls onto. Whereas round tubing is easier to tuck up against interior walls/roofs and it doesn't present sharp edges to whack your head & body against.

    Furthermore, for the part of the car's structure around the driver's body, round tube cages are safer for collapsing. Welds are prone to cracking & breaking more than bends. It's better to have a cage made out of a few tubes + many bends, rather than many smaller straight sections of tube all butt-welded together. Advantage: round tube. The OEMs don't use round tube for the passenger "safety cell" but they make elaborately-shaped stampings that curve and bend and overlap at joints. It creates redundancy at the joints and "give" when it crumples, more like a round-tube cage than a bunch of square/rect sections.


    Most pro touring cars are perimeter frames + rollcages because most of them (at least on this forum) are GM-based. GM loved perimeter frames so much that they even put half-frames onto their compact cars in the muscle era. Ford & Mopar were much friendlier to unibodies back then. The PT builds on those Ford & Mopar unibody cars mostly retain the stock rockers & subframes. That's just for practicality. It's simply easier (and usually lighter weight) than custom-fabbing all that stuff. No need to reinvent the wheel.
    ... significantly wrong ... round tubing is quite a bit stronger giving it quite an advantage as far as structural integrity
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  9. #9
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    ... significantly wrong ... round tubing is quite a bit stronger giving it quite an advantage as far as structural integrity
    Define 'significant' for this application.

    Compare a wire-frame type box welded together out of straight pieces of round tubing, and another one made of square tubing. In the big picture there won't be a dramatic difference in how strong & stiff they are.

  10. #10
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    Quote Originally Posted by raustinss View Post
    ... significantly wrong ... round tubing is quite a bit stronger giving it quite an advantage as far as structural integrity
    Quote Originally Posted by mikedc View Post
    Define 'significant' for this application.

    Compare a wire-frame type box welded together out of straight pieces of round tubing, and another one made of square tubing. In the big picture there won't be a dramatic difference in how strong & stiff they are.

    If the cross sectional area of the round and square tubes, the material, and the moment of inertia of the two are the same, there will be NO difference in in strength and stiffness.


    A 2" square section tube with a given wall section will be stiffer and stronger than a 2" diameter tube withe the same wall section. The cross sectional area and moment of inertia will both be greater for the square section.

    Yield strength and Young's modulus of a material, the features that control strength and stiffness of any cross section, are properties of the material, not the cross section shape.

    Both round and square tubing are constructed essentially the same way. The sheet is rolled into profile, and resistance welded closed.
    Last edited by Twentyover; 02-23-2020 at 05:57 PM. Reason: Removed statement now believed to be incorrect
    Greg Fast
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  11. #11
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    In the application of round tubing frame vs a frame made of square tubing ... the round tube frame would be stronger , as in its resistance to deflection or deformation
    Spinnin'my tires in life's fast lane

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  12. #12
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    Quote Originally Posted by raustinss View Post
    In the application of round tubing frame vs a frame made of square tubing ... the round tube frame would be stronger , as in its resistance to deflection or deformation
    What is the engineering basis for making the claim? We studied this stuff in statics when I was a freshman in college, first principles do not support your contention.
    Greg Fast
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    1970 Camaro RS Clone
    1984 el Camino
    1973 MGB vintage E/Prod race car
    (Soon to be an SCCA H/Prod limited prep)

  13. #13
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    The basic shape.. a circle is stronger than a square ...maybe I'm not using some proper engin-nerding language
    Spinnin'my tires in life's fast lane

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  14. #14
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    The round vs. square tube debate is a fairly complicated topic with no fast answers regarding one being "better" than the other. It all depends on the application.

    This link sums it up nicely: http://www.super7thheaven.co.uk/roun...ronger-square/
    - Ryan

  15. #15
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    Well first off let me start by saying I have enjoyed reading through everyone's responses to my original post! This is exactly the type of discussion that I was hoping to spark. I figured I would get some good input from my fellow engineers AND technically minded people if I posted it in this section. Anyway, I do want to make sure we don't start up a round vs. square tube debate. Circling back to one of my original statements, I have my assumptions as to why we typically see latter frame / perimeter style chassis in pro touring builds. My opinion is there are two main reasons:

    1) Packaging, most of these old cars were originally a perimeter style frame or uni-body architecture from the factory. A lot of folks would rather NOT tear out all of the floor panels and replace them with sheet metal if it is not needed. It is extremely time and $$$ consuming to do so. To do a full on "space frame chassis" would almost certainly require cutting out all of the original floor panels. Two of the main structural elements to this style of chassis are the center section or "backbone" and "side members". Neither of which will fit under a factory floor.

    2) Maybe I going out on a limb here, but I assume most of the "high end" builds which get a custom chassis are fabricated by folks with experience mostly in drag and or circle track racing. Quite simply its just different from what most fabricators are used to.

    Now, lets get right down to the point. The real reason I started this thread is because I am currently underway in building a 67 Camaro. Here is a link to my build thread,
    https://www.pro-touring.com/threads/...7-Camaro-build

    I am to a point where I am wrapping up the sheet metal fab / replacement and am starting in on the mockup and fab of the chassis and suspension. I plan to use a combination of C4 components for the front suspension and C5 IRS components for the rear suspension. I am going for "smiles per dollar" with this one.I fully realize that there are better (much much more expensive) options out there. Anyway, the front section of the chassis will be using more traditional 2 x 3 in rectangular tube frame rails as the foundation. I do plan to add some additional structural members that will tie in to the firewall and or roll cage for additional torsional rigidity. For the rear section of the chassis, I am planning to use C5 rear suspension control arms and uprights and a 5th gen Camaro rear differential. Obviously this will require custom axles. One of my main goals is to use as many factory components as I can to keep cost down and allow for easy serviceability in the future. Keep in mind this will be a 90% street driven car.

    Now to the good stuff. I am seriously debating on the design of the rear chassis. Traditional rear frame rails (rectangle tube) vs. a "space frame" style rear section, most likely comprised of 2 x 2 in rectangle tube. Since the IRS suspension requires mounting points for both upper and lower control arms, plus mounting the rear differential, I think that a space frame rear section has a lot of advantages. Also, I am very certain that I can fabricate a space frame rear section from 2 x 2 in tube that will in the end be at least as robust (if not stronger) as traditional rear frame rails.

    By going with a space frame style rear frame section, it will essentially be a triangulated truss style structure. My initial design concept is to use 2 x 2 x 0.125" mild steel tube for the rear chassis section and tie in the rear down tubes from the roll cage (traditional round tube main hoop and rear down tubes).

    So what does everyone think? Is this a viable concept or am I completely off the deep end?

    I should be mocking up the front end over the next couple of weekends and will then start on the rear chassis / suspension. Please note, I realize this will require significant modification to the rear wheel tubs and trunk floor plan. It also may require running a fuel cell instead of a gas tank. I am okay with both of these features.

    Please keep the comments coming. I am really liking how this conversation has begun and hope to get some good technical discussion / debate going.

  16. #16
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    For the tubing comparison I would suggest looking at what is actually available versus what is listed on someone's website. Then you can compare the cross sectional properties.

    To simplify the comparison there's a decent program called MDSolids that I find handy. One of the modules offer Section Properties.
    https://web.mst.edu/~mdsolids/