Round Tube vs. Square Tube
There is ongoing debate about the use of round vs. square tubing in frame design. Most of it is from people who are selling one or the other. The bottom line is that it costs more to produce an original style round tube frame because it’s more difficult to design and manufacture. Many companies have taken the easier path of making square tube ladder style frames. We felt our challenge was to make an original style frame stronger without losing its inherent beauty or period correctness. Good engineering means form and function. Once we committed to building a round tube frame we found some pleasant surprises.
* Round tube weighs less than square tube.
* Round tube is available in stronger steel. There is no rectangular 1020 DOM tubing, it’s all 1010. 1020 DOM is about 30% stronger than 1010.
* In torsion, (exactly the kind of stress that cross members are subject to) round tubing is much stronger than square tubing. Makes sense why there are no square drive-shafts, right?
* Under vertical bending loads, square tubing is stronger, but since the round tubing is available in higher grade steel, the advantage square stock had in vertical loads is diminished.
* Round tubing helps make a lightweight ladder frame, with excellent vertical and superior torsional load resistance.
* A well-designed round tube frame is the hallmark of professional chassis engineering. They are complex, requiring intricate jigs, difficult to design and they are usually made from better materials. They are also really beautiful.
In the kit car world, square tube frames are more common because they are simple to design, easy to make, and the materials cost less. They can be made strong, but to do so means you’ll likely end up with a brutishly heavy slug of a frame. The 1020 DOM 4” round tube that we use for our main rails and cross members costs $10.30 per foot. Similar square steel tubing (commonly used) costs $1.98 per foot.
For a more comprehensive review of the principles involved you can read one of the best books ever written about racing materials and engineering. The author is Caroll Smith and the book is entitled “Engineer to Win”. You will probably feel that you got your moneys’ worth after reading just the dedication and short two-page introduction.
One final note: If you think we’re making too much of a deal out of just a car frame, maybe we are. Just about any design will technically do the job. The thing is though… The frame is the most elemental part of the car. Unlike an engine or suspension, it’s hard to modify and upgrade later. Not only that, the frame is a huge part of the personality, character and soul of the car. Part of the reason you will love this car like your own child is that elegant frame… designed and built by people who see a race car chassis as functional artwork. It will be more than just a good servant to you in your quest to build your own world class sports car.
