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Old 02-25-2009, 11:58 AM   #1 (permalink)
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Tube Frame Design

If a guy would want to construct a tube frame chassis for a road vehicle, where could he get guidance for its design? I've looked on Amazon for books, I've googled, but haven't found much. There's books on suspensions and what not, but nothing really for the actual frame itself. One thing I have uncovered is Finite Element Analysis. I think the AutoCad Pro program can perform FEA. Is this what you need to calculate strength of a particular design? AutoCad Pro is darn expensive, though...

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Old 02-25-2009, 12:11 PM   #2 (permalink)
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For a general overview I would recommend looking at dune-buggy frame plans.
Building your dune buggy. Custom building instructions for you!

You could take an existing design and alter the layout for the wheel size, ride height, and interior configuration you're aiming for, or just use it as a reference for how the structure needs to be laid out to handle the drive train and suspension element stresses while protecting the occupants.

If you want free Solid CAD try Alibre Xpress
http://www.alibre.com/xpress
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Old 02-25-2009, 06:59 PM   #3 (permalink)
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There are also books about race car fabrication that will deal with tube frame design at least in one section. I am guessing that the books about building a "Locost 7" (that's a home-built design inspired by the old Lotus 7 car) would give you some basics, at least for that specific design.

This book looks like one of the "Locost" books:
Amazon.com: How to Build a Cheap Sports Car (Motorbooks Workshop): Keith Tanner: Books

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Old 02-25-2009, 08:09 PM   #4 (permalink)
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Autodesk Student Community

There's free AutoCAD for students. Have no idea about the FEA.
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Old 02-25-2009, 08:51 PM   #5 (permalink)
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Besides just the design of the chassis, the material selection is very important. What works for a race car with a 2 year life is very different than a street car that lasts 10 years and 100,000 miles.

I had it lucky I had my fellow teachers at the college here plus help from a local university for the information I wasn't able to find otherwise(structural engineering and the like). But if you are not going to college or working at one just showing up asking questions will probably not get you much help unless you can get someone there interested in your project. There are enough 'experts' on the internet that I would probably base most of my research on books and very little on web based information if you can't interest an engineer in helping. Some of the frames I have seen people build online are pretty scary.

Also don't ever use chromoly, structural steel is much better for road use, trust me. Using any tools like Autocad can give you a false sense of security if you don't have experience building things with it. It is one of the more annoying programs to use. I say just jump in and start building. When in doubt run over it with a truck or beat it with a sledge hammer and see what breaks. That usually gives pretty good results. But relying on computer tools to do your testing is nowhere near as good as testing out ideas yourself.
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Old 02-25-2009, 09:52 PM   #6 (permalink)
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Why is structural steel a better choice than chromoly for road use?
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Old 02-25-2009, 10:11 PM   #7 (permalink)
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"4130 Chromoly or ( Chromium Molybdenum ) has a 0.30% carbon content... thus you will have a higher tenancy for stress cracking or metal fatigue, specailly around the welds and transition where stress loads are high. Chromoly still is considered technically a "low carbon alloy steel." But 1020 steel ( Mild Steel ) is also a "low carbon steel" but it has a content that is 0.20%. The carbon makes the metal easier to weld, but the more carbon content makes it more brittle and thus lower the tensile strength of the metal. However, these metals both have a very high tensile strength."

Grabbed that from some random search result. It gives you the basic idea. A race car gets ran hard occasionally and retired after 2 years. The loads it has to handle are much higher than normal driving but the constant vibration and use of a street car will make it fatigue just as quick as racing frames. Unless you plan on replacing it every few years you will start seeing stress cracks show up all over it and after 5 years of use it will be so weak that any impact at all will probably rip the car apart.

The only bad part is you are probably going to double the weight of the frame you build by going to a structural steel frame. The good part is the frame will outlast the car. I guess everything is a tradeoff so it all comes down to what you want out of the car and how you plan on driving it. I am just trying to make sure you get all the information so you can make good decisions when building your ultimate car. It is a lot easier to build a car than most people think, it just takes a huge amount of time. Hopefully you will have lots of fun doing it
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Old 02-26-2009, 11:13 AM   #8 (permalink)
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The classic on the subject is "Racing and sportscar chassis design" by Costin and Phipps. It's from the 60's (I think), but has absolutely everything you'll need to know about design, materials, suspension, you name it. Most libraries have it.
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Old 02-26-2009, 04:31 PM   #9 (permalink)
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Dang. Another $100 book to put on the wish list.
Thanks, davidgrey50. That's what I was looking for.
I've also been wanting to get Hucho's book through inter-library loan again, so I guess I'll be going to the library tonight...

Coyote X, I have much respect for you after watching what you did with the Metro. I would do well to just jump in, as I do have the tendency to over analyze. But planning and designing will keep me occupied while I save my pennies for a welder. Speaking of which...do you prefer oxyacetylene or MIG? I learned oxyacetylene back in A&P school over 15 years ago, so I think I could dust off those skills fairly easily. Never tried MIG, but from what I understand it's not too tough to learn.
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Old 02-26-2009, 08:08 PM   #10 (permalink)
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Keep in mind the metro was car #2 I tried building. The first one was a bit more rough It does take a bit of practice getting it right but the good thing is the metal is not that expensive really so it won't cost much to abandon a car and start over.

For welding I say use what you like. As long as it is a good weld it really doesn't make that much difference for most things. I used a gasless mig for 90% of my frame and a stick welder for the rest of it. I would have preferred to use a stick on all of it but the mig is so much easier to get a good weld with it was a matter of convenience to just use it.

Off the top of my head here is a list of stuff I have for metal work. I will probably miss something but it will be a good start.

Welder - lincoln pro-mig and lincoln ac buzzbox stick welder
cheap chop saw from Lowes
Angle Grinder came with it in a package deal
Tubing notcher from harbor freight with hole boring bits from Lowes
Bender from harbor freight
alignment magnets from harbor freight
hobart airforce plasma cutter
lots of jack stands probably 12 of them total. Useful for holding stuff up. Can also weld them in place to keep stuff steady.

That is about it and it has done a pretty good job for what I use it for.

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