Tube chassis question
 

To start off I didnt know where to post this so I figured I would try here. I am not familiar with the strength of aluminum box tubing so I am writing in hopes that someone here is. I want to make a tube chassis for my car out of aluminum instead of steel to save weight. My question is would Aluminum 6063-T52 Bare
Square Tube 1.5" x 0.125" prove to be strong enough or will I have to go with a larger size like 2.5"? The design that I am going to go with will be similar to this, however out of aluminum. http://www.thesamba.com/vw/classifie....php?id=615731

And I guess for that matter how much should I brace a Karmann Ghia to support an ej20 t pushing a 915 transmission? I assume a kafer brace minimum. How solid are those of your cars who have done such a swap or something of greater power? Thank you very much for your time alone to read this and especially thank you to those who can help.

Hey Bloodline,

There's a reason 99.9999999999999999999999999995% of chassis builders build their chassis from steel and not aluminum....

Use Chromoly tubing if you're trying to save weight, but aluminum will get you injured / killed

I did not know that but hey, its why I asked and should probably thank you greatly now, thank you. What is it about aluminum that is a great risk factor for death? I decided to save up for a 915 or greater transmission and as far as the brakes go I got one sweet deal on a local guy parting out a 944 n/a and not knowing what its worth. :)

You can flex steel quite a bit before it lets go, and it tends to do so slowly, aluminum can't handle fatigue that well and then just snaps off suddenly. Steel is just more "tough". I think if you get into really exotic aluminum and castings you can get a lot more out of it, but for general fabricators, it's probably not the way to go, to many possibilites for something to go wrong or just let go.

Jason

IIRC, aliminium also 'work hardens' and then starts cracking, no?

That makes sense, I was wondering why no one else was running aluminum when it is about one third of the weight. Thanks for the information everyone I definitely appreciate it.

There's no reason why a frame should not be made out of aluminium but it requires the design capability of a fully fledged chassis engineer to factor in the loads, strength, fatigue values etc, etc. Commercial aircraft are built mostly of aluminium and they are subject to substantial forces. There were early failures until the engineers understood more about metalurgy.

evilC

Aluminim might weigh about 1/3 of what steel does but it's also just about 1/3 as strong. Typical numbers might be like 200MPa (megapascals, million kilograms per square meter) for steel but around 70MPa for aluminum. Plus aluminum work hardens (as stated by zeroaxe) and gets brittle. Additionally, as volkdent mentioned, aluminum fatigues even at extremely low stresses, where steel has an endurance limit (the lowest stress that contributes to fatigue) of something like 100MPa. Of course, all of these numbers change with the actual alloys chose, but for illustration they work well.

If you want a safe tube frame chassis, I would say just look to the FIA specs for their rollcages (since I would hope you will have one of those too if you're having to build a chassis). The specs have been develeoped for a lot more abuse than you will hopefully see, and hold up to bad crashes every day.

Peace of Christ

I planned on a full roll cage. I got over my head in dreams of a going straight to a 320hp + sti engine and forgot about my current financial situation. I will though at least start off with a 227hp sti with a 915 transmission. How should I brace my car for it? Or must I build a tube chassis? I just assumed that that would be to much power for a vW chassis with only kafer braces in the rear.

Agreed. It's probably more power than the chassis can take without bending not good parts. But, With the factory pan in place you could build a rollcage that would tighten everything up pretty well, and if you extend the cage back around the engine you could put in Pretty good engine bracing as shown here:

http://bader-racing.de/pics/gallery/...TUNGSTEST2.jpg

(Thank you Mr. Bader for your picture)

Could I get away with installing something like this? http://www.autopowerindustries.com/I...g/DSC_2840.jpg

Sweets, that makes things a lot quicker and smoother than I expected.

This looks like it would be a good start for you, but it's bolt together. Bolt together rollcages are a bit dodgy in my book, if you want strength they need to be fully welded. Since you have a Ghia though, I haven't found many good cages for a Ghia. I forgot that Ron Lummus Racing sells complete tube frame chassis for a Ghia. http://ronlummusracing.com/Race_store.htm

I was kind of thinking about doing a complete cage like the one Bader has. However I was thinking about cutting out the vw rear torsion and mounting a 944 complete torsion set up and having it bolt up to the cage. I don't know for sure how to run front suspension if I decide to go away from the b/j beam. Does anyone know any pros/ cons to mounting the 944 in the rear. The reason I want to do this is because I can simply adjust how it is mounted to adjust the ride height I want without working around the camber/ caster issues VW's have when lowered to a certain extent. I realize I will probably have to flare the rear out on the ghia to fit wheels under it.

Remember that the Bader Ghia's rear tubing that you see is only/mainly there because the 911/993 engine uses a (sort of mandatory) rear engine hanger, unlike a bug/vwghia where the engine only hangs on the transmission.
The added 'cage-effect' was used more a s a 'by-product' of the 911 engine install IIRC ;)