Hello!

I'm building my hybrid car.
And im calling it hybrid because it wont have 1 use which makes everything more difficult.

I 'm trying to make the ultimate track+road use car for MY needs.

Half/rear roll cage isnt something that its a MUST if you dont want homologation for track race and ofc you "dont need" it for road use car.
However...The benefit + the reason that i want a rear roll cage is..

Extra stiffness
Appearance
4way Harness
Some extra protection if ANYTHING happens

The reason that i want not to do that with Steel/Chromoly is the weight of it..!

The weight of a steel rear rollcage will be about 25kg but if i go with an aluminum one will be half (12..5)kg.
I wouldnt mind the difference of those 12.5kg if there was at the bottom of the car but since most of the kgs will be above the mid of the weight ration i want to avoid them ASF..

So this is the reason that i want to make an aluminum cage.
I'm thinking of making it with 45mm X3,25mm(wall) MAIN..and rest with 40mmX 3.25(wall) aluminum(6082) pipe.

Do you think that there is ANY way that this wont be a gain and will be a loss in terms of safety?

If a car(MINI) without a rear cage is 0 in terms of safety....and a car with chromoly rear cage is 100 in terms of safety.. (theoritical numbers just for the conversation)
Will the Aluminum rear cage will be any plus in terms of safety... like +20?+40?+60? or might be a minus like -20, -40,-60?

I m not making the cage for the extra safety and only as i said... BUT i dont want to feel an aluminum pipe in my head after a crash...!

I can accept the safety level of the car and i can feel that a steel cage is much better in terms of safety...
But i CANT accept to put something in the car that might be fatal for me instead of HELPING me or at least do NOTHING..


Thoughts?

 

I'm not a materials scientist by any means, but I can tell you as a mechanical engineer that you're going to have a lot of issues both manufacturing and implementing your roll cage. But the short answer to your questions is this: "If no one's done it yet, there's probably a reason why."

Aluminum overall has about half the tensile strength of steel. I don't know the forces involved in a crash but I know it is enough to at least deform a steel crash structure (talking either the roll cage or the unibody of the car). This means an aluminum roll cage will definitely at least deform if not be completely destroyed. And we can all imagine the dangers of having unsecured sharp metal flying all about the cabin. As machinists say "You're the softest thing in the shop car."

But even leaving out the possibility of a crash, I don't think an aluminum crash structure will give all that much stiffness either. And not because aluminum itself isn't strong enough, there are plenty of high load suspension components made of aluminum. But welded aluminum pipe is very prone to fatigue fracturing at the welds. My guess is that the body flexing the roll cage enough will mean you're constantly doing repair welds or god forbid miss something and it bites you in the rear later.

There's also the problem of affixing it to the car. You can't weld aluminum to steel, so bolting it in is going to be your only option. And then again in the event of a crash you have to think of those steel bolts ripping through aluminum plate with high enough forces involved.

You'll have the same problem attaching a harness to an aluminum cage. High enough forces, the aluminum is going to give first.

Finally, even leaving materials out of it; please don't put a cage in a car you intend to drive on the street. It makes it a much less safe car for daily driving. I know you intend on having a harness, but with car safety gear intended for racing applications, it's an all or nothing affair. You'll need to be wearing a helmet as well. Harnesses are also not designed with the same intention as a seatbelt and airbag. A harness has one job, keep you exactly there you are in the car no matter what. Seatbelts and airbags are designed to absorb impact and leave you (hopefully) uninjured. So even in a low speed crash, you're actually going to be hurt more by a harness.

 

@thebombardier I really appreciate your time spent writing this msg.
And im very glad that you didnt focus in one parameter and tell me the bad-good with your mind.
Before i tell you my verdict / conclusion of waht im going to do , let me reply to your sayings .


Porsche did it actually many years ago but it was a short road for it..


For sure the stiffness is not going to be the same,,,however some stiffness there is, i mean...if you compare it to nothing.

This not an issue since a proper cage have welded "Towers" on the chassis and there like "male/female" the pipes go inside and then you have extra bolts. This thing is impossible to move.
However as you mention about the welding strenght and that time after time it must be welded again and again is something that i havent thought about it and you are right





The seatbelts works as harness -4way seatbelts two....They want to keep you as STEADY/Close to the seat and have zero absorb because the main thing is NOT to move from the seat and have Extra G force from yours body weight...The airbag is there to "catch" you from that flex/bend that will do your body after an impact.
Helmet is a no need in terms of safety since the cage is rear and you are not going to hit it by anyway with the cage.


So after thinking it a lot talking with mechanics /friends etc etc...

Its REALLY pain in the *** make the cage Aluminum, Hard to bend without a scratch,not welding that good and because NONE can guarantee that Aluminum wont fail or have a negative impact in terms of safety i will do it from still 45X2.5mm the main and 40X2.5mm the rest.

I will make a simple design in order to be as light as possible, have my harness and know that in any accident that wont be fatal and actually might save me.. HOPE i will never find out if cage was a save or not though.

Car is not daily and im not intend to ride it in roads with a lot of cars put others people lifes in danger.