Car 54's Cage

#1

I was looking at pics from the Summit race and noticed that Car 54 has an X that reinforces the rear down tubes. I’ve been told by our regional director and some tech stewards that a single diagonal is fine, but if someone were to protest the X, it would most likely be ruled to be "for stiffening purposes only," and thus disqualified. Can anyone shed some light on this? I would really want to put the full X in my cage and need some ammunition against their arguments.

#2

That doesn’t sound right - I’ve seen a ton of cages built with a complete X to reinforce the down tubes, and nobody in their right mind would protest that. Build your full X traqrat!

-Vic
SO #325

#3

I would never protest you, but I cannot see how the X’s do anything for safety. The main roll hoop protects the driver, and the main hoop diagonal keeps this from crushing at the driver’s head, and the rear down tubes keep it from racking over forward/back. So, the main roll hoop has all the buttressing required to be safe with just the above three elements. The additional X’s only seem to increase the car’s tortional rigidity, which is not supposed to be the purpose of the cage.

Sasha

#4

Ex36 wrote:

[quote]I would never protest you, but I cannot see how the X’s do anything for safety. The main roll hoop protects the driver, and the main hoop diagonal keeps this from crushing at the driver’s head, and the rear down tubes keep it from racking over forward/back. So, the main roll hoop has all the buttressing required to be safe with just the above three elements. The additional X’s only seem to increase the car’s tortional rigidity, which is not supposed to be the purpose of the cage.

Sasha[/quote]

stiffer cage = stronger cage = safer cage.

I’ve never understood how anyone would protest someone under that rule.

#5

Sasha,

Really? I DO NOT mean this to sound insulting (because I dont know how else to say it) but you obviouslly aren’t a mechanical engineer (or have never taken statics or strength of materials). If you think the bare minimum is safe enough, then why put a cage in at all? Why add more than one door bar? The B pillars keep the roof above your head, the A&C pillars keep it from racking front to rear… :wink: Well, at least to 1.5x the static weight of the car (which is what a roof has to perform to for street standards.) Thats not stong enough in a racing accident, so they mandage more strength by adding additional material. The minimum is simply a guideline. The minimum will still fail in a big enough accident. Adding more increases your safety. Heck, add enough tubes and you could drive a M1A1 Abrams tank over the cage and keep it intact (no so w/ the bare minimum).

Adding ANY tube will incease the strength of the cage is some dimension. As a by product it also stiffens. The trick is getting the best of both w/ minimum weight (since a 4000 lb SE30 may be indestructible it sure wont be competitive.)

In my ~10 yrs w/ NASA, Ive never seen a cage that would obviouslly fail 15.6.2 "Intent". (but I haven’t seen them all either). Its quite easy to prove w/ software that tubes add measurable strength. What scruitineer is going to say your cage is too safe/strong? Thats an open invitation to an injury lawsuit unless they hold a PhD in engineering and have done analysis on YOUR car and cage.

Personally, I’ll prepare my car to above minimum standards because thats what Im comfortable with. I have more than the bare minimum race suit, have the newest 2" FIA seatbelts, wear a HANS (for a year and a half now), have a built in fire bottle and not just a 5lb extinguisher, etc.

Best,
SMD

#6

I asked my Rocky Mountain NASA director about this, and he said that the X is fine, but that if there were a second X half way down the main hoop back to the rear points, then he would be inclined to say that is for chassis stiffening (i.e. 6 total tubes to the rear points).
cheers,
bruce

#7

SMD,

I have degrees in physics, aerospace engineering, and architecture. Why do you have to attack me personally? Unlike you, I was never demeaning to anyone on the forum.

I stand by my original contention. I’m not arguing that the X between the down tubes doesn’t make the cage, and consequently the car, stiffer. I’m arguing that it doesn’t provide appreciable benefits for driver safety, because the X will primarily help keep the down tubes from getting out of square relative to one another at their bases near the shocks (which is beneficial to the chassis), but won’t do much to help prevent the main hoop from crushing. Therefore the X’s primary benefit is to stiffen the car torsionally, as I said originally. There’s a lot of voodoo in cage-building, as far as I’m concerned, and it seems like many cages are built with a "more tubes the better" mentality (which is NOT engineering). I said I wouldn’t protest such a cage and I agree that NASA is in a tight spot when it comes to protesting cages and their exposure to lawsuits. Everyone should do what they feel comfortable with. I’m not going to lose a race to someone because they have an X where I don’t, and I also don’t feel that I’m likely to die because I don’t have one. I do, however, think people should design their cages thoughtfully and within the spirit of the rules. The original post essentially asked for opinions on cage design and I offered mine without implying that anyone was ignorant.

Sasha

#8

This is exactly my point. Anyone with an engineering background won’t protest me, but I get the feeling from talking to the regional powers-that-be that I probably will get protested and lose. I wish they’d just let us run as many bars as we want so long as we stay within the 6+2 points rule. You can only make the car so stiff before the weight gains start to penalize you.

#9

Sasha, I DID NOT mean to attack you personally (I even stated so). Im just an engineer w/o the eloquent writing style required to make it appear any different. Luckily I know that and can appologize before hand. My wife sometimes says Im "tactless" - I just go tell her to sit on her ass and please stop talking;)

I do think the X in the rear support plane helps. You can do an analysis in any FEA software or something simpler and more oriented to the task like GRAPE. What is you car is tagged in the rear driver wheel area, compromising (kinking) the rear support? After all the car is only THIN sheet metal. Then the car rolls as a result of the contact - there is no significant rearward brace. An X is partially redundant and helps distribute the load the the other side.

I agree the "more tubes is better" mentality exists beacuse many in the field dont have the technical background (like you and I) to understand what is really going on. But in all fairness it has been analyzed so many times by people who DO engineer it and most come up w/ the same conclusion - more tubes are better;) Building a cage that looks like one from a WRC or DTM car isn’t a terrible example to mimic.

Another argument for the full X is if you carry a passenger/instruct. Dont they deserve symmetric design and protection? (Im a little hypocritical here - I kinked my driver bars into the door but not into the passenger door - it was simply too much work for a car that will rarely have a codriver.)

eg: There was an interesting discussion on pelican about door X bars vs nascar bars:


While not perfect, it uses real engineering to analyze and improve upon the problem. The most interesting fining is that adding a 5th tube in the X significantly reduced the intrusion. Im not saying that it’ll help in all cases but it really makes you think.

SMD

#10

traqrat, Go get GRAPE (its free), do some simple analysis w/ and w/o an X, put the results in your cars log/notebook. If someone protests, to can rightfully claim a reduced deformation of XYZ%. If you (or someone else) has done the analysis to quantitatively prove that its stronger, I dont think the protester will have a leg to stand on.

SMD

#11

here is a link to the grape software for the curious. It is Canadian so it must be good (guess where I got my engineering degree B) )
http://www.grapesoftware.mb.ca/

cheers,
bruce

Post edited by: leggwork, at: 2006/04/07 22:56

#12

SMD,

I forgot to take off my ninny, sensitive architect’s hat and put my dusty engineering cap back on; no offense taken–you are well-mannered compared to some of my colleagues from my old NASA (aerospace, not racing) days. :wink:

What you say is true about protecting the passenger; I never gave it a lot of thought, although I do have door bars, an FIA harness, and an FIA seat for a passenger even though it won’t be used much once I get my competition license, so I value their safety, too. There is also some value in the redundancy of a rear X, but I feel comfortable with the design NASA has recommended for the most part.

I have, however, made the following additions to my cage compared to the NASA diagram: knee braces up high between the main hoop and the halo, an extra bar up in the halo under the middle of the sunroof so I don’t rely on the flimsy sunroof panel to bear any blunt impacts, and a door bar design that includes two nearly parallel bars with lower knee braces and verticals so that I’ve almost made an inverted kingpost truss. I like x-brace door bars (but not NASCAR bars because they don’t help the cage function as a system), but my cage builder said some rules that require two door bars would DQ an x because the point of intersection only counts as one bar.

Sasha

#13

Sasha, Good to know we’re on the same page.

I’ll be adding a diagonal over my head too before the next race. It was in the plan all along, but I decided to delay it until the seat was in the final position. I was REALLY worried about headroom, but have that problem whipped now.

Im suprised NASA doesn’t require a dash/knee bar. W/o it, your feet (and the whole front of the car) are quite vulnerable if you get hit in the front side. I guess its the burdon of the driver - if they want more protection they’ll add it.

The rear/mid engine porsche guys have discussed adding a lower horiz tube between the front (& rear) supports across the floorpan (they have no tranny/driveshafts to get in the way). No easy way to do that in the E30. I’ve thought about a horiz bar under my calves and a plate that ALMOST touches the tranny tunnel (so its not a 9th point). It would need to be restrained to prevent from bending/sliding in an accident though (and making the problem worse). But, at this point its still just a whole lot of thinking - I still have other stuff to get done.

People need to remember its not all about the design either. Fabrication skill goes a LONG way in making it as safe as it was designed to be. I’ve seen cages by pro/high dollar builders that fail to do 360 deg welds or just plain fail to get good welds. Scary really.

SMD