Geo

Egress is important, but if the bars are not effective in stopping a car in a side impact, it's a moot point. Personally, I'd be worred about that first, egress second. Too many side bars are not terribly protective.

Geo

Oh, very clever. I'm going to file that one away.

Cory M

Here are a couple of pics of similar bars on pro-built cars, the pics are of the Prodrive Aston Martin and a car from the European Touring Car Championship, note how the tube extends all the way to the mounting point on the floor, probably to avoid putting any bending loads into the other bars. I wonder if these bars would be legal in NASA or SCCA because the rules basically say that any redundant supports that are there to increase the stiffness are illegal. Anyway, good job on the cage. Since it's been discussed here lately, what method did you use to notch your tubing?

fatbillybob

Here are my non-professional comments. First, I'm also doing my cage now and I'll be next for the Rennlist grilling. I like the cage. It looks good but I wonder about too many tubes. I think the dash tube should be over the steering column even if you have to move the column. I don't like kneebreakers and the dash tube makes for great support for the column and for tabs for your side nets. I would delete the apiller tube set. I would gusset the apiller bend with the same thickness as the tube and at least 1 tube diameter in each direction. I wouls also tack or fab 16g plate with holes in it to the a-piller tube to existing a-piller. Don't forget there is already DOT protection in the stock A-piller and your goal is to protect your head area in a roll over the lower libs are very far away. Also, how did you mount the harness bar and do you intend to have a passenger? If no passenger then I would deleate the passenger side harness bar and make the most critical diagonal rollhoop bar one piece welded from hoop corner over your head to the passenger side foot of the rollhoop. It is also important that your seat be placed in the driving position to make sure you have positioned the harness bar correctly especially for HANS use. Also the side x-bars look just like x's. That's o.k. but you get more protection if the X might be asymetical due to the outline of the seat in the car. That means the doorbar starts high at the shoulder and ends at the left front apiller foot and the other part of the X may be at the junction of the dash tube (OR JUST ABOVE THE BEND IN THE APILLER THIS SOLVES YOUR PROBLEM WITH LESS BARS) to the rear rollhoop foot. If you don't like to use the x-bar to support the you can run a tube from the top of the windshield tibe to the top of the dash bar tube right smack in the middle for a NASCAR look. Finally, I also like sill bars to connect the seat to the cage (that may be "too many bars" to some).

Benton

Sorry, but you either lost me on those comments, I disagree, or you didn't look at the pictures much. The X in my cage is just as you describe; the tube from the foot of the main hoop was notched at the bend in the front tubes for strength. I'd rather not trust "DOT" strengthening at 150 mph.

Too many tubes? ?

I'm not trying to be rude, but some of the stuff you have stated sounds like you are just repeating what you have read on message boards. Granted, some of it is true and good info. Please don't take that in the wrong way.

Yes, this car will have passengers. Yes, I did mock up the seat for the harness bar...

Personally, I don't think my a-pillar tube is too intrusive with the seat I am running. However, I think a better solution would be to install a bar as John has described, in case I change to a halo seat in the future.

As for chassis stiffening, I don't think the bar I currently have tacked in serves much purpose for stiffening. It does, however, strengthen a weak area. Safety is the purpose of that tube.

Geo

Before you make anything permanent, consider the reason for that tube from the top of the a-pillar to the bottom of the front support. The top of the a-pillar is absolutely the weakest part of the cage. If you run the tube you have and the cage collapses, you're going to meet your maker and those tubes near your head won't help. The main hoop is supported in two ways: first and most importantly the diagonal from your head to the bottom of the main hoop on the passenger side; secondly the rear supports. As I said, if the cage comes down around your head, those little tubes won't stop anything.

But the a-pillar tubes will reinforce the weakest part of the cage.

Consider carefully what you do before making anything permanent. I'm not saying anything is absolute here. And ultimately it's your neck on the line. Just consider the trade-offs carefully.

Personally, I would have installed a-pillar tubes if I could have fit them in my car and still be able to get my fat heinie in and out of the car. But, the 944 has a considerably small door opening.

Benton

So George, I'm confused. Are you recommending that I remove it? I think I will anyway and do a bar like John described. I appreciate all of the input, guys.

Geo

No. I'm trying to explain what the a-pillar tube is doing for your cage vs what the "head tube" (if you will) is doing.

The weakest part of the cage is the top of the a-pillar. This can be made quite strong, but the driver must be able to get in and out so compromises are made. The door openning is the toughest part of the cage to design around. We try as much as possible to triangulate a cage, but that is virtually impossible in the door openning.

So, having said this, the tube you currently have in place supports the weakest section of the cage. Look at your cage on the passenger side. the openning for the door is a pentagon. While some Pentagons are strong and powerful, they aren't in a race car cage. Ideally, a straight tube from the top of the a-pillar to the mounting plate at the base of the front supports will triangulate this area and add significant strength. In some race cars you find other methods being used to strengthen this weak area. Perhaps the best example that comes to mind is the factory BMW Motorsports E46 chassis. It has this huge curved gusset that adds considerable strength there. Very light. Not something the average shop can reasonbly do beause of the curvature along the bend.

The head tube is there to support the side tube of the cage basically. Ideally it would tie into the top of the a-pillar but that would block the ingress and egress of all cars I can think of. So, most of the terminate in the middle of the side tube. Terminating any tube in the middle of another tube is something we typically prefer to avoid because you just don't get the strenth you would get by terminating at a node. In fact, the termination of this tube can create a bending moment on the side tube in an impact. Once bending begins, strength goes away in big numbers.

As a result of all of this, I think the a-pillar tube will add more strength to your cage overall and thus protect your noggin better. And one of my points was that with the strengh of the main hoop (properly designed and executed as your's is) combined with a reinforced a-pillar tube (from node to node preferably), if any part of that cage begins to fail, you're going to meet your maker anyway and those head tubes won't matter. They are simply not going to strengthen the cage as much as the a-pillar tube.

Clear as mud? If so, I'll pare it down some, but I wanted to give you the full explanation.

That said, I've said before and will say again, cages have compromises. Choose your compromises. Consider what's driving them and choose carefully. It's your life, so make sure you are comfortable with your compromises. I made compromises in my cage. I'm sure John will tell you that he weighs compromises in every cage he builds. We all do.

Lastly, I cannot install the a-pillar brace because it will restrict ingress/egress too much, but now I think I may install the head tubes before I paint my car. Best compromise for me.

Benton

Ok, now I definitely see what you are saying. That was pretty much what I was thinking all along (what you said). I was confused in your first post regarding that tube, but you clarified it. I'm going to go down and mount my seat, close the door, and see how easy it is to get out with a helmet on. If I can fit well, I'll keep it. If not, I'll remove it. Once again, thanks for your time!

Geo

Oh, and one more note about the "head tubes" for those playing along at home... If you install them, but don't install the lower end at the node where the horizontal main hoop brace attaches (and hopefully other tubes) as John did, you're really limiting their effectiveness. Look at both examples John provided. Tying to that node will give them maximum strength.

Cory M

so what method did you use to notch your tubing?..

RedlineMan

Adam;

That's just pretty slick!

Red;

I did not mean to sound like your tube was dumb. Far from it, as the pro built cars show. However, they have other revised parameters that make these tubes less intrusive. Mostly that is highly revised seating positions from stock. Most of these cars move the driver (and often the engine) far to the rear of the compartment along with pedals and an extended column, much farther than in the street version of the car. This moves the driver farther from the limiting angle of the a-pillar, gives room for seat halos and such, and preserves a fairly acceptable occupant openning. Having distance from the halo to the slanted a-pillar is absolutely key.

Your tube obviously adds tremendous stiffness to the weakest portion of the cage as George points out. Even a tube fairly closely placed to the exisiting bent side hoop would dramatically increase support here. As you can see in the pro built cars, they are placed far less afield from the side hoop than yours. You might be able to move it closer at the top and maintain decent ingress/egress. I would certainly pursue it.

Geo;

Don't sell head tubes short. Having a properly arched head tube really does add a lot of strength to the front area. If there is no center support between the side hoops, and the center roof is crushed down between them, the side hoops will be drawn inward and failure of the entiure fron t segment will be dramatically hastened. Havcing a properly arched head tube spreads loads out in different directions and makes an important contribution to safety. They are surprisingly tricky to bend, but they ARE worth it.




Pic 1 is a 944. The previous head tube on the bolt in cage had the arch placed horizontal! Don't know what strength they thought that would provide?
Pic 2 & # is a Rabbit Ice Racer. It has a very flat roof, so I dropped the side hoops down a bit at the top of the windshield to try and get more arch for the head tube. The corner bracing adds a lot too.
Pic 4 is a 930. Corenr braces are webbed with sheetmetal like Porsche does it on some cars. Looks nice. Strong too.

Regarding door bars; Straight line bars are actually easier to make strong because they immediately load in tension when hit. Bending a tube is much easier than stretching it. Unlike "NASCAR Bars" that will travel inward as far as they project outward unless they are EXTREMELY over built. Most of these done on sports cars are NOT built strong enough.

Geo

I haven't given up on the John. I'm going to take a look at it. The problem is that the 944 openning (as you know) is very small. It doesn't look small because of the area at the front, but it's useless. With the relatively tall X tubes, space is really at a premium for me, especially considering my fat heinie.

Adam Richman

Geo, I am going to bet that the most crucial bar to aide in stopping a car in a side impact is the bar that runs [straight] parallel (and in close proximity ) to the rocker between the a-pillar and main hoop mounts. I see a lot of cages without this and after seeing a particularly bad IT7 wreck a few years ago, the absence of this tube makes me cringe (irrespective of nascar style bars or plain x door bars). I think my take is what I perceive you and John saying, that the side bars need not be protective from a "side" impact to be protective in a rollover.

I have a top bar in my cage (like the one pictured in John's #3 pic and shown below) for instance as well as gussets in each corner of the windshield (like his #4 pic - shown below)/dashbar openings (not shown) - these certainly may ironically get in the way of something coming at them perpendicular but their purpose is to stiffen these large rectangular openings that cannot otherwise be bisected (triangulatated) by tubing like the other parts of the cage. If you look at the door bars, obviously its a different train of thought from the X but we have essentially all 3 corners gusseted and ironically, how its shaped, it can both contain the window net mounting and be quite unobtrussive to egress (that may be my size too however).



Thanks! I was happy with how that worked out On a hopefully related creative note, I also found that retired lap belts and d-rings make for good towing eyes.

I am not sure if you are referring to a particular class (at least w/ the SCCA and since an SCCA logbooked car from my experience can be logbooked for NASA racing - both sanctioning bodies) but this is not a rule or suggestion I am familiar with in the GCR. Actually, most sections of specifications for roll cage construction state that any number of tubes may attach to a mounting plate or another tube.

Geo, on the heiny thing and worries of egress; can you not drop in from the top of the bend (at or forward of the bend toward the front of the car) directly (vs. a more perpendicular to the ground) to the a-pillar mounting junction or does that present an issue for visibility? It seems like it would not take too much away on egress unless that's a plane you swing your feet through (seems like you would not). If not, the great part of that side brace above (behind/next to) the head is that it shouldn't come into play - kinda like a free brace in regards to the gresses

I still wonder for the guys that run the X door bars (and believe me, this is not a critique, rather a curiosity), how close are your left shoulder and left forarms to these bars normally? Just looking at it makes me think (expecially in a 944 which I am familiar w/ the proximity in my street car) you would be pressed right up against the cage. Are you positioning the seat at an angle away from the bars or just off center from the pedals??

Geo

Adam, we're saying the same thing on the a-pillar brace. Ideally it would attach in the bend (and thus supporting the bend) and straight-line to the front mounting node. That would be really solid. On the 944 it would take up too much space for my heinie because the a-pillar slopes back so far and the door openning isn't nearly as large as the 928.

As for the side "head" bars I am gonig to look at that again. I am still concerned about the bending moment caused by attaching the upper part in the middle of the tube. Question for the engineer types: Given that tube would attach to the middle of top side tube, would a force on the very top of the a-pillar be more likely to start a collapse because of this bending moment on the tube. In some ways it seems to me that it creates a levering action on the tube that might (absolutely not sure) start that tube bending where left along it might distribute the load better. God, I hope that makes sense.

Regarding the legaltiy of "extra" tubes in a cage, as far as the SCCA is concerned, knock yourself out as long as you don't exceed the allowed number of attachments and follow the other rules.

Lastly, in my car with my Evo2, the shoulder "wing" will be just above the X tube and the main body of the seat will be about 4-5" away from the X. My biggest fear there is not intrusion, but the flex of the seat allowing me to contact that tube. Otherwise, any intrusion anywhere along those tubes and I won't have to worry about it for long. I'll be meeting my maker. But, I think the strength of my set-up is relatively speaking, quite high. You see, I do have that tube along the rocker you were talking about. I put it there because of fear of some small car (like a Miata or RX-7) getting a nose under the X.