camber799

Ahhh. Yes, you are right, sorr, I did misread you and couldn't agree more.

tinman944

38D.
I have written this since the start.
There is no Horizontal bar in John's cage.
That is the bar that goes from one side of the car to the other. mounts between the main hoop.
Plus added to it the cross bar is too far towards the center of the car.
If the cross bar was to be mounted as close to the drivers side of the car I would say it was a little better.
John is trying to convince us that the bar he has mounted on the support legs is making up for this cross bar.
You need to go back 9 pages and look at the photos. put your self in the drivers seat and look at all the metal that is between you and the only support in the main hoop.(The cross bar.) Your body is going to absorb all that energy.
Now this is in the worst case accident. a side impact.

Larry Herman

As a side note, do I take it then that the roll bars which have a curved horizontal bar (to allow for greater seat travel) are not nearly as strong as one with a straight bar? So if I were to get a custom cage built, the main hoop should be set back far enough to allow for a straight horizontal bar.

tinman944

yes Larry I you are right the bent bar is not a strong as a straight one.
If I was to have a CUSTOM cage built I would have it fit to me.
That is the great part of having a cage built for YOU.
Some drivers need that bar bent to allow there seat to go back far enough.
I prefer to set the main hoop back far enough to fit the driver.
A lot of folks like the main hoop hidded.
There are ways to hide the main hoop behind the B pillar for tall driver.
I have in the past moved the petals forward to help.
In my track car I set the main hoop at the B pillar and moved the petals closed to me.
I have short legs but I wanted the weight of the car as close to center as possible.
It really depends on what you are going to use the car for.
When you use your car as a daily driver with multi drivers it is hard to fix everything to one spot.
A freind of mine has this problem. We have talked about installing a cageinto his car to fit him. The problem is he is over 6 feet and his wife is around 5 foot.
It is almost imposible to build a safe car to fit both.

Mike S.

That's correct Larry. I'll try and follow up with some quantitative info later.

That is perhaps the most direct and simplest approach. On a related note, I also suspect that crossing the horizontal bar and main hoop diagonal is done to obtain a meaningful increase in the buckling strength of both members. These guys can experience a tremendous compressive load in a side impact, roll, etc...)

Mike

Geo

Indeed. That is why I added another tube across the base of mine. A bit of overkill, but it will keep the bottom of the main hoop from collapsing inward in a side impact. Stronger still since the bottm of my door X ties there as well to give multiple load paths.

To expand upon what has been written in response to Larry's question, whenever possible, all tubes should be straight between the mounting points. I have two curved tubes in my car (compromises that I mentioned earier) for the following reasons:

1) The top front tube is curved at both ends. Not the strongest, but the trade-off is that it tucks this tub up into a little trough of sorts in the roof and gets it out of the way of my little monkey brain. Had I installed a straight tube there I would have had to place it entirely too close to my head for my liking or safety.

2) My dash tube is curved at both ends. Again, not the strongest, but the alternative would have been a knee-capper (just isn't going to happen in a cage I build) or none at all.

I'd rather that these two curved tubes not be at the same general area in the cage, but IMHO these compromises were necessary. Whenever possible keep all tubes straight and the shortest distance between two points. If you do this and tie tubes together at a single point or node, you build an incredibly strong cage, even if it isn't particularly fancy.

Mike S.

Quick "back of the envelop" cals of a 3" arc/bowed vs. straight tube (1.75" OD X 0.095" wall, Steel)and setting buckling concerns aside....this particular arc/bowed tube will start to permanently deform at 1/7th the value of a straight tube when loaded in the axial direction (a la side impact).

Mike

Mike S.

Geo...this is likely obvious to you and many others, but when a tubes function is being asked to handle an axial load like the dash tube does, and it has the potential to either buckle or permanently deform (yield), one should always be mindful of what direction (and amount) this bar will/may travel. Bends in a tube in this application will indicate the general deformation or collapse direction. Collapsing away from important things (human body or mechanically important) is on the design checklist to recognize and consider.

Mike

RedlineMan

Bob - finally, we can understand most of the exceptions you have held;

Commonly refered to as a belt bar. True - this bar is specified in most race rules I have seen, and I don't have one. I'm not necessarily building a race car. Never said I was. I am taking part in a creative exercise. If I were building a race car, it would be there because the rules say so.

This is not to say that this bar is the Holy Grail either. Firstly, there is usually no stipulation but one as to how it must be rendered. I could bend it around like a pretzel, and as long as it connected both sides of the main hoop (the stipulation) it would meet the letter of the rules. No of course no one would do that, but... Secondly, the only force that this bar would counter to any significant extent is one parallel to its direction, and originating close to it termini. Any force coming from any other angle or location will activate it to a very much reduced extent.

For those who do not understand, here you are speaking of the main hoop diagonal brace. In the strictest, simplistic, literal sense, this statement has merit. It is a convention. In reality, the statement offers no proof of just how much stronger it is than the way I have done it. Move the terminus how much closer to the upper node? I've seen them rendered farther. How far is too far away? Purely subjective. Logical... but subjective, and highly speculative as well.

Not true. Not even close. I must assume you are speaking of my belt bar assembly? Wouldn't make this claim in a million years. You are.... um... fabricating! It holds my belts, and I happen to think it is visually appealing. NOTHING more.

I happen to feel that I have many side intrusion features that are never built into the average generic cage.

First - pillar beams. The main hoop is inside the b-pillar on purpose, one being that it eccentuates the natural strength of the door post. Further, the fact that it is substantially integrated into that post creates a beam structure of not insignificant cross section, and therefore strength. How much? Subjective. However, beams are very strong, far stronger than single tubes.

Second - Sill tubes. No one seems to feel these are worth much. I disagree. For reasons stipulated earlier (to which no one took exception afterward) I feel they certainly offer a lot more protection (and other benefits) than a cage without them.

Do these features out-do a simple horizontal cross tube? Good question.

If it were simply a function of having mass of metal between the outside air and the driver, my design wins over a simple cross tube. Mine will protect over a wider area and from more different angles than any horizontal cross tube ever would. However, I don't feel it is simply a matter of mass. One horizontal cross tube -vs- beams and sill tubes. Which is better? That is a good question.

Again, you were doing so well. This is hairball. Unless I was aware of and factored in all of the features of a design, and had the dimmensions and software to do the FEA on this, I would refrain from such wild statements. This is exactly the kind of stuff that will cause folks to dismiss ANYONE who utters it. At least it would me. I can understand a difference of opinion. I can even understand being wrong. This is absurd.

If I were building a pure race cage, and to the extent he should care, I would be making Bob a lot happier. I'm not in either case. I'm having fun, building a "piece of rolling sculpture" for my own amusement. This car was no where near a good candidate for a stock class race car to start with. If I want to race, I can toss in a few tubes and be ready to meet the rules. Do these keep me safer? I think not, to any great degree.

Tinman has a lot more experience than I do. He's built a lot of stuff. This could mean that he is a far better fabricator than I will ever be, or just as easily that he has been bodging up a lot of tubing for many years [EDIT - I certainly make no value judgement toward the latter]. I'm just doing what I do, and sharing a project. I might do it completely different tomorrow.

I would never reject constructive criticism... if it were offered.

Geo

Check!

38D

That to me is the key. The reason convetion has people build cages a certain way is it is 1) effective and 2) easy. Unless you can run the actual computations on a different design, it is speculative.

Btw, I had a bolt in Safety Devices cage in my former 914/6, and just about ever tube had a bend in it. The cage was then welded in, and very simple door bars were added. I hit the wall hard (head on) at Lime Rock and rolled. The cage was not even bent in the slightest. In this crash the unibody deformed around the cage, rather than the cage failing/bending.

fatbillybob

Mike this is an interesting point. I understand you. If you have a straight dash tube and get hit in the side perfectly the tube will yield and buckle somewhere hopefully not on you. However, being perfectly straight and impact perfectly straight on the bar can take the most force before yielding. If you put a small bend in the center of the bar pointing forward I suspect in a similar impact the bar would collapse toward the front and away from the driver. That is a good thing but takes less force to deform. So how do you decide on the compromise...bend or no bend?

Interestingly there was a pro on rennlist who advocated bending door bars do in a front crash they bent away from you. I do not buy this idea.If your doorbars are bending from a frontal impact you have more problems than what direction they will bend.

tinman944

I explained this in my PM to you

tinman944

Damn it that did not work out.
I will try again

tinman944

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