M758

John,
My point on rules was certain classes limit attachment points on cages. This is to prevent super stiff cage work that they consider to be beyond the intent of the class. PCA GT however only cares if the cage is safe. Attachment points are a minimum rather than a maximum. That is were I was talking about the "no rules" aspect of this cage.

Bends in general make the structure weaker. I don't consider an arch to be a "bend". Sure it curves, but the bend so gradual to have very different properties that a normal bend which is more often a much tighter radius. An arch is a wonderfully strong design element when loaded in the proper direction. Rather weak in other directions. The tough thing about putting arches in a roll cage is that there are so many different directions of load that benefit in one direction weakens the other and weakend is just bad.
Most upper cross bars in roll cages are not arches anyway. They are straight or have two sharp bend on each end. This structure does not perform the same way as a proper arch does.

I STILL wood attach that bar direct to the side of the chassis. You are so close and by doing that you have a SIGNFICANT increase in strength of the cage as well as chassis stiffness.

RedlineMan

Now, that I'll buy...

A much less sweeping generalization. You certainly can't predict exactly where a load will be coming from, but you assume very generally and make the best of it. These are not true archs, indeed, but they possess some of the basic properties of one. If the first impulse of the cage is to transfer force down any of its lengths in compression, it has performed the valuable service of spreading load away from the impact, even if only for a split second.

Because of this, I try to put the bends in my header tubes nearer the center than the ends.

kurt M

Rather than bend and rebend and rebend I use PVC plumbing drain pipe for mockups. Heat it with a heat blanket that is used to bend elec conduit, a heat gun, a cheepo tank top propane heater(my winter time favorite as I get to heat the shop too) or a torch fittied with a flame spreader. while warm and pliable you can bend it to the shape you want with your hands and then cool with a wet rag to freeze it in place. With little effort and little cost you have a full size template to match up in metal. You can mock right down to the fish mouth rough outs. You can keep the templates for the next time as well.

Or do the bling tunner thing and skip the make it from metal part completly. I saw a fake roll cage kit made from PVC. Cut to fit, glue in place and wrap in roll bar foam to finish. Mount little fire bottle on pass side A pillar and you are ready to cruz wit d boys. (it might not pass Tech)

Geo

That depends upon the sanctioning body. Some limit the numbers of bends you can make in various tubes.

Perhaps on a 944 (relatively speaking it's very stiff from the factory), but on most cars, a well designed and executed cage should significantly stiffen the chassis. However, according to Jon Milledge, even the 944 car be stiffened up noticeably with a well designed and executed cage.

John, that's potentially a scary statement. While there may be some green inspectors, I'd wager most of them have seen more cages up close and personal than you or I ever will.

I'm not an engineer, but I'd wager that "arched" tube would collapse much more quickly. The tube is already bent! An arch is only stronger parallel to its axis.

I'm really surprised you're not welding the upper side tubes to the coachwork. You've already stated you weren't building for any rule set. You've weakened the coachwork by cutting it out. I thought you were cutting it out so you could weld the tubes into it. When I first saw it I thought "how clever" but if you're not going to weld them in, I think you're doing yourself a disservice. Welding them in place would make the structure much stronger overall.

RedlineMan

I am not questioning the knowledge of the inspectors. I am wondering about those that might stick so resolutley to the rules as to be zealots. If it is a highly competitive group like National/Regional SCCA, then I can see the need for being more stringent. If it's just local weekend warrior stuff, I think that is a bit silly.

OK, assuming that to be true, then explain the "NASCAR door bars" that everyone seems to think are God's Gift? How are they not also blighted by this statement?

Point 1 - While I am not being **** about meeting every single point of a rule set, neither do I want to stray too far. Point 2 - The close proximity of the tube and roof structure render them virtually one to a large degree, even though they may not be directly integrated (I will be attaching at the main nodes, which I KNOW will make you happy ). Point 3 - The strength of the unmodified roof relative to the tube structure is paltry, and I therefore do not see huge gains to be had in integrating them. Point 4 - It makes it a lot harder to pad!

One aspect of this project has been trying to limit it so that I could actually complete it before things get busy. Having said that, I have in the end done a lot more at every juncture than I intended to. It may yet happen.

kurt M

I disagree, you are back to the two guys on one leg or one guy on two. You loose any geometric value the roof line had.

Well said. bends are often limited for good reasons. This is not conformity think but real world design for max overall build with min weight.

Geo

Understood.

IMHO they are. I don't think NASCAR bars are God's gift in any way. Mass for mass I believe you will find they are weaker than an X for the very same reason. That is why my car has an X. The strength of NASCAR bars comes from the number of tubes and additional mass as well as the attachments in a traditional set that attaches to the rocker or frame rails.

Actually John, I fear that folks will think NASCAR bars are the ticket and try to mandate them. I don't want them. And I'm very safety minded.

Oh come one John, JUST DO IT!. I think it actually will increase the strength considerably. It will really be a cool solution. As for padding, just drill small holes above and below the tube to fish ty-wraps through to hold the padding. Come on John, do it. You're almost there. Take the few extra minutes and really tie that cage in. The only possible reason I could see for not doing it is if you get a loon in tech who wants proof the tube is complete on the other side (and that can be accomplished with an inspection hole).

Do it John.

Do it.

RedlineMan

You know, G...

When I first envisioned this, I had in mind to completely hide the side hoops by fairing them in with sheetmetal. You may note that the NASCAR boys do this. I imagine it is for safety (nothing to snag), and aerodynamics (window opennings create a TON of drag. There is also a lot to be said for having a more uniform flatter surface for safety reasons.

On the other hand, I thought the scruts would never go for it, so I let it go.

You're a pain.

RedlineMan

Regarding bends;

The main reason for the bends in the side hoops and header tube is to tuck things in tight to the chassis for headroom, space efficiency, and to reduce the uneven shape of the impact zones. This latter point is not addressed often enough!

Funny... I note that the formed sheetmetal beams that these tubes are complimenting (door openning (or aperture) and header beams) are not straight, but arched.

Hmmm....

fatbillybob

Re: side head tube to roof structure...as Geo said Do It John Do It! How about an in dash dash tube to support the A-pillers? O.K. I'll shut up now.

Geo

I'll drink to that!

I'll have to buy you a beer when we meet someday.

kurt M

No need to make flashing, just slather on some Bondo.

I am going to be a pain too.

Dont drill holes, use a high bond contact cement to fasten the padding.
All kidding aside I understand the desire to have everything tight and to have as much clearance as you can get but to me from here it looks like you compromised one system to install another that does the same thing. Rather than gaining 2X you had to start with a sawsall induced deficit and much of your added metal is only making up for what was lost to the sawsall. There must have been a way to have a nice tight clever photogenic install without negating the original strutures value. Complement rather than compromise.

RedlineMan

Hey, K;

The drill holes suggestion was in the sheetmetal fairing around the tube, not the tube itself, I'm sure.

I hear what you are saying about the structure, but I feel you are over playing the importance of it. I could take a sledge hammer and with one blow cave in the side of the roof to some degree. I'll get my teeth chipped to Hell if I try hitting the cage like that. To compromise a structure that is so relatively weak is not a big loss in my opinion.

If I fair it all in with sheetmetal, it will be stronger than either alone.

Now... if I could find a really long lasting contact cement...

Larry Herman

Do it! Do it! Do it! Do it! Do it! Do it! Do it!

RedlineMan

Larry...

That took you FAR longer to edit than it was worth. Trust me on this...