norval you need 1 5/8 mild steel with inside thickness min .118 on a cage. For cm you need.058 for 1 1/2 or 1 5/8 .083 depending on what bars.Usually you get .134 ms drawn on mandral ms so it passes sonic test for thickness when it is bent .

 

Just get the mild steel one, chrome moly is just for all out race cars like big632, on a street car it's jut for bragging rights..just like aluminium 6 piston calipers (weird norval how you have opposing views on the 2, a chrome moly cage where in all reality it doesn't serve any real purpose over the mild steel one, yet no better calipers LOL ) Seamless cold drawn mild steel will get the job done and won't cost an arm and a leg or be a total PITA to weld (you could gas weld the 4340, I assume you have a gas welding kit?)

As for chrome moly being stronger than mild steel, yes it is but is it also tougher? No. Tensile strength is just the max. tension a material can take before it snaps, the yield strength is where the material keeps deforming without any increase in load on it. This is plastic deformation, beyond the elastic region. Permanent deformation is the result. Before the steel snaps when you pull it apart it forms a neck where the material has gotten thinner. The amount of reduction of area is a measure for toughness, the greater the difference (just before snapping) the tougher the material. There's also elongation, the amount of stretch before it snaps.

here's the numbers for mild steel and 4340:
4340:
elongation 28%
reduction of area: 52%

mild steel:
elongation 38%
reduction of area: 62%

So, mild steel is tougher than chrome moly. This is important in a cage, the fact that the material has more "give" in it means that it will absorb more impactand thus dissipating energy.

Apart from that it's hard, so difficult to cut, saw, drill - if you drill it and the drill goes blunt you have to temper the material because it will get brittle from the friction heat > out comes the ox/ac torch again - it needs tempering after bending, cutting, welding...anything that

I don't see why you would want a chrome moly cage.

 

Why don't you start out with a simple roll bar for now & leave room to complete a cage later?

Here is an example from www.ioportracing.com

 

That one does not have bars tying into the front section of the frame, near the suspension. I think that's one of the things Norval wants to have.

 

I've been thinking about adding a cage/6 point roll bar to my convertible - for safety reasons. The windshield/A pillar has essentially zero strength and would pancake in any kind of rollover-ouch!!

I agree with the comments advocating the use of DOM - mild steel versus chrome moly. TT hinted at the heat affect zone (HAZ) issues you will have with the welds. With a "tempered" or heat treated/ hardened metal the HAZ will be weakened back to its annealed state. The cage is only going to be as strong as the welds and HAZ areas. So you may want to look at the properties of the alloy you are considering in its annealed state (to determine whether you'll need to heat-treat it back to a stronger state) as well as the properties of the weld material. You will want to pay attention to yeild strength as well as tensile strength for the reasons cited. I have not done the math yet as to strength/ material needed to reduce flex and protect on impact/rollover. I'm hoping that 1 5/8" DOM will do the trick. Hopefully I'll be able to report back in a couple of months with those results.

Regarding the location of the frame tie-in points, I've been assessing my/our frame(s) for weakness/flex - both from a reaction to engine torque and a handling perspective. It would be a waste to put in a cage that only improved on protectiveness. The two major weakpoints are 1) at the kick up point behind the seat and 2) near the footwell where the frame narrows to accomodate the front suspension. Providing "triangulation" is the most efficient way (least material/weight) of providing strength. For the kickup I've been thinking that tieing the "rollbar" into the kickup 5-6 inches up and then running at least 2 bars off of the rollbar. 1 bar would connect back near the number 4 body mount, and the other bar forward to the footwell area.

Like Norval, I don't want to cut up the interior or give up the ability to take the body off the frame. I want the car to still be a comfortable daily driver (when desired) and as well safe for "competition". I've been trying to figure out a way to connect the cage to the frame in a manner that provides the required strength, but can be unbolted/slid out (that would easier with a convertible than a coupe). I am considering a two-piece cage similar the IOport illustration where the front could be removed for daily driving use.

Regarding the frame tie in near the footwell I was thinking that it might be best to weld a section of rectangular "boxed" material to the end of the tube below the floor and then either weld or cross bolt the boxed material to the side of the frame.

I'm up early to go to Carlisle - so no busts regarding post time/day please. Gotta hit the road!!

 

big632 that is very generous of you offering to send the tube knotcher and chassie book. I might take you up on the book later in the year.
We have a state of the art knotcher at work. It is a $12,000 knotcher that uses a large milling cutter and you just dial in the angle and the diameter of pipe and it does a perfect job.
We have 3 race teams in the university, one in a Indy car, one off road team and one solar team. Each year they build a new car from scratch and each has a budget of over $100,000. I see a truckload of slicks come in yearly, just not my size.

I will go with mild steel. You convinced me.
I was reading the GM chassie book last night and they have drawings and dimensions on the cage for a crovette and they used 1 1/2 tubing .095 wall????
D.O.M. STands for drawn over mandrel. Found it last night on the internet.
Thanks Big632

 

Thank you Twin turbo for taking the time with that long post. I agree NOT to go with Chrome moly tubing. It is a welding nightmare, we do alot of chassie building at work in chrome moly and the best solution for welding it is mild steel filler wire and tig, both would be available to me but I am convinced it is not the way for me to go. I have also priced DOM mild steel tubing and it is $5 per foot CDN.
In ANYTHING on our cars were are looking for elastic limits, not yield points. Yield points are too late. Elastic limits are what is used in design. An elastic limit is the point at which if we go beyond we plastically deform the part and it never returns to normal.
Every time we torque a bolt we are reaching towards the elastic limit but not exceeding it. Go beyond it into the plastic zone and the bolt is ruined.
The 6 calipers?? I was just convinced they are needed. I am not convinced Chrome moly is needed anymore so I am going mild steel. I will use the standoffs like in another of your posts suggested.

 

Twin turbo is right. I will not go half way. I want to tie the front and back together so tubes will go as far forward and backward as I can get them. On the mustang I am almost touching the tail lights.
I will tear the interior out completely and start over so that is the time to do a complete job. The winter is long.

 

Red73vert. It is bothering me alot that the body could not come off very easily if something happens to the frame. I am just resigning myself to mating the 2 together for life. I can always cut the frame out.
I am definitely goind with standoffs. They will be welded to the frame and pass through the floor where the main cage will be welded on. In my mind the have them pictured and I can make they look really nice and all welding of the cage will be in good locations for complete welds and at the same time I will keep it neat. I will fiberglass them in and reinsulate the floor.

I work in a welding research center/part of the university and we have the knowledge to do good welds, well at least on paper we have the knowledge. I also have a friend but I will not call on him but he is availabe that has build at least 10 complete chrome moly indy chassies complete with many many tubualar A arms as spares in case of accident. He would do any tig welding I want.
But being me I will do all my own welding and since I agree on mild steel I know I can do picture quality welds.
Tie in points in the back will be as far back as possible and still get a good spot on the frame and in the front as close to the a arms as possible.
I also need to add a spreader bar at the same time.
I will not bolt to the frame, bolts always have a little give. I will weld everything and hope the frame never needs changing.

 

Just a quick lesson on numbers.
Take 4340? The first number 4 means moly. Moly prevents brittleness . Any steel beginning with 4 or 8 means moly a good thing.
The 3 stands for chrome and nickle so from that number we know it is an alloy steel containing these 2 elements.
The last to number, 40 refers to carbon content. In this case it is .4%.
So anyone knowing how to read the numbers just looks at any steel and can tell you the makeup and what those elements added to steel does.
When designing anything we always worry about yield points, not ultimate tensile strength, not percent elongation. These are all too late, In a bridge it is great, it is the last line of defence before the bridge falls down. Sure it is sagging due to plastic deformation but again the bridge is doomed.
Yield point is an elastic band, stretch it over and over , millions of cycles and it always returns to normal, elastic range, Just exceed the elastic limit just ONCE and it plastically deforms and never can be put back to normal. Every bolt has a torque value, it is below the elastic limit but close to it for maximum clamping force.
We calculate/test for yield point, ultimate tensile stength, % elongation but want to stay away from them all. Reach either of them and the part has failed. The yield point ALWAYS come first and that is the one to stay away from, you can get close but never exceed it.
I know this is getting off topic but it might be of interest to some of you.

 

Sounds like you got everything handled. I will send you the pics to give you an idea of how you might want to run the bars. Mine is different than pics of cages.In a vette we run a wraparound bar behind the seats, not a straight bar.I know yours wont be the same since its not back halfed or tubbed.DOM was in my previous post.1 5/8 .134 is the best choice. take care.

 

Big632 I have a number of race shops within a few miles and I am on good relations with them all. I can drop in anytime and use/borrow their equipment. I return it promply, clean and grease it well, lots of thankyous and I often put a generous donation in their coffee fund. It is great for me, the shop knows I am good to their equipment, take care of it, return it promptly and while they say no to the donations I insist.
Both of us gain a little from the borrowing. Their knowledge is free like on this forum.
I do think I have it under control now that you guys have set me straight.
Thank you.

 

I picked this up from NASAProRacing.
It is copied from their site

The most frequently asked questions regarding roll bars and cages have to do with the different types of materials used in their fabrication. Generally, there are three different materials used: ERW, DOM and Chrome-moly. ERW stands for Electric Resistance Welded and this is a mild steel that is made from a flat sheet, rolled up into a tube and welded. DOM stands for Drawn Over Mandrel. This tubing is actually ERW tubing that has gone through an extra step to even out the imperfections and make it stronger. Chrome-moly is an alloy of steel which incorporates both Chromium and Molybdenum in the process and is stronger still. NASA’s rule book is very clear about what tubing is required for different weight cars and different classes. The rule book will tell you what is allowed and narrow down the choices.

 

Big632, what chassis book would that be? I've got some chassis books, maybe there's one I should add to my collection? I've got some Race Car engineering books, a Herb Adams book, a Stock car chassis book and some on trans ams and corvetets + the early chevy power manual.

Norval, am i understanding that youa re getting new calipers?? I'd get some GT mount patter ones (5,25" spacing) since all the big calipers seem to have that (some have the 3,5" spacing) and this means that you can run a lot of different calipers.

I may have to get a tubing notcher, they don't sell those things here but I will have to have one shipped then..why they don't sell it here is beyond me (well, they have a 2000$ one but i'm not gonna spend that, just looking for a nice one w/ an angle plate that uses hole saws), I do all the notching by hand now by making patterns with a protractor and laying it over the pipe. A lot of work but it gets the job done...slowly.

 

Norval, am i understanding that youa re getting new calipers?? I'd get some GT mount patter ones (5,25" spacing) since all the big calipers seem to have that (some have the 3,5" spacing) and this means that you can run a lot of different calipers

Not a chance twin turbo. I still think my stock calipers are more then adaquate.

Our work notcher is overkill. It was $12,000 and you just dial the pipe diameter and the angle you want and a large milling cutter cuts the proper angle. Since it is available I will use it but with a hand grinder and a little patience you can do almost as good. I wouldn't spend too much on one. I can also borrow the hole saw type if I want.
I was using some plastic pipe and laying the various bars out, it still bothers me cutting up the car and putting bars in the interior.

 

so that's a typo then?

 

Yes it is a typo. I feel they are NOT needed other then bragging rights.

 

wow...lightning fast response

 

twin turbo and norval. The book is Door Slammers by Dave Morgan.Lamplighter productions,1 800 331 8014.

 

Just have another problem. I just found out that I can buy the chromemoly tubing for $40 CDN per 20 foot length so price is no longer a problem if I decide to go that route.
My sons company buys large quantities of it and I can have any amount I want cheap. The goes also for mild steel tubing. He has dies for 1 1/2 and 1 3/4 but no 1 5/8's. ON monday he is going to give me samples of his tightest radiuses on both these dies. Dies come in large and small radius.
In the past I used stainless, 1 5/8's schedule 40 which means 1/8th wall. That cost $1.05 per foot if I went that route.
What do you guys think about stainless?? The mustang is stainless, sure polishes up nice.
We have a specialty stainless tubing shop a few miles away that specializes in bending/welding/selling this tubing cheap.