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Discussion Starter · #1 · (Edited)
Roll Cage Design 101

Roll Cage 101

I got inspired to write this article after seeing countless ill-conceived designs and poorly executed fabrications on the internet. This article is intended to be a guide to the basics of cage design. I don’t claim to know everything in the world about building cages and chassis, and I am not a metallurgist, but I am a CAD designer, fabricator and welder. I am always interested in learning more, so I welcome corrections and addendums to the following guidelines. I would like this thread to be a sticky, and I will edit this post, incorporating any good suggestions that are made.

Material:

There are three basic choices for material.

Mild steel tubing is made from sheet that is rolled and welded. The alloy is typically 1010 or higher. This material is not as strong as the others, but is totally acceptable with a proper design. It is even preferred by some for its tendency to bend before breaking.

DOM steel tubing is manufactured the same way as mild, including the welding. The alloy is typically 1018 up to 1026, the higher the number, the higher the carbon content. DOM means Drawn Over Mandrel, which “trues” the tube and hides the weld, giving it more accurate dimensions. DOM is about twice the cost of mild, and almost as much as 4130.

4130 chromoly steel tubing is a true seamless tube, with chromium and molybdenum added for strength and light weight. 4130 is very expensive and is used most often in big budget builds. It requires heat treating after welding to achieve maximum strength. Some say that if a 4130 chassis is not heat treated, it is no stronger than the other steels. 4130 suspension components should definitely be heat treated.

The size of tubing is determined by the weight of the vehicle, or the class it will be raced in, along with other factors. In general, it would be wise to use a minimum of 1.5 x .120 for lightweight vehicles like sand rails, 1.75 x .120 for mid sized vehicles like pre-runner trucks or Jeeps, and 2 x .120 for the heaviest vehicles like trophy trucks or huge 4WD buggies.

Bending the tubes:

The first rule of bending tubing is that no deformation is allowed. The bend must be smooth with no scoring or ovalizing of the tube. Do not use a pipe bender to bend tube, because the dies do not fit correctly. The HF “kinker” is infamous for a terrible bend on tubing.

Notching the tubes:

There are countless methods of notching a tube so it will fit tightly to another tube prior to welding. This is also called a fish mouth. The most common way to notch is with a hole saw, which is often done in a Tube Notcher tool. Cheap ones are available from HF, and high end models are available from several manufacturers. There are methods of notching on a mill or lathe, and there are also expensive dedicated machines for notching that use end mills or abrasive belts. A proper fitting and tight notch are extremely important for a strong weld joint.

Designing the cage:

There are many design rules that should be followed whenever possible.

The most important rule is to make triangles, not squares, and especially not artsy-curvy designs. Look at any structure in the world, whether it is a bridge, a crane, or a cantilevered sign, and you will see triangles everywhere. This principle should be applied as much as possible to a chassis or cage design. Every tube should be one leg of a triangle if possible. This is especially true of the primary structural tubes. The peripheral tubes like bumpers can be more “artistic”. It’s a good idea to build crumple zones into bumpers to absorb crash damage. It is better to crush a bumper than to damage the main chassis structure. A bolt on bumper makes repairs much easier than a welded on structure.

Bends are not your friends. Some bends are unavoidable in a design, but they should be minimized, not maximized. Even a perfect bend is weaker than a straight tube. Bends should never be mid-span, or unsupported. The apex of a bend should be a node point or junction for at least one other tube, and gusseted unless several tubes meet at the node. An example of a node is the center of an “X”.

It is advisable to gusset corners, especially when building a bare minimum cage. This can be done with triangular plates welded into the corners. A stronger method is to weld a 6-12” tube diagonally in the corner, similar to the letter A.

“T” junctions are called a dead tube junction, as one tube dead ends into another. This should be avoided whenever possible, because the dead end tube will bend the other one when the loads are along the dead tube.

“A” pillars should not be leaned back too far, unless a second A pillar is added to triangulate it. Otherwise it can collapse into the passenger compartment. The B pillar will be strongest when near vertical. It is always safer to double up on the A and B pillar on heavier vehicles. All cages benefit from a vertical tube in the windshield area. An inverted “V” like this /\ is even stronger.

The B hoop should have an “X” built into it, or at the very least a diagonal or a V. If the A and B hoops are inverted U shapes, the “spreader” tubes that go between them should intersect the apex of the bends for greatest strength, and they should be straight. The roof area should have a V or X built into it, depending on overall design. The B hoop needs to have rearward supports, typically at a downward 45 degree angle. If the B hoop does not have an X, then these tubes definitely should.

No tube should EVER terminate into sheet metal, like a floor or firewall. The A and B pillars should pass through the floor and weld solidly to the frame rails or tubes. If necessary, it is acceptable to weld a plate to each tube, above and below the sheet metal, and use four bolts to connect them together.

Most exo cages seem to be designed to protect sheet metal more than the vehicle occupants. Which do you value more? If an exo cage is a must for you, try to incorporate as many of these design features as you can, especially in the cab area.

Some say that any cage is better than no cage. This may be true in some cases, like flopping over at zero mph. But what if a failed hill climb attempt results in looping out and a triple endo down the hill? What if an off camber slip results in six barrel rolls down a rocky slope? What if one of these tumbles comes to a sudden halt against a large tree or rock? It is in these worst case scenarios when you need a properly designed cage and chassis, fully triangulated, and expertly welded. A marginal cage could collapse, doing more harm than good.

If you are not 100% sure that you can make a strong high quality weld, leave it to a professional. In general, TIG welding is considered superior to MIG welding, but a proper MIG weld is completely acceptable and just as strong. Tube splices and repairs should always be sleeved for strength and rosette welded, never butt welded.

The more you integrate these “rules” and suggestions into your cage, the stronger and safer it will be.
 

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Looks like you hit it pretty good. I am almost finished with the cage in my jeep. It is atleast driveable again. Now I still need to gusset and triangulate a few areas to improve the strength. Good article.
 

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Scott F said:
Roll Cage 101

No tube should EVER terminate into sheet metal, like a floor or firewall. The A and B pillars should pass through the floor and weld solidly to the frame rails or tubes. If necessary, it is acceptable to weld a plate to each tube, above and below the sheet metal, and use four bolts to connect them together.
This is the only section I have an issue with.

If part of a cage is to pass through the body and be welded to the frame, then all of the cage down bars should do the same thing. Otherwise there should be some sort of isolation between the frame and cage to allow each part to flex as a whole. Allowing part of the cage to be mounted to the body which has rubber bushings between it and the frame and part to be welded with no allowance for flex will end up fatiguing the frame where it is welded, and ripping the body where it is mounted.

There needs to be a systematic approach to how things are mounted.

Welding all points to the frame is best for a comp rig which will see multiple rolls each time it goes out, but a trail rig which see's road time, would best be served by a cage mounted to the frame with some allowance for flex to keep from fatiguing both the frame and body.

Mine uses large sandwich plates to attach to the body, and bushings at each frame attachment point.

Your thoughts?
 

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Discussion Starter · #4 ·
I agree with you for the most part. Perhaps I should have elaborated more. It is best for a cage to be welded directly to the OEM frame rails, whether there is a sheet metal body or not. If the cab is rubber mounted, then the tubes should pass through to the frame, with no mechanical connection to the sheet metal. If the cab is solid mounted, then additional strength can be gained by welding or bolting to the cab. Sheet metal should be welded to the cage, not vice versa.
 

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That is a matter of opinion.

I think most full bodied trail rigs are fine with a cage attached to the body. Welding the cage striaght to the frame is a surefire recipie to fatigure both the cage, and the frame.

I don't think there's any question that a frame-tied cage can be stronger, but there are definite disadvantages to that design if not executed correctly.
 

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My preference for mounting a cage on a full bodied truck is to mount the plates over the body mounts. I used All-Pro's cage on my x-cab and created larger mounting plates that spread the load over the front and rear body mounts. I think this gives a bit of the best of both worlds. Just an opinion from a web-wheeler.
 

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Scott.

I'll agree that welding the cage directly to the frame is best for a strictly off road rig. One that see's road time in my opinion should have some sort of isolation to allow for flex and vibration. Basing this on total lifetime since a trail rig will be driven alot less than a street legal rig. All factory frames flex, no way around it. Welding the cage to a factory frame is going to stiffen it at the junctures but will still be stressed by it trying to flex, not to mention vibration over time. These stresses on the HAZ of the welded points will lead to eventual cracking/failure. This is in addition to multiplying stresses on a frame since the flexing will be limited to a shorter section of the frame rails and the forces acting upon it are going to be the same.

I'm no engineer, so if I'm completely off track here, show me how these things work out.
 

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Discussion Starter · #8 ·
Thanks, I was going to add this. If the tubes terminate at the body mounts, it mostly eliminates the sheetmetal tearing issue. This is a quite acceptable solution for those who don't want to weld straight to the frame, but want most of the strength benefit.


rockota said:
My preference for mounting a cage on a full bodied truck is to mount the plates over the body mounts. I used All-Pro's cage on my x-cab and created larger mounting plates that spread the load over the front and rear body mounts. I think this gives a bit of the best of both worlds. Just an opinion from a web-wheeler.
 

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Discussion Starter · #9 ·
My mindset when writing the article was more towards a full time off road or competition vehicle, particularly a full tube chassis. I agree that if an OEM frame only had a 4 or 6 point cage welded to it, it would increase the flex in the unsupported frame rail sections. But many cages extend forward into an engine cage, and rearward into a bed cage. These portions are typically welded straight to the frame. In that case, I would say that the cab cage should also go straight to the frame. I should add that boxing in a C section OEM frame rail adds a lot of strength.

kwrangln said:
Scott.

I'll agree that welding the cage directly to the frame is best for a strictly off road rig. One that see's road time in my opinion should have some sort of isolation to allow for flex and vibration. Basing this on total lifetime since a trail rig will be driven alot less than a street legal rig. All factory frames flex, no way around it. Welding the cage to a factory frame is going to stiffen it at the junctures but will still be stressed by it trying to flex, not to mention vibration over time. These stresses on the HAZ of the welded points will lead to eventual cracking/failure. This is in addition to multiplying stresses on a frame since the flexing will be limited to a shorter section of the frame rails and the forces acting upon it are going to be the same.

I'm no engineer, so if I'm completely off track here, show me how these things work out.
 

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No prob.

My only concern in making the distinction between DD and comp rig was having JoeTJ reading this, welding his cage to the frame and thinking he was good to go after putting 50K miles on his junk with no reguards to fatigue. As is often the case, the weekend warrior and the pro have different needs and considerations to take into account. Might want to ammend the title to make that distinction clear.

Good write up.
 

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Also, not an engineer, but a former racer of lightweight road things.

In formula racing, for the purpose of safety, you will often see a formula car "dissassemble" itself during a wreck. The extraneous weight of the tires/suspension, engine, etc. will seperate after a major impact. The reason is to lose weight and therefore momentum. The driver "capsule" should remain intact to protect its occupant.

Translating this to 4wheeling, might it not be safer for the cage (with seats and harnesses attached to it) to seperate from the frame? That would allow the occupant capsule to slow sooner and get away from the heavy parts of the rig. It would also make the cage stronger, since during a rollover it wouldn't have to support the weight of the heavy stuff on the bottom.

I could be pissing up a rope here, and this would NOT be for rigs that have the belts and seats tied to the body, and the cage tied to the frame. Also, the cage would still be tied to the frame, but with a failure point that would allow the cage to remain intact if it seperated from the frame. I guess I would look at it as a fuse.

Looking back over the metamorphisis that rockcrawling and 4wheeling have gone through, I am seeing more different types of technology. Ten years ago, people built stuff with no regard for weight, just stength. Now, proper building involves balancing wweight with strength, as well as clearance balancing with CoG. Maybe cage design should start reflecting standards and practices of other motorsports.
 

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I just want to eloborate on the details of 4130 ChroMo.
You said "chromium and molybdenum added for strength and light weight".
To be a little more specific, these are added for strength, yes.
Also they make a material that is easier to control for secondary processes, such as heat treating or annealing.
The lighter weight is not contributed by proxy of the Chr or Moly but by the fact you can run thinner wall materials and acheive the same strength as a heavier wall thinckness mild steel tube.
After welding ChrMo as in cages or links, it's important for the material to be drawn down (annealed) before heat treating.
The welding process will make the welded joint brittle, much little stainless steel. Annealing will draw the temper down to an even scale and heat treating will make it hard evenly at a desired scale.

I fully welded CroMo cage may be a bit scary with brittle joints on tall granite slabs. But they could be drawn down at home with a rosebud and temp stick.
 

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In this months DIRTsports magazine has a great article on understanding tubing for cage building, after reading the article you would probably feel chromoly is the was to go.
As for formula car theory, it mostly applies to later model monocoque designs, and not as much for old tube style cars as far as energy dissipation goes.
Having the seat move independently of the seat belt mounts is defenatly a problem, that’s why many late model cars have the belts mounted to the seats.
 

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Discussion Starter · #17 ·
I was alluding to the Indy tub concept above when I talked about incorporating crumple zones, and I thought about what you are saying below. After further consideration, I think that we do not need breakaway components. The reason is that we do not crash into walls at 200 mph, so we don't need to absorb that much energy. Even high speed desert race cars don't usually have those type of sudden impacts. Our deceleration forces are a fraction of those Indy and F1 crashes. I believe the main benefit of the Indy cars breaking apart in a crash is not mass reduction, which may be a secondary benefit, but it is slowing down the impact event, thus reducing the G forces that the driver is subjected to. The remaining tub is what keeps the driver in one piece.

As for your last sentence, I consider desert race cars like Class 1 and Trophy Trucks to be the pinnacle of chassis and cage design in the off road world, and they are the yardstick by which I measure all designs. One area they lack in are non-structural crumple zones. This could be because they would add weight or complexity, but I think some sacrificial tubing in the front and rear, and also the sides, would be beneficial to any off road vehicle.

JeeponRock said:
Also, not an engineer, but a former racer of lightweight road things.

In formula racing, for the purpose of safety, you will often see a formula car "dissassemble" itself during a wreck. The extraneous weight of the tires/suspension, engine, etc. will seperate after a major impact. The reason is to lose weight and therefore momentum. The driver "capsule" should remain intact to protect its occupant.

Translating this to 4wheeling, might it not be safer for the cage (with seats and harnesses attached to it) to seperate from the frame? That would allow the occupant capsule to slow sooner and get away from the heavy parts of the rig. It would also make the cage stronger, since during a rollover it wouldn't have to support the weight of the heavy stuff on the bottom.

I could be pissing up a rope here, and this would NOT be for rigs that have the belts and seats tied to the body, and the cage tied to the frame. Also, the cage would still be tied to the frame, but with a failure point that would allow the cage to remain intact if it seperated from the frame. I guess I would look at it as a fuse.

Looking back over the metamorphisis that rockcrawling and 4wheeling have gone through, I am seeing more different types of technology. Ten years ago, people built stuff with no regard for weight, just stength. Now, proper building involves balancing wweight with strength, as well as clearance balancing with CoG. Maybe cage design should start reflecting standards and practices of other motorsports.
 

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Discussion Starter · #18 ·
Thanks for the clarification. Perhaps I should have said added for greater strength, resulting in lighter weight through the use of thinner wall tubes of equal strength. I thought the mass per cubic inch of 4130 was a little less than mild steel.

I agree with your thoughts on working with 4130, but I doubt many people use those techniques. Some say that if you were to anneal a 4130 chassis, it would be no stronger than mild steel. I have yet to hear someone say that they did this, then heat treated an entire chassis.

JeepinDoug said:
I just want to eloborate on the details of 4130 ChroMo.
You said "chromium and molybdenum added for strength and light weight".
To be a little more specific, these are added for strength, yes.
Also they make a material that is easier to control for secondary processes, such as heat treating or annealing.
The lighter weight is not contributed by proxy of the Chr or Moly but by the fact you can run thinner wall materials and acheive the same strength as a heavier wall thinckness mild steel tube.
After welding ChrMo as in cages or links, it's important for the material to be drawn down (annealed) before heat treating.
The welding process will make the welded joint brittle, much little stainless steel. Annealing will draw the temper down to an even scale and heat treating will make it hard evenly at a desired scale.

I fully welded CroMo cage may be a bit scary with brittle joints on tall granite slabs. But they could be drawn down at home with a rosebud and temp stick.
 

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Good read.... I have to disagree with a tiny part of it.... the outside radius of a bend has a stronger impact point than straight tube... So if you back flip onto a rock and impact it at the bend it doesnt dent the tube as much or deform it as much and in say a spreader bar. I rear ended a .120 hrew tube bed with my .120 wall hrew bends on my front clip and crushed his straight tube in and dented it... straight on no funky angles... my 120 degree bends have chipped paint on them.. so any way straight is not always stronger... depends on the super crashes moves you pull.. ;) there is no perfect cage...
Mike=
 
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