Roll Cage Design 101

Extracted from DezertRangers.com forums



I got inspired to write this article after seeing countless ill-conceived designs and poorly executed fabrications on the internet. Here in California we have a wealth of information and experience in cage design. Even kids who are hacking on their trucks in their mom’s garage with a 110V MIG can do a good job with the many fine examples in this part of the country. This information is directed more towards those who are new to off roading and have no clue, and the four wheelers who haven’t seen desert racing technology, and don’t know any better.

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. The one area they could improve in are non-structural crumple zones. This could be because they would add weight or complexity, but I think some sacrificial tubing or energy absorbing bumpers in the front and rear, and also the sides, would help reduce injuries in endos and other severe crashes.

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 please post up with any corrections and additions to the following guidelines.

Material:

There are three basic choices for material.

Mild steel tubing is typically made from sheet that is rolled and welded. The alloy is 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 usually 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 and the stronger the steel. DOM means Drawn Over Mandrel. It is a process, not an alloy or type of steel. The DOM process “trues” the tube and hides the weld, giving it more accurate dimensions, which also strengthens the tube through cold working. DOM is about twice the cost of mild, and almost as much as 4130. DOM is considered the best choice for most builders, since it is the strongest mild steel option, and it does not require the expertise of 4130 methods.

4130 chromoly steel tubing is usually a true seamless tube, with chromium and molybdenum added for strength. This allows for a lighter design, with a thinner wall as strong as a thicker wall mild steel tube. 4130 is very expensive and is used most often in big budget builds. It requires heat treating after welding to achieve maximum strength. 4130 suspension components should definitely be heat treated for greatest strength and benefit.

The size of tubing to be used 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.

To learn more about tubing specifications, visit http://www.astm.org/.

A few more facts:

CREW = Cold Rolled Electric Welded
HREW = Hot Rolled Electric Welded
ERW = Electric Resistance Welded
CDS = Cold Drawn Seamless
DOM = Drawn Over Mandrel
HFS = Hot Finished Seamless
CDBW = Cold Drawn Butt Welded (Continuous)
W&D = Welded and Drawn

11 GA = .120 12 GA = .105 13 GA = .090 14 GA = .075
16 GA = .060 18 GA = .048 20 GA = .035

1-3/4" x .095 wall is 1.59 lbs/ft
1-3/4" x .120 wall is 2.09 lbs/ft
Steel = .2843 lb/cu in Aluminum = .0975 lb/cu in

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.

On vehicles with sheet metal bodies or cabs, the A and B pillars should pass through the floor and weld solidly to the frame rails or tubes. No tube should ever terminate like a “T” into sheet metal, such as a floor or firewall. If necessary, it is acceptable to weld a plate to each tube, on each side of the sheet metal, and use four bolts to connect them together, but only if the cab is solid mounted to the frame. Otherwise the sheet metal can tear when the cab flexes on rubber mounts. The least desirable arrangement is to keep the rubber mounts, and tie the cage into the mounts.

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 just butt welded.

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