Design of roll cage

Motoiq.com
Motoiq.com

In order to avoid from bending which is more worst then compression and tension, tubes should be arranged to form triangles with the major loads applied at the intersection of tubes.

In comparison to straight tubes, bent tubes have more chances to get buckled. It is seen bent tubes are better than butt welds (which should be avoided), but still not as good as node formation.

Add diagonals in roll cage if it is already made and is too flexible. Diagonals work best if it is connected to major load points ie spring suspension mount.

Add additional cross members to the chassis.

Some of the tubes are AISI 1018 , AISI 1020 , AISI 1022 , AISI 4130

For fire wall and other roll cage member, bent steel tubing is better than welded lengths of tubing. You don’t have to worry about heat stress to the tube.

Bent should be done accurately according to analysis else it will result into negative effect. Do not use pipe bender to bend the tubes because the dies do not fit correctly. HF “kinker” is infamous for terrible bend on tubing.

Harbor freight pipe bender
Harbor freight pipe bender

A proper fitting and tight notch are extremely important for strong weld joint. End mills and lathe are most common notcher machine. Some expensive machines are end mills and abrasive belts.

Even though nodes are the strongest point, too many nodes are not allowed at one point (3-5 tubes). Maximum tube node should be selected as shock mounting place it helps in proper distribution of load.

Shocks and lower A-arms are the most stressed segments of the car. In places where you can’t use tubes to form a node, you can try metal plate with flared holes that can be pretty rigid although round tubes offer material in all 3 dimensions for added strength.

The apex of bend should be node point or junction for at least one another tube, and gusseted unless several tubes meet at the node. Never leave bent tube unsupported.

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. when using plate as a gusset, never have sharp angles try to curve it because it tend not to crack as much in tension.

Different types of tube

Mild steel

Mild steel also known as carbon steel (Containing a maximum of 0.29% carbon ) or plain steel. Typically, it is stiff and strong. Carbon steels do rust easily, so they must be painted or primed. They are cheap so they are the normal choice for most fabrications. Mild Steel can be easily cut, drilled or welded to meet your requests, making it suitable. Mild steel tubing is typically made from sheet that is rolled and welded. The alloy is 1010 or higher. It is no strong as compare to others but it has tendency to bend before breaking.

DOM Steel tubing (Manufacturing process)

Drawn Over Mandrel Steel tube is manufactured in the same way as mild, including welding. The alloy is typically 1018 up to 1026, Higher the number, the higher the carbon content and stronger the steel. Dom is a process which hides the weld giving it more accurate dimensions, which also strengthens the tube through cold working. Its cost is double in comparison of mild.

4130 Chromoly

It is usually a true seamless tube, with chromium and molybdenum added for strength. It is lighter with thin wall but as strong as thick wall steel tube. It is expensive and needs heat treatment after welding to achieve maximum strength.

The chromium content is approximately 0.8-1.1%. The carbon content is nominally 0.30% and with this relatively low carbon content the alloy is excellent from the fusion carbon content the alloy is excellent from the fusion heat treatment. The actual breakdown of 4130 alloy steel is as follows:
Carbon 0.28 – 0.33
Chromium 0.8 – 1.1
Manganese 0.7 – 0.9
Molybdenum 0.15 – 0.25
Phosphorus 0.035 max
Silicon 0.15 – 0.35
Sulphur 0.04 max

4130 low alloy steel is used as structural steel in aircraft engine mounts and welded tubing.

It can be easily machined in the normalized and tempered condition, as machining becomes difficult in fully heat treated condition because of increased strength.

Formability is best in the annealed condition for which the ductility is very good.

4130 is a steel and as such is not corrosion resistant. In corrosive environment the alloy should be given a protective coating.

4130 alloy is noted for its weldability by all of the TIG.

Heating at 1600 °F followed by an oil quench will harden the 4130 alloy. For best results a normalizing pre-hardening heat treatment may be used at 1650 to 1700 °F followed by the 1600 °F soak and oil quench.

Forge at 2200 °F maximum down to 1750 °F.

Hot Working-4130 in the annealed condition has excellent ductility. Thus it is usually not necessary to do hot working to form parts. If hot working is needed it can be done in the range of 2000 °F to 1500 °F.

Cold Working
Cold working by conventional methods is readily accomplished on this alloy.

4130 (and most of the other low alloy steels) may be annealed at 1550 °F for a time long enough to allow through heating of the section size. It should then be cooled in the furnace at a rate of less than 50 °F per hour down to 900 °F, followed by air cooling from 900 °F.

Tempering is done to restore some of the ductility that may be lost after the hardening heat treatment and quench. Alloy 4130 is tempered at between 750 °F and 1050 °F, depending upon the strength level desired. The lower the tempering temperature the greater the strength.

The 4130 alloy is a through hardening alloy and should not be case hardened.

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

“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.

When tying the cage to the stock frame areas, try to spread the load by incorporating more surface welds, this can be achieved by using plate or some type of boxed gusset. Reason for this is so the tube is less likely to rip out and on “C” channel frames box in or run a tube between the “C”.

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.

Tubes should intersect at an angle 90 degree normally. Dimple dies plat is good for making plate more rigid. Displacing material from the center of mass improves stiffness, stiffening areas of large deflection will improve an overall part’s strength. So in effect you could have a lighter and stronger part overall if you use a dimple die correctly(Dimple die installation video). Cavitation is a good way to tie two plates together to strengthen the load laterally.

References-Ref 1Ref 2Ref 3MIG welding

Chromoly dealers link 

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