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Cromoly

Motorsport Fabrication Fundamentals

Relevant Module: Materials > Mild Steel

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Discussion and questions related to the course Motorsport Fabrication Fundamentals

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Dear Team,

SO i dont understand if there is more carbon in cromoly which also mean it has less flex but it is also harder to break, then why is joint need to have gassets and what materials will gasset need to use? Mild Steels? to inprove flex? so it doesnt just BREAK apart?

best,

chris

Gussets add strength and rigidity. They do this by distributing the specific load or stresses over a larger area.

If a roll cage is also tied into the pillars of a car with gussets it provides extra rigidity and strength than just being attached to the floor.

As for material selection this gets harder, as welding different types of steels together has its own set of problems.

chromoly plate is what I would use but it all depends on what you're trying to attach it too. Trying to weld chromoly to an aluminum frame is not going to work due to their differences. So, you will need to look at what filler rods you can use to suit the application. There are a few tig rods you can use for joining mild to chromoly, ER80S-D2 being one of them.

Great stuff Jeb.

Christopher, like Jeb said, using high quality material doesn't necessarily avoid the need for, or advantage, in gusseting.

A lot of motorsport authorities will also require that for a cage to be certified it has to have gussets of a certain dimension in specified locations, these will have to be homogeneous with the material used in the rest of the cage so that they have the same behavior under load.

Motorsport Australia Safety Cage Requirements

It's not so much the "area" as the "leverage" on the structure's joints.

If you have, say, a 500mm (18") long tube welded to another, fixed, tube , if you apply a side force of 500N (110 lbs) then that's going to be a bending moment (bending torque) of roughly 250N.m (165 lb.ft) at the join. But if you use a 100mm (4"") gusset (or bracing tube) that brings the effective length of the tube to 400mm (16") at the new join, which reduces effective bending moment to 200N.m (~129 lb.ft), a 20% rduction.

If you consider the other end would also have a gusset, that may bring the effective tube length down to 300mm (12"), reducing the bending moments proportionally.

The other thing to consider is the intermediate section is also effectively much shorter, in the plane(s) where the gusset(s) are used, and will be therefore much more resistant to bending forces applied to it directly from the gusseted joints, or in compression.

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