Chromoly vs mild steel for a space frame chassis
For the same tube size, chromoly (4130) and mild steel are equally stiff. Chromoly is much stronger, which only saves weight if you also use thinner walls, and it is harder to weld well. For most kit and track cars, mild steel is the sensible choice.
The key fact: stiffness is the same
Stiffness depends on the material's elastic modulus, and almost all steels sit at about 200 to 210 GPa. Swap a mild steel tube for a chromoly tube of the same size and the chassis is no stiffer. Torsional stiffness comes from the tube sizes and the triangulation, not the grade of steel.
Side by side
| Mild steel (e.g. CDS or ERW tube) | Chromoly (4130, normalised) | |
|---|---|---|
| Stiffness (elastic modulus) | about 205 GPa | about 205 GPa |
| Yield strength | roughly 250 to 350 MPa, depending on the tube | roughly 435 to 460 MPa |
| Density | 7.85 g/cm³ | 7.85 g/cm³ |
| Welding | MIG or TIG, forgiving | TIG preferred, clean prep and careful heat control |
| Cost and availability | Lower, easy to find | Higher, fewer suppliers |
Typical published values; check the mill certificate for the tube you actually buy.
When chromoly makes sense
- You are chasing weight and will use thinner walls to take advantage of the extra strength.
- Your rules allow or require it, and you have a welder experienced with it.
- Crash structures where strength, not stiffness, sets the tube size.
When mild steel is the better choice
- A first build, a kit car or a track car, where cost and easy repair matter.
- Where the design is limited by stiffness, which is most chassis: same stiffness, lower cost.
- Where your series specifies a particular steel (for example, many roll cage rules call for cold drawn seamless carbon steel with a minimum tensile strength).
Always check your series' current rules for the exact material and tube sizes required in roll cages and safety structures.
Spend on the tubes that matter
FrameScore shows which tubes add the most stiffness per kilogram, so you know where a bigger tube helps and where a lighter one will do.
Score a space frameTube sizes guide