Space frame triangulation: why every bay needs a diagonal
A space frame is stiff because its tubes form triangles. Every rectangular bay without a diagonal can fold like a parallelogram, and the frame then relies on tube bending and joint stiffness, which is far weaker.
Why triangles
A triangle is the only shape whose angles are fixed by the length of its sides. Load a triangulated frame and each tube is pushed or pulled along its length. A steel tube is many times stiffer in tension and compression than in bending, so a fully triangulated frame is stiff for very little weight.
Leave a bay open and the load has to go through the welded joints and the tubes bending. That is why one missing diagonal can cost a large share of the whole chassis stiffness.
Where diagonals matter most
In a twist test the load runs from the front suspension mounts to the rear ones. The bays on that path matter most:
- Front and rear bulkheads, where the suspension loads enter the frame.
- The floor and the sides, which carry the twist from one end to the other.
- The cockpit opening, the usual weak spot: the driver needs the space, so stiffness has to come from deep sides, a stiff transmission tunnel or a triangulated floor and firewall.
- The engine bay, often left open for access. A bolt-in diagonal keeps access and restores the load path.
Rules of thumb FrameScore checks
| Check | Rule of thumb | Why |
|---|---|---|
| Tubes meet at a node | Centre lines within about a quarter of the tube diameter | An offset turns an axial load into bending at the joint |
| Diagonal angle | Steeper than about 20° to 30° | A shallow diagonal carries very little of the shear |
| Unsupported tube length | Under about 35 × the tube diameter | Long, slender tubes buckle in compression |
| Gussets | Where a bay cannot take a diagonal | Stiffens the joint, but far less than a full triangle |
These thresholds are working rules of thumb drawn from FIA Appendix J Art. 253-8, AWS D1.1, AISC Design Guide 24 and Derek Seward's Race Car Design, to be tuned with your own experience.
Common mistakes
- Adding heavier tubes instead of a missing diagonal.
- Stiffening the end that is already stiff while the cockpit stays soft.
- Suspension mounts in the middle of a tube instead of at a node, which bends the tube every time the wheel moves.
- Diagonals that all run the same way, so the frame is stiff twisting one way and softer the other.
Triangulate your frame in the browser
Sketch the frame node by node and let the Frame Builder add diagonals to every open bay, then check stiffness and weight and export STEP for your CAD.
Open the Frame BuilderScore an existing frameRelated: Chassis torsional stiffness: what is good, and how to measure it