Kit car chassis tube sizes: what to use and why

FrameScore guide · updated 23 October 2026

Most kit car and Locost style space frames are built from tube around 25 mm (1 inch) across with walls of about 1.6 mm (16 gauge). Going up in diameter adds far more bending stiffness per kilogram than going up in wall thickness.

Diameter beats wall thickness

A tube's resistance to bending rises steeply with its diameter, while its weight rises only in step with the steel used. The figures below are calculated for plain tube, one metre long:

Tube, mm	Weight, kg/m	Bending stiffness (second moment), mm⁴
25.4 × 1.6 round	0.94	8,500
25.4 × 2.0 round	1.15	10,100
31.8 × 1.6 round	1.19	17,300
38.1 × 1.6 round	1.44	30,600
25 × 25 × 1.6 square	1.18	about 13,700

Going from 25.4 × 1.6 to 25.4 × 2.0 adds 22% weight for 19% more bending stiffness. Going to 31.8 × 1.6 adds 27% weight and doubles it. Square tube figures assume sharp corners, so real tube is slightly lower.

But a triangulated frame mostly pulls and pushes

In a fully triangulated frame, most tubes carry load along their length, so their stiffness depends on cross-section area (roughly, weight), not diameter. Diameter still matters for three reasons:

Buckling. Long tubes in compression buckle; a larger diameter resists it.
Open bays. Wherever a bay can't be triangulated, such as the cockpit or engine bay, the tubes around it work in bending.
Joints and mounts. Suspension and engine mounts load tubes sideways; a larger tube spreads that load.

Round or square?

Square tube is easier to cut, jig and weld for a first build, and flat faces make brackets simple. Round tube gives more stiffness for its weight and is the usual choice for roll hoops and race cars, but needs fish-mouthed joints. Many kit cars use square tube for the base frame and round tube for hoops and braces.

Where to use bigger tubes

Main and front roll hoops: follow your series' rules for minimum size and material.
Around the cockpit and engine bay openings, where triangulation is compromised.
Main longitudinal rails carrying suspension and engine loads.

Elsewhere, a smaller or thinner tube in a well-triangulated bay usually saves weight without costing stiffness.

Find the tubes that earn their weight

FrameScore ranks every tube by how much stiffness it adds per kilogram, so you can upsize where it counts and slim down where it doesn't.

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