WARNING: This is going to be a bit nerdy.

People often ask what steel we use for our frames. The short version:
a custom‑manufactured Taiwanese air‑hardening steel, engineered specifically for Sour.

Zur Einordnung: Reynolds 631 has a tensile strength around 800–900 MPa, while 853 reaches up to roughly 1200 MPa thanks to heat treatment.

Our tubing sits almost exactly in between these values.
And because we control the alloy spec, the drawing, and the butting profiles, we tune it precisely to what our frames need — not what a catalog offers.

Designed for how Sour bikes are actually ridden

We don’t buy standard tubesets.
We design our own and have them produced exactly as we want them. That gives us total control over:

• wall thickness

• diameter

• butting transitions

• ductility vs. stiffness balance

• shaping and forming behavior

And because the base material is close in performance to 631/853, we get the ideal mix of strength, dent resistance, durability, and ride comfort — but optimized for ATB, MTB and rough real-world riding.

Why ductility matters more than pure stiffness

Steel frames are naturally stiff enough for today’s geometries.
What really counts off-road:

Ductility

A more ductile tube survives impacts better than an ultra-stiff, thin-walled one.
Reynolds 631, for example, is known for “tough, durable and comfortable frames particularly suitable for long distance riding, ATB and BMX” thanks to its 800–900 MPa cold‑worked strength and air‑hardening properties.

We tune our material specifically toward this kind of behavior.

Impact toughness

853’s strength increase in the heat‑affected zone gives it high dent resistance — our material mirrors this, without forcing us into standardized butting.

Controlled flex

Better tracking, better comfort, better traction.

Heat-treated where useful — not as a philosophy

Depending on model and tubeset:

• Non‑heat‑treated tubes where ductility and forming flexibility matter

• Heat‑treated tubes where we want stronger dent resistance or safely thinner walls

Unlike catalog systems, we apply this tube by tube, not “whole tubeset at once.”

About dents (let’s be real)

Steel dents when walls get too thin — especially at modern MTB diameters.
Reynolds notes that heat‑treated tubes can be drawn thin but require smart butting and reinforced zones to survive off‑road impacts.

Our approach:

• no ultralight “race” wall thicknesses

• reinforced butting in impact areas

• diameter and wall tuned per frame size

• durability > meaningless weight savings

A Sour frame should last, not impress on a scale.

The big advantage: custom butting + true size-specific tubing

This is where your material truly sets you apart.

Custom butting profiles

We define:

• thickness at each end

• mid‑section thickness

• transition lengths

• deflection & flex behavior

No catalogue compromises. Every tube fits its exact purpose.

Custom diameters & wall thicknesses

Not “the closest available” — but exactly what we need for a given model.

Real scale sizing (size-specific tubing)

Every frame size gets its own tubing spec:

• XS → thinner, smaller diameters for comfort

• XL → reinforced zones & larger diameters for equal ride feel

• No size rides too stiff or too soft

Most brands scale geometry.
We scale the tubes — and that makes all the difference.

TLDR:

Our custom Taiwanese steel:

• sits almost exactly between 800–900 MPa (631) and ~1200 MPa (853) in strength

• matches the air‑hardening behavior that makes those alloys so reliable

• gives us full freedom in butting, shape, thickness and forming

• is optimized for ductility, durability and real off‑road riding

• uses heat treatment only where it makes engineering sense

• avoids dent‑prone thin walls

• is size‑specific for consistent ride feel across all frame sizes

It isn’t “steel like Reynolds.”
It’s steel exactly the way we want it — engineered for Sour, not borrowed from a catalog.

That’s it! Thanks for reading!

PS: This is for our german speaking Followers. Our big boss Chris had an interview with Antritt at Detektor FM about 32” wheels. If you guys want to give it a listen: