Steel, deliberately
At Sour, steel is not a stylistic choice.
It’s a design decision.
We use steel because it allows us to define frame behavior precisely — under load, over time, and across a wide range of riding conditions. Material choice, tube selection and heat treatment directly influence how a frame transfers forces, responds to repeated stress and ages through use.
Rather than relying on predefined tube sets or single‑solution approaches, we work with heat‑treated 4130 chromium‑molybdenum steel and apply it deliberately. Tube diameters, wall thicknesses and profiles are selected based on the demands of each frame, size and intended use.
Stiffness is introduced where structure and control require it. Compliance is allowed where it improves traction, comfort and predictability. Durability and fatigue resistance remain central throughout — not as marketing terms, but as engineering requirements.
Steel gives us the freedom to balance these factors without compromise.
That balance is the foundation of every Sour frame.
Tubing as structure, not inventory
We don’t treat tubing as a catalogue of upgraded options.
Every frame is built as a complete structure, with each tube chosen for the role it plays within the whole. Highly loaded areas prioritize stability and longevity. Less critical sections allow controlled flex where it benefits ride feel and control.
This approach ensures consistency across different riding styles, surfaces and time in use. It also allows each model — from gravel to trail to suspension platforms — to express its character without abandoning a shared material logic.
Steel remains our foundation because it rewards careful work, deliberate decisions and responsibility in construction.
HT‑41 Explained
Our approach to heat‑treated steel
HT‑41 is our internal designation for how we work with heat‑treated 4130 chromium‑molybdenum steel in frame construction.
The base alloy itself is well understood. What defines HT‑41 is not a proprietary material formula, but the way the steel is processed, selected and applied within a frame. Heat treatment, tube geometry and placement determine how the material behaves once built — stiffness under load, resistance to fatigue and long‑term reliability.
HT‑41 represents a system, not a single tube set.
Heat treatment and application
Heat treatment allows chromium‑molybdenum steel to achieve higher strength at lower wall thicknesses, while maintaining toughness and predictable behavior. This gives us flexibility in how we distribute material across a frame.
Rather than pursuing minimum weight in isolation, heat treatment is used to refine load paths and structural balance. Areas subject to repeated stress cycles are reinforced accordingly, while less critical sections remain lighter and more compliant.
The result is not a uniform stiffness profile, but a deliberate one.
No one‑size‑fits‑all tubing
HT‑41 does not describe a fixed list of tubes.
Tube selection varies by:
- model
- frame size
- intended use
Diameters, wall thicknesses and shaping are chosen to support the character of each frame.
A long‑distance gravel platform places different demands on structure than a short‑travel suspension bike — even if both are built from the same alloy. HT‑41 gives us the framework to respond to those differences without abandoning material consistency.
Consistency over spectacle
Our approach to heat‑treated steel
Our focus is not on pushing extremes.
HT‑41 prioritizes predictable ride behavior, durability under real use and consistency over time. Fatigue resistance, alignment stability and reliable performance matter more to us than isolated performance peaks.
Care in material application is perceptible in the finished frame — in how it tracks, how it responds under load and how it continues to feel after years of riding.
Why we name it
We use the term HT‑41 to make our material approach transparent.
It’s a way to describe the system we work within, the decisions we take responsibility for, and the standards we apply consistently across our frames. It’s not a claim of exclusivity — it’s a record of intent.
HT‑41 is how we apply steel, deliberately.
