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Some Things About Our Steel

As you may know, we build our bikes from steel. We always have. We use steel for a lot of reasons that we won’t go into detail explaining here, but the crux of it is that steel offers an nice balance of design flexibility, ride quality, cost effectiveness, durability, repairability and environmental sensitivity. That’s why we call ours ‘Natch (slang for ‘naturally’), as in “Duh, why would you choose anything else?” Quite a few people have asked us over the years, “Why does Surly use generic, off-the-shelf tubing instead of the fancy brand name stuff?” The answer is that brand-name tubing is off-the-shelf tubing.

4130 cromoly is just one variety of steel. The number is a designation of the Society of Automotive Engineers (SAE) that indicates the metal’s components. Basic high-tensile steel is made of iron and a little carbon. By adjusting the amount of carbon and adding small but precise amounts of other metals different properties can be achieved. Adding chromium and molybdenum (cromo, get it?) yields a high strength-to-weight ratio. Funny car frames are famously made of 4130 steel for the same reason it’s chosen for bicycle frames: it results in a steel that can be formed thin and light but that remains strong and compliant (flexible but resistant to deformation and cracking). There are other things that can be done to the resulting material, like heat treating, a process in which the metal is brought to a specific temperature and cooled at a controlled rate to realign its atomic structure, thereby further strengthening it.

At Surly we have had long and fruitful relationships with our frame manufacturer and tubing suppliers. Through these relationships we have continuously worked to develop the right tubing for each frame we offer. We develop tubing not only for each model but also for the frame sizes within that model. Sometimes there is tubing available that suits our needs, sometimes what we want doesn’t exist. For example, our new Instigator 2.0 is comprised entirely of tubing specifically designed for this frame. The main triangle’s tubes are internally butted and some tubes are externally tapered, flared like a trumpet and ovalized to add stiffness and strength without adding weight. On a heavy-duty bruiser like the Instigator, it is a serious consideration to keep the weight down wherever possible without sacrificing strength. Our Long Haul Trucker’s tube diameters change across the size spectrum, and the placement and length of the tapers and butts within the tube vary from size to size. This is done to ensure that the right amount of material (no more, no less) is used in each welded joint. This way we can ensure a responsive ride while keeping the tube ends sufficiently thick to keep them from melting away during welding.

A frame is more than just its tubing, however. We work with our suppliers to develop the best way to build each frame, including determining the best weld sequence and choosing the best type of welding rod, tube mitering and fixturing for each weld. The chainstay yoke just behind the bottom bracket of our Krampus, ECR and Instigator 2.0 frames is formed by pouring molten steel into a mold. We collaborate with our suppliers on details here too, such as how it is poured, how the mold is manufactured and more. By paying close attention to all these details, from tube design and manufacture to welding, to heat treating and even finishing, we address important steps in producing not just bikes, but Surly bikes…durable bikes that deliver nuanced, lively performance appropriate to their intent. Name-brand tubing companies such as Reynolds, Columbus, True Temper and Dedacciai make good stuff. They offer frame builders a wide selection of tubes to choose from. But their tubing is not better, not in the sense of being the best option all the time. It’s nice to see their logo on a frame, but we wanted you to know that you’re not being shortchanged just because you see a Surly ‘Natch decal by the bottom bracket. In fact, we go above and beyond to exceed what any off-the-shelf tube can offer, specific tubing for the needs of specific models and sizes.

We just thought you’d like to know that.

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