Richard greets all curious and passionate cyclists – technicians. Today, I dare to start from scratch.
At the end of the summer of 2022, when we showed RoxoRR to the world, the most common questions about it were: “Is it made of rebar (in Czech „roxor“)? Will it withstand riding? Will it not bend or crack? How can it ride? Reinforcing steel is made of „mud“, after all.” Surprisingly, RoxoRR not only still holds up but also rides well! Why?
Let’s try to discuss it a bit scientifically in the following paragraphs. We will find that we will encounter the same questions as in the case of any other frame. For example, is it really the material that affects the frame’s properties the most? Is frame stiffness the most important factor? Is steel really as comfortable as it is often claimed? How much does the diameter and profile of the tubes matter? Is a round tube or square beam better? What about wall thickness? Can the size of the frame or the sloping also plays a role?”
The topic is too extensive for one article to cover, so we will divide it instead. What you will read is the result of observations, tests, and searching for relevant sources, preferably scientific papers that are not burdened from the beginning by the author’s subjective belief that a stiffer frame is better and titanium is more comfortable than carbon composite. To avoid confusion – for example, frame tests are regularly published in dozens of media outlets. However, evaluating a frame that I know is made of “High-Mod-Pro” carbon (paraphrasing), which is 17% stiffer than last year, puts a lot of pressure on testers to filter out the placebo effect. And despite all efforts, it sometimes fails. It is not the fault of the testers, human psychology simply works this way. Moreover, it should be about objective evaluation (i.e., based on measurements) in real-world use because none of us ride in a laboratory. A laboratory-measured value means nothing until its impact on everyday use is proven. Simply put, the well-known saying, “the more stripes, the more Adidas!” does not apply here. Want an example? In a discussion about frame stiffness, my friend mentioned a certain Italian brand known for its very stiff frames. I countered that, according to measurements, these frames are relatively (compared to the competition) not very stiff. How is this possible? It turns out that these frames have the stiffest seatposts! This illustrates how incorrect impressions can arise. Conversely, it is a lesson that the touted stiffness of the frame is not actually essential because the bikes of that brand are undoubtedly not bad.
Alright, let’s start with what RoxoRR is actually made of. And we will also talk about what other steel frames are built of. But first, we need to clarify two basic material characteristics that we will work with – strength and elasticity. Strength is generally the ability of a material to resist being split into two parts (fracture, crack), in other words, to resist forces that break the cohesion of the material. Elasticity refers to the ability of a material to regain its original dimensions and shape when the applied load is removed.
If we load the material from which the frame is made (for example, by bending a steel tube), the applied force will create internal stress in the tube material. The magnitude of the stress depends on the size of the force and the amount of material in the tube (i.e. its cross-section). The material of the tube will strain (stretch on one side of the bend and compress on the other). As we increase the force, the stress will increase (it should be noted for accuracy that this is usually the case, but we will not complicate the explanation with that). If the stress does not exceed a certain value (called the elastic limit) during loading, the tube will still return to its original shape and dimension after the force is removed and will not be permanently deformed. This is called elastic deformation. If the elastic limit is exceeded during loading, permanent, plastic deformation will occur.
The stress distribution in the bent steel rod is shown in the image (yellow). It can be observed that the highest values of stress are reached in the areas farthest from the so-called neutral axis – that is, the imaginary axis on which there is no change in length during bending (the material neither contracts nor elongates).
Bending of steel rod
The stress level in a material, at which the material begins to deform permanently (meaning that a tube, for example, will remain bent even after the force is removed), varies for each alloy (including steel, aluminium, titanium, and other metals). This value is referred to as the elastic limit (yes, it can be a bit confusing when discussing the elastic limit as a strength characteristic, but that’s just the terminology). It is expressed in Pascals (just like pressure). In practice, for metals, the characteristic that is measured and reported as a material property is the yield point instead of the elastic limit, which although not exactly the same, can be considered equivalent for our purposes. For metals, exceeding the yield point of the material results in permanent deformation of the structure and therefore its failure. Nobody wants to ride a bike with a bent frame. That’s why the yield point is the most important value of interest! It is not the ultimate strength, which is the highest stress that the material can withstand before failure (cracking or fracturing).
The properties of metal alloys are usually determined by a tensile test using a universal testing machine. The result is a curve that shows the relationship between stress and strain in the material – the machine measures the force required to stretch the material to a certain length. The stress in the material is then calculated from the cross-sectional area of the sample and the applied force.
In the image below, the vertical axis represents the value of stress in the material, while the horizontal axis represents the value of strain, which is the elongation of the sample. We can see that the value of “E”, which represents the elastic limit, and the value of “K”, which represents the yield point, are close to each other and are therefore often used interchangeably in practice. The ultimate strength is marked as point “P”.
It is evident that the frame should not undergo permanent deformation during normal operating conditions. Therefore, it must be designed and manufactured in such a way that the stress on the material does not exceed the yield strength. And just to clarify (because nothing is black and white), even if the material of the frame as a whole deforms only elastically, certain areas where plastic deformation occurs at the microscopic level can be found. These areas are often referred to as stress concentrators. Typically, these are joints, bends, or holes. These can then become a source of fatigue cracks in the future, which are the most common type of failure in steel, aluminium, or titanium frames.
The first metal used for making frames was steel – an alloy of iron with carbon and other elements that determine its properties. These properties are also significantly influenced by the mechanical and thermal treatment of the tubes – such as rolling, annealing, quenching, etc. Therefore, two tubes made of the same steel alloy may have significantly different mechanical parameters. However, with the quality and processing of the material also comes an increase in its cost, so the “best” steel tubes are also expensive. As we will show below, however, their use in a frame is definitely worth it.
Czech technical standards divide steels into nine classes. For high-quality frames is used at least ČSN 15 130 steel, which is the equivalent of the European designation 25CrMo4 or the American AISI 4130. If you are interested in framebuilding, you have certainly encountered one of them. This steel already has very good mechanical properties and is also used in the construction of ultralights, racing buggies, etc. Its yield strength begins at 400 MPa (i.e., 400,000,000 Pa) and can reach up to 800 MPa (depending on further treatment). It also has good vibration damping compared to other steels, which is important for bicycle frames. However, cheaper steel frames or scooters may use steel with poorer properties that is cheaper. The manufacturer then replaces the lower strength material with thicker tube walls, but this increases the weight of the frame. For comparison, construction steels used in many things around us have a yield strength of 235 to 460 MPa.
In Europe, there are three manufacturers producing tubes specifically for bicycle frames: British Reynolds, and Italian companies Columbus and Dedacciai. They use “regular” 25CrMo4 steel for their basic tube ranges, and have their own alloys with high or very high strength for their more expensive tubes. The strongest “framebuilding” steel on the market, Reynolds 953, has a yield strength of at least 1450 MPa, which is three times that of steel commonly used in industry! To illustrate the importance of heat treatment of the material, we can take the example of two tube ranges, Reynolds 853 and 631, which use identical steel. However, due to different heat treatments, the 853 tubes are 53% stronger (yield strength of 1000 MPa vs. 650 MPa).
Comparison of the yield strength of Reynolds and Columbus steel
The benefit of using a stronger material is evident at first glance – it is sufficient to achieve the necessary strength with less material, resulting in a lighter frame. As an example, let’s take a steel tube with a yield point of 400 MPa, an outer diameter of 31.8 mm, and a wall thickness of 1 mm. If we were to manufacture it from a steel with a yield point of 1000 MPa, we would only need a wall thickness of 0.4 mm to achieve the same strength. Such a tube would then be 2.5 times lighter.
However, it is not possible to thin out the material along the entire length of a tube because more material is required at the ends where the tubes will be joined. This is due to both the areas of stress concentration and the heating of the material during welding or brazing, which can also reduce its strength (although occasionally it can have the opposite effect). Additionally, welding such thin tubes is extremely difficult! Therefore, the main frame tubes have a wall thickness of 0.4 mm to 0.6 mm, with thicker walls (0.6 mm to 0.9 mm) at the ends. This is known as “butting,” and it creates double-butted tubes. If the thickening is only at one end of the tube, it is called single butting. Manufacturers no longer use triple butting today. Nevertheless, this means a significant weight saving thanks to stronger steel because, for example, on a tube with a diameter of 31.7 mm and a wall thickness of 0.8 mm at the ends, thinning the wall to 0.5 mm in the middle saves 50-60 grams, which is not insignificant. On the entire front triangle, this could mean a weight saving of up to 150g. High-strength steels also show better resistance to so-called cyclic loading (alternating stress from tension and compression or bending), which is precisely the type of load encountered when riding a bike. Moreover, the best steels from Reynolds and Columbus are also corrosion-resistant, which means a unique look for the frame made of pure metal and even weight savings because the frame does not need to be painted.
Cross-section of a double-butted tube
But a stronger material doesn’t just mean lower weight, but also higher elasticity, which is the material’s ability to deform and return to its original shape after the load has been removed. The “ease” with which a material can bend is described by an attribute known as the modulus of elasticity (not to be confused with the elastic limit, which indicates the material’s strength as we discussed earlier). It is a material attribute that is generally defined as the relative resistance that a material opposes to deformation. With increasing modulus of elasticity, this resistance increases, and thus the force required to overcome it also increases. Simply put, you need to exert more force to bend such a tube – the tube is therefore stiffer. It is again expressed in pascals, or in practice in gigapascals (GPa).
The modulus of elasticity of steel under tension during static or ‘very slow’ loading (the modulus of elasticity of metallic materials is typically determined by a static tensile test) reaches some of the highest values among all known alloys, averaging 205 GPa. Its value is relatively constant for most types of steel, regardless of their strength (yield point, ultimate tensile strength, etc.), and ranges within ± 15 GPa from the stated mean value of 205 GPa. This means that a stronger steel is equally as flexible as a less strong steel. An exception is certain high-alloy steels, where the modulus of elasticity can drop below 140 GPa. However, these steels are not used in cycling. On the other hand, the modulus of elasticity is almost independent of the heat treatment of steel.
In the graph below (again using a plot from a tensile test), we can observe the graphical representation of the modulus of elasticity in the elastic deformation region (represented by the blue line). The steeper the line (meaning the higher the angle between the line and the x-axis of the graph), the stiffer the material, as a greater force is required to produce a given elongation of the sample.
If we can use less material for manufacturing a tube by using a stronger steel, its elasticity increases proportionally with the decrease in material. Therefore, it makes sense to invest in better material for constructing a steel frame as it brings lower weight and higher elasticity (thus providing greater comfort). We will discuss in one of the future instalments of our series what elasticity exactly means for riding. Alternatively, we can state it the other way around: if we have two identical steel tubes differing only in the strength of the material, they will bend just as “easily” (we need to exert the same force for the same deformation). However, we can bend the tube made of stronger steel more without it permanently deforming because it can withstand higher stress. Consequently, a frame made from it can “work” more.
As mentioned earlier, the mentioned values of the modulus of elasticity apply to cases where the applied force remains constant or changes slowly. However, that is not the case for how the frame is actually subjected to loads. After the initial loading caused by the rider’s weight, the loading becomes dynamic (pedaling, impacts from the terrain, etc.). In these cases, the dynamic modulus of elasticity of steel is 20-30% higher than the value obtained from a static tensile test. Therefore, the frame material behaves as if it were stiffer. On the other hand, if the tube has already been plastically deformed, even by a very small amount (e.g., 1%), there is a significant reduction in the modulus of elasticity, possibly by tens of percent. This applies not only to a frame that has been crashed but also to cases where the tube is intentionally bent to achieve a specific frame design – such as enhancing comfort, or tailoring the aesthetics of the bike. Bending also affects the wall thickness and, to some extent, the cross-sectional area of the tube. A bent tube is then “softer,” meaning it has a lower modulus of elasticity than a tube in its original shape. Consequently, it can be bent more easily. This flexibility can contribute to a smoother ride experience, absorbing vibrations and impacts from the road or trail.
The additional bent seat stays from the Columbus Cento tube set
The RoxoRR frame is made of reinforcing steel, which is commonly used in the Czech Republic and has a minimum yield strength of 420 megapascals. In this regard, it is certainly not weaker than the steel frames that many of us are familiar with. However, compared to a “regular” steel frame weighing around 2 kg, the RoxoRR frame does have significantly more material, weighing over 5 kg. By considering this simplified reasoning, we can conclude that the RoxoRR frame will not be less sturdy than a “typical” frame. Its material is not inferior, and there is plenty of it, contributing to its robustness.
However, we must realize that material is only half of the “success” (i.e., frame construction). And let’s also state upfront that it’s the “lesser half.” The other half consists of the cross-sections of the beams (typically tubes) from which the frame is made. And also the construction, if you will, the “shape” of the frame. Their influence is ultimately much greater than the material from which it is made. Why this is the case, and how RoxxoRR stands in this regard, we will discuss in the next installment.