1: Stainless steel is heavy. NOT TRUE for bicycles. This comes from the use of stainless in other industries. The fatigue strength of stainless relative to it’s ultimate tensile strength (UTS) is higher than non-stainless steel. This means that when you are building structures where the loads could be close to the fatigue strength you need to use a higher safety factor – meaning use more material, therefore heavier. Now, that assumes that you are comparing grades of stainless and non-stainless which have comparatively similar UTS. That is not the case with the materials used in bicycle tubes – Reynolds 953 and Columbus XCR have higher UTS than non-stainless grades. The other factor is that the load cases in a bicycle frame in normal use should be much lower than the fatigue limit, so the safety factor is not such an issue.
When it comes to comparing stainless steel and steel with titanium, then titanium is lighter (less dense) but what you actually need to think about is the strength to weight ratio and the stiffness to weight ratio and the moment of inertia of the tubing available – in which case stainless steel is more favourable (Owen has written another piece on that topic here and at some point I should write one explaining moment of inertia and why tube thicknesses and diameters are important)
So the actual statement should be “Structures made from stainless steel in some industries may be heavier but this is not the case in bicycle manufacture”
2: Titanium is stronger by a factor of (insert your favourite number). NOT True for bicycles. As with the previous discussion we have to consider grades of materials. Both materials are alloys and there are many grades of titanium and stainless steel, useful for different applications. (Titanium is an element but when we refer to titanium in bike making we are actually talking about an alloy of titanium and steel is an alloy of the element Iron, and Aluminium in bikes is an alloy of the element Aluminium)
Now, titanium alloys are generally stronger than common stainless alloys (304, 316, 316L), but the higher grades of stainless used in bike frames are stronger or in the same ball park as the higher grade of Titanium that is used (6al-4v), and are much stronger than the lower grade of titanium that is used (3al-2.5v). Even if this were not the case, we have to think about the application on the bike, and things like the strength to weight ratio, and there is a whole discussion around whether we are talking about the design of the tubes, or the design of the frame. So the actual statement should be “The grades of Stainless steel used in bicycle manufacture are at least as strong as, and may be stronger than the highest grade of titanium used.
3: Stainless steel is difficult to weld. TRUE but… Compared to normal steel there are some additional factors to welding stainless steel. The first is that the material is different, but being different does not equate to more difficult, and stainless is used in so many applications and welding is the primary method of joining so it shouldn’t be a limiting factor. I believe it’s also required to back purge the frame when welding in steel, so it’s more complicated, but difficult and complicated are not the same thing. These complications also apply to Titanium. What is more interesting is that in bicycle making stainless tubes are a lot thinner and thinner materials are more difficult to weld. Now in my opinion this is an issue for the person making the frame, their competence and confidence and this should not be something you should need to worry about if you are buying a bike. So it’s true but shouldn’t be a factor in your decision if you have confidence in your bike maker.
4: You can’t braze stainless steel. NOT TRUE – but I understand where this comes from. Stainless steel can not be brazed using brass/bronze as with non-stainless steel but it can be brazed with other brazing alloys, primarily silver alloys or nickel silver alloys. Nickel silver melts at a higher temperature than even brass, which is not desirable, and I’m not sure if it’s used or recommended for bike frame tubing. I use silver alloys, which melt at a much lower temperature, which is actually more desirable in any case. The disadvantage is that it’s very expensive. Another disadvantage is that the alloys behave differently to brass when molten, and are much harder to control – so it’s much more difficult. The UTS of the silver alloys and brass alloys are relatively similar and therefore result in joints of similar strength. So, it’s not true, but it’s more difficult and as with welding, it doesn’t need to be a factor if you have confidence in your bike maker.
(there is a whole other discussion to be had about the comparative difference between lugged construction, fillet brazing and TIG welding but in summary you can consider them to be technically equivalent if correctly executed.)
5. The ride quality… This is such a complex subject. The ride quality of any bike is dependant on the material, who it’s built for, whether it fits, the experience of the builder and hundreds of conscious and subconscious decisions made during the build, what components are used and how well they are assembled, the conditions of the roads, whether you like the bike or not, etc etc etc. If we just want to talk about the difference in ride quality between materials then we need to have a long discussion about spring rates, Hooke’s law and Young’s modulus and I think we’ll leave that for a future blog post.
What’s more important here is that your bike builder builds something that fits, and is designed as well as possible for how you are going to use it. If you pick a framebuilder who’s style you like or have some affinity with, and they have a good understanding of your requirements then you should get a ride quality that is perfect for you, irrespective of which material they use.
This article was written by Owen at the very great Donard Bikes. Owen builds wonderful bespoke frames from his workshop in Northern Island.