First of all the stainless steel tubing used to make bike frames (Primarily Reynolds 921, Reynolds 953, Columbus XCR and KVA tubing) is very
different to “normal” stainless steel that some people may be familiar with for example 304 and 316/316L grades of steel. There are also different considerations when using it for a bike frame compared to when using it in construction and architectural projects. In this post I’ll focus on the use of bike grade stainless steels within a bike frame.
There are basically two material properties that are interesting when it comes to bike material selection, strength to density ratio, and stiffness to density ratio.
For simplicity think of strength as how hard it is to dent the frame and stiffness is how hard it is to bend it.
Now from a pure material physical properties perspective:
- Steel is the stiffest material.
- Reynolds 953 is the strongest material that bikes are made from (It’s actually used for armour plating!)
- Titanium had the highest strength to density ratio, but now Reynolds 953 is about the same.
- Carbon has the highest stiffness to density ratio.
- Stainless steel and normal steel (for example Reynolds 853 or Columbus spirit) have the same density and stiffness.
- Aluminium – who cares? (well OK, from a mass manufacturing point of view it’s interesting, but for custom bikes not so much)
With Titanium there are different grades used for bike making, primarily 6al-4v and the softer 3al-2.5v, and there are different grades of carbon fibre and aluminium.
The really clever bit is when you start making bike tubes from those materials. If the material is stronger, then it’s possible to make thinner tubes which won’t get damaged easily, therefore you get a lighter bike. If it’s stiffer then you’ll get better power transfer when you are pedalling. The framebuilder’s job is then to select the best tubing and materials to make a custom bike perfectly designed around the rider.
Taking all of that into account, it should be possible to make a stainless bike that is lighter than normal steel bikes, but retains that unique ride quality, and it should be as durable as Titanium, and stiffer than Titanium.
This article was written by Owen at the very great Donard Bikes. Owen builds wonderful bespoke frames from his workshop in Northern Island.
Check out Owen’s other article “Stainless Steel – Myths & Legends“