Crafting Titanium MTB Pedals - The Designer's Journey: The Beginnings

Throughout my engineering career, I've had many opportunities to design some amazing creations, from motorsport parts, experimental go-kart engines, heavy semitrailers, and recently large agricultural equipment. But titanium is one material on my bucket list that has always managed to avoid my grip.

Rewind to 2020, I jumped onto a mountain bike after a long hiatus. Australia was on the COVID journey and exercising was one of the few reasons anyone was allowed out of the house, albeit, alone, so mountain biking became more a mental health outlet for me, more than a physical one. So on one of the many rides, talking to myself about all things life, I somehow got onto the topic of titanium and MTBs.

There is a reason titanium isn't a material most engineers are going to get the opportunity to design something useful with. Its applications are limited to areas where cost is the least of concern and performance or one of its unique attributes is paramount. Even then, if you happen to be John Hammond and can spare no expense, there are challenging material properties that don't always make it the super-metal that it is often portrayed as.

Credit: World Atlas

Is it just bloody rare? Is that why it is so difficult and expensive? Well not really, it is actually the 9th most abundant element in the Earth's crust, if you exclude non-metals, it is the 7th most abundant (let's skip over Silicon's metalloid status). What makes titanium usually cost-prohibitive is it requires an enormous amount of energy to refine compared to other metals, needing a process called "Kroll". Titanium isn't naturally found in its metal form, this is not uncommon for metals, they are often found as oxides in the Earth's crust and titanium is no different. Titanium oxide is often found in black sands called Rutile or an iron/titanium mineral called ilmenite, weirdly pure titanium oxide is white and frequently used as white paint pigment and food coloring, go figure. What makes titanium oxide special is it is incredibly stubborn, it doesn't want to let go of its oxygen atom.

Rutile-rich sand. Credit: James St. John

Along came William Kroll in the 1940s and he figured out an improved method to break titanium's bond with oxygen. First, the oxide is "chlorinated" with chlorine at 800-900°C to produce titanium tetrachloride, this is then combined with liquid magnesium at 800-850°C to produce magnesium chloride and pure titanium sponge. This sponge can then be crushed and remelted into various usable ingots, extrusions, pipes, bars, etc. Easy-peasy right? Oh did I mention that titanium violently reacts to the atmosphere when heated which means any process like remelting, casting, forging, welding, etc needs a specially controlled vacuum or inert working environment? Not too hard yeah? Oh, also, it's not the easiest material to machine either!

Credit: Toho Titanium Co Ltd

So, does titanium even make sense as a material for an MTB component? Well, what are the goals of an MTB? Something as light as reasonably possible (it is human powered after all, not talking to you E-bike fanatics) that is durable enough to withstand the various forces an MTB sees, jumps, impacts, drops, vibrations, etc. Mass-to-stiffness ratios are also important, you want a bike that you "feel" in control of, not a floppy, flimsy mess. So how does titanium stack up? I will be specifically looking at the workhorse of titanium alloys, Ti6Al4V, or grade 5. (It is important not to get confused with commercially pure titanium, which has dramatically different properties.)

	Ti6Al4V Grade 5 Titanium Alloy	4140 High-Tensile Steel	6061-T6 Aluminum Alloy
Density	4430kg/m3	7850kg/m3	2700kg/m3
Tensile Strength	900-1100MPa	930-1080MPa	240-280Mpa
Young's Modulus	115GPa	210GPa	69GPa

Roughly, Grade 5 titanium is as strong as a high tensile steel for 45% less mass, but also 45% less stiff (It's just a coincidence they are the same number here). So for applications where you are after the strength of steel for less weight and you can handle a bit of extra flex/elasticity, titanium is a good choice. On the other hand, Grade 5 titanium could be used to add rigidity and strength when compared to an aluminum alloy, being 400% stronger, 170% more rigid and only double the mass.

This led me down the path of looking at titanium as a suitable material for MTB pedals, titanium prices are slowly declining, so how feasible could it be to develop a set of pedals that use titanium for its properties, not just a gimmick, and still be within the realms of sensible for cost? That's a story for another day! Cheers, Patrick Ampt