Materials

Titanium is a very good material for bike frames, but it isn't magic. A lot of what's said about it, that it rides softer, that it's the lightest, that it lasts forever, comes from marketing rather than engineering.

This page sets out what titanium actually brings, which titanium goes where, what it asks of the person working with it, and why Sturdy chooses it.

What titanium actually brings

Titanium's real advantage is strength for its weight, not stiffness. On stiffness per kilo, it sits level with steel and aluminium.

Typical published values, rounded. For titanium, the yield strengths are the minimums the certification standards require; tested batches usually come out higher. Exact figures depend on the alloy and its condition.

• Stiffness per kilo: titanium sits between steel and aluminium on both stiffness and weight, so per kilo there's little in it. How stiff a frame is comes from tube size, wall thickness and length, not the metal.
• Strength per kilo: well ahead of common frame steels and aluminium alloys. That's what allows thin walls on bigger tubes, until manufacturing, rather than strength, becomes the limit.
• Fatigue: like steel, titanium has a stress level below which it can take effectively unlimited load cycles. Aluminium doesn't, so it's designed for a finite life. The benefit only holds if the design and workmanship keep stresses below that level.
• Corrosion: titanium forms its own protective oxide layer, so it can go unpainted and shrug off salt, sweat and road grime. Interfaces still need care: threads can seize, and aluminium parts in contact with it can corrode.

Carbon doesn't fit in a table row. Its properties are set by how the fibres are laid up, so it can be the lightest and stiffest option, and it behaves very differently when damaged.

Stiffness is a design choice

Stiffness isn't strength. How stiff a tube is depends on the material, its diameter, its wall thickness and its length. Make a tube bigger and thinner and it can keep the same stiffness for less weight, until the stress in that thinner wall, or the thinnest wall that can practically be made, sets the limit.

So no metal rides a certain way. Ride quality comes from how the whole frame is designed: the right stiffness in each part, with no part stressed to failure, and the feel the rider wants.

Not all titanium is the same

"Titanium" on a bike usually means one of two alloys, each supplied in different conditions. Choosing the right one for each part matters as much as choosing titanium at all.

• Grade 9 for tubes: cold worked, it's strong enough that at the thinnest wall you can practically make, weld and trust, stress isn't the limit. Grade 5 tube would add cost and difficulty for no real gain.
• Grade 5 for machined parts: it doesn't take kindly to cold forming. It work-hardens and loses ductility. Machined from solid, where nothing has to be drawn or bent, its extra strength earns its place.

The condition matters as much as the grade:

• Cold worked and stress relieved: drawn cold to raise its strength, then heat-treated just enough to relieve internal stresses while keeping most of that strength.
• Annealed: heat-treated to soften it fully. Lower strength, more ductility, easier to form.

Which is right depends on where a part goes and what's done to it on the way. A tube that's shaped needs different properties from one that stays straight and works hard. Welding changes things too: the heat of every weld alters the material next to it, and the design has to allow for that.

Sturdy uses all of these, each where it suits. Exactly what goes where is part of the craft, and it stays in the workshop.

What titanium isn't

• Not soft-riding by nature. A titanium frame rides however it's designed to ride. Tube size, wall thickness and length set its stiffness, so it can be made stiff or supple.
• Not automatically the lightest. For the same stiffness, it's about level with steel and aluminium, and carbon frames are usually lighter.
• Not indestructible. It can last a very long time, but a frame is only as good as its design and how it's made. In the titanium frames Tom has repaired, cracks start where the material was mistreated: a poorly purged weld, tube overworked while it was formed, or a badly drilled hole.
• Not all the same. Grade, condition and what's been done to it all change how it behaves.

What it asks of the builder

Titanium is famously difficult to work with because it punishes poor process. Most of that difficulty is invisible in the finished frame, which is why process matters so much.

• Welding: hot titanium reacts with the air, so the weld and the metal around it must be shielded with inert gas, inside and out, until it cools. Contamination you can't always see weakens a weld.
• Cleanliness: it has to be scrupulously clean before it's welded.
• Forming: bend or shape it too hard and it work-hardens and loses ductility, leaving a hidden weak spot where a crack can start.
• Drilling: an overheated drill leaves a hard, brittle burr at the edge of the hole, a ready-made starting point for a crack. Every bottle boss hole matters.
• Machining: it holds heat at the cutting edge and work-hardens, so it needs rigid setups, sharp tools and patience.
• Threads and interfaces: titanium threads tend to seize, and aluminium parts in contact with it can corrode. That's why Sturdy uses phosphor-bronze threads and bushes at the thru-axle and the seatpost clamp.

How designs are tested

Sturdy frames and components are designed to exceed the ISO 4210 load cases. ISO 4210 is the international safety standard for bicycles, and its frame tests are a benchmark meant to represent real riding: pedalling, pushing and pulling at the front, weight on the saddle, a falling mass and a falling frame.

Why Sturdy works with it

None of this makes titanium magic. It just makes it the right material for the frames Tom wants to build.

• Fatigue: with the design and workmanship right, a titanium frame can take the loads of riding, year after year, built to last decades.
• Corrosion: it needs no paint to protect it. Salt, sweat and winter roads don't eat it, so a frame can stay in service for a very long time.
• Balance: it sits between steel and aluminium on stiffness and weight, which makes a well-balanced structure possible, and its strength relative to both is very high.
• It's good to work with: it punishes poor process, but done properly it's much nicer to work with than the alternatives. It has to be clean to weld, but it stays clean, with no rust or scale. It doesn't suffer the porosity aluminium is prone to. And it machines predictably once you respect it.

It's also expensive to buy and slow to work, which suits a small workshop making frames one at a time.

Titanium has no single best number, but no weak link either, if you know how to work with it. That's why Sturdy uses it.