Titanium Framebuilding - Part 1(a) - Material Selection

Before I even get started writing about material types and sources, I want to preface this (again) by giving some context that should be, but probably isn't, obvious. Everything I'll be covering is assuming a "normal" shop situation. Milling machines, files, lathes, TIG welding, and various cutting tools are the assumed tools. There's 3D printing now, but it's not economically accessible to everyone yet, and more importantly, I don't know enough about it to write about it. Maybe Tyler and Jaime from Firefly want to educate the public about the merits of 3D printed ti bits. They know their shit and I trust in their abilities/resourcefulness.

Also, I'm a high school graduate with only part of a mechanical engineering degree completed. I'm not an engineer. I've been making titanium frames since 2006, worked for a contractor to an aerospace company (as a financial analyst) immediately preceding but have had at least one foot in the door of the bike industry since 1993. You don't have to believe what I'm writing and I encourage you to learn more for yourself if you're interested. There's a lot of material out there. Also, I wasn't an English major which will be obvious moving forward. In fact, I feel confident in stating that I'm not a great written communicator. If you take issue with the way in which I've written anything, keep it to yourself because I already know.

When we're talking about framebuilding and titanium, we're generally talking about what's referred to 6/4 (6% Aluminum, 4% Vanadium, (Grade 5)), 3/2.5 (3Al/2.5V (Grade 9)) and CP (Commercially Pure, (Grades 1-4)). Those are in order of hardness with 6/4 being the hardest and CP being the softest. As in any industry, the material choice starts with the task at hand, what's available and what condition is it in. We need the individual components to surpass a safety threshold when put together in the system that is a bicycle frame. At it turns out, the material market has partially sorted this out for us. The material used in bicycle frames is often the same material used in the aircraft industry, in fact, a good deal of it is purchased from the aircraft surplus market. There are certainly some mid-sized and larger manufacturers that purchase material directly from the mill, but for economic reasons, the smaller builders don't. I'll get back to this. (MAKE SURE TO GET BACK TO THIS). If you look at what's readily available, it's mostly 6/4, 3/2.5 and CP. If you look closer, you'll see that 6/4 is usually found in billet or rod form and rarely in tubing. 3/2.5 is usually found in tube form and rarely (if ever) in billet or rod form. CP can be found in tubing, rod and billet form. That's the reason you see most structural hard parts (dropouts mostly) machined from 6/4...it's a matter of the strength being up to the task AND material availability.

When it comes to tubes, theoretically, you can choose between 3/2.5 and CP. Those of you who have been around for a while will say, "wait, didn't Moots use 6/4 seat stays on a fancy pants road bike a few years back?" Yep, they did. I believe they were half inch diameter by 0.02something thin tubes. I'm sure they weren't easy to come by at the time either. But my understanding is that they don't anymore so you can safely assume that either they were seeing more failures than they would have liked OR something happened in the supply chain (cost went up, or they weren't available anymore. Another of you might say, "didn't Kona make mountain bikes with 6/4 chain stays back in the late 90"s?" Yep again. My understanding about that one is that it was a right place/right time kind of situation and after a year or two of availability those particular tubes were reCLASSIFIED by the US government because they wanted them for space projects and didn't want to compete for access or pay more than they had to (anyone who knows anything about government will find this laughable.) Anyhow, getting back to it...you have 3/2.5 and CP available. You need to look at the mechanical properties of each, decide whether or not you want to put dentists kids through college while simultaneously insuring that your financial future will be ruined and you will quickly realize that 3/2.5 is the only way to go here.* To further complicate the decision making process, 3/2.5 is fairly commonly available in either annealed form or CWSR form. Within this alloy, those are two types of tubing have very distinctly different mechanical properties. Annealed generally has about 15% elongation, 70ksi yield strength (the force required to permanently deform) and 90ksi tensile strength (the force required to break). Through cold working, CWRS generally has about 10% elongation, 105ksi yield strength and 120ksi of tensile. So CWSR is the strength winner here, by quite a margin.

*This doesn't mean that 3/2.5 CWSR is absolutely necessary for every tube on the bike. I'd argue that in order to be safe, you're better off with CWSR in the front triangle, but in the rear triangle, there's some redundancy. If you break a tube in the back end, the failure isn't as critical and is less likely to lead to an immediate and spectacular crash. What's also true is the fact the chain stays often need to be manipulated (the fancy word has been "swaged" but what they actually mean is squished) to clear tires, crank arms and chainrings. An unfortunate byproduct of all the cold working and stress relieving is that the tubes are more brittle (remember the elongation? it's in play here.) So put a tire clearance bend in a 7/8" x 0.035" tube and then try to squish it down to 16-17mm to fit in between the tire and chainring and you have a 50/50 chance of cracking it before it ever becomes a frame. There was a highly esteemed builder of ti (and steel) bikes on the East coast who had an unfortunate run of chainstay failures because of this very issue...and the fact that they were sold "high strength" tubing from a supplier on the West coast. I guess the East vs. West coast feud broke out of the music industry in the 2000's. In the case of stays, you can make the argument that a more appropriate tube would be an annealed one, or even a CP one. In either case, they're fine so long as the wall thicknesses are appropriate for the forces they'll see.

Then there's the case of weld wire. Again, various alloys are available but I think there's really only one choice...and it's Grade 23. What the hell? I thought we only used three different types! Grade 23 is just a special version of 6/4 called 6/4 ELI. The ELI stands for Extra Low Interstitial. Interstitial refers to the space between atoms of the intended alloys. Generally, we're talking about oxygen, nitrogen, hydrogen and carbon. Remember those elements, they're important later in the series. The interstitial elements interrupt the crystalline structure of the alloy and embrittle it so some degree. The degree of embrittlement is dependent on how much of these interruptors are present. In the case of standard Grade 5, the max allowable percentages of O2, H2, C and N2 are 0.2%, 0.015%, 0.1% and 0.05%. In grade 23 the percentages are dependent on the specific certification but are about 0.13%, 0.0125%, 0.08%, 0.03%. The idea being that basically, you make every conceivable effort to prevent contaminants during the weld process and decreasing by any margin is a win.

I haven't even gotten to country of origin, material certifications, falsified material certifications and what the hell to do about any of it. It's time for dinner. Part I(b) coming eventually.