Gerblefranklin, you’re on the right track. The key parameters for frame-making are:
Density
The mass per unit volume.
Tensile Strength, Ultimate
This is the force required to pull apart a bar of metal divided by the cross sectional area of the metal. The link for titanium says that it’s ultimates strength is 49,900 psi (pounds per square inch), or about 50 ksi (thousand pounds per square inch). That means that it would take a pull of 25 tons to rupture a 1" x 1" bar of titanium. Similar numbers for ordinary structural steel (bridge I-beams for example) are from 30,000 - 50,000; and for exotic bike-tubing steel around 160,000 to 200,000.
Ultimate strength isn’t that important on a unicycle because the frame is usually bent and ruined before it breaks.
Tensile Strength, Yield
This is the force required to permanently deform a bar by pulling on it. For brittle materials it’s the same as the ultimate strength, since brittle materials crack before they deform.
Elongation at Break
This is how far the bar stretches before it breaks. Again, not so important since most of this elogation is from ductile yeilding. It’s useful as an indicator of how the metal will fail, though, so it has some use in judging how safe the frame is. Frames that simply snap are dangerous.
Modulus of Elasticity
The modulus of a material is important for determining how stiff the frame will be. Think of it as if it were the stiffness of a spring - high modulus materials, like stiff springs, require a lot of pull to stretch even a little bit, whereas low modulus materials stretch very easily. Carbon fiber is a very high modulus material; rubber is a very low modulus material.
Note that all of these frame-making metals have about the same specific elasticity. That means that if you just vary the wall thicknesses of the tubing all of them will make frames that ride about the same. Since aluminum and titanium are lighter than steel they can be formed into tubes with larger diameters to make much stiffer frames.
Also note that aluminum and steel have about the same specific strengths, but titanium is twice as high. That means that the titanium frame is more likely to hold up under abuse.
All of these materials have an elongation of over 6 or 8 percent, which means they aren’t brittle materials and are unlikely to snap into sharp-edged pieces that will puncture your leg.
Thanks for all this info. Did you derive this information from the links I gave you?
I plan on making a few steel hunters exactly like the original, and then trying one out of titanuim. One hitch I’m runnign into is that the person who runs the shop I may be using says her shop isn’t set up to do titanuim. She says I need a vacuum chamber for some forms of titanium. But on the other hand, I talked to Steve Howard, and he says that I can TIG titanium just as I would steel.
Now, assuming I’m still going to make a titanium hunter-like frame, can you tell me how much larger diameter the tubes need to be? I originally said 50", but would 25% work? I’d still like to keep the tubing small.
Also, do you know of anywhere I can get the materials for a titanium frame? Steve Howard said I would need brand new High-speed steel end mills for the bearing holders, and I assume this applies to the brake bosses. He said that they’d be worn out pretty quickly. Also, where do I get grade 5 titanium? I doubt I can just drop by the scrap yard and pick some up.
Thanks for all this advice, and I apologise for jacking hte other thread, although by now it seems dead.
Yup. First or second year engineering schools stuff. It’s called Mechanics of Materials. You’re math is probably good enough already for you to read a textbook on this topic. They probably have a few used copies at the bookstore of the local engineering school. If you run into anything you don’t understand one of your math or physics teachers can help.
Steve is going to be a MUCH better source of information on metalworking than I am. Aerospace engineers just come up with the design specifications. We don’t actually know how those specifications are turned into parts.
Titanium LOVES oxygen, so your shop person wasn’t far off. But Steve is right - the inert gas displaces all the air and allows the molten titanium to stay a metal instead of burning into an oxide.
One of my engineering 101 professors explained the reactivity of different metals in this way. He said, “A freshly cut surface of steel will oxidize in a few days or weeks, and will keep oxidizing until the steel is gone. Freshly cut aluminum oxidizes in a seconds or minutes but the oxide coating is so tight that it stops the reaction. Titanium is the same way, but it forms the oxide coating in a few milliseconds.” It’s safe to say that no one ever really sees freshly cut titanium, and that molten titanium will react furiously with air if you ever give it the chance.
The Russians built a couple of subs out of titanium during the cold war. No one was sure exactly how they did it. It turns out they built the subs in a giant airtight building filled with argon. All the workers wore space suits.
I’ll have to get back to you on that. My sense is that 25% would be fine, but you are going to be limited by whatever tubing size you can find.
You might call Lightspeed or some other titanium bicycle frame manufacturer and ask them where you can get materials. They might find your project amusing and offer some help.
Steve is telling you something quite valuable. Titanium is reactive, right? Very reactive? Those top-end cutting tools are going to get dull very, very fast because the titanium will weld itself to the cutting edges as it is being cut. Those welds will round the edges of the tools in no time, either because tiny titanium blobs will coat the edge, or more likely the titanium will pull chips of steel off the edges.
Steve is also cluing you in on the possibility that you may need to spend a lot more money on tools than you will on materials. What Steve is saying is don’t waste your time with cheap tools because they will become useless almost instantly. The really good stuff will cut titanium for a minute or so before it goes dull.
The machinists who worked on the SR-71 (a mostly titanium aircraft) used to go through a truckload of tools just to get one assembly built.
There are a couple of processes that have been invented for working titanium without welding or cutting. It’s so reactive that if you just get two parts hot and press them together they will diffuse into each other and be virtually welded without ever melting. Titanium also has this neat property that it will slowly flow or into a new shape if you get it hot and apply enough pressure. I think the process is called superplastic forming. If it’s not too hard to do perhaps you could make the bearing holders this way by “mushing” them on to a 40 mm steel form. It might be cheaper than buying a few sets of cutting bits.
So to recap, I know just enough about making things to be dangerous. Listen to Steve. He’s a pro.
How hot do I need to get it to do superplastic forming? Would 500 degrees farenheight do? Then, I could just stick it on an open flame and press until it was formed. How much pressing are we talking about. 200p.s.i? 1,000p.s.i? Also, how much are those kinds of cutting tools? Would a slow enough speed running the bits enlongate their lifespan? Also, how does one make a 40mm cut on a non-CNC milling machine? Do I need a 40mm end-mill?
This is a fascinating discussion about framebuilding. But I want to go back to the original question of building a Hunter frame out of aluminum. The engineering aspect of doing this is only one part of the equation. You have left out “why a Hunter frame?”
Purpose of the Hunter design:
Fatter tires require wider frames. Wider frames bang into your legs. Remember, this design was developed before and during the time people started using 3" tires. Making a wide frame (for 3" tire) out of fat tubing would make for a very wide outside width; more chance to interfere with your knees.
Good quality bicycle tubing comes in wide sizes, a little too wide for a good unicycle frame, and narrow sizes, for the rear triangle on the bike. The Hunter (and Telford) design is based on doubling up on this skinny tubing to maintain strength, while keeping the overall width down.
Conclusion: Making a Hunter-style frame out of wide tubing doesn’t make a lot of sense. If you’re going to use wider tube, a different frame design should be considered.
I think this is part of why the Wilder frames are shaped the way they are. They essentially have rectangular tubing on the fork legs. Wide front-to-back for strength, but narrow side-to-side to keep the overall width down.
For titanium, you can probably do a more standard fork design. It might not look as cool, but it should be easier (and cheaper) to build.
For aluminum, it would be nice if you could fine some ovalized tubing. Otherwise, you’re just going to have to live with a wider frame. This may not be a problem, depending on rider height. I think the shorter you are the more problems you’re likely to have with a wide frame.
I got to see Kris Holms latest prototype that he was riding on while he was in Telluride for the Mountain film fest. It’s an alluminum frame made with oval tubing that has a flat crown like his others but it tapers in at the top of the vertical legs so it’s not as wide of a flat top to reduce leg scrapes. It seemed very stiff and also light, I hope it makes it to production it was a sweet frame. It would be great to see a low cost production alluminum frame muni available. Kris Holm has done great things making high quality munis more affordable, I hope he keeps it up.
That’s originally why I decided to try to use titanium rather than aluminum. Were I to use aluminum I’d make a larger triangle than on the average hunter, and then make the rear tube be large diameter and the front tube only about 5/8" diameter. That’s just a it more complicated than I wanted.
As for using titanium, you saw Dan Heaton’s titanium muni frame at moab I assume. It was of the conventional design. i got to ride it both days and my knees got to know the crown of that fork much more than I wanted. The frame was simply too wide. A Hunter style one wouldn’t be.
I was thinking of cheating around the bearing holders. What if I made steel bearing holders, and then gave them an adaquate sleeve to fit onto the titanium. That would avoid the expensive materials and tools costs of making them out of a block of titanium. Or I could use aluminum, and also make strap-on brake bosses that simply clamp on like a seatpost clamp. That’d cut machining and fabrication costs a ton. The problem would be getting te aluminum bearing holders to fit the titanium without developing slop.
Does anyone know of somewhere that I can get ovalized titanium tubing? If I can get that, i could make a KH-style frame. It’d be easier to attach brake bosses and bearing holders to that kind of frame, not to mention it’d be cheaper.
Both the Pashley and Miyata unis have decent lollipop bearing holder designs that work well in practice. The Miyatas are lighter by far than the Pashleys but you might make a hybrid design. The easiest thing to do might be to use a set of Miyatas which are easily obtained from uni.com. You do have to get a bearing that matches both holder and axle, though.
I don’t mean lollipop. I mean a main-cap bearing holder that has a press fit onto the frame tubes. This would be better because it has the advantages of main-cap. The miyatas suck. Dan Heaton’s muni frame uses them, and there’s tons of slop in there.
What I’m describing is like having the top of the top bearing holder part press fit permanently onto the frame. Something where there’s a small block of titanium at the base where the entire bearing holder bolts on.
Yea I don’t see any reason to build a titanium frame and then use crap lolipop bearing holders. You should talk to Ed at creative geiko he builds with titanium and makes alluminum bearing holders that clamp to the titanium tubes, infact he doesn’t weld anything to the titanium tubing the crown is also aluminum. I’m sure he would be willing to work with you, and he could also get you the titanium tubing you need, or Steve Howard could also machine you something.
The Miyata and Pashley bearing holders aren’t crap. They are quite good. I don’t know about Dan’s bearing holders, but I’ve had nothing but good results from the Miyata and Pashley. The design is not as easily accessed, but the pressure on the bearing housing is ideal for smooth tracking. And thousands of people have put huge amounts of stress on the Miyatas.
Yea crap is a bit of an overstatement but why would anybody build a titanium muni and then use miyata lolipops. Definitly not the best way to go, besides pashley you don’t see any munis or trials unis being built with them and the general opinion is that main caps are much better. Miyatas are an exelent freestyle and street uni and pashleys are plenty sturdy but it’s been the same for an eternity when are they going to realize that people are useing 24"x3" tires.
> The key parameters for frame-making are:
>
> DENSITY
> TENSILE STRENGTH, ULTIMATE
> TENSILE STRENGTH, YIELD
> ELONGATION AT BREAK
> MODULUS OF ELASTICITY
If you are considering aluminum, it is also necessary to include some
sort of fatigue analysis. Stresses on aluminum must be kept
significantly lower than yield strength to prevent cracking from
fatigue.
