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Titanium axle questions

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Hopefully, someone out there is a machinist with some metallurgy knowledge and can help me out. I'm a machinist and have found a source for Titanium at semi reasonable prices. I'm confident I can duplicate an axle accurately out of the stuff, since some of the guys I work with that have experience with Ti tell me it machines like stainless. However, my concern is strength, if I make it exactly like the stock rear axle, it needs to be hollow, and big enough to fit the stock triangle stand on one end. Will that create a stress problem for a non-steel axle? Sure hate to build a trick axle only to wad up my bike and my body because the axle snapped like a twig. If it's strong enough, I'd also like to make more stuff out of it, since I have access to the tooling and machines at work, and I work nites, which gives me quite a bit of free time. Any suggestions?

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It sounds like you have some concerns. The question that you are asking depends on some factors. Just to start do you want the axle to be the same strength as the stock one? Are you 150 pound person that mostly runs on the flats, or are you 350 flat landing freeride jumps. Also what type of titanium are you going to be using?

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I really want the axle to be at least as strong as the stocker. I weigh about 165 and ride a litle bit of everything, playride at some local tracks, trails and dunes. The material I've found is called 6-4 Titanium, which I've been told is good quality and fine for structural stuff.

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I wouldnt worry too much about making the Ti axle work with the Triangle stand....

I dont know what the wieght differential is between Steel and Ti. But Id think if I could get it 10-15% lighter, that would be good enough for me...at that point, Id stop gun drilling it...

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If you want to be a little intuitive, you can find the shear modulus of the steel used in the axle, then match that to the shear of the the titanium alloy you whant to use. Most lilely they are not going to be the same. There are calculations to find a comprable shear by changing the thickness of the axle. What you are asking is normally a complicated engineering process. Most likely when a new component is made it is engineered then tested. I am assuming that you do not want to do just that. On the other hand you could just make the part just like the stock one, and it might not fail.

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Finding the sheer limits of the Ti would be easy enough, but what about the stocker? Is it 1045, 1018, 4140, or what? See why I'm concerned? I know enough about metal to know you can't just say, "it's steel" and be done with it, there are so many steel compositions with such a varied sheer and tensile strength that I simply can't take it for granted. As far as just making it to the stocker's dimensions and crossing my fingers, that's not going to happen. Shawn's advice may be a step in the right direction, I don't know why that never occured to me.

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thats one hell of a big might... :cry:

Ain't that the truth! As far as the wieght, Ti is nearly as light as aluminum, but as strong as steel. I doubt I'd ever feel the diference, but being a machinist and a detail geek, I can't help but wonder about making such stuff.

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Another thing to consider. The axle will wear fast at the point of contact with the bearings if not coated with a teflon, or Ti Nitrate, or something better. The increased surface hardness still doesn't stop the problem, just slows it down. The wear time is proportional to the agression level of the rider.

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I am not implying that the part will not fail, or that it will. What I am saying is that you might never have a problem with it. We are not talking about the space shuttle here. If you wanted a sure fool proof way of doing this, you would have no chance. otherwise,for insurance you dont drill the center out as much and drill the end open more for the stand. You could also inspect the part after use, such as regular mainteance.

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If the complete designation number for your "Ti 6-4" is Ti-6Al-4V which is the closest thing I can match it up to in the properties of titanium alloys it has a yield strength of 150ksi which is the strongest alloy on the chart the highest aluminum yield strength is 73ksi. I would confirm the designation number of the ti with your supplier and also the properties of it he should have these numbers. Option B mock up a test if your made of money put a stock axle in a press with a gauge and press till it breaks then do the same with the ti. Then you'll know first hand how strong your axle is compared to stock! But if you do have Ti-6Al-4V the material is over twice as strong so you should have no problems at all. :cry:

Edit Note: Ti-6Al-4V with 150ksi is Quenched and Aged at 1000 Deg F. Ti-6Al-4V that is only Annealed has 120ksi

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why dont you just duplicate a another titanium axle. Go buy one and dont put it on, just copy it and then go back and return it.

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Titanium is an animal all its own. It does machine similar to stainless, but not just like it. It tends to curl off in long hot chips and isnt as easy to get a smooth cut with. It eats cutters relatively fast and sharp carbide works best. TiNi coated cutters last a while longer. The company I work for uses Ti almost exclusively. Like has been said already, if you're that concerned(and you should be on something important like an axle), get an aftermarket one and take dimensions off that and make a few of your own, then return the original. I made some Ti paintball gun parts and valvespring retainers in my spare time(and custom jets in my sig, but those are 1/4" hex brass).

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As has been mentioned above, without the exact steel and titianium alloys it is impossible to tell for sure. Just to give a rough idea typical stainless steels and titaniums have a stiffness of around 2.9x10^7 psi. That means that if a given force is applied to them they will both deflect about the same amount.

What is probably a more relavent number is the yield strength, or how much stress the material can take before it undergoes irreversible deformation. Here, the alloy and heat treat is very important. The values for this vary greatly. For example, a 304 SS has a yield strength of 42000 psi and a 430 SS has a yield strength of 64000 psi (both in annealed form). Titanium 6-4 has a yield strenght of about 142,000 psi. Based soley on these numbers you should be fine. Again, these are average numbers and can vary widely based on heat treat and alloy.

Another consideration is fatigue. Titanium at room temp generally performs well under fatigue. A factor that affects this is the surface finish of the material. Considering that Ti is not the easiest material to machine, I would recommend making the OD of the axel a bit oversized on a lathe and then grinding it to the final dimension to give a nice surface. The rougher the surface, the more prone a part is to fatigue failure.

I would feel comfortable building the axel to stock dimensions with Ti. But its not me crash testing it either.

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Thanks for the input, that's kind of what i'd hoped to hear, I guess. But as was pointed out, I'm not sure if it'd be worth the risk. Realistically, I'm pretty easy on my bike (that is, I'm not as worried about lap times as I used to be) and very picky on maintenance. Checking the axle for cracks or wear every couple weeks would not be too much. Thanks to all of you for sharing your experience, I'll let you guys know how it turns out.

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