titanium, steel, or others?
titanium, steel, or others?
The answer to this is directly related to the specific hardness of each material.
Aluminium is the softest material that I am aware of in use as cogs.
Ti is the hardest, and steel sits nicely (and inexpensively) in the middle.
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Steel can be hardened much more than titanium. I'd say steel is the best material.Originally Posted by Bike_13
I've used the DuraAce ti cassette. It wore out pretty quickly. I went back to the all-steel Ultegra for increased durability. Aluminum cassettes are special for racing only. They wear super quick.
Chain rings seem to be made of aluminium fairly often.
You want a cassette that wears out fast? Most of the non-steel cassettes will fall into that category. If you absolutely need to shave a few tens of grams you can buy an aluminum cassette that will be good for about 1000 miles; titanium versions are not that much better for longevity.
It also depends on the size of the cog. On Dura-Ace cassettes, only the largest few cogs are ti, because larger cogs wear more slowly than small ones (all other things being equal). A titanium 11-tooth cog wouldn't last very long at all.
That's why you can make chainrings out of aluminum without any trouble. They're just much bigger. Steel and titanium chainrings only make sense when it comes to granny gears on mountain bikes.
Campy used to make an all aluminum freewheel. It was light, but it barely held up long enough to ride out of the bike shop parking lot.
I think the manufacturers are not trying all that hard to be honest. Stuff that is light, wears out fast and costs alot makes these companies lots of money. There are Ti and aluminum alloys that are harder than the steel they are making cogs out of now. Maybe the cost is prohibitive, but it could be done. There are some car engine blocks made of aluminum alloy that last as long as cast iron blocks, for example. Such material may be too brittle for cogs, but there literally hundreds of alloys available to manufacturers.
Ally blocks usually have steel liners, or even ceramic ones.
As a machinist I have machined many different alloys of aluminum and steel and have yet to see an aluminum alloy that is harder then any of the steels I have cut. All of the titanium I have worked with is harder to machine then soft steel but it is actually softer (will dent easier).I think the manufacturers are not trying all that hard to be honest. Stuff that is light, wears out fast and costs alot makes these companies lots of money. There are Ti and aluminum alloys that are harder than the steel they are making cogs out of now. Maybe the cost is prohibitive, but it could be done. There are some car engine blocks made of aluminum alloy that last as long as cast iron blocks, for example. Such material may be too brittle for cogs, but there literally hundreds of alloys available to manufacturers.
Does anyone know if the steel cogs are heat treated?
D'oh?Originally Posted by dooley
???Originally Posted by sydney
Pretty much all steel is heat treated. If you're talking about how hard it is, I'm no expert, but I would venture to say that the cogs are on the upper end of the hardness spectrum. They don't have to be especially tough, so there's no reason not to harden them...Originally Posted by Coda1
Not really.Originally Posted by ivan_yulaev
What???Originally Posted by ivan_yulaev
True, but some don't. I believe some manufacturers use an alloy with alot of silica in it.Originally Posted by dooley
Originally Posted by Coda1
I would think they would be, but I have no idea. It would be really simple and cheap to case harden them by quenching in oil or water, so I would expect that they are. Anyone have access to a brinnel hardness tester? I'd be currious to know how hard steel cogs are.
Nope. AFWIW, case hardening isn't a simple oil or water quench.Originally Posted by darkmother
If you want something that wears fast, go buy a dura ace cassette.
I am not a materials expert, but my basic understanding is: titanium alloy is stronger than steel; pure titanium probably is not as strong as steel; titanium alloy is a lot harder to machine because it is so tough (and that is why it is more expensive); pure titanium is a lot easier to work with but is not used very much; and, you probably do not have to worry about excessive wear of titanium cogs unless you start to see sparks flying from your freewheel.
I have some materials background. Shimano will not give out any information but is is fairly easy to deduce. Titanium. I have noticed that on a few dura-ace cassettes, there is rust on them. Since the cassettes are made in Japan, looking at viable titanium suppliers, they are using Russian titanium. The Russians do not have the technology (triple-vacuum melting) that we do - therefore the "purity" of their Ti cannot match that of US and therefore, that is one explanation for the rusting (impurities in the titanium). Next, looking at machinability, and cost... looking at both of these factors, one can deduce that Shimano is using CP2 (Commercially Pure grade 2 titanium) which is non-hardenable. Shimano claims Nickel-plating on their Dura-Ace steel cogs. Without actually Rockwell testing for hardness I would deduce they are using a mild inexpensive steel/non heat treated and using the nickel plating (because it is less expensive than using and heat treating a higher quality steel - knife makers do this as well). The nickel plating increases wear resistance and insulates the steel from the elements, preventing rusting. That is my 2 cents :-)
titanium is not stronger than steel. period. its strength to weight ratio is better, but htat's irrelevant in cogs (same size). Titanium is hard to machine becuase of its properties (bad cconductance etc) not because it is tougher or harder..
Why not? It can indeed be that simple.Originally Posted by sydney
CP titanium has only 35,000 pounds or so tensile strength. Also, by being soft, it is "gummy" and tends to gum-up the cutters on milling machines. Titanium alloys approach the strenghts of some of the better stainless steels. We have heat treated Beta-C titanium to RC50 - which is approaching 260,000 pounds tensile strength. I believe the highest strength metal is Nitinol 60 (a Nickel/Ti alloy). We have heat treated to RC70 ! And it was not brittle. I think I have the only remaining supply :-)
Depending on the steel being hi or low carbon.Originally Posted by darkmother