Weight, durability and shifting performance are competing factors in cassette sprockets. Sprocket durability is a function of its metal properties (hardness and strength), tooth width and profile. Metal properties affect weight, while tooth width and profile affects shifting.
Durability
The surface hardness of titanium is less than steel. This suggests ti cogs wear faster than steel; but the durability picture is much more complex.
Firstly, note that our titanium inner chainrings outlast steel by a significant factor - as much as double. The reason is found in the tooth profile. Steel cogs (including chainrings) are stamped. Stamping is a crude process that only approximates a proper tooth profile. In addition, the surface that contacts the chain roller is left very rough and pitted. In contrast to stamping, teeth can be machined very accurately so that the engagement with the chain roller is extremely efficient.
| Figure 1. Stamped tooth
profile Chain roller contact area is uneven across width of cog. The surface is also rough and pitted. |
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| Figure 2. Gear cut tooth
profile Chain roller contact area is perfectly formed. The surface is smooth. Chain and sprocket mesh is very efficient. |
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Figure 1 illustrates the cross section of a stamped tooth profile. As the stamp pushes through the metal sheet, the shearing of metal is reasonably smooth to about midway through. The metal from the last half of the tooth width is simply torn away, leaving a poor surface and larger hole. Initially, only about half the cog width contacts the chain roller, until it is quickly worn to the same loose dimensions of the other side.
Figure 2 illustrates the cleaner profile of a machined cog. The tooth is accurately cut across its full width. The surface that contacts the chain roller is also very smooth. Our teeth are gear cut, which is a very specific process to making teeth, discussed on the "what is gear cutting" page.
Granny ring teeth profiles can be made to hold the chain roller very securely. A good design, combined with accurate machining, makes for incredible durability. Titanium is extremely good in this application because it is stronger than steel and work hardens with use. Aluminum lacks strength: aluminum teeth deform very quickly, requiring frequent replacement and worse, contributing to chainsuck.
Rear cassettes benefit from more efficient tooth profiles possible through machining, but shifting requirements entail other compromises. Rear cassettes shift under load, so their engagement with the chain roller must be carefully designed. Rear cassettes wear out because of chain stretch. Illustrated below is the positioning of a chain roller with a perfectly pitched chain (12 links = 12 inches) and a chain roller from a chain stretched 1/16 inch over 12 inches (on a 17 tooth cog). A new chain loads the tooth at the base; pressure and abrasion is well distributed over the full height of the tooth. Very quickly, as the chain stretches, it loads the tooth further up concentrating pressure at the weakest part of the tooth, near the top.
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Figure 3. Chain roller in sprocket tooth |
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Figure 4. Blowup of chain roller
contacting tooth Roller A represents positioning of chain roller from new, unstretched chain. Roller B represents position of chain roller in chain with 1/16" stretch (over 12"). It rides 0.5% higher on tooth. The dynamics of loading on the cog tooth is dramatically changed (for the worse). The shape and height of the tooth in the circled area is critical in determining both cog shifting and durability characteristics. |
The shape, height and positioning of that top portion of the tooth, circled in figure 4, has a great impact on the shifting performance of cassettes. Stamped cogs are made thin and loose fitting here: it makes for good shifting and gives a large tolerance for variations in the stamping output. However, durability is sacrificed. We are able to provide equivalent shifting performance while using a slightly wider, better fitting tooth because of the perfect accuracy of gear cutting. That's why the durability of our titanium cassettes is near steel, without sacrificing shifting.
Shifting performance
Shifting performance is measured by the ability to shift into the cog you want, quickly, even with a load on the chain. Many factors are involved in achieving good shifting.
Ramps are the most visible factor. Our ramps are CNC machined into the cog. The teeth preceding and following a ramp are beveled to assist the chain shifting. Ramps and bevels reduce cog tooth thickness but are necessary for great shifting performance.
Spacing is critical to indexed shifting performance. Spacing out by .002 inches per cog will have noticeable impact. We provide precision spacers, cut to +\- .0005 inches, matched to cog thickness in every set for perfect indexed shifting.
Timing the teeth and ramps between adjacent cogs is another factor. When replacing single cogs, it is important to identify which cog set it belongs to; there are, for example, three different 21 tooth cogs. It is timing requirements that limit the choices in customizing cogsets.
Tooth profiles have significant impact and are the most technical aspect of manufacturing sprockets. Theoretical models of timing and profiles are only the start. The processes used to place the design into metal play a vital role. Different materials require different methods and designs. And there is no substitute for field-testing: our titanium cogs have been refined in over four years of use
This page updated March 30, 1997
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