Chainring versus cog wear: riddle me this:

[Dietmar]

O.k., this is something I've been thinking about recently in the shower: We all "know" that our cassette cogs wear out after a while (rule of thumb seems to be something like "replace the cogs together with your nth chain replacement", with n being somewhere between 3 and 5 or so), but chainrings last (almost) forever (well, given proper drivetrain maintenance, in particular, assuming the chain is replaced before it is stretched too much). So the question is: Why is that? After all, the chainrings are just aluminum alloy, while our cogs are usually Cro-Moly steel, and if they're "just" titanium, people will start worrying about premature wear. Nobody would trust aluminum cogs to last for more than a ride or two...


Now, before you shoot off your answer too quickly, consider this:

• The forces on the cogs are exactly the same as the one on the chainrings.
• The absolute relative velocities at which the chain rollers engage the teeth of the cogs and the ones of the chainrings are the same as well; the cogs rotate faster, but the speed (rate of rotation times radius) is identical.
• If we consider a 53 chainring first then, yes, for every revolution of the crank, the most often used cogs (say 13-18 or so) make somewhere between 3 and 4 revolutions. But there's at least four or five cogs that are used more or less regularly, plus five or six more, so the number of "chain miles" that run over each of the individual cogs versus the "chain miles" that the chainrings see should be quite similar.
• Assuming that (at least for a flatlander) the picture doesn't change too much if we bring a second chainring into the picture (which I am using probably less than 20% of the time; if you're in the mountains, things will be different, of course, but then you'll use all of your ten cogs more evenly, too), we still find that the rear cogs see about as much interaction with the chain as the chainrings.
• Yet (see above), even though the chainrings are made of aluminum, they wear out slower than the steel (or titanium) cogs.

So, what's the answer? Does anybody know why cogs wear so much faster than chainrings? Or could we use aluminum cogs and be just fine???

 

[alienator]

O.k., this is something I've been thinking about recently in the shower: We all "know" that our cassette cogs wear out after a while (rule of thumb seems to be something like "replace the cogs together with your nth chain replacement", with n being somewhere between 3 and 5 or so), but chainrings last (almost) forever (well, given proper drivetrain maintenance, in particular, assuming the chain is replaced before it is stretched too much). So the question is: Why is that? After all, the chainrings are just aluminum alloy, while our cogs are usually Cro-Moly steel, and if they're "just" titanium, people will start worrying about premature wear. Nobody would trust aluminum cogs to last for more than a ride or two...

The load on the cogs is distributed over fewer teeth. Also the cogs experience greater friction--same load, different surface area--than the chainrings. I'd say those are two big reasons.

 

[Dietmar]

The load on the cogs is distributed over fewer teeth.

Is it? I must say I 'm not at all an expert on this, but I thought I saw a claim somewhere that for cog-chain systems, just about all of the load is carried by the first two teeth or so, at least if the chain fits well. Does anyone have more definite information on this?

Also the cogs experience greater friction--same load, different surface area--than the chainrings.

Why is that? Because of the above (fewer teeth carrying load)?

You know, maybe my intuition on this is just wrong, but I feel the steel cogs should have a lot more resistance to wear than the aluminum chainrings, and they stilll wear faster. So whatever effect leads to more load/friction/wear on the cogs should be quite big, say, maybe by a factor of two or three at least. I just have a hard time seeing that kind of a difference.

 

[Wheel Right]

Is it? I must say I 'm not at all an expert on this, but I thought I saw a claim somewhere that for cog-chain systems, just about all of the load is carried by the first two teeth or so, at least if the chain fits well. Does anyone have more definite information on this?



Why is that? Because of the above (fewer teeth carrying load)?

You know, maybe my intuition on this is just wrong, but I feel the steel cogs should have a lot more resistance to wear than the aluminum chainrings, and they stilll wear faster. So whatever effect leads to more load/friction/wear on the cogs should be quite big, say, maybe by a factor of two or three at least. I just have a hard time seeing that kind of a difference.

Many factors like tooth profile also come into play here. The fact that load and friction is distributed across more teeth on the Chainring versus the cogs is absolutely true. The majority of the stress on the chain may be at the leading transitional unsupported links but that is not the same as friction and load on the rings. The physics of this is akin to friction knots where ropes are wrapped around different diameter objects (like tree trunks). The holding power of the "knot" increases with the diameter of the tree (assuming the same number of "wraps") but the friction per square centimeter decreases with the diameter of the object (tree).

All this being said. I personally change aluminum chainrings more often than my steel SRAM Red cogsets. I use a 180mm crankset, and weigh 100kg. I wear out chainrings in high torque events like up hill accelerations/sprints in the big ring. I have yet to wear out a SRAM Red cogset. They are bomb proof. I used to destroy DuraAce 7800 cogsets more frequently than chainrings . . .

If you would like to have a more durable/longer lasting cogset, the SRAM Red cogset is bombproof!

 

[Dietmar]

Interesting. Thanks for sharing your insight.

If you would like to have a more durable/longer lasting cogset, the SRAM Red cogset is bombproof!

I am running 11-speed Campy Super Record right now, which has six of the cogs in titanium. I'll see how it goes. I have had people telling me to get the Chorus cassette (which is all steel) because of wear, but at that point nobody really had any substantial mileage on those cassettes. In my case, so far so good.

I'll come back here to complain if the cogs last less than 10,000 miles...

 

[Akadat]

The riddle is the cost of the parts vs the value of your time. "If in doubt, throw it out" is a useful rule unless you have lots of spare time where you can eke out the most from every part.