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On a bike there are three kinds sprockets that the chain comes into contact with: the chainrings, cassette, and derailleur jockey wheels.

It seems that the standard practice is to replace the cassette for every 2-3 chains or so. Chainring replacement is heard of less commonly, and it seems they last much longer than cassettes. Why is this so?

In fact, it seems counterintuitive since each (given a 2x10 setup) each chainring is used on average five times more than each rear cog.

I can perhaps think of a few possible reasons:

  • Rear cogs experience more wear since they are smaller, so there is much more force on each tooth. (However, this doesn't apply to mountain bikes, where several of the rear cogs are bigger than the smaller front chainring) (Edit: in fact, this really isn't the case for XC mountain biking with a lot of climbing, in which much if not most of the time is spent with a front:real gear ratio of less than 1:1.)

  • There is more wear and tear from the chain pulling on the teeth (rear) vs the teeth pulling on the chain (front)

  • Front chainrings are made out of more durable material.

Finally, there are also the derailleur jockey wheels, which no one ever seems to replace, except when replacing the whole rear derailleur. I assume that's because they aren't a "load bearing" part of the drivetrain like the cassette and chainrings are.

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    Cassettes vs chainrings is mainly a matter of tooth count -- fewer teeth means more rapid wear. And jockey wheels have very little load on them. – Daniel R Hicks Jul 24 at 18:42
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    Road chainrings are aluminum, so that's actually a lot less durable than steel or even titanium for the cogs. It really is that the chainrings have so many more teeth. Consider that that for performance road bikes, consumers have usually had a 34t small chainring and a 30t big cog. As you pedal, you load the top span of the chain much more than any other span, so I agree with Daniel that the jockey wheels have little actual load - which is why most are plastic and they still last a long time. – Weiwen Ng Jul 24 at 18:59
  • If you have a single chainring in front with an EVEN number of teeth, you can prolong its lifespan by always installing the chain in the same "phase" in relation to the chainring, i.e make sure the teeth of the chainring that go between chain's INNER plates, ALWAYS go between inner plates, as this article explains: sheldonbrown.com/chain-life.html – Robert Lee Jul 24 at 19:47
  • I think tooth count does apply to mountain bikes. In my case my cassettes always wear first on the middle cogs. the ones that are indeed smaller than any chainring. Larger cogs almost seem completely fine against middle cogs that look completely trashed. This may be due to frequency of use though. (I like to climb a lot and spend a lot of hard-pedaling time in the first 4 cogs of a 9 speed cassette.) – Jahaziel Jul 24 at 20:31
  • @weiwen ng Although cogs are made of durable materials, they are much thinner than chainrings, especially on 10 and 11 sos cassettes, which is a reason they wear out quickly even with high tooth counts – Andrew Jul 24 at 21:34
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To expand on Jahaziel's comment, and address your point that you have 5x as many cogs at the back (though anywhere from 3x to 11x is common):

You don't use all your gears equally.

Most pedalling on most bikes suited to their riding conditions is near the middle of the cassette, with the more extreme sprockets for the steepest climbs or fast descents/flats. You only need to wear out one commonly used sprocket to need a new cassette. In my case it's always the 3rd or 4th, counting from the biggest, depending on the bike, though if I get to the point where I can feel that, wear is very visible on some others. This will have fewer teeth than (most of) your chainring(s).

Tooth count has a double effect - fewer teeth in contact to spread the load, and each tooth encounters than chain more often.

On all my bikes, though probably least the MTB, the cassette gets dirtier than the chainrings - more abrasive grit.

I suspect a further effect of sprocket size beyond tooth count: a worn chain/sprocket combination that doesn't mesh well is likely to do more damage when the chain is bent more.

The jockey wheels last longer despite being small and plastic for two reasons: much less contact force against the chain to cause wear, and they don't have to be very good to do their job

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Why are cassettes replaced more often than front chainrings [?]

They have fewer teeth (at least on road bikes, not so much on MTBs with super small rings) so each tooth sees many more load cycles .

and derailleur jockey wheels?

Jockey wheels are not transmitting any torque.

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