Why are cassettes replaced more often than front chainrings and derailleur jockey wheels?

Question

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.

Answer 1

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

Answer 2

As a chainwheel/sprocket wears, the "engagement point" wanders away from the contact point with the chain. The chain touches the wheels with its topmost teeth relative to the chain direction, but the teeth pulling on the chain are further away - 2, 3, 5, 10 20 etc. teeth along the circumference. Before that, the chain rollers simply lay in the troths of the teeth and are pushed upwards and out of engagement, the further away from the "pulling" teeth, the stronger. This is usually no problem while you are using the chain that is wearing together with the cogs, this starts to matter when you put a new chain on. In the front, with the 42, 53 etc. teeth, having, say, 10 teeth pushing the chain up instead of pulling, there are enough teeth beyond to do the pulling. On the rear, on the, say, 14 teeth cog, all the 7 teeth touching the wheel are pushing upward - and the chain may skip. Thus, it is not necessarily a matter of different wear, it's a matter of noticing it. You can use a front chainwheel that is worn to all hell without noticing (at least until you start getting chain suck), but a worn cassette will make itself known by skipping.

There are also differences in how quickly either of the components wear, but the main reason is whether you notice or not. That does not mean that you SHOULDN'T change the front when it's worn, even if it does not skip.

The jockey wheels are also called "idler wheels", which in itself explains why they don't wear quickly. They're idling. They do nothing (in comparison). They transfer no torque, and the only wear are the normal abrasion or forces exerted by shifting or by non-ideal chainline.

Answer 3

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.