My chain broke so I brought the bike in and the guy told me "Well you don't have the old chain, so we have to replace the cassette also. They go together. If you don't replace the cassette then the chain will skip when you pedal hard."

Is there any possible truth to this, or is it just a scummy attempt to turn a $20 job into a $75 job?

For reference, the chain and cassette were both only about a month old, and had both been installed at the same shop - although they didn't seem to remember these facts when making the case for needing the new cassette.

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    I figure 3-4 chains per cassette, but I don't wear my chains out. It's hard to say how worn the chain was, and if you let it go too long it accelerates the wear on the cassette. Mar 15, 2018 at 1:24
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    Well, I recently had this problem and got this advice, but I told the workshop to replace just the chain anyway. Lo and behold: I can barely apply pressure to the pedals without the new chain skipping. To be fair, the old chain and casette were about 10 months old, used throught the winter, and not really maintained
    – Suppen
    Mar 15, 2018 at 7:56
  • It is very easy for a skilled bike techie to determine if a cassette/sprocket is excessively worn. If it's ambiguous by sight or on lifting the chain from the cog, then there is a cog wear gauge that can be used. Mar 15, 2018 at 12:41
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    If the guy at the shop said that it's complete BS. There are gauges to measure sprocket wear and tooth wear on cassettes and chainrings. Chains of the correct speed number can be 'married' to cassettes of that speed number. The width between the sideplates of the chain is relevant. See here under width: en.wikipedia.org/wiki/Bicycle_chain
    – Carel
    Mar 15, 2018 at 13:28
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    Surely on a month old part you should expect the shop to replace and repair completely free of charge? No part should fail after a month of normal usage! Mar 15, 2018 at 15:10

10 Answers 10


At our shop, we suggest a new cassette every 2 times you replace your chain, IF you stay on top of your maintenance schedule. With 99% of customers this would mean a new cassette every 1-2 years depending on how much you ride it. If you are a commuter and ride everyday, your timeframe for replacement gets much smaller.

We would also assess the condition of the bike. If it looks like it has been neglected, we can, with high confidence (we do hundreds of these bikes a year), tell that you would need a new cassette as well.

What I tell every customer in your situation is as follows:

A chain that is not cleaned regularly will attract and hold dirt. That dirt will act like 50-grit sandpaper on everything it touches. When the chain stretches out, it will wear the cassette equally. If we try to put a new chain on a worn out cassette, the teeth on the old cassette will match the old worn out chain and won’t properly hold the new chain. We recommend replacing the cassette for every 2 chains so we can guarantee shifting performance from our tune. If replacing the cassette is something you want to hold off on we can try to put a new chain on the old cassette, however we may find that we can tell what your favorite gears are and it may skip under load.

You can always ask them to just put a chain on, check to see if they can make the adjustment, and call you if it is skipping or poorly shifting.

However, since your chain and cassette were only a month old when the chain snapped, unless you have a bent/broken tooth on your cassette, it should be good if everything else is adjusted correctly.

As far as the snapped chain, it may come from manufacturer defect, improper installation, or you may need to revise your shifting strategy.

You want to make sure you’re not cross-chaining. This will put a lot of stress on the components and will wear things out quickly and possibly snap your chain. Also, your bike doesn’t have a clutch, so you have to back off on power when your shifting. Right before you’re about to shift, let off pedaling a bit and wait for the gear to change. This will increase shifting speed and accuracy.

We do see a lot of customers resolve shifting problems with proper information on shifting. (If you’re shifting the proper way, then disregard. It’s more for people who will view this post later.)

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    Problem for the shop is if the customer chooses the cheap option and is unhappy with the result the shop stands to get a poor reputation though no fault of their own.
    – mattnz
    Mar 15, 2018 at 1:04
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    Both chain and cassette were less than a month old, and both were installed by the same shop (although they did not seem to remember when making the case for the new cassette). Does that change anything?
    – bigjosh
    Mar 15, 2018 at 2:05
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    That changes things a lot, please edit your question to add the information.
    – mattnz
    Mar 15, 2018 at 3:27
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    @Rider_X a majority of people dont properly maintain their equipment. This “rule” is designed for those people. It isn’t a hard rule. If I put my chain gauge on, fully max it out and it still has play, I’m going to inform you and give you the option to do some preventative maintenance. We’d never push components on customers if they don’t need them. I often tell customers no to things like “I think my bike needs new tubes” when in fact they just need to be pumped up. Even though they would be happy to buy new ones. Being a reliable and trusted service center will bring you more money.
    – wski
    Mar 15, 2018 at 19:22
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    @Rider_X we also use Ultegra chains and cassettes unless the customer asks otherwise. When you use lower end components, they are typically more robust (in exchange for lower performance and higher weight) and you can extend the service life a bit.
    – wski
    Mar 15, 2018 at 19:38

No, it shouldn't be necessary to replace a cassette after a month's use. However, unless you've had bad service from this shop in the past, I'd give them the benefit of the doubt and assume that it was a mistake, not a scam.

Bike shops deal with a lot of badly maintained bikes so I guess they're pretty much used to giving the "you need a new cassette" spiel to people who need new chains. Most of those people probably do need a new cassette, because chains lengthen over time and dirty chains reprofile the teeth of the cassette.

Did you remind them that the cassette was only a month old? If not, most likely, the person on the shop floor just didn't know that, so made a recommendation based on the general state of your bike. If you'd agreed to have the cassette replaced, you'd probably have got a phone call from the mechanic saying that, actually, your cassette is fine and they won't replace it unless you specifically want a new one for some reason.

On the other hand, if you did remind them that the cassette was new, I would expect them to immediately apologize and say that it wouldn't need replacing unless it was damaged when the chain snapped. At that point, they'd most likely phone you for authorization if the mechanic determined that a new cassette was needed. If you reminded them and they continued to insist that you'd need a new cassette, I'd look for another bike shop.

  • This seems the most pertinent answer; not so much focused on the technical aspect than the behaviour of the shop.
    – AnoE
    Mar 15, 2018 at 12:39

If you only purchased the cassette and chain one month prior, it is highly unlikely that you substantially wore your cassette and chain out. If you rode 2000+ km in poor conditions in that month, then its possible.

Most people ignore chain maintenance and do often wear the cassette too much for a new chain to mesh correctly. As the chain wears, the pitch subtly changes, which in turn changes the pitch of the cassette. Eventually, the pitch of the cassette changes enough to make it incompatible with a new chain. Many local bike shops (but not all) take advantage of this to always recommend a new cassette, especially if your bike looks worn or old. They simply assume the cassette must be worn too.

Shops can also have disdain for customers on the lower end, usually assuming the worst. The price of $75 for a chain and cassette suggests a lower end setup. Often employees simply want you to buy something and move on, so they may not take the time to ensure what they are prescribing is actually needed as it is all viewed as "cheap".

If you had worn your cassette out, there will be signs. Here is a good summary


not worn worn

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    I do see some wear (one side sloping) on the fourth and fifth smallest rings; presumably that is still in reasonable limits?
    – PJTraill
    Feb 18, 2019 at 20:34
  • I'm curious about this too. My cassette mostly looks like the one in the top picture, but I was told it needs to be replaced. I went ahead and bought one as it's a cheaper cassette, but now I'm wondering if I actually need to replace it. Jul 24, 2019 at 17:07
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    @steezeburger check all the cogs, some times one gear gets excessive used and is more worn than other gears. You can always check by running the old cassette with the new chain and see if it slips under high torque or makes excessive noise. Even if it doesn’t slip, but there is a fair amount of wear on the cassette, the chain can wear faster. It’s a black art, so it’s hard to diagnose without pictures. Also shops will be extra cautious to avoid litigation.
    – Rider_X
    Jul 24, 2019 at 22:36

I suspect the advise you have been given is in your interest as much as in the interest of the bike shop (but cannot be certain your not being ripped off)). In their situation I would advise the same. You can always ask another bike shop if you don't trust the one you are using.

The chain and cassette wear together. If the chain was very worn, and you put a new chain onto an old cassette, at best it will wear the new chain faster than it should. At worst it will not engage properly and can skip or you may get poor shifting or even break the new chain. You can see if a cassette is badly worn, but one that is not worn as much, but worn enough to affect the performance of the new chain can be hard to detect (there are tools and techniques I won't go into here).

Its recommended to measure the chain for wear, and when it gets past a certain point, replace it. The cassette should be replaced every second or third chain. However, this is just one strategy of many. Another is to ride until its unusable and replace both cassette and chain.

In you specific case we cannot tell if how worn the cassette is, however because the chain broke, it is more than likely due to excessive wear. If the bike shop replaces only the chain, and it skips and does not change gears well or breaks the chain, they have to fix it, and stand to get a bad reputation.

  • "The chain and cassette wear together." This would seem to imply that there would be a wider matting surface on both a worn chain and a worn gear. Since the space between links/teeth does not change with wear, would this not imply that a worn chain or gear would actual mate better with a new counter part than if both were new? What am I missing here? Thanks!
    – bigjosh
    Mar 15, 2018 at 2:07
  • Your wrong in thinking the space between links/teeth does not change with wear. The chain it gets longer because the roller pins wear down. Look up "Chain Stretch", the common term even though its not stretching in the strict sense.
    – mattnz
    Mar 15, 2018 at 3:07
  • @bigjosh every chain link basically has 2 little bushings that allow it to rotate freely. The interior of these bushings wear out and become slightly larger. When you multiply this small change in diameter by the length of your chain (typically 110-116 links), you get pretty significant chain lengthening. I have seen close to an inch of lengthening on some of the really worn ones.
    – wski
    Mar 15, 2018 at 4:21
  • Oh, this. My local bike shop told me it's too late to replace chain only and advice to drive them until it is fully worn and replace both.
    – user35278
    Mar 15, 2018 at 7:12
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    Since this answer was posted, the question has been revised to include the information that the chain and cassette were only a month old. Mar 15, 2018 at 8:42

No, it is BS. I used to follow this advice, which seems like most bike stores give, but I am now on my 3rd chain with same cassette and everything is fine. For the first few days there might be some skipping. But once the new chain and cassette adapt to each other, there will be no issues. That is unless you really do have a worn out cassette. Keep your chain and cassette constantly clean and you will get many years out of the cassette before it really does need replacing.

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    Calling it "BS" assumes motivation that you don't know. IMO, it was more likely a mistaken assumption from the shop that most people who replace their chain also need a new cassette. Oct 15, 2018 at 11:02
  • Another point is that noone really remembers how far their bike has travelled on a chain. Strava makes this easy if you remember to enter the updates, but for the bulk of people they won't know if the bike has done 500 km or 5,000 km.
    – Criggie
    Oct 17, 2018 at 0:26

If your cassette was only a month old then a new chain should be fine working on it in my opinion. There should be no way that it could be so worn unless you've done a few thousand miles in the month! A good shop should be able to look back through their records of jobs to see that the cassette was replaced... then they could see just a new chain was needed.


If you keep your drivetrain clean and properly lubed it will last longer. If you replace the chain before it stretches too much the cogs and rings will last much longer, maybe even through 4 or 6 chains. If you have one of the blue Park chain checking tools, this can tell you exactly when your chain is starting to get seriously worn. Past .7 on the chain check tool, and your cogs and rings are going to start wearing out. If you replace he chain before this, the cogs and rings will last a long time. If you are riding expensive components this is even more important. A SRAM Red 22 cassette is $275-300. A front ring is $40 or $100-215 (small, large). A Red 22 chain is $35-40. I have ridden 22,000. miles on about 5 chains and my cogs are still good. The front big ring is getting a bit worn and its time to replace that, just looking at it and it seems to not shift as well. But the rear cogs and the small ring seem and look fine. Given that there are 11 of them and mostly the middle 7 get the most use its not really surprising. Now if I had let the chain go, even once, past the 1 mark on the chain wear gauge, I'd be replacing all of it. OUCH!

So don't be a doofus and wear it all out in one shot. Totally an expensive mistake.


I get along fine by having 2 chains and rotating them about every month or so with about 80 miles ridden per week. I get a new chain about every 6-8 months If there is clearly wear on the casette, then I will change it. I go by my personal rule that if any one thing is worn then it is more likely it will wear other things around it faster.

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    Not sure if regularly rotating between chains is worth the effort. I simply replace the chain when the 0.75% side of the chain wear caliper easily slides into the chain. This is usually the case after 2 to 3Mm (10 and 11 speed derailleur shifter, 65kg rider, all weather, usually relatively high intensity training).
    – Michael
    Oct 15, 2018 at 10:57

I was once told the same – you need to replace the chain and the cassette – when I could hardly afford the bill, but the mechanic was right. I had really over-used the chain which had stretched badly, as it lengthens it cuts a new profile in the front rings and cassette.

The stretching is caused by two factors: 1) actual stretching of the two side-plates and 2) wear in the bearings causing them to come looser and allowing each link to move a miniscule amount further apart when under tension. There are lots of those links and tiny variances add up to something significant.

I learnt my expensive lesson and since then care for the chain by keeping it clean and lightly oiled and discarding it when 0.5–0.8% stretched. As the links are spaced half an inch apart just count out 10 pairs and the span will be 10 inches long. A stretch of 1% will measure an extra 1/10 inch, and you should never use a chain with more 1% stretch. An extra 1/16 inch has about 0.6% stretch and that's when I start looking for a new chain.

Buying the best chains you can find and washing every 100 miles in winter, 200 in summer works for me, I get at least five chains per cassette. This is with Shimano Dura-Ace 9-speed chains (CN-7701); chunkier chains should last longer so long as you keep them clean and lightly oiled.


if youre going to buy the same chain brand with the same length and size as the old one then i dont think the cassette needs to be replaced too. Maybe the mechanic doesnt have the same chain type that you have. If you could buy the same chain brand new then i think its better. Im using Shimano olive chain for my fixed gear. 50:17 gear ratio.

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    Chains and cassettes - as long the chain matches the number of sprockets in the cassette - are completely interchangeable. The manufacturer makes no difference. Mar 15, 2018 at 1:18
  • although shimano chains are not in the same league as the rest of their kit and in my experience break very easily.
    – Amias
    Mar 15, 2018 at 11:58
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