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I have a Raleigh MTB "Sports" with 6061 aluminum frame that was bought new in 2006 or 2007. It's undergone various repairs over the years and I'm still decently happy with it.

However, recently it's been skipping when I pedal — suddenly the pedal will just jerk down and the chain seems to slip past a few teeth. I've concluded that the crankset probably needs to be replaced.

This is my first time doing so. It's a 3x7-speed with 28" tires. The original crankset appears to be called Sunrace Fluid Drive.

I'm not looking for anything fancy. I'm a casual biker. It just needs to work and not skip. And be shippable to Canada.

What do you recommend for a simple solution? (Also, if I'm misdiagnosing it, that's welcome too. But I did already replace the rear derailleur and the chain recently.)

Pictures (chain loose because derailleur's not on at the moment)

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    Can you add a clear and well-lit photo of your crankset and your cassette? So we can see the wear on the teeth. Also try a chain gauge on your chain, or if you don't have one, grab two adjacent links and feel for play. If there is perceptible play your chain is elongated and needs replacing too.
    – Criggie
    Jun 6 at 21:57
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    @Criggie Good idea. Added some images to the post. The chain is brand-new (and was bought after the slipping problem started, so it neither caused nor fixed it). Jun 6 at 23:02
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    That middle ring is totally gone!
    – JoeK
    Jun 7 at 6:43
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    @ChrisH OP mentioned the chain is new. Maybe it is not worn out? I am aware a worn out cassette will not be good with a new chain (as OP is experiencing) but isn't the chain wear independent from the cassette? I feel that in general the stretched chain is wearing out the cassette, rather than the contrary. So I would preserve the chain (if OP didn't cycle that much on it)
    – EarlGrey
    Jun 7 at 11:00
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    @LukeSawczak if it's really new, and not just much newer than the worn parts, you should be fine keeping using it. Used with worn components it will wear quickly, so there's a chance it's suffered more than you'd have expected while tracking down the problem and parts. (I'd missed your "brand new" in the comments - it should be fine)
    – Chris H
    Jun 7 at 11:05
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The SunRace crankset is cheap. They are still available and probably have an identical specification. The cheapest types have non removable chainrings. The steel used to make them is not particularly hard wearing and they will wear fast with a worn chain. My experience is that you can get slipping from the chainring, though these usually wear more slowly because they are larger than the sprockets at the back.

Given the low cost of the components used for your entire transmission, it makes sense to replace the cassette/freewheel, chain and chainset all at the same time, this time around; then it should give you a decent amount of use again (taking into account that derailleur cables and pivots can wear etc affecting performance) and ride pretty near to new, as far as the gearing goes. This could be (2021) £50-£100 in parts and no more than 1hr labour for a shop.

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  • Looks like a 48-tooth outer chainring. From the era of the bike, probably a 7-speed freehub using a cassette, not a 7-speed freewheel. Probably a square-taper bottom bracket.
    – Armand
    Jun 6 at 23:28
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    It looks like if you could get the bolts out you could replace the middle chainring. But a cheap square-taper (almost certain, but check) crankset won't cost much more, e.g. Shimano Tourney which should be easily available.
    – Chris H
    Jun 7 at 8:55
  • Have ordered new Shimano cassette, crankset, and chain. If it fixes the problem (how could it not) I'll accept this answer. Thanks! Jun 7 at 11:20
  • Hi @LukeSawczak Hopefully all will go according to plan. It looks to me, since you have put up the photis, that the rear sprockets are the freewheel type. Hopefully you end up with the right kit. The Shimano craks should fit on the same length bottom bracket as the sunrace but you will probably need to adjust the front gears quite heftily, else get the "ideal" length bb too.
    – JoeK
    Jun 7 at 12:10
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However, recently it's been skipping when I pedal — suddenly the pedal will just jerk down and the chain seems to slip past a few teeth. I've concluded that the crankset probably needs to be replaced.

This conclusions is most likely false. A chain does not slip on front chainrings.

There are two reasons why the chain could seemingly slip:

  • First, a chain can slip on rear cassette sprockets. This most often occurs when installing a new chain after the old chain has worn cassette teeth in a manner that cause them to worn only on the old chain. However, I suspect if not changing the chain for a long time and letting it wear way past the recommended wear limit of 0.5%, it might be possible that the old chain could start slipping. I have not experienced old chains slipping because I always replace my chains at 0.5% wear, and occasionally notice the chain slips so the cassette sprockets need to be replaced too.

  • Secondly, the freewheel in the freehub body could slip. This most often occurs in cold conditions on freewheels that have been lubricated with thick grease. The cold causes the grease to become even more thick, so thick that it prevents the pawls from engaging the freewheel ring. The fix is to remove the grease from the freehub body and install a light oil in its place. Unfortunately, freehub bodies can be difficult to open.

Since you said you replaced the chain but did not say about replacing the cassette, I suspect your slipping is of the first type, and the fix is to replace the rear cassette sprocket set.

The rules of drivetrains are:

  • A new chain may not engage to a worn cassette
  • A worn chain may not engage to a new chainring

So when replacing any drivetrain component, you may in some cases need to replace everything behind it too. Example: if you replace a chainring, it might necessitate replacing the chain, which can in turn necessitate replacing the cassette.

I would never replace crankset or chainrings because of a slipping chain. The best chainrings (7075T6 / 7075T651 aluminum alloy) can last hundreds of thousands of kilometers. The only reason to replace a chainring is if the teeth have been practically worn away, or if you experience chainsuck, i.e. chain not disengaging from the chainring (happens only with long cage rear derailleurs and small chainrings, typical of MTBs -- does not happen on road bikes).

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  • Thanks! Very thorough explanation. At least one of the front chainrings is badly worn (the bike has been ridden quite a lot over its 14-15 years, though probably not hundreds of thousands of km). Would that make it worth replacing? But to your main suggestion, if I'm hearing you clearly, my best bet is to simply buy a new cassette of a similar type, such as one of these? Jun 6 at 18:49
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    "This conclusions is most likely false. A chain does not slip on front chainrings." Did you look at the middle chainring? I'd be surprised if you could crank through a full rotation without the chain slipping.
    – DavidW
    Jun 7 at 18:41
  • @DavidW In fairness, there were no pictures yet when this answer was posted. Jun 8 at 0:13

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