My chain slips despite new parts and tuneup

Question

I use my bike for commuting, and so my knowledge of bikes is enough to make to work and back.

I have a trek 4500 mb. I purchased it used, and from the very first day I have had this issue where the chain slips/jumps. Which means, peddling is far from smooth.

I've already:

1. tuned up bike (several times)
2. replaced chain
3. replaced cassettes
4. replaced crankset
5. replaced derailleur (rear)

But the issue persists. Slipping happens every day, at least once a day, but often when I create more torque.

What could the issue be? I like this bike, but the random slipping is rather dangerous as sometimes it happens when I'm standing. I have 2 bike shops near me, neither can give me answer or fix the issue.

Answer 1

I presume all the new drivetrain parts were replaced close enough together that they were all new or new-ish at the same time. If on the other hand just the chain was replaced, the cassette or rings were highly worn, you rode anyway for some time (say a few hundred miles), and then replaced those too, it's possible you could have put enough wear on the chain in that time to keep the problem going. You can measure the chain wear with a ruler to test this.

Misaligned rear derailer hangers and/or cable housing friction can both cause this and unfortunately can be missed by shops. If that were what's happening, shifting would be poor too.

In an extreme case, rear frame misalignment can cause this. You'd probably be able to see it though.

The other thing that can do this is issues with the ratcheting mechanism inside your freehub or freewheel, which can also go unserviced at shops. The common scenario is the lubrication at the pawls gets gummy or depleted, keeping them from popping up with adequate force to engage properly with the driver or drivering (arbitrary terms for whatever toothy bit the pawls mate with). Or a pawl or pawl spring can be broken or out of place, creating the same problem. Then sometimes as you pedal, just the tip of the pawls might be engaged with the teeth, which feels solid until it slips under load due to incomplete engagement. The sound and sensation is very similar to chain slippage.

If that is what's happening, one problem is hubs can't take too much of it before the pawls or driver get damaged and can never work right again.

You could diagnose this by subbing in another wheel to test, or by getting someone to watch the skipping as it happens and confirm its the freehub turning and not the chain slipping. With experience one can make pretty good educated guesses about what's happening inside a freehub body by manually back-spinning it with the cassette off. Sometimes you hear tinkling or crunching inside and it's less of a guess.

Fixing it depends entirely on what hub it is. Usually the first step would be get some lube inside and see what happens. Some hubs make it easy to take the whole thing apart, but many do not.

Answer 2

Daniel R Hicks had the right idea in the comments. (It should have been an answer).

I replaced an old, barely skipping chain with a brand new one, and it skipped a lot more, right away. I finally figured out that it was too long (by 3 links), but by this point, my two most commonly used sprockets had been worn down by all the slipping.

When I was looking around for this same answer a while back, I found most people talking about worn teeth/sprockets, narrow chains, and minor adjustments. I think a properly sized chain is the best foundation for avoiding this problem, and should precede all the other troubleshooting steps.

Edit: There is a commonly held belief that a stretched chain causes a lot of wear on cogs simply by virtue of the increased spacing in the chain. This makes only a little sense. While an increased inter-link distance will put more stress on an individual tooth as you pedal, it's only causing a very minor and slow amount of wear compared to a chain that is slipping. (The movement plus friction of a slipping chain is going to remove a lot more material from more teeth than a static pressure on individual teeth or a tiny amount of single-link slippage).

I think what's going on when a chain stretches is the following:

1. A tiny amount of cog wear occurs when a chain begins to stretch. It does not cause slipping (beyond a small amount that happens on an individual link basis). This tiny amount of cog wear doesn't cause slippage, but it sets the mechanism up for step two.
2. When the chain becomes long enough from the stretch (or a new and excessively long chain is installed), a number of things can allow step one above to lead to extreme slippage. The rear derailleur may not be at an angle or tension that will hold downstream links to the cogset in a way that prevents anything more than single-link slippage. A new longer chain won't be straight, and any misalignment in either the front or rear derailleur will cause the chain to skip off the cogs. Etc.
3. As soon the chain is too long to contain slippage to one link at a time, and the whole chain slips at once, massive amounts of wear occur. The cogs become unusable a lot faster than they would with the same inter-link distance but a shorter chain.

This is all just my theory of chain slippage, but it checks out with the rates of how mechanical wear occurs. It also explains why or how anybody can have those pictures of completely worn cogs while still able to bike (if your chain is the right length for most of the time you wear down your cogs, it'll all be gradual, individual link wear). If chain length weren't a major factor in slip, as soon as the teeth wore down a little bit, the effect would be the same mechanical situation as an increased inter-link distance, the chain would slip like crazy, and they wouldn't be able to bike any more to produce that massive amount of wear.

Answer 3

You left a list of the parts you have replaced. You did not mention the bottom bracket bearing. As it wears out, it wobbles more and more. The misalignment can cause derailer to malfunction. I had a used bike with a ruined crankset before that needed a new bottom bracket bearing. It's fine now. You need a special tool to remove the old bearing but it's not that expensive. Hope this helps. :)