New chain jumps under load

[Custom24]

Hi
I've Google'd this, but that's just confused me.

Specialized Hardrock 24 speed Mountain Bike (Hard tail). This is all my own doing (no LBS involved)

My old chain (the second chain on the bike from new) was worn >1% according to the chain tool, so I replaced it. I bought a KMC x8 .93 SP chain this time, which I think is very similar to what came with the bike originally. I measured it and counted the links versus the first chain that came with the bike.

Now (instantly after replacement), the chain is jumping when in the smallest sprocket on the cassette, when under load. I can see it happening on the bike stand if I use the brake as a load. It's not trying to move sideways or change gear - it looks like a link is "hopping" upwards off the cassette momentarily.

I thought the problem was indexing at first, so I replaced the gear cable which needed doing anyway and adjusted the indexing stops and tension so that everything is perfect on the stand

When cycling, the smallest sprocket on the cassette is unusable - it is hopping continuously (which makes me think not a stiff link, because it's constantly doing it). It is also happening to a much lesser extent in the next sprocket up, and even a little on the one up from that, although I can't see those jumping on the bike stand.

The peculiar thing is that the first chain lasted about a year before the chain tool said it needed replacing, and the second one only lasted about 3 months. I wonder if the second chain has chewed something? The second chain was also a KMC 8 speed, but I can't remember which one (except it was cheaper).

What to try next? If the answer is to replace the cassette and front chain rings, then I'm tempted to put back on the previous chain and just cycle on it and replace the whole lot at once when something fails terminally. It wasn't causing me any problems at all apart from the chain tool telling me it was worn.

Thanks
Mark

 

[Shut Up Legs]

The chain's jumping sprocket teeth because the old chain and cassette were subject to wear together, but now you have a new chain (with different link spacing to the old, i.e. not as stretched) and old cassette, so the chain links don't line up with the cassette cogs' teeth properly. This is particularly noticeable with the smallest cassette cog because of its small radius. The solution is to replace the cassette also. Depending on how much you use each front chain ring, you may also need to replace the more frequently-used ones, because you'll have the same issue with new chain link spacing not matching the old chain ring teeth spacing.

Sorry if that wasn't the answer you wanted .

I've been told that if, after buying new chain, front chain rings and rear cassette, you rotate between 3 chains for the life of the cassette, you can make the cassette & front chain rings last over the lifetime of 3 chains. What you do is swap the 1st chain for chain #2 after about 2,500km, then swap for chain #3 after another 2,500km, then repeat this once more for each chain. You then get roughly 15,000km of use out of the cassette. I've never tried this myself, because I'm just lazy: instead I just replace chain, cassette and front chain rings together when the chain starts jumping.

So one alternative solution for you is: put the old chain back on, use it until it starts jumping, then replace chain, cassette and front chain rings together. When you do that, you could then give the chain-rotation method a try.

Also, how much distance you use each chain for depends on how you use it. If, like me, you ride with a lower cadence than average and ride more hills than average, the chain's life span may be shorter, and so you would need to rotate chains after a shorter distance than the above-mentioned 2,500km.

Hope that all made sense .

 

[Custom24]

Thanks victor. It totally makes sense, and is what I was expecting, although not hoping! The smallest sprocket is separate to the rest, so there is also the option of replacing that, although I prefer the idea of putting the previous chain on. I have some spare jockey wheels already, so I don't need to worry about the derailleur.

 

[Amanda P]

Victor's right. And you're right: you could, if you wanted, replace only the sprocket that's causing the trouble (if you can find a replacement). Actually, it's likely that all the sprockets are separable - they're usually only held together by three soft rivets, easily drilled out - all they do is hold the sprockets together for convenience, you don't need them.

If only the smallest sprocket is causing skipping, it's very unlikely that the front chainrings will be badly worn. As they're much bigger, the pressure, and the wear, are spread over more of the teeth, so they wear more slowly than sprockets.

Your strategy is one I've used: if a new chain is skipping, run the chain and the cassette into the ground together and then replace them both at the same time. But have the spares on the shelf ready to go.

Jockey wheels do wear, but won't cause skipping - just friction or noisy or slow shifts.

 

[Custom24]

Thanks
Is there a loss of power transmission efficiency using a worn system, if it is not yet skipping?

 

[Custom24]

Actually I did not realise front chain rings were so expensive, compared to cassettes. I am not sure which chain rings I can replace independently of the cranks either. So I am not sure my plan to run things into the ground is a great idea...

 

[Amanda P]

If your chain rings can't be removed, there's a reasonable chance that they're steel, in which case it's even more unlikely they'll be badly worn.

Are the chainring teeth even, or are they worn hooked on their front edges like shark's fins? If they're not visibly shark-finned and a new chain isn't skipping on them, stop worrying: they're not worn. (You'll normally replace five or six cassettes in the lifetime of a chainring).

Beware, though, that some chainrings have occasional teeth that are a funny shape or only half the height of the neighbouring teeth, or are hardly there at all. That's not wear, that's there from new to allow the chain to move smoothly from one ring to the next. So have a good look at all the teeth. (This is true of sprocket teeth too).

A worn chain isn't going to be noticeably less efficient than a new one. But it won't shift as nicely, and will wear the other transmission components faster than a new one - principally the sprockets.

Is it rusty? Are there any stiff links or protruding rivet ends? If so, there's an increased danger of it snapping. Trust me, you really don't want that happening. But if it's just a bit worn, only enough to make one sprocket skip, it's probably not going to snap any time soon.

The thing about chains is that they have to be kept lubricated. But the lubricant tends to be sticky, and bits of dust and grit stick to it. Mixed with the lubricant, this becomes a kind of grinding paste which wears away the inside of the chain's pins and rollers, so that each link effectively becomes a tiny bit longer than it started out.

This dust-grit-and-lubricant grinding paste on the outside surfaces of the chain also serves to grind away at the sprocket (and chainring) teeth (but mostly the sprocket teeth because there are fewer of them in contact with the chain per pedal revolution, and the pressure from the chain is spread over fewer of them too).

You can minimise this by keeping the chain scrupulously clean, removing all excess lubricant (what's left on the outside of the chain isn't doing you any good) and so on*, but if you use the bike, the chain will wear. I look upon chains as a consumable item, and expect to replace them at least once a year or every two or three thousand miles, depending on how much and where the bike's been used - bikes used in wet salty winter weather get through chains much quicker.

*Search the forum for "Mickle method" to see how best to do this.

 

[Custom24]

Thanks for the time to write that detailed reply.
I use the Mickle method, but after about 5-10 minutes, sore feet from crouching, and the paper towel still coming off black, I generally stop. I've just bought a Barbieri chain cleaner, so that might be better.

My usage pattern and lazy cleaning routine is very hard on components, so I am happy to treat them as consumables too. I do a muddy off road commute all year round (apart from in the ice), 6 miles each way every day, and then most weekends I go out off road as well.

The chain I'm going to put back on for the moment is fine. I will double check, but there are no stiff links, certainly no rust, and no rivet problems.

I've so far not bothered with anything other than 3 in 1 oil on the chain and GT85. When I replace the drive train next, I will invest in some wet and dry Finish line or similar. Don't know how much difference that makes.

Is the "new" chain I have OK to use in the future, perhaps as Chain number 3 in the Victor rotation method above?

 

[Davidc]

I agree with much of the above, certainly you need a new cassette.

As for using chains in rotation - just messy storing them. Buy a new chain when the old one's at 0.75% wear and put the new one on when next convenient. A cassette should last 2 chains, sometimes 3.

I predominantly use the middle chainring on my triples and that does wear out eventually despite being steel. The last one lasted 9 chains. It is worth keeping a note of chain, cassette and chainring changes.

Finish line dry lubricant is great stuff. Do shake the bottle well though.