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Chain jumping at random / load increase, when at the smallest cogs
When the chain is at the smallest cogs both rear and front, then some times (depends which mood the bike is in that day), the chain jumps when I go from soft to harder pedaling. It does not happen very often. The feeling is like it goes on top of the cog teeths, but I can not be 100% sure as I haven't managed to see it happen. When I operate the derailer with my hand it feels like it not tensioning the chain as much as I would assume it needs to (too much rotational friction), and that tension is significantly better when the chain is at larger cogs (be it larger rear or larger front). The bicycle is used for aprox 3 months doing maybe 45 minutes per day, so even if it was not expensive, it shouldn't be wear ?. It is some years old though, not being used prior to now, so maybe some spring inside is not quite like new any more, or maybe the friction of the upper spring joint (the lower one feels fine) is not like it should. It's a Shimano Altus I think. I also have spring tension problems with regard to getting it to pull down to the smallest wheel, but that's another problem, or simply dirt in between the spring feathers maybe. I just cleaned it (taking it off), but the tension problem felt the same as before also after cleaning. As an alternative cause I've asked myself if the angle of the chain being not to straight when at these smaller front and rear cogs could be an issue, but because some days it's just fine, I think the derailer upper joint tension (or friction) is the problem. It's been becoming slightly worse lately, while the temperatures are going lower. The part of the derailer that has this tension mechanism seems like it's not made for service / repair ?. .... I have read a significant number of postings telling that this is not caused by the reason I state I think it is, but by various other reasons, but I've ruled out all those other suggested reasons. The bike was not expensive though, so dirt and early wear can not be ruled out, and I'm definitely using the smaller rear cog the most (mostly only that one), but I fail to see any obvious wear. I currently think that this Shimano Altus is too cheap and only tensions like it should when it's new. But given that I have close to no experience with this (Just started using the bike to work for the first time in my long life), general experience tells me I should listen to the experience out there :-) ...... One guy in another posting concluded the chain was too long from the factory. But why I didn't have this problem before then ?. I may differ from most people with respect to having a preference for using high? gears with low leg rpm. When I watch others they mostly pedal at maybe 2 to 3 times the leg rpm of what I'm using.
The big question is if the chain is jumping between different gears, which indicates the shifters being out of adjustment, or if the chain is just jumping over the top of the teeth, which indicates that your chain and cassette are worn. A possible third option is that the ratchet inside your freehub on your rear wheel is failing prematurely.

Chain angle, the springs on the derailleur, etc are all important to some degree. But it's somewhat unlikely that anything you describe is causing your problem.

It can be hard to diagnose these things because they only happen when your riding. You can try to have someone jog along side you to see if they can spot if the chain is jumping and how. You can also measure your chain stretch (google it) to see if it is showing signs of wear.

If the bike's 3 months old, can you take it back where you got it. May be warranty issue or at least they may help you diagnose for free even if it is normal wear.

On pedaling, your body is most efficient turning the pedals somewhere between 60-100 rpm. As you ride more, your ability to spin faster will improve also. There are times to crank at slow rpms, but generally all this does is make you more tired and wear out your knees. It's a very common thing for newer riders that at first they tend to ride in too high a gear for the speed they're going.
Thanks :-). If the lower rear cogweel is worn (or full of dirt), why then is it not jumping when I user the larger front cog wheels ? ... If some individual chain joints are damaged (maybe because of this jumping), then maybe, in combination with low tension and worn and/or dirty lower cogwheel, then maybe, as it needs to take some narrow curves back there. ... Or if the smallest rear cog wheel is maybe worn already (that's soon I think), it's alowing the top side chain exit to far towards the next cogwheel ?. ... The derailer and chain I just cleaned very properly, but I didn't focus on the casette. Sorry about my english vocabulary, it's kind of limited with regard to bicycle parts. I'm focusing on using the muscles around my knees to take the load in order to avoid putting strain on the knee joints, but it's definitely something I should bear in mind as I just passed 50 :-). Thanks.
Slow reps like you are doing are not helping save your knees at all. They are putting more strain on your knees, not developing muscles around the knee directly. The very act of pedaling puts effort directly through the knee joint. So better to work at a higher rep level and develop gradually. Otherwise, you are asking for knee issues.
I could not agree with RB more!
Too many folks forget that bicyling is an arobic activity and not a weight-bearing exercise. With improved Cardio fitness comes the improved muscular fitness. You have to first be able to pump blood to those muscles, yes? Don't mash. Spin.
Improper use of sporting equipment accounts for the majority of people with good intentions that wind up quitting before they have a chance to improve.

At 48, I'm right behind you, finn. Smile
Wheelies don't pop themselves. (from a QBP fortune cookie)
Thanks for the muscular advices. I have now bought and replaced : Chain, Casette (rear cogweels), and Derailer (rear gear shifter mechanism). Now all the problems are gone. Before It didn't always manage to shift down from the innermost rear cogwheel to the one below, and the spring/derailer quite often was not capable of pulling it back to the outer/smallest cogwheel. Those problems are not there anymore. When I push/pull the derailer mechanism by hand it's now moving with ease (no unwanted internal friction), and the springs feels like they have the tension they should have. The new one is not "Altus" but "Actera"? or something like that. I failed to see any obvious wear on the parts I removed, and the old chain felt o.k. So what caused the chain jumping I do not know. I do have a couple of theories though. As previously mentioned insufficient spring tension and/or to high friction causing to little chain tension is still a suspect, causing chain jumping underneath the smallest rear cogwheel (possibly in combination with dirt). Additionally I've asked my self, due to the nature of the evolving of the problem, if the diagonal angle of the chain when at the combination of the smaller front cogwheel and the smaller rear cogwheel, in combination with minimal wear on the smallest rear cogwheels (as I've been using almost only that), and in combination with microscopic chain wear, and in combination with dirt ?,... may have caused the chain to try to jump up on the adjacent? rear cogwheel ?. As the jumping happened when I increased the load, thereby initially increasing the tension of the upper part of the chain, causing it to be bent inward as seen from the rear cogwheel. But that would be a very tiny intermittent jump if so, and the feeling I got when it happened was a bit like loosing the chain tension for a fraction of second, a feeling I would more likely get from the derailer failing to enter the chain properly underneath the cogwheel. So sorry, but no final conclusion. Simply dirt is also not to be ruled out. From now on I will try to avoid using almost only the lower rear cogwheel to avoid putting all the wear almost only on that one, in the hope that this time it will work a bit longer than 3 months :-). And maybe I need a bike stand to make it easier to keep it clean back there.
"As the jumping happened when I increased the load, thereby initially increasing the tension of the upper part of the chain, causing it to be bent inward as seen from the rear cogwheel."

When you say bent inward do you mean tilting to the side toward the next gear? That should not happen in any gear or situation if things are setup properly. Chain should be in the same vertical plane in all gears Now, if your derailleur hanger was out of alignment it could cause that.
Painting pictures with words is a challenge. I have 3 chain weels in the front and 7 in the rear. If we're viewing the bicycle from the rear and above, and look down on the chain on the innermost front chain weel and on the outermost cogweel of the rear cogset, then the chain points straight on the legs around both the rear and fore cogs, but has a slight diagonal angle from the rear/aft to the fore, thus when applying pressure to the pedals the upper half of the chain will be slightly bent in the horizontal plane where it exits from the rear cogs (and also where it enters the fore cogs). And it was only when the chain was in this max diagonal gear combination that the skipping/jumping occured. And consequently my logic concluded on this in combination with worn outermoust rear cogs as a maybe possible cause. The only argument against it being that the skipping felt more like it entered on top of the cogs and remained there for half a rotation of the the smalles rear cogwheel, takin my thoughst back to the lower half of the chain along with lack of chain tension, and possibly also dirt around the outermost rear cogs. Derailleur adjustment not having been a part of the jumping/skipping problem I feel more than 90% sure of.
Maybe this one is related : http://forums.bikeride.com/thread-5127.html
And maybe this one is related as well : http://forums.bikeride.com/thread-5153.html and that it was cleaned during service ;-). ... And this one as well could be related : http://forums.bikeride.com/thread-5215.html
Now I had this problem over again, with another, brand new bicycle, that I bought to have one with winter tyres. It came after aprox 4 weeks of to/from work. Over a period of maybe 3 months. (using the other one when it's above freezing). This time it came suddenly, and was a bit worse, and happened on all gears. So I managed to reproduce it in the garage, being able to see what the problem was. The feel of the chain jumping was almost exactly the same as last time. So likely the same cause. There was one! single joint! on the chain, that were happy to rotate/bendt the one way, but that got stuck (high friction in the joint) after being rotatet/bendt the other way. Possibly from sand/dirt/dust from the road, as it has been exremely dry her for 1 month. Salty air (close to the ocean), and the bicycle standing outside, may have contributed as well. So when the chain passes the two small lower entry weels the chain link first gets rotated one way, then the other way, and then that specific joint stayed in a 90 degree bend, lifting the chain out from the center cogwheel, causing failure of re-entry of the chain, and it looked like ti caused som de-formation on the lower cogs as well, but that then as a consequence of the chain-joint-stuck problem. The solution was to clean it with 5-56, and running the chain in sharp angles (Both ways) over a screwdriver, making shure all the joints moved with ease. I do not lubricate with oil chain lubrication as that makes the dirt/sand stick to the chain, and in my theory will increase the rate of wear, more than a bit of rust. Sorry for the complicated explanation. A picture would have been better. One can easily imagine that this kind of fault could happen more often on the outer/smaller cogwheels, as the chain gets more bent before entry on those gears.
Your problem is actually a common one, though not always that extreme. You are describing riding in a "cross-chain" combo (small-small or large-large). It is typically at least noisier due to the large entry angle at which the chain leave the rear cog and engages the chainring, as the chain side plates can catch on the teeth. In your case it's possible the angle is large enough (because of a not-optimal chainline or short chainstays) that the chain when under pressure catches on the next largest chainwheel (especially if there is a large difference between the two chainwheels). There are commonly pins on the chainwheel that could catch the chain during such times.

Due to normal tolerance the chainwheels are not in a perfect plane and the chain is not perfectly straight, which in combo with different amounts of stress (during which times the chainwheels can flex) would explain the intermittent nature of the problem. If you are exerting an especially high stress on the drivetrain it's also possible that the frame is flexing, which would make the angle slightly worse, and the problem greater.

Note also that riding the small-small combo will lead to very premature wear, as for a given gear ratio the pressure on the drive train is the same but the chain passes over the rear cog many more times per mile. It's much better for noise and wear reasons to ride the next chainring up with a larger cog. Due to overlap in all multi-gear bikes you can almost always find a similar gear ratio. The cross-chain positions should not be ridden routinely, and certainly not ever under high pedal pressure.

Finally, poor derailleur position below the cogs could not have been the cause of your problem. The chain as it enters the cog from the pulley is under almost no tension, and the tension does not change at all when pedaling harder.

p.s. The tight link problem you noted has a very distinctive rhythm in most cases, as a particular point on the chain makes a complete revolution that is dependent on the length of the chain and the number of chainwheel teeth engaged, so for example with a 112 link chain and a 39 tooth chainwheel the tight link comes around a little less than once every 3 crank revs. The most common fix for a tight link is to flex it back and forth sideways.

As for chain lube, the original lubrication on the chain is quite good so one can run without additional lube for some time, but running a chain with no lubrication at all will indeed wear the chain much quicker than otherwise.
p.s. Although poor initial position below the cog is not a cause it is possible for extreme flexing of the frame to essentially tension the shift cable more and change the derailleur position.
Here's a picture of what may happen when one (or more) of the individual chain joints doesn't flex with ease. It explains the problem I had better than all of my words I think.
(07-02-2014, 12:44 PM)finnstru Wrote:  Here's a picture of what may happen when one (or more) of the individual chain joints doesn't flex with ease. It explains the problem I had better than all of my words I think.

It's nice you provided a photo example of the problem you describe, but again a stiff link does not match up with the OP's symptoms.
Are we talking Cross chain, High to High and small to small rings?

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