Chain jumping at random / load increase, when at the smallest cogs

[finnstru]

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.