I just bought a BTwin Fit 5, all stock, fresh from the distributor, Decathlon. Took it for light pacing at the store and things seemed OK. About 100 metres out the door, I shift up to the top end of the range, pedalling firmly but not hard, and it promptly throws the chain to the outside.

Returning to the store, the mechanic puts the chain back on, adjusts the tension on the derailleurs and tells me not to use the top three or bottom two gears (i.e., the 3 highest resistance and two lowest resistance shift positions in the drive train's range), or the chain will throw again. I asked why, but unfortunately, I'm a foreigner and don't speak the local language well enough to understand any of his explanation. Please note I definitely did not misunderstand the bit about not using the gears; this was crystal clear. 

Wild speculations on what he may have said about the reasons for this:


Chain tension or play due to the lateral shift between the inner and outer gears of the front and rear cassettes when shifting
Chain length
Something about the derailleurs


Again, there's a substantial language barrier and I have zero confidence in my understanding of what he said about why I shouldn't use those gears.

If you're not familiar with Decathlon, it's a sports department/box/super store. Definitely not a high end or dedicated bike shop. The house mechanic struck me as less than an expert, to put it gently. I've definitely read some reviews online griping about the inability of Decathlon staff to correctly set up the drive trains of new bikes. However, it's important to understand that this quality of expertise is representative of (or better than) that of every shop within an hour's travel.

This advice seems utterly mad to me. What on earth could be the reasoning behind this? What can I do about it? Aside from the obvious of taking it to a better mechanic, as that presents substantial difficulty for me on a number of fronts. The bike shifts without a problem, including before the throw. I did test out every gear during my test ride. I haven't yet experimented with trying to reproduce the problem yet; I'll do so in the next few days most likely.

The bike has a Shimano Sora groupset and Shimano Deore shifters. I'm an experienced rider but no mechanic; I haven't done anything beyond basic maintenance myself. I can provide additional pictures and am game to make relatively minor adjustments, but even just clearly nailing down the issue would be considerably useful.







Additional pictures, shifted to big-big.








Question: Possessive form (of) with compound nouns
Answer: As I understand it, your question about the sentence


  This is a main difference between the personality of good people and bad people.


involves determining whether "the personality of good people" is being contrasted with "bad people" or with "[the personality of] bad people." Structurally the sentence permits (with a little logical fudging) either reading, though I suspect that the author intends the second one.

That being the case, the biggest weakness of the sentence as written is that it seems to favor a logical reading that finds the intended contrast to be between "the personality of good people" (where "personality" is obviously a singular noun) on the one hand and "bad people" on the other. We can start to overcome that problem by changing personality to personalities:


  This is a main difference between the personalities of good people and bad people.


At this point, we still have (from a strictly logical reading of the sentence) a contrast between "the personalities of good people" on the one hand and "bad people" on the other. But beyond that, we have a plausible set of parallels attaching to the stem word of: "good people" and "bad people." And whereas before that of linked problematically to a singular personality, it now links to the plural personalities—a better platform for the parallelism. And we can strengthen the parallelism of these phrases by adding the word  of before "bad people":


  This is a main difference between the personalities of good people and of bad people.


Now the parallel phrases "of good people" and "of bad people" attach to the stem word personalities—and a reader cannot logically misread the intended contrast as being between "the personalities of good people" on the one hand and "bad people" on the other, without ignoring the of immediately preceding "bad people."

As other answerers and commenters have pointed out, there are alternative ways to make the sentence unambiguously present a contrast between the two types of personalities. For example, you could say


  This is a main difference between the personality of good people and the personality of bad people. 


But the shorter form that I suggest is every bit as clearly parallel and yet (to my mind) somewhat less tedious.

Conversely, if you wanted to establish a contrast between "bad people" on the one hand and "the personality of good people" on the other, the simplest way to proceed would be to flip their order in your original sentence:


  This is a main difference between bad people and the personality of good people. 


Either way the things that the sentence is seeking to compare are clearly defined, and readers won't be tempted to misread the writer's meaning.



As a side note, I acknowledge that the form "the personalities of good people and of bad people" may suggest to some readers that the author is comparing multiple personalities of good people to multiple personalities of bad people, whereas the original wording seems to take the view that good people have one unitary personality and people a very different unitary personality. If this change runs afoul of the author's intentions, we could revise the wording (again) to say something like this:


  This is a main difference between the personality of good people and that of bad people.


where the new parallels are "the personality of good people" and "that of bad people," both attached to the stem word between.