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.