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.

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Additional pictures, shifted to big-big.

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    Check the part number under the Deore shifters; If it contains "M6.." they're 10sp shifters which will pull the wrong amount of cable for the 9sp rear mech, and have the wrong number of stops. If the part number contains "M5.." they've merely been setup incorrectly. – Emyr Oct 13 '14 at 15:03
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    Presumably he was advising against "cross-chaining". But even when you're cross-chaining the chain should not come off THAT easily (assuming you were not abusing the bike by, eg, shifting under extreme load). It's mainly not advised because it causes excessive wear and noise, and the problem is usually only significant on a 3x front. – Daniel R Hicks Oct 13 '14 at 15:35
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    Two things. First, the whole shifting thing can easily be self-maintained, I think it is definitely worthwhile learning how to check and adjust this yourself. Second, I have knowledge of Decathlon, admittedly from a while back, but found them pretty good. I'm just making the point that it's the individual mechanic, not the shop. I generally view Decathlon goods as not top-notch, but nevertheless great value for money, – PeteH Oct 13 '14 at 15:43
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    BTW, that chain does not appear to be very well lubricated. – Daniel R Hicks Oct 13 '14 at 19:46
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    After reading your question, all the answers and all the comments, I've come to the conclusion that either you just need to adjust the derailleurs (see youtube), or something is damaged and you shouldn't be paying for it. – Carey Gregory Oct 14 '14 at 5:01

Not using the upper and lower gears is a very effective solution. Stupid, but effective.

Traditionally one would simply use the limit screws (at the rear derailleur, often marked L(ow) and H(igh)). Shift to the lowest/highest gear (front and rear) and tighten the screw so that it only allows the mech to move ever so slightly over the edge of the largest/smallest sprocket (Having the screw too tight makes it hard to shift to the lowest/highest gear).

More detailed and always a good read: http://sheldonbrown.com/derailer-adjustment.html#rear

  • It is indeed effective to avoid using those gears - the mid range performs just fine - but the idea of effectively losing 18.5% of my gears puts quite the bad taste in my mouth and seriously lowers the usefulness of the bike, making this advice ultimately useless to me. I want to get my ride in proper shape, not learn how to put up with an obnoxious problem. The link and advice on tightening the limit screws are helpful though, thanks for that. – Esoteric Screen Name Oct 14 '14 at 13:25
  • I've accepted this as the answer because it addresses the actual issue, which was not cross chaining (if you think it was, re-read the question more carefully, though I can understand why that was the first thought of many users), and because it offered actual advice on what to do, as opposed to just telling me to search. Adjusting the derailleur limit screws fixed the problem, thanks @linac! – Esoteric Screen Name Oct 17 '14 at 1:56

You have a double front, right? The usual advice is to not shift into the highest 3 gears in the rear cassette when in front the chain is on the large chainring, and to not shift into the smallest 3 cogs, when the chain in front is on the small chainring.

This prevents 'cross-chaining', which wears the chain fast, produces noise and difficult shifting. Furthermore, some derailleurs may not even support those extreme combinations, when the chain is crossed.

On a side note, the chain should not be dropping in any gear combination, probably the limit screws should be touched a bit, as linac describes.

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    Other than excessive wear in the most extreme position, this is really out of date advice today. Modern drive trains shift fine into all gears. His problem is incorrect adjustment. – Rich Wagenknecht Oct 13 '14 at 20:56
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    @EsotericScreenName The advice from the shop is utter bullshit. You should be able to shift into any gear combination with a functioning drivetrain. Whether you should use some of them or not is another debate, but you should be able to shift into them. – Carey Gregory Oct 14 '14 at 5:16
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    Why is a twice incorrect (1. triple, not double 2. legacy advice) answer voted top? Here's what Shimano has to say : When the chain is in the position shown in the illustration, the chain may contact the front chainrings or front derailleur and generate noise. If the noise is a problem, shift the chain onto the next-larger rear sprocket or the one after. link – Vorac Oct 14 '14 at 6:45
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    @CareyGregory has put my reaction to this advice quite succinctly. On a new bike, there is absolutely no reason I should expect the chain to throw (and throwing is what I was told to expect at the shop, not just wear or rough shifting) when shifted to top gear. Vorac, I don't understand the relevance of your latest comment, can you please elaborate? – Esoteric Screen Name Oct 14 '14 at 13:13
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    @EsotericScreenName, I began wondering if really cross-chaining hurts and decided to quote the manufacturer. To be honest, I was living with the assumption that cross-chaining is a major evil. Btw recently I learned that running disc brakes with worn out pads could be no problem at all - a friend wore his brake pads so much, that the pistons of the caliper were grabbing the disk, and yet the pistons and the disk are OK. – Vorac Oct 14 '14 at 13:26

Crossing chaining hasn't been any problem at all since the invention of bushless chains 20+ years ago and wasn't even a real problem back in the ancient days. It's a persistent myth that just won't die...

Your bike should leave the shop capable of shifting into any combo of gears possible and riding any amount of time you like in that gear. At most I would accept a slight extra noise in the extreme combos(big/big and little/little), but those components were designed to work in all the gear combos.

If you are mixing/matching components or running odd size gears, you might run into some problems with derailluer capacity, but any bike stock from the shop should be able to use all it's gears.

  • Unless the factory made a serious error, the components themselves shouldn't be an issue, as the bike is totally stock and brand new. I agree that a bike should be able to use any gear without issue, and have never had any problems remotely like this in the past, which is why I'm so flumoxxed by this advice. – Esoteric Screen Name Oct 14 '14 at 2:16

Are all of those pictures in the same gear? Big in front and 4th in rear?

On the picture of the derailleur it is pretty much maxed out giving chain length. Notice it is swung way forward.

When you go small small it is the opposite. The derailleur has to take up the maximum amount of chain - swing back.

Most likely your derailleur will not take the entire range so you have to stay away from big big and stay away from small small.

There are a lot of opinions on cross chaining. I agree with this cross chaining. I almost never to small to small but I will go big to big a lot as on the flats I only need it for a few strokes accelerating from a stop. The small gears on the cassette tend to wear out first so I like to use the big gears when I can.

  • Yes, all the pictures are taken in the same gear. I'll try and get more up tonight showing different gears and derailleur positions. – Esoteric Screen Name Oct 14 '14 at 2:30

Further to Blam's answer about the position of the pulley (which takes up the slack in the chain), my hybrid/touring bike was recently serviced by a very experienced French shop mechanic of racing bikes.

He put on a chain which was too short (i.e. it pulled tight and jammed) if, when I rode it, I cross-chained it onto both big rings: and when I told him that (that that's what happened when I test-rode it) he said, "You're not supposed to do that." He did replace it with a longer chain though.

I'm a bit surprised there's a Decathlon near you but no bike shop. Any bike clubs?

  • There are actually many local bike shops, however, they are extremely low end. They aren't what you would envision for an LBS, but small hole-in-the-wall places where you can go to buy a cheap piece of steel. Even at the better ones which carry actual brands the staff don't understand proper fit (or just really want to make a sale). For example: "why do you want a road bike? Just get a mountain bike, they are better" or "we don't have big frames, just ride a 52cm and put the seat up". I'm 187cm with long limbs. Bikes are a highly fungible commodity here, used for cheap transport. – Esoteric Screen Name Oct 14 '14 at 2:44

What your mechanic said, is true for most drivetrains.

And there's absolutely no point in using extreme combinations, such as 1:8 or 3:1, as they are doubled by 2:somethings (first number meaning the front gear, second for rear gear). All you need is all 2:x plus probably no more than 1:1, 1:2, 3:7 and 3:8.

Here's a ratio calculation I made for a 11-32 rear and 24-42 front cogsets. Colors depict identical or nearly identical ratios (red borders mark "forbidden" combinations"): ratio table

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    What? The extreme high and low ends of the gear ranges are not effectively duplicated in the middle. And I use the top gears extensively (and the bottom on really steep, long climbs). If you don't, fine, but the middle third of the range is not "all I need". – Esoteric Screen Name Oct 14 '14 at 2:59
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    Esoteric Screen Name wrote: > The extreme high and low ends of the gear ranges are not effectively duplicated in the middle. Try looking at it this way: On the crank, the big cog-wheel is on the outside, and on the wheel the smallest cog-wheel is on the outside, so in extreme top gear the chain is on the outside (not crossed over), and in extreme low gear the chain is on the inside (not crossed over). These extreme-ratio combinations are OK, as long as the chain length and tensioner range are adequate. Mathematically the overall gear ratio is proportional to the number of teeth on the driving – j77h Oct 14 '14 at 8:12
  • @j77h You have misunderstood; perhaps I wasn't clear. I'm referring not to the mechanics or engineering of the gears, but to their feel and use while riding. "Extreme high and low" means top and bottom gears, where pedalling resistance is highest and lowest. The particular conversion rates of muscle effort to speed at either end are not present in the middle of the gear range. – Esoteric Screen Name Oct 14 '14 at 13:20
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    I think this is a point that lots of folks are missing. Back when you were lucky to have 10 speeds, you kinda wanted every one of them, but with 27-30 (and sometimes even 40) few people make any real use of every gear combo. Mostly one will pick a "range" with the front and then adjust within that using the rear, but the "extremely mediocre" gear combos (big/big or small/small) will not be used to any real extent because they're in the middle of the range and duplicated (often twice) by other "more convenient" or "more natural" combos. – Daniel R Hicks Oct 14 '14 at 20:28
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    @EsotericScreenName - If the mechanic actually said that and meant that, he was stupid. – Daniel R Hicks Oct 17 '14 at 2:22

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