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I purchased a new Mountain bike a month ago and noticed after a week or so the gears were starting to slip, this was expected because of stretch in cables and wear in etc so I took it in for a tune up at the place of purchase and when I got it back the gears were still slipping in places.

I then took the bike to a local mechanic and he said that I shouldn't be using the gears on the high extremes ie the two smallest cogs at the back because of the angle of the chain. I will admit I am no expert but in all my life I have not heard this, is this true?

Thanks

Thanks to all that answered! and to add to my question to clarify some points, i have 3 chain rings on the front and the slipping sensation is more like a clunk from the front crank when i put pressure on the pedals, its not a constant noise. this happens on a flat surface so its impossible to use this gearing on an incline. It only happens when i have the chain on the small front ring and either of the two smallest back cogs.

I also get a 'ghost' gear shift when i use the middle front chain ring and either of the two smallest back cogs.

There seems to be polarising opinions about cross chaining which i will look into but either way i think i should be able to use all my gears if i want too. yes?

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    Google for "cross-chaining" and see what you learn. – Criggie Jul 21 at 7:48
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    Do you have more than one chainring in the front? This makes a drastic difference - if you only have a single chainring (as common on most newer MTBs), there is no such thing as "cross-chaining" as suggesten by @Criggie. – anderas Jul 21 at 8:21
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    What do you mean by "gears slipping"? There's no way a chain should be slipping (or skipping) on a new bike. Do you mean grinding noises that sound like the bike wants to shift of another gear, or your bike won't stay in a particular gear? shouldn't be using the gears on the high extremes ie the two smallest cogs at the back because of the angle of the chain Bah. That might make your chain wear a bit faster. For a mechanic, though, it can be real hard to properly configure a bike so that it can be cross-chained like that without chain rub or ghost shifting... – Andrew Henle Jul 21 at 14:36
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Using the smallest chainring with the smallest sprocket or the largest chainring with the largest sprocket should indeed be avoided. It’s called “cross chaining”. It’s less efficient and causes slightly increased wear and noise. On the small-small combination it can cause chain slap (because the chain will be almost slack) and on the large-large combination the chain is almost taught, so getting something like a twig caught in your chain can easily cause the rear derailleur to snap.

However cross-chaining is not very bad and it should work just fine. Just avoid it if you can.

Of course if you only have a single chainring it can’t be avoided and you should just use the gears when you need them.

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    However cross-chaining is not very bad IMO "cross chaining is evil!" is pure cargo-cultism bollocks. I've often seen (and done) selection of cassettes such as 11-28 for road races for the very purpose of being able to stay on the large chainring the entire race - that means every climb is done cross-chained. on the large-large combination the chain is almost taught, so getting something like a twig caught in your chain can easily cause the rear derailleur to snap. Getting something caught in your chain is going to snap your rear derailleur off no matter what gear you're in. – Andrew Henle Jul 21 at 14:30
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    I respectfully disagree with Andrew. While we should avoid cross-chaining, there are times in the heat of battle when this simply escapes one's attention. Or, if it's a short climb, one may wish to cross-chain rather than do a front shift. Also, in my experience, getting stuff caught in your drivetrain is not common enough to blanket recommend against cross-chaining. If it were common, wouldn't we see more broken 1x drivetrains? Especially on MTBs? – Weiwen Ng Jul 21 at 16:58
  • "I shouldn't be using the gears on the high extremes ie the two smallest cogs at the back" I don't think the mechanic was talking about cross chaining. I've also had a mechanic tell me that I should outright avoid my 11 and 30 cogs on the back because they increase chain wear, regardless of which chain ring. Doesn't seem to be a very popular opinion in general though. – Hannes Hertach Aug 12 at 8:34
  • @HannesHertach: It probably does increase wear slightly (and decreases efficiency) but you (probably) have those gears for a reason. What’s the alternative? Not using them at all and installing spacers instead of the cogs? :D – Michael Aug 12 at 8:46
  • @Michael I agree, it probably wears the chain more but I don't really care enough to actually avoid using the gears. Interestingly the mechanic also mentioned something about newer DI2 systems having a setting that will prevent you from using extreme gears? Unfortunately I don't have DI2 to test this. – Hannes Hertach Aug 12 at 9:28

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