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I'm very new with bikes, I just got my new bike since I was a kid like 25 years ago, I bought a Trek Marlin 7 2020 which comes with a 2 chainring crankset and 9 rear gears. I've been hearing that is not good to use the small chainring with the smaller rear sprockets and neither the opposite the biggest chainring with the biggest sprockets but some people says that with a 2 crankset it's safe to use all gears with any of the sprockets, so I'm very confused... In your experience what would be your advice? Thanks a lot!

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    Just a point of information: I think that when you say “change”, the correct English word is cassette cog. When you say small crank, I think you mean the small chainring on the crank. As Criggie pointed out, it’s better not to use the small chainring and smallest cog (aka small-small combination), or the big chainring and big cog. However, it’s not going to damage anything. I think your small-small combo will have enough chain rub that you’ll shift out of it anyway. – Weiwen Ng Dec 27 '19 at 7:17
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    @WeiwenNg: Big/big could cause damage to the rear derailleur if the chain is too short. – Carel Dec 27 '19 at 10:43
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This is "older" advise that used to be gospel but these days has been engineered to be less of an issue.

The chainrings and cassettes are constructed in opposite directions. That is the big chainring is on the outside, and the big cog in the rear cassette is on the inside nearest the midline of the bike.

Upshot, when your chain is on "big-big" meaning the biggest chainring and the biggest cog, your chain will be doing a zig-zag, with bends where it leaves the cog and where it arrives at the chainring. Same goes for the "small-small" combo.

From SE

Realistically, this is far less of an issue than it used to be with better engineering. Your chain may wear if you crosschain occasionally, slightly faster, but not measurably so.

Given that your bike is new, its engineering will be of modern standards and not that of 80s bike transmissions.

If you're grinding up a hill, you should be in the smaller chainring. If you're on the flat you should be in the bigger chainring. But a few seconds in big-big when pulling away from a red light won't upset anything.

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    With a Shimano 11s double road system, the small-small combo has quite a lot of chain rub. It may be technically usable, but it’s loud enough that I shift out of it naturally. I believe my Campy 10s drivetrain was the same. Big-big is/was Ok on both. I’m under the impression that SRAM may engineer their FD cages to not produce as much rub. Also, current generation 9s Shimano may be more permissible of cross chaining. Last, with compact cranksets and wider range cassettes, I actually find myself in the big ring most of the time. – Weiwen Ng Dec 27 '19 at 7:15
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    @WeiwenNg concur with all that. Basically its not the problem it once was. – Criggie Dec 27 '19 at 7:24
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    While more common on road shifters some incorporate a "trim" function. This allows a small front derailleur adjustment to minimize rub while cross-chaining. – mikes Dec 27 '19 at 12:29
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    I'd add that cross-chaining adds more friction into the drivetrain so you don't want to do it for a long time or distance, but as Criggie says it's not a problem if done occasionally. – Argenti Apparatus Dec 27 '19 at 12:43
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    If you look at the chain angles of a 1x12, there not much different to a crossed 2x9, yet in the hills, mountaineers spend all there time on either the top or bottom couple of cogs. i.e. modern 1x is almost always 'cross chained'. – mattnz Dec 28 '19 at 1:19

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