couldwheels

With chain on "small-small" gear ratio, I get chain slap if I back pedel. Sometimes, the chain will move to the next lower gear if I continue backpedeling. If I don't back pedel on the high gears, the bike is perfectly fine. The chain is not sagging either, it looks straight to the eye.

I'm not experiencing any other problems on my new used bike, it shifts fine, drivetrain don't make any rubbing noise, just the chain making that usual "anchor's away" noise, a lot less louder ofc.. Does this mean the chain is too lose or should I just leave it alone?

dsaul

The chain may be catching on the shift ramps/pins of the large chainring. You really should not be using the small/small combination. Don't back pedal, bicycle drivetrains are designed for forward pedaling. Shifting to another cog when back pedaling is completely normal, since the derailleur can only control the lower run of chain. There is nothing to keep the top run of chain on the correct cog, when pedaling backwards.

JohnDThompson 

As @dsaul suggests, the small-small gear combination is best avoided. That combination, along with the large-large combination, is the most extreme deviation from ideal chain line, which leads to more wear and the unintentional derailment issues you mention. The reason it doesn't happen when pedaling normally is that the derailleur holds the chain in proper alignment with the sprocket. When backpedaling, there is nothing to ensure that the chain is aligned with the sprocket, so it can easily derail. The chain-slap happens because there is nothing to maintain tension on the chain when backpedaling. In normal pedaling, the spring in the rear derailleur maintains tension on the lower run of the chain, and your pedaling the crank maintains tension on the upper run.

So, to prevent these issues:

1) Avoid the small-small and large-large gear combinations.
2) Avoid backpedaling.

70sSanO

While I’m no longer an advocate to absolutely avoid a small-small (especially in light of 1x systems and more flexible chains), it should not be a high use combination.

As for the back peddling issue, there are some possible reasons, there is a lack or much less spring tension on the chain to prevent it from slapping; since the friction in the severe chainline angle is greater than the tension holding the chain taut.

The other aspect is that RD’s function is from below the cogs. The chain is guided to line up with the cogs from the guide pulley. Back peddling engaged the cogs without the use of a guide pulley, and the more severe angle causes the chain to shift to a less severe angle.

I find a slight back peddle can helpful to align a peddle going into a turn, but it really has no purpose, other than the rider just likes to back peddle.

John

HillRider

Despite the fact that neither big-big or small-small should be used regularly, it is essential that the chain be long enough to allow shifting into big-big. If it is not and you inadvertently shift into it a lot of damage to the drivetrain and possibly the rider can occur. If the chain is long enough and gearing is such that small-small uses up nearly all of the rear derailleur's "wrap capacity" then the chain will be slack or nearly so in that combination. Short answer: don't use small-small.

tyrion

Sounds normal to me. As others have mentioned, riding in the small-small sprockets is not a common thing. You should be able to ride in small/small and big/big, but it's not optimal and generally only done when not paying attention.

Iride01 

Small/small is when the DR has the least amount of spring tension left to try an keep your chain properly tensioned for when you are pedaling forward and trying to shift. Backpedaling just makes it harder for your DR cage at that point to keep the slack out. The forward travel of the bottom run of chain is pulling the cage forward so the top run goes slack. And along with the "small/small bad" thing that others mentioned, you are worsening the situation by backpedaling.

couldwheels

Thanks everyone, sorry for not giving all the information. I actually have 1x drivetrain.

The chainring is small at 34T that's why I call it "small-small". Maybe it doesn't matter on a 1x?? My previous bike was a MTB with triple crank.