Smaller big ring and chain length
 

Replacing a 50t with a 46t. I didn't plan on shortening the chain. Is there any reason I should?

Nothing major; minor: less wieght, and a very very small chance of improved shifting.

Short answer is "You'll be fine."

Your little/little sprocket combination hasn't changed. If your derailleur was taking up all of the chain slack before it will continue to do so. There's actually a chain sizing philosophy that says you should use the longest chain that your derailleur will take up the chain slack.

Your big/big combination has gotten smaller. Since your chain was long enough to safely cover the larger combination, it will obviously handle this.

Lots of people say that a new sprocket deserves a fresh chain. If your old drive line parts have extensive wear a new chainring might not mesh with the old chain and, if you replace the chain, it might not mesh with the rest of your old stuff.

Thanks, I figured as much but didn't know if there would be any repercussions with the excess chain. The bike is basically new so not worried about discrepancies in wear yet.

If the chain is too long after your change the ring you'll hear a new noise, when the front is on the big ring and the rear is on the smallest gear (high/high gears). It will be the top of the chain rubbing on the bottom of the RD. This happens because there's too much slack in the chain and the RD can't handle it all. If this doesn't happen then most likely you're just fine.

I had this happen to me just yesterday -- which is the only way I know this.

I had a chain that was sized a bit too large but working fine on my bike with a standard double 53/39 crank. I changed to a compact crank (50/34) and now the chain was WAY too long and rubbing on the body of the RD and when in high/high I heard the zipping noise of the chain rubbing the RD.

I did some checking on YouTube and found a chain sizing video from Canada where the guy in the vid said, if the chain is too long it will rub right here and pointed right to where mine was rubbing.

I shortened my chain by 2 links to the proper size and the noise went away and everything works correctly.

But as I say, if yours isn't rubbing in that high/high combo then you should be fine.