Generally, Shimano tends to be conservative in their ratings, and they tend to be the first to introduce a new standard. 16T seems to be it for now, but I welcome larger, as I like as few duplicate gears as possible, and 16T sure is a ton better than old 10T on 52/42, though 14T on 53/39 was also common. Of course, 1X has zero dupes, so additional development on wide 2Xs may have stopped.

Here's a long discussion of sort of the same thing

https://www.bikeforums.net/general-c...-era-best.html

Concur. But as I indicated, my drawing was crude, and only to give a visual. You’ll notice that I only drew five gears. But the bike I had this problem on was a seven gear bike, and the interference happened on the smallest two gears.

Dan

I noticed the effect you mentioned when I got a bike with 50/34 chainrings (my other bikes had 53/39). When in the small ring, In the 3 smallest cogs the chain was scraping on the big ring. So I'd avoid those gears. I basically used it as a Cross Chaining Alarm.

I have one bike with a 50/34 and another with 52/36. What I've found is that on the former, I end up shifting the FD a lot, whereas on the latter I don't. That may be at least partly the cassettes I'm using, not just the rings.

I think it’s a no-brainer to go 50/34 up front from everything you’ve said. You will still be fine on the descents and have a little more leeway on those steeper climbs. Ideally you would want to go even lower to get your cadence up on those 8% climbs, but 1:1 is a reasonable ratio for 8% grades at the power levels you are talking about.

If it was just scraping on the smallest cog, perhaps, but on 3 smallest cogs indicates you have a chainline issue, the chainrings are too far in. I have 50/34 and chainstays toward the short end, and get no scraping on 34-11, however mine is 7 speed cassette (8/9/10 are all just a skosh wider), 130 OLD, 43.5mm chainline, and also the large ring is "dished" so the teeth are slightly outboard of the mount at the crank spider.

Agreed. Two less teeth up front make it harder to whistle. :innocent:

i’m going to try 52-34 first, since the little ring is very inexpensive. if any problems shifting, i’ll change the 52 to a 50 as well.

OP has the latest Shimano 2x12 drivetrain. Since at least the earliest Shimano 2x11 drivetrain, the chain brushing the big chain ring while riding in the small chain ring and the smallest 3 cogs (a) has been unavoidable, and (b) does not indicate a chain line issue.

See this 2015 Shimano dealer manual (DM-FD0002-05-ENG.pdf) at page 4.

i don’t think the chain touches the big ring when it’s in 36-13. i’ve used that gear on a ride or two when i forgot to charge the battery and while it sucks as a highest gear it didn’t sound awful. i do think it might brush against the FD cage.

da di2 won’t shift into 36-12 or 36-11, so i’m not sure if it would touch when cross chained like that.

Who cares if it does? No good reason to use even the smallest 4 or 5 sprockets of a 12 speed cassette when in the small chainring. I rarely venture more than halfway down the cassette before switching to the big ring.

did (basically) the opposite on one bike - went 46-34

Shimano does not offer that combo - but the front shifting is amazing - snaps up and down almost like rear shifting

suits me better - but I probably ride at around half your speed :)


old - 50-34 :

https://cimg5.ibsrv.net/gimg/bikefor...de05c89aeb.png


new - 46 - 34 :

https://cimg1.ibsrv.net/gimg/bikefor...a81785c9c8.png

the 46t big chainring mounts in a different fashion - had to use the funky chainring attachment cap / nuts *** to mount the chainring :(

*** each attachment cap / nut is unique :(

One reason is you're paying attention to something else, like riding, the other riders around you, cars, etc., rather than constantly looking down at your drivetrain. IME, with more modern drivetrains (10 speeds and up) it's easy to find yourself way up the cassette in the big ring or way down the cassette in the small because it is just so darn smooth. It's also because there's a lot of overlap, of course, but I find I don't mind that. So the chain rubbing on the big ring when in the small ring and the third smallest cog is a handy reminder.

Older drive trains (6-8 speeds, DT or STI), I find myself much more aware of which ring I'm in because I'm always about to run out of low gears.

I know that my SRAM AXS locks out the smallest cog, but not sure if it allows the second smallest. Never tried it. I automatically get back on the big ring after the first 3 or 4 upshifts in the small ring. I might stay on the small ring for a few more upshifts if I know there is another hill coming straight up, but I never get down to the last 3 smallest cogs on a 12-speed.

I'm still a mechanical kinda guy, so the only lockout I have is "Cripes! What's that noise?" ;)

My "climbing" bike is geared 50-34t/11-34t and my "fast" bike is geared 52-36t/11-32t. Either way, I still climb with both bikes. I utilize my lowest gear more often with my 52-36t, but I generally enjoy the faster speed output. The 50-34t is easier to climb with and I still have a few extra gears to spare before going into my lowest.

only reason is when your battery dies and goes to RD only mode for a few hours. no other reason to use those combos.

nice ! i assume the same nuts will attach the 34 as the 36!!

No excuse for a dead battery 😂

while there is no excuse, my explanation is that the thing lasts so long you forget it even has a battery! months and months.

Right up until it says, "Hey *******! I have a battery!" by being totally discharged.

A cross-chaining alarm! I like this.

I always found it interesting how some cyclist cross-chain without noticing it and/or without being bothered by it. I mean, the chain is literally screaming for help. Surprisingly (not), these riders usually have drivetrain that looks like a stove pipe and when you explain them that the actual color of a chain and cassette is light gray, that's the facial expression they make:

https://cimg1.ibsrv.net/gimg/bikefor...40d7bcf003.jpg

They're probably just using the grease the chain came coated in....

too soon! too soon!