Chainring sizing?
 

Question about Ultegra di2 12sp chainring sizing.

Now I've got 52/36 chainrings. Is it ok to just change 52t -> 50t so ended up 50/36 combination?
Chain needs to be shorter but does it need any other changes?

I think that I will benefit more 50t in my flat area and weak legs 😄

On modern Shimano road cranks, you'll have issues finding just the one 50 ring at a economical price. At least I think you will have a hard time. Even getting the common 50/34 large and small combo together won't be inexpensive so maybe just get a 50/34 crankset. If that 50/34 crankset is Shimano and the 36 ring of the old fits onto the 50 ring of the new, then try that combo and see if you like it.

For the 2 tooth difference in size, you may or may not see any difference in your shifting performance requiring a shorter chain. That should be an easy fix anyway that you can do once you find out if you have real issues.

Just because you are in flat land, doesn't mean you should be in the big front and any of the 3 smallest cogs for your normal cruising gear. Those are for when you are pushing hard to go fast. Or at least they are for me. I also have weak legs and use a 52/36. But still I miss my 53 ring sometimes in my rolling terrain.

You don't need to shorten the chain, unless you want to save a few grams. Otherwise you're better off leaving it as it to preserve its integrity.

You will want to drop the FD to keep it close to the new outer ring in order to maintain crisp shifting.

Now the hard part......
Index outer chainrings have pickup pins and shift gates phased to the inner ring. If your new 50t is not Shimano and/or made to pair with the 36t you MIGHT lose some shift smoothness. Even so it should still be OK.

Good tips. Hole crankset is not as much higher price than I thought.

Nice to hear that combination is good to go and nice tip that of course I need to index FD.

Not familiar with those pickup pins. Have to research what is purpose of them.

Here is Shimano's explanation of how the front shifting is designed to improve shifting. This is from the very beginning of SG-X so some differences have been made since. MTB triple in this example.

https://cimg8.ibsrv.net/gimg/bikefor...b7ff71b4dc.jpg

It includes pins (or hook, plates,...) on the inner side of the chainrings (opposite of the blue arrows) to pickup the chain to move it to larger rings.

Additionally, the rings are 'clocked' relative to each other so the distance between teeth on 'matching' rings is spaced such that the chain can engage easily, without riding on the tip of the teeth.

Other design elements are shaping of the teeth to release or pickup the chain, as applicable.

So, essentially using rings that are not designed to be 'paired' may have an effect on shifting. How much? ...

This seems like a pointless and expensive change. You have TWELVE gears in back - there are no gear combinations you are truly lacking. 2t is very, very little - the difference between a 26t and 27t cassette cog.

Count me as agreeing with Kontact.

It's just not enough change to warrant spending more than a few bucks on. The real question turn on why you'd bother. The difference is essentially a half step between the smaller cassette sprockets, so the only reason I'd bother is if I chronically found myself bouncing between a gear slightly too high and one too low,

Otherwise, if you are never using the outer with the outer few cassette sprockets, you might want to do something more material, ie. going down to a 46t outer, or something in that range. That's about 15% or equal to 1-1/2 cassette sprockets. Assuming that sounds right, consider that you'll definitely and to lower the FD and check how far you could, or buy a new one suitable to use with the new combination.

So, take the time to think about what you really want, then consider all the options and implications.

I made a similar change with a Campag Chorus 53/39 crankset, swapping the 53 for a TA 50 from Peter White. The reason for reducing the chainring was that it put me down into the middle of the “corncob” region of the cassette for normal riding. With the 53 I hardly ever used the smallest couple of sprockets — the 50 essentially shifted everything down one sprocket. It was a good move for me and there was no deterioration in shifting. I lost a little top end, but I didn’t miss it. TA rings are excellent quality.

I undestand your point but road cycling is full of unnessesary upgrades and this one could be one 😄

My point is to get smaller cassette jumps in my comfortable area (cadence and speed) without massive cross chaining.
Mostly I'm using bigger ring and many times found myself using 19t 17t in back. Cadence 90 it is little over 30km/h. Almost never needed 11-13t sprockets and also 36t is rarely needed as my area is totally flat.
In ideal situation I just want 18t in back but that is not possible.

But you are right. I have to consider all option. Maybe even 1x could be one option with 46-48t chainring

This is exactly why I want to make some changes.

Seems like for what you are asking, you need a rear cassette with a smaller range from high to low. You'll just have to accept the fact that the 11-13 don't get used much. You won't find many cassettes that don't have them.

But when you need them, you might regret not having them. If you are noob to cycling, then your legs will get stronger if you ride more than once a week. 3 times a week I feel good. 4 to 5 times a week I feel like superman.

This sort of "problem" made sense when you only had 8 cogs to choose from. But with 12 speed you are going to have extras at some part of the range, and are unlikely to have dead spots anywhere in the middle.

Buying individual rings can indeed be expensive - what I replaced Campagnolo rings, it was often cheaper to buy a whole NOS crankset than just rings (with the added bonus of eBaying the surplus crank arms). My standard drivetrain is a 53/34 with a 12-25 10sp cassette (12-17, 19, 21, 23, 25). The advantage of this is that I use all of the sprockets, rather than bringing along a rarely-used 11 for the ride. The only time this setup is a disadvantage is on a fast descent with a group - I'm spinning out on the 12 while the rest of the group is still powering on their 11s - however, this is a rare enough instance and not sufficient justification to ditch an otherwise excellent (for me) cassette choice If I'm soloing, if I spin out on the 12, I'm already going plenty fast - OK to coast. If/when I upgrade to 12sp (where an 11 sprocket is unavoidable), I'll probably switch to a 48/32 crank - still slightly more top end than my current setup, but I really don't need a 50/11 gear.

There are some lovely cranks availble for equal or cheaper than chainrings; 50/34 hollowtech II style, 110 BCD, $65 with steel rings with dents in place of lift pins, and $75 with aluminum rings with lift pins, and both with external bearings at that price.

53/34... say, that's a nice jump, above what I thought was current max of 16 teeth, I was wondering if I could do that on my crank as noted above, I would bet it would need big ring with lift pins. Aftermarket 52 rings seeing now on amazon, good price, no lift pins, flat and no dish to the outside as current 50, my guess is those are for 1X cranks.

I changed from a 52 to 50 through lack of availability of a replacement 52 ring. This is DA9100. It made very little difference to ratios, looks worse and I’ve never managed to quite dial in my FD. And I had to move the little adhesive plate on the seat tube that the small screw contacts to hold it steady (may not apply if you have a clamp-on FD)

Basically I wouldn’t bother and would train more to get used to that 17T ;)

What's the world coming to?! Then again, I'm guessing that crank is 4-bolt assym, AND, that aftermarket rings to fit that weird spider are nonexistent. See, that's why I went with 5x110mm BCD, a very old standard. Plus I think it looks better.

It does. These asymmetric cranks are an abomination :)

If you really want to do this right. Set up an excel spreadsheet and lay out the ratios with gear inches and cadence.

You can use columns for inner and outer rings, ratio, GI, etc; and rows for cassette cogs.

Then based on your current routes and gear combinations used, you’ll be able to compare current to potential.

I’m all for setting up your bike how you want it, but a lot of people over-estimate the effect of 1 or 2 teeth.

But I agree that 11-13 is a horrible gap. I’m not familiar with 12 speeds, but can’t imagine any 12 speed cassette with that gap.

John

This site does that with no effort.
https://www.bikecalc.com/archives/gear-inches.html

Ultegra 12 is:
11-12-13-14-15-16-17-19-21-24-27-30T
So no gaps until 17-19.

Or 11-12-13-14-15-17-19-21-24-27-30-34

For years I have been using the gear calc on sheldonbrown, similar results to above. But recently, someone posted about:

gear-calculator.com

And I must say, it offers the additional advantage of a visual representation of the spacing, which is also "normalized" to (I think) a logarithimic scale of "development", meaning, for example, 1 tooth between 11/12 does not equate to the same visual representation as one tooth 20/21.

sorry - just checked back in and realized I had typo’d. That should have been a 50/34, not 53/34….😟

Ah, that's more like it. 50/34 is really taking over the market for doubles, very versatile and with little sacrifice, and I think less duplicate gears. Most of these systems are designed with some margin, for example, I've read that the capacity on some rear derailleurs is rated conservatively, they can accommodate a bit more, and mine shifts so well with the 50 steel ring, I'll bet it could go a bit larger with an aluminum ring with lift pins. We'll see what's available come time I need a new ring. But the price was so good, I bought a whole backup crank with rings and bearings while they were cheap on amazon. Still available on ebay in that style. My hope is that in time, someone will produce the same crank, hollowtech II style, 5x110 BCD, with threaded holes inboard for a triple low, for those that need a bailout low, but can't fit a huge 1X cassette, or would prefer not to.

I assembled my own 10 speed by mixing 2 sets, 12-13-14-15-17-19-21-24-28-34

If I ever go to 11 speed, id probably do the same arrangement as my 10 speed but add a 16t.

Miche offers you the option to purchase your own selection of cogs for 10 and 11 speed cassette.

12 speed is too expensive and rare for touring, but if I did, then I might choose the same arrangement as what you've listed, as most 12 speed are on spiders and can't be mixed. Otherwise I'd swap the 11t for a 16t.

44/24 double chainrings

Edit:
Here's the Miche pimato K12, possible combinations
https://miche.it/en/wp-content/uploads/sites/2/2023/07/miche-schede-pignoni-primato12sh_m20230828.pdf

First position is either 11-12, 14-15, or 16-17. If you want 12-13, I would imagine that you could look for spare Shimano 12-13 and lockring.

Edit 2:
Seems Shimano does not have a 12t first position on their 12 speed cassette. If you have a hyper glide freehub, maybe it's possible to take the 12-13 from an 11 speed, although the pitch will be off by maybe 0.3-0.4mm, possibly negligible.