alexk_il

I bought M785 cranks and two M785 (40t and 24t) chainrings. I already see 24t might not work well with FD, might end up swapping it to the standard 28t. Anyway I already see an issue of the chain slightly rubbing against the inner side of the outer chainring when in 24t-11t combination.

The chainline seems to be correct. I use 11sp on the rear with 11 speed Shimano chain.

Do I need spacers for the inner chainring if I go with 28t instead? What size?


​​​​​​Update:
Problem solved, click here

Andrew R Stewart 

Welcome to crosschaining and why its a ring/cog combo that has been suggested to avoid for any duration. The 16T jump is a big one and as the tooth difference gets larger the ill effects of crosschaining becomes worse (as does front shifting ease too). Still worse further if the larger ring has any tabs or pins sticking out (that aid shifting). If you increase the spacing between the two rings the chain might begin to get stuck either on the tops of the small ring's teeth or caught between the two rings, when shifting off the large ting.

The solution that most follow is to not use the combos that have this crosschain created rub. Some will compare the actual gear ratios in the cross combo and other ring/cog combos. Often their's a near duplicate of ratio that uses the large ring and one of the more middle cogs. Using this combo will reduce the wear that crosschaining increases, be quieter and offer more shifting to a harder/easier ratio options.

But its your bike, your ride and your choice as to how you take care of your bike. Do know millions before you have faced this too and some never even are bothered by any rubbing. Andy

Iride01 

Though you said your chain line looked okay, it only takes a fraction of a mm to make it rub or not rub.

What type BB do you have? There are spacers that go on the drive side and even the non-drive side depending on the width of your BB.

https://si.shimano.com/en/pdfs/dm/FC...002-17-ENG.pdf

Look at pages 12 - 13 and figure out which is your situation. If your BB is press-fit, look around page 21.

alexk_il

Bingo!!! Using Shimano BB80, looks like one spacer is missing on the driving side.

Thanks.

KerryIrons

It would be a surprise if the chain didn't rub the big ring when on the small-small combination. Often this happens in the two smallest cassette cogs. It's not something to fix. Or put another way, the fix is to not ride in that gear combination.

70sSanO

This is a real cat and mouse situation because you have so many cause and effect results. As Andrew R Stewart pointed out, as you increase the space between the chainrings you increase the chance of the chain getting caught between the chainrings. But there's more to it, the primary concern with a 40/24 is getting smooth shifts with that gap between the chainrings. If you are, then tweaking may throw some of that out the window.

With a non-index FD lever shifter, you can set the high low stops and usually bridge that gap and still maintain proper chain/cage clearance for big/small and small/big. With an index FD shifter the FD cage will move a set amount. Depending on the additional spacer, it is tough to say if you'll be able to maintain the same chain clearance. BITD, a slight cage profile tweak could help, but poorly executed could ruin an FD.

If you are running a triple FD, you don't have to worry about the amount of travel, If not you need to make sure the FD will extend out far enough.

If after all of that you really want to try and slightly shim the 24t, I would use ultra thin shim washers. That along with "slightly" moving the chainline out might help, but you may induce poor shifting in the process.

Can't guarantee when you will get these, but I have used them in my 126mm OLD tweaking to get 8/10 speed cassette in an aluminum frame, cantilever brake arms, and an 8 speed chainring fudge using a thumbshifter.

https://www.ebay.com/itm/16457318490...Bk9SR9KxrLyeYQ

John

alexk_il

Nope. I just incorrectly looked at 68mm diagrams, mine is 73mm and the bike was set up correctly.

Next step - will try 28t chainring first, then spacers.
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alexk_il

I have successfully installed 42/26t+10sp on my other bike without any cross-chaining issues whatsoever, all combinations work fine and the shifting is smooth.

I didn't expect 40/24 +11sp to be different. Too early to give up though. 🤦🏻😂

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Iride01 

So it sounds like you might be bumping up against the issue KerryIrons mentioned. And I have always felt this is one of the reasons that Shimano cautions against using the small/small combinations in some of it's literature that many people point to as their proof for the mantra and chanting "don't cross chain or your bike will blow up and other bad thing befall you!" <grin>.

If it works on another bike it might be a difference in the length of chain stays that gives less of an angle. Perhaps you can play with spacers and get it dialed in, but you will also be fighting with the front DR cage too.

And it's very likely that you have virtually the same ratios available to you in the big ring that you do in the small front and two smallest rear cogs. And getting back to those people that shout "don't cross chain", I use to not give them any credit, however since I've gotten Di2 and run it with full synchro shifting, I'm never cross chained at all when in the big or the small ring. And so far my chain at over 5000 miles doesn't show any wear. So maybe the don't cross chain people were on to something, but their scare tactics turned off a lot of us... at least it did me.

HillRider

The real answer, even if it does work on other bikes, is to not use that small/combination. The same gear ratio is available using the middle chainring or perhaps even the big chainring and a more appropriate cog. That level of cross chaining is not good ever and totally unnecessary.

alexk_il

I used to always have cross-chaining issues when my bikes were serviced by professional bike shops. One larger retail and small local. I suspect their business model doesn't allow them to spend more than 10 minutes on tuning the gears, while I personally prefer spending a couple of hours working on my bike over watching Netflix.

Anyway, I suspect the issue here is related to m785 crunkset being designed for 10 speed. One extra cog on the back seems to be enough to change the chain angle and cause rubbing. Also, m785 outer front chainrings is desinged for MTB and are thicker than road chainrings on my other bike.

I guess there are few options available for me:
1. Remove the 10->11 speed spacer on my rear hub to push the cogs inward. I accudently did it the past on the same bike and the same outer chainring (1x) without any issues

2. Add 1-2mm spacer on driving side of the BB

3. Add spacers to 24t (or 28t) ring.

Or maybe all of the above. 😂

70sSanO

I think I understand why you want this setup. When I setup my mtb's, I approached it as two 1x drivetrains. Of course I was running a 2x7 (24/34) with a small cog of 13t so my extremes were less extreme.

But the logic was simple, run in the 24t for climbing and on an unknown trail where things can change drastically around a bend, and run in the 34t on flat. slightly uphill, and downhill. I ran every gear combination, regardless of any duplicates, because it was basically stupid to go through the shifting gyrations on a short downhill just to go back before another climb. When I added a 40t (8speed), in my old age, chain slap became an issue (non-clutch) and I am more careful about running the 24/13. On my road bike, this is pretty much a non-issue as I don't extreme cross chain except in dire situations.

My first rule is to determine the gearing I "need" for the terrain, my ability, and how I like to ride. If you don't need a 24t and are fine with a 28t, it makes no sense to limit your speed with a 24t ring if you never get into the small ring/big cog. If you need the 24t chainring, it makes no sense to install a 28t just to squeeze out another gear. And if you want to get technical, a 28/13 is basically the same ratio as a 24/11. So if you are happy with a 24/11 you'll be just as happy with a 28/13, and never shift into the 11t, unless this is more about being able to make it work than it is about the riding.

John

alexk_il

Thank you for your support. My main reasons for this setup are:

• Plenty of uphills and dowhills on my commute to work
• I like to be relatively fresh when I arrive to my office
• My FTP is likely to be below 200Wt. I'm not in the best shape.
• I am doing my best not to exceed 150 bpm heart rate when I exercise, health reasons. I can go up to 175 if I push it, but I just shouldn't

Therefore I need the extra wide gear range to climb hills with as little effort as possible

So, my setup is 11-42 on the back and 40/24 on the front. 24 seems to be impossible though, it is way too low for the FD to push it up, even though the FD/RD are specced for 16t difference. Will change it to 28t.

As for the crosschaining, of course I can avoid these gears, but making them work is fun.

KerryIrons

My point was that chain rub in the small-small combo is common. If everything is the same except the switch in chain ring size then this is confusing. If the chain line is different then that might be the explanation.

sean.hwy

the 2x M785 chainring combination 38-24T 38-26T 40-28T

https://bike.shimano.com/en-EU/produ...0/FC-M785.html

While you can push the boundaries and swap out cogs don't expect to shift as nice as OEM. For the last several years I have been running 11-40T on my 2x grx. It does not shift as good as the factory recommendation of 11-34T. It shifts more than good enough for me and I know what I did. If it rolled off the factory floor like this I am sure a lot of people complain and think sram was better.

As other have pointed out several times try not to cross chain.

alexk_il

I fixed it, finally. I ended up with 1mm extra spacer at the drive side of the BB; also changed the small chainring from 24t to 28t to make everything smoother. The 24t was shifting up and down without any issues, but the chain was still slightly rubbing the pins in the inner side of the outer ring.

I suspect 26t would have worked too, I but can't be bothered to check. Maybe will do later if I get bored.

Anyway, the final setup now is:

• ​​​​​​XT 11-42t on the back
• XT 40-28t on the front.
• XT FD/RD are driven by R7000 brifters through a pair of Jtek shiftmate adapters.

All gears/chainrings combinations now work without any noises, rubbings or any other cons associated with cross-chaining. I can shift between 28t and 42t at any of the 11-42t gears without any issues.
​​​
I guess the "cross-chaining is bad" myth was created by bike mechanics that can't afford to spend an extra couple of hours to tune the front derailleur.

Kontact 

[QUOTE=alexk_il;22734280
1. Remove the 10->11 speed spacer on my rear hub to push the cogs inward. I accudently did it the past on the same bike and the same outer chainring (1x) without any issues
[/QUOTE]
How did you get the lockring to tighten down properly on the cassette if the body is longer than the cassette? What do you think that spacer was there for?

It isn't a myth, and has nothing to do with the front derailleur, as you discovered when:
Cross chaining is something every bike should be able to handle, but because of the chain angle is noisy and puts extra wear on the tips of chainring teeth. That's all the "bad" anyone cares about.


If I'm following your solution properly, you put a spacer that the crank spindle isn't designed to handle on drive side cup, changed your chainline and gave up on the gearing you wanted. But you're smarter than a lazy bike mechanic.

alexk_il

No idea why, but it tightens up properly on my particulat wheel with or without the spacer. Anyway, I did not remove the spacer, all is set up as it should be.


That's not what some other people are saying. Just try asking a question here about rubbing in small-small or large-large combo and see how many people will tell you "welcome to the world of cross-chaining".

Anyway, I have three bikes at home, all are setup to work smoothly in small-small or large-large combos. Took a lot of time to achieve this though, way more than a standard LBS charging £40 per hour is willing to invest into a quick bike service job.

Not exactly. M785 crank was not designed to be a 40/24t, should be 40/26t. I was pushing it to work out of specs. I am sure I can make 24t work with another 0.5mm spacer either on BB or between the chainrings. The XT M786 derailleur is happy to shift it, it's just the crank/frame geometry that for some reason doesn't allow it without extra spacers.

I can definitely make the 26t it was designed for to work too, but si​​​​nce I already had 28t available I decided to use it instead. With 42t at the back I will hardly notice the difference, so not really giving up on anything.

sean.hwy

40-28T like I mentioned before.

I installed the M785 crankset on my ibis. I run it with 11-36T with my road wheel set and 11-40T with my gravel wheel set. I did not have any of the issues you described.