Chain Sizing 1x11
 

I am due to get the last of my 1x11 gear set today. What do you think is the best way to determine the best length for a 40T front, 11x40 rear and a medium cage Shimano XT?

Try it on the bike not the internet.

bend a section of a spoke into a C shape to estimate by looking before cutting

and using the quick link.

put it on the big cassette cog.

Size it using the big-big + 1" (or in your case only-big+1") technique. That assures you will be able to use your largest rear cog safely. Here is the technique taken from Sheldon Brown's web site (emphasis is mine):

"The best technique for setting chain length is to thread the chain onto the large/large combination, without running it through the rear derailer. Mesh the two ends on to the large chainwheel so that one complete link (one inch, -- one inner and one outer half-link) overlaps. In almost all cases, this will give the optimum length.

Hi Paul,

There are a variety of ways to calculate the optimal chain length but, in the end, it is always best to test fit the chain and make an accurate measurement. I prevented a big mistake that would have ruined an expensive SRAM PC 1091R chain for my road bike, a 2008 Team Fuji with a carbon frame+fork and a SRAM Force drivetain.


How to calculate the chain length
I started by calculating the chain length with the following equation:


Where: C (chainstay) is the chainstay length in inches measured along the length of one chainstay from the center of the rear wheel axle to the center of the bottom bracket spindle. F (front) is the number of teeth on the largest chainring. R (rear) is the number of teeth on the largest cassette sprocket. D (derailleur) is the rear derailleur adjustment (use 0 inches if there is no rear derailleur, use 1 inch if a normal road derailleur, use 2 inches if a SRAM CX1 rear derailleur or equivalent).

In my case, C = 16-3/16 inches (16.1875 inches), my 1x chainring has 50 teeth, the largest sprocket on my cassette has 28 teeth and I'm using a short-cage (normal) SRAM Force rear derailleur. Therefore:


You have to round the number to the nearest inch which meant that I needed a 53 inch chain. Each link on a bicycle chain is 1/2 inch long so 53 x 2 = 106 links. I needed a chain that was 53 inches long having 106 links. I subtracted 1 because SRAM chains include a removable PowerLock link. So the chain would need 105 links plus the PowerLock link.


Big-to-big estimation
But I wanted to be sure so I also used the technique described by HillRider above. I wrapped the uncut chain around the biggest sprocket (28t) on my cassette and my chainring (50t) without using the derailleur. Where they met, I added on inch (two links). This produced the exact same result: 106 links.

But I had a feeling that this might not work. So....


Accurate measurement
I decided to accurately measure the optimum chain length. This is accomplished in similar fashion to the easy "big-to-big estimation" except the rear derailleur is included. Begin by clicking your rear derailleur to your biggest sprocket. Needless to say, it is important that your rear derailleur be properly adjusted before doing this. This is especially important if you changed the largest sprocket size on your cassette. Before I began, I counted out 106 links on my uncut chain an attached a twist tie at the 106th link so it would be easy to spot. Next, I mounted my chain around the largest sprocket (28t) and threaded it through the derailleur guide wheels. Then I wrapped the other end around my chainring (50t). Finally, I pulled the chain ends together until the unmarked end reached the 106th link with the twist tie. And I discovered that 106 links wasn't enough!!!

Why? Because it pulled the cage of my rear derailleur so tightly that the teeth of the large (28t) sprocket could catch on it. Very bad!!! I needed to add two more links (you must always add links in pairs since one is narrow and one is wide), bringing the optimal chain length to 108 links (54 inches).


Verification
Based on the above, I cut my new chain to 107 links (remember, I have to allow for the PowerLock link), mounted it on the bike, closed the PowerLink and tested it. It seemed to work fine. Just for kicks, I took it back off and tried a 106-link chain that I had for another bike. It proved that 106 links wasn't enough because the teeth of the 28t sprocket lightly tapped on the cross-guard of my derailleur cage because the chain was pulled too tightly.


Explanation
Why did both the chain length calculation and estimation methods fail? Answer: Because my road derailleur maxes out at 28t. It is the largest sprocket that it can use. But in order to accommodate such a large sprocket on a road bike with a short cage derailleur, the rear derailleur's "b-screw" must be screwed in all the way in order to get required distance of the top guide pulley wheel from the large 28t sprocket (6 mm on most SRAM rear derailleurs). This effectively rotates the rear derailleur backward, requiring a slightly longer chain. Neither the chain length equation nor the big-to-big estimation take this possibility into account.

You may not run into the same problem that I did. But another forum member reading this thread may. So, when there is the slightest doubt, accurately measure the chain by running it through the rear derailleur and around the largest sprocket and chainring. It takes a bit of strength to pull the chain against the rear derailleur spring and there are various tricks to help with this (like a bent coat hanger wire). But it might just save you from ruining a good chain by cutting it too short like I almost did.


Kind regards, RoadLight

There may be something quite different with SRAM rear derailleurs but I've used the big-big+1" w/o going through the rd sizing technique with literally dozens of Shimano road and MTB rear derailleurs and a couple of Campy rear derailleurs and it always works. I have no problems shifting into the big-big gear combination after the chain is joined and there is no hesitation or interference with the derailleur pulleys. I've used it with cassettes with largest cogs from 24 to 30T so that isn't an issue either.

Big-big+1" ... however ... if you think there's any chance you'll want to switch to a bigger chainring (42T or 44T) or bigger max cog you can add another couple inches and everything will still work fine. Your RD (even a short cage) probably has plenty of excess capacity on a 1x, so sizing a little long has no down-side and leaves a little room to grow.

Hi HillRider,

Perhaps you're right about SRAM. I've used the big-to-big estimation for years (as well as the equation) and it has only failed once.

But the conditions that I'm describing require a rear derailleur that is being used at the precise upper limit of its spec with regard to max cog size. 24 vs 30t are irrelevant if they were not the largest cogs that their respective derailleurs are designed to use because it is only at the extreme upper limit of the derailleur where its b-screw must be used aggressively.

Besides, there are so many other factors to consider. The only way we could make a fair comparison between different manufacturer's rear derailleurs is if we removed all other variables. The bikes would need to have the same chainstay length, same hanger, same cage type (short, medium or long), et cetera. These other factors could have mitigated your observations. (For example, my hanger may be a little shorter than most---I don't know---which caused me to use the b-screw adjustment more than you.)

Regarding the OP, I'm not sure what the max cog size is for his Shimano XT rear derailleur with a medium cage. But 40t sounds very large for a medium cage length---I would have expected such a large sprocket to require a long cage. Be that as it may, it sounds to me like Paul may be working near the upper limit of his rear derailleur so it would be prudent to use a little extra care when determining the optimum chain length.

Kind regards, RoadLight

Hi Kopsis,

A 1x drivetrain does simplify things considerably and probably does reduce the problems of a too-long chain---a little. However, some of the more demanding riders probably won't like it. The setup that I described above, is a 1x also. But you have to remember that most 1x drivetrains also use a wider range of sprockets on their cassette. This removes some of the 1x advantage regarding chain length and a single chainring.

Whether or not it makes sense to add links back on a chain depends on the type of chain. Some have reusable pins and adding back a few more links is no big deal. Others require a special replacement pin---again, no big deal as long as you have the required pin. But some of the high-end chains are not so easy. For example, they may use hollow pins that cannot be replaced, making the addition of more links in the future problematic. Unless you are willing to use multiple master links (or PowerLinks) and I don't favor that.

It depends, too, on how critical the rider and/or mechanic are. I can think of many situations where I would not be willing to use a chain that had links cobbled back onto it if I can avoid it. Racing, long-distance rides, extreme climbing, to name a few.

If the OP anticipates changing the max chainring or max sprocket in the future, my advice is to buy a new chain when he does. Why not have multiple chains that are the exact right length, one for each setup that the rider/mechanic plans to use. That would be the ideal way to deal with it.

Kind regards, RoadLight

Sizing for 1x is super easy. Anything between long enough for the largest, and short enough so the Rd can take up all the slack on the smallest is fine.

Within that range, some people (including me) prefer going as long as possible, so there's room for a larger cassette if it's ever wanted. Other bias to the short end to save weight.

There may (MAY) be slight performance differences based on chain length, with better shifting happening if the RD cage is near vertical on the 3rd or 4th sprocket in, but it's a subtle difference.

OP is running an 11-speed XT which has a 42T capacity in the shortest (medium) cage version. Running an 11-40 cassette with a single chainring uses 29T of capacity leaving an additional 13T (6.5 inches) of chain the RD can take up. So adding a couple inches to the big-big+1 length is guaranteed to work fine with no negative effects (other than four links of added weight). This is exactly what I do on my 1x10 CX bike -- X7 short cage RD (30T capacity), 11-32 cassette, 4.5" excess capacity, chain sized big-big+3" allows me to bump the cassette up to a 36T max or the chainring to a 44T max without touching the chain.

No 11-speed chain that I know of has reusable pins, so lengthening an 11-speed chain after breaking is always going to require an additional master link (or Shimano connecting pin). No real harm in doing that, but no real advantage either when you can just leave a little extra on the chain in the first place.

Yup. Big/big +1 will give you the shortest chain that will safely work with that gear combination. The arm of your derailleur cage will be pulled almost horizontal when using your biggest rear cog.

Roadlight, I used your method and it worked perfectly. Thanks!

Good job! Enjoy your new setup. I'm like my new 1x drivetrain very, very much. :-)

I measure by threading through the derailleur while in big-big. Seems like the easiest "no fail" way to me. You can be sure it works with your components.

What's the downside ?

Depending on the RD and the amount of cage tension, it sometimes takes three hands to do it this way and truly get to the shortest allowable chain (those long cage MTB RDs put a healthy amount of tension on the chain when fully extended). Big-big+1 should get you the exact same length (the +1" is allowance for the RD pulley routing) it's just a little easier to do since you're not trying to hold tension on a freshly lubed chain while you measure.