Originally Posted by
Paul Barnard
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?
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:
Chain Length (inches) = ( 2 x C ) + ( F / 4 ) + ( R / 4 ) + D
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:
52.875 inches = ( 2 x 16.1875 ) + ( 50t / 4 ) + ( 28t / 4 ) + 1
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