The culmination of 20 years spent creating and refining, X.0 is the pinnacle of the SRAM derailleur line. Every gram possible was shaved. The finish matches the look of the X.0 shifters. Forged alloy plates and links, save the Grilon P-knuckle. Machined and heat-treated oversized link pins, sealed bearing pulleys, and a new carbon cage holding the jockey wheels in place. This 20th anniversary edition features a special gold b-bolt and graphics.

SRAM has specific Total Drivetrain Capacity (TDC) ranges they use to figure out when to use which size derailleur. TDC is a number that is the sum of the tooth differentials on both the cassette and chainrings. Let's say you have an 11-34 cassette. take 34 subtract 11 and you're left with 23. Then you have a 44/32/22 combo up front. Take 44 subtract 22 and you're left with 22. Add the differential in the rear to the differential in the front, 23 + 22 and you get 45, which is TDC.

TDC Expressed as an Equation

You might be wondering what will happen if you go with the wrong size. If you have less capacity in your derailleur than you need, you'll end up having to choose between having the chain loose on the small ring and smallest cogs or having the derailleur break on the big ring and biggest cogs. If you have more capacity than you need, you'll just be running more chain than necessary.

The SRAM X.O derailleur has 11-34t minimum-maximum cog sizes and 22t max difference in chainrings. Total capacity is 45t for the long cage, 37t for the medium cage, and 30t for the short. The derailleur works with both eight and nine speed cogs and shifters and is compatible with all 1:1 shifters. Choose long, medium or short cage. 203g/ 197g/ 192g.