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My SRAM x5 rear derailleur got caught on something and broke. I went to order a new one and learned there are different cage sizes. I don't know which size cage I currently have. While trying to figure out which one I need I figured out that I need to know my Total Drivetrain Capacity. All of the sources that I have come across say

Maximum Chainring Difference + Maximum Cassette Cog Difference = Total Drivetrain Capacity

Not sure what the formula is when you only have 1 chainring. The answer to this question appears to just be

Maximum Cassette Cog Difference = Total Drivetrain Capacity

Is that correct? So in my case it would be 34 - 11 = 23? So a medium cage would be plenty? The sources I have come across say if in doubt, get a large. I would rather have a smaller cage though to hopefully not have it catch on something again.

My current setup
Crankset: alloy arm with 34T single ring
Shifters: SRAM x4 9 speed
Rear Derailleur: SRAM X5, 9 Speed
Rear Cogs: SRAM PG950 Cassette, 9 Speed, 11-34T

SRAM x5 medium cage specs
Largest Cog: 34
Chain Wrap Capacity: 37

SRAM x5 long cage specs
Largest Cog: 36
Chain Wrap Capacity: 45

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    My bike is a 2017 Diamondback line. Everything stock except the chainring. rei.com/product/116941/diamondback-line-275-bike-2017 – KRob May 28 at 2:57
  • Do you think its possible that your existing 34T big cog might get replaced with anything larger? Or are you perfectly happy with the gearing as it sits ? – Criggie May 28 at 3:06
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    @Criggie No, I am happy with the gearing as is. – KRob May 28 at 3:12
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    Just a thought: is the derailer itself broken, or is the derailer hangar broken? The latter is a piece of aluminum that’s bolted to the frame, and it is replaceable - because when you crash, the hangar tends to break before anything happens to the derailer or the frame. It takes a bit more than a crash to kill a derailer. Yours looks like it might be Ok. – Weiwen Ng May 28 at 10:30
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    Seeing your new photos, it does look like the plates of the cage got bent. Ouch. If it's just that the plates are bent, then you could think about trying to bend them back using a vise and seeing how it shifts. If the pulley got ripped out of the cage, then probably some threads are damaged and bending the cage back would be futile if so. If you don't have a vise, you could ask a bike store if they can bend it back, but considering labor costs, it could be more cost effective to get a new derailer. – Weiwen Ng May 28 at 17:56
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You have it right. Between medium and long cage, medium is the one to get here.

As you say, you only need a capacity of 23, which is short cage. SRAM is still listing a short cage X7 on their site, but I couldn't find anyone selling or distributing it. If you can track one down and if you're staying with this gearing, that would be more ideal than medium cage. But the medium X5 is fine too. Sadly the short cage 10-speed GX with a clutch is not compatible.

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