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I have an 11-42t 10 speed cassette and a 42t narrow wide chainring. I cannot work it out definitely whether a Sram long cage rear derailleur will work with it.

I think that this one should work but if someone clever could confirm and/or recommend something better, I would really appreciate it: SRAM GX Type 2.1 10 Speed Rear Mech.

It's a road/gravel/commuter bike with bullhorn bars that I'm building myself. Its the first time I've built a bike and has been a steep learning curve which is part of the reason I'm doing it, buts its very fun.

I know there are loads of similar questions on the internet but I've trawled through hundreds and there seems to be so many inconsistencies and exceptions in the answers.

  • +1 as per answer below from Nathan. But you might also find the Roadlink by Wolf Tooth components handy. It extends the range of a rear mech. wolftoothcomponents.com/products/roadlink – OraNob Nov 1 '16 at 22:22
  • I edited the title based on the assumption we're talking about a SRAM bike here. The conversation gets pretty different for a Shimano gravel road bike. – Nathan Knutson Nov 1 '16 at 23:33
  • @OraNob On their website it seems to say that 11-42 is not supported. Also although its a very simple addition, I would rather not have it if possible, just to be a bit more minimal. – Paddy Livingstone Nov 2 '16 at 15:31
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    @NathanKnutson I have a SRAM crankset and was planning on a SRAM rear derailleur, but if a Shimano one would be easier or better then I don't have any allegiance to SRAM or anything. – Paddy Livingstone Nov 2 '16 at 15:35
  • Follow-up question. Digging around and just wanted to confirm that a 11-42 or 11-40 10-speed would work with SRAM GX 2.1 in a 2x configuration. – Gerard Jun 2 '18 at 16:31
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One confusing thing here is that SRAM themselves, that I know of at least, have never made or warranted a 10 speed 11-42 1x setup. So you're not going to hear from them or their product documentation that any SRAM RD will work here.

There are two questions. One is capacity, which is the total amount of slack the RD is capable of taking up. Different cage lengths are what gives you your different amounts of capacity. SRAM doesn't publish actual capacity numbers like Shimano does; they make you infer it from looking at their Compatibility Map documents. Because you're using a 1x, the capacity you need is simple to calculate: it's large cog tooth count minus small cog tooth count. For 2x or 3x, you're adding to that number the difference between the large ring and the small ring. So by looking at the Compatibility Map, we see that you actually only need a medium cage, since their capacity is sufficient for use with a 14t difference in front (42/28 double) plus a 21t difference in back (11-32 cassette). So at least 35t total capacity, whereas you only need 31t. Long cage would still work, but you don't get any benefit from it here other than versatility if you want to change things later, and the drawbacks are weight, ground clearance, and arguably shifting performance.

Note that sometimes things can get awkward when one flirts with maxing out the total capacity of a rear derailer on a 1x, because then you're riding with the cage pulled all the way forward when in your big cog, which tends to add friction and decrease shifting performance. Here it should be fine, this setup with medium cage is more or less a known quantity (Wolf Tooth and OneUp both say it's a go with their 42t conversion cogs, for example) but in other cases I'd be cautioning that putting it all together is still going to be a bit of an experiment. Also note that chain growth is a factor that has to be managed on some full suspension bikes.

The other issue is largest cog clearance. SRAM says the max for their 10-speed derailers is 36. In practice lots of people run them with 42t cassettes, either ones that come that way or are modded with aftermarket large cogs. It's possible that to get the pulley/cog gap required you may need to swap in an extra-long B-tension screw, which is just an M4x0.7 bolt. Note that SRAM 1x setups need kind of a lot of clearance here. I'm always a little baffled about the physics involved that makes a bigger gap work better, but it's true. In theory since you're putting the clearance out extra far here, shifting performance may suffer at the other end of the cassette, but in practice that doesn't seem to be much of an issue.

  • I know that we're not supposed to use the comments to say thanks, but, thanks. – Paddy Livingstone Nov 2 '16 at 15:22
  • So the result of this is that I should be able to use a SRAM medium cage derailleur, possibly requiring an extra long B-tension screw. And the shifting may still not be perfect but should be very useable. At least that's my understanding. – Paddy Livingstone Nov 2 '16 at 15:25
  • @PaddyLivingstone You got it. Also it may be worthwhile to just read through everything on this page, it will give you a pretty good understanding of what your options are (I'm not plugging Wolf Tooth; there are other companies that make similar stuff): wolftoothcomponents.com/pages/1x-gravel-setup . – Nathan Knutson Nov 2 '16 at 16:08
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I just tried the configuration you are considering: 11-42 10-speed with a SRAM GX 2.1. I had read several online posts claiming this worked flawlessly. However, on my bike the derailleur barely cleared the 42t cog. Shifting to/from the 42t cog was terrible. A longer b-screw didn't help. To make this configuration work, I installed a Wolftooth Roadlink. This is not the standard/intended use of the Roadlink. The Roadlink is intended to enable road derailleurs to work with large cassettes, but it works fine with the SRAM GX 2.1.

  • @user36625- what is the cage length of your GX rd? – Paddy H Aug 10 at 4:30
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I tried with a SRAM X7 2.1 Medium Cage and have issues changing from 42 to 36. With Apex 2x10 converted to 1x10 and 40T RaceFace chainring. All other gears work/shift fine. Think I'm going to upgrade to a GX 2.1 Long Cage. This bike has this combo.

  • Did the long-cage work? At the moment, your answer just describes something that doesn't work, but there are infinitely many combinations like that. – David Richerby Sep 22 at 11:55
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    I ordered the GX off Amazon. Will update later... – David Sep 23 at 16:01
  • @david I'm looking forwards to the update - please do let us know how you get on with an edit to your answer. – Criggie Sep 24 at 11:38
  • Well, I installed the long cage GX with a new shifter cable. Works great after getting the B-screw in the sweet spot. Spacing from the guide pulley to the cassette seems the same, the angle from the chain seems a little different, going from medium cage to long. I'm betting if I had a 11-40 cassette the medium would have been fine. Hope this help somebody. – David Sep 27 at 17:09

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