Sram MTB 2x RD for road set up?

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illskittlz
Posts: 39
Joined: Sat May 10, 2014 5:39 am

by illskittlz

I am building up a 2x gravel bike and want a wide range cassette. I believe the Sram road RD's can go to a 36T but if I wanted to go bigger, like a 40t, can I use a MTB RD for that? Or would a long cage CX1 work instead with a 2x set up?

Trying to see what my other options are instead of using a Wolf Tooth road link.

grover
Posts: 1302
Joined: Mon Jul 26, 2004 1:06 pm

by grover

SRAM WiFli road derailleurs go to a 32T cassette.

SRAM 10speed mtb derailleurs go to a 36T cassette and are compatible with SRAM 10 and 11 speed road shifters. These are the models that use 'EXACT ACTUATION'. The models that use 'X ACTUATION' are not compatible.

SRAM don't make a derailleur that has capacity for bigger than a 36T cassette and also 2 chainrings by their specifications. That said many people had success using SRAM 10speed mtb derailleurs listed as only accepting a 36T cassette with cassettes up to 40 or 42 when Wolf Tooth and other companies started bringing out their conversion kits. They often needed a longer B-tension bolt and sometimes a different inner cage plate.

Any of their 1x derailleurs with X-Horizon absolutely will not work with two chainrings. They rely on exact chain length to set the gap from jockey wheel to cassette due to the offset jockey wheel and horizontal (fixed) parallelogram. By using two chainrings the chain length is effectively changing.

I'll also point out that a 42T ring with 10-42 cassette is exactly the same range as 36/46 rings with 11-36 cassette.

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illskittlz
Posts: 39
Joined: Sat May 10, 2014 5:39 am

by illskittlz

grover wrote:SRAM WiFli road derailleurs go to a 32T cassette.

SRAM 10speed mtb derailleurs go to a 36T cassette and are compatible with SRAM 10 and 11 speed road shifters. These are the models that use 'EXACT ACTUATION'. The models that use 'X ACTUATION' are not compatible.

SRAM don't make a derailleur that has capacity for bigger than a 36T cassette and also 2 chainrings by their specifications. That said many people had success using SRAM 10speed mtb derailleurs listed as only accepting a 36T cassette with cassettes up to 40 or 42 when Wolf Tooth and other companies started bringing out their conversion kits. They often needed a longer B-tension bolt and sometimes a different inner cage plate.

Any of their 1x derailleurs with X-Horizon absolutely will not work with two chainrings. They rely on exact chain length to set the gap from jockey wheel to cassette due to the offset jockey wheel and horizontal (fixed) parallelogram. By using two chainrings the chain length is effectively changing.

I'll also point out that a 42T ring with 10-42 cassette is exactly the same range as 36/46 rings with 11-36 cassette.



Thanks for the reply. So what about if I use a roadlink with a sram road mid cage? Can I get to a 40t in the back? It seems like lot of people have done so successfully on youtube, but I am also wondering about durability or other possible issues by doing this.

grover
Posts: 1302
Joined: Mon Jul 26, 2004 1:06 pm

by grover

I've not done it.

If I were in your position I'd first try a 10 speed mtb derailleur with the 40tooth.

Some frames work, some don't, depends on how long the stock hanger is. But the mtb derailleur is going to have a better chance of working than the wifli road derailleur.

If it doesn't work then get a roadlink.

Make sure that your cable routing from the frame will work with the direct entry of mtb derailleurs though.

I'll also point out in case you aren't aware. It's not the cage length that makes a derailleur compatible with a bigger cassette cog. It's the geometry of the parallelogram. The cage length is to take up the chain slack. Which is more on wider range cassettes hence the need for longer cages.

To calculate your required cage length.

(big ring minus little ring) plus (big cog minus little cog) equals tooth differential
eg (46-36)+(36-11)=35 tooth differential
for sram
long cage - 38 or more
medium cage - 32 to 37
short cage - 31 or less
so the example would require a medium cage. if that example changed to use a 40 tooth cassette the tooth differential would be 39 and a long cage derailleur would be required (which is not available in a road derailleur).

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