[swoodbrn]

I switched my rear derailleurs and now I have a bunch of chain slack - why? I swapped out my Shimano LX M-whatever-rapid rise rear derailleur for an XT conventional one. Both are "long cage", or "SGS", yet now I have a bunch of chain slack. I measured and determined that the LX has a slightly longer cage (distance b/t guide pulley and tension pulley axles). I was expecting them to be the same. Should they? Did they send me the wrong part?

So, now I need to shorten the chain?

Thx for the assist.

 

[Jayem]

No, cage lengths are not always the same. If the pulleys are in a different location or a different size they'll also have an impact.

 

[Hecubus]

LX I seem to recall having a rather longer long cage than is normal on others.

 

[Tracerboy]

If you got chain slack then shorten the chain. A general rule of thumb for chain length is having it in the granny gear and the highest in the rear and make sure the chain is just tensioned enough where the chain doesnt rub the top cog on your derrailleur cage.

 

[swoodbrn]

Thanks for the advice. So, I actually had to take out 2 links in order to get the chain below the guide wheel when on the small rings. That surprised me - that simply going from LX to XT would require that much shortening, but perhaps my bike was put together with its chain too long to begin with (that WOULDN'T surprise me, as I've been consistently losing confidence in the boys at the LBS where I got it).

Anyway, it's pretty taught now when on the big rings, but I guess that's alright? I saw in Zinn's that the one way to determine proper chain length, when using a typical long cage derailleur (see my initial post where I question if my cage is 'typical'), is to run the chain around the large rings, without going through the derailleur, and use enough chain to overlap the ends by 1 link - mine can now overlap by about 1/2 a link. Potential damage from a chain that's too short??

 

[Jayem]

One thing that throws a monkey wrench into this is if you have a full suspension bike. Most full suspension bikes have chain growth, which means the chainline has to get longer as it goes through the travel (the axle gets further from the bottom bracket). This requires more slack in the system, and prevents some people from using short-cage derailers, it also requires you to set the chain length accordingly.

 

[swoodbrn]

Yes, I guess I'll have to make sure I don't bottom out while using the big rings, lest I snap the chain or mangle the derailleur? Shouldn't really be a problem, I've only got 4" of travel, the SGS is supposed to be a long cage, and nobody rides on the big rings anyway. Right?

 

[swoodbrn]

FYI - took the bike out with the new XT rear derailleur on, and two fewer links in the chain and it worked like a peach!! NC mountains in the winter - rain, snow, mud - never missed a shift, never slipped a ring. So long, rapid-rise XL...