enderconka

Our long cage dreailleur is broken. I found the shorth one in the market as a replacement. Do I have to shorten the chain as well when I change it_

Regards,

Ender

fietsbob

Greater length between pulleys will take up slack, that otherwise will sag *, if you get the identical RD , depending on the design
and your mechanical problem solving skills,

it may be possible to swap the cage parts..

* Math ... add both, largest tooth counts, subtract both smallest, that difference is the slack take up requirement.




[in the 80s, I bought a long cage replacement and swapped it onto a short cage rear derailleur, so i now have both..]




..

tagaproject6

Did you have long cage derailleur to accommodate the size of the cassette? If that is the case, you may want to check if a short cage RD will work (this is assuming your cassette is a 32).

SkyDog75

If you put a shorter cage derailleur on the bike, don't shorten the chain to accommodate the derailleur. The bike's gearing determines chain length, not the derailleur. The chain needs to be long enough to wrap around the biggest chainring and biggest cog, otherwise something will get damaged (badly) when you try shifting into that combination. You can potentially trash your rear derailleur, derailleur hanger, wheel, and even the frame.

Long cage derailleurs exist to handle wide gearing ranges. When you shift from your biggest chainring and/or cassette cog to smaller ones, the chain would hang slack if it wasn't for the spring-loaded cage on the rear derailleur. The longer that cage, the more slack in the chain the derailleur can take up, and thus thus the wider the gearing range the derailleur can handle.

Rear derailleurs have published specs, specifically "wrap capacity", that tell you what gearing range they can handle. To determine the wrap capacity you'll need for your bike's gearing, add the difference in teeth between your largest and smallest chainring to the difference in teeth between your largest and smallest freewheel/cassette cog. Then pick a derailleur whose wrap capacity meets or exceeds that number.

For example, let's run the numbers for a 34-50 compact double crankset and a 12-28 cassette on the rear wheel:
(cassette cog difference) + (crankset chainring difference) = wrap capacity
(50-34)+(28-12)=32
You'd need a derailleur whose wrap capacity is 32 teeth or greater.

Retro Grouch 

My philosophy is that if I've got the part in my junk box I'll bolt up almost anything and see if I can make it work to my satisfaction. If I'm BUYING parts I'll always hold out for stuff that's designed to work with my application.

It sounds to me like you're buying a component. If it was me, I'd keep looking for a long cage derailleur.

JohnDThompson 

A general rule-of-thumb is that a long cage derailleur can handle any drivetrain set-up a short cage derailleur can handle, but not necessarily vice-versa. If your drivetrain spans too wide a range of gears, the short cage derailleur may not be able to wrap enough chain to work on a wide range drivetrain.

MikeyMK

The chain should really be made to the length for optimum derailleur tension. I always adapt a chain to the chosen set-up.

The derailleur can handle quite a range. A short cage should handle any 'road' bike but if the chain is made up for a long cage then it might be a bit long for a short cage when you're on the smallest cogs front and rear, so you'll want to remove links accordingly to keep it tensioned.

My range is pretty broad; 11-36T rear and 39-56T front, and yet i only have a 'medium' cage. I can roll 36/56 without breaking anything, and 11/39 with a taught chain (at 117 links) and that's bearing in mind the chain-stay lengthens with suspension compression.

So on road ratios, going from long to short cage, i expect i'd be taking links out to keep the chain taught.

ThermionicScott 

*taut

Kapusta

If the chain was the correct length to start with, you should definitely NOT shorten it.

The conventional wisdom of chain length is that it should be just long enough to accomodate “big-big” (being in the biggest chainring and biggest cog).

This will not change with a shorter cage.

Google “chain length big big plus one” for more explanation.

Retro Grouch 

I agree - strongly.

If you think that you have troubles now, try accidentally shifting into the big/big with a too short chain. Try not to do that farther away from your car than you're willing to walk back.

Viich

No one has really mentioned - the issue with the small cage is that it doesn't pull as much chain. This means the chain may go slack if you shift into small-small gearing, which may provoke a chain drop. No where near the problems you get if you have a too- short chain.

Kapusta

This had been mentioned several times so far.

Viich

I meant that no one had described the effect - that the chain going slack in small-small is all that happens. Fietsbob hinted at it, but the discussion quickly went (not incorrectly) to why shortening the chain too small for big-big is a bad idea.

SkyDog75

Fourth post:

"Long cage derailleurs exist to handle wide gearing ranges. When you shift from your biggest chainring and/or cassette cog to smaller ones, the chain would hang slack if it wasn't for the spring-loaded cage on the rear derailleur. The longer that cage, the more slack in the chain the derailleur can take up, and thus thus the wider the gearing range the derailleur can handle."