Bicycle Mechanics - Long or short Rear Derailleur Cage?

What exactly dictates whether you need a long or short rear derailleur cage?

chain wrap capacity and maximum sprocket size.

there's actually short, medium and long.

I always try to use a long cage if I have one. Longs always shift well and the biggest reason for shorts is to save weight. In my case weight saving is not that import compared to ease in shifting. Roger

I always try to use a long cage if I have one. Longs always shift well and the biggest reason for shorts is to save weight. In my case weight saving is not that import compared to ease in shifting. Roger
+1. Long cage rear derailleurs allow a greater range of gearing at a very small penatly in weight and possible loss of face with snobby riding partners.

There are those who claim short cage rds shift slightly faster but an give no reasonable explanation why that should be so. The jockey pulley that actually does the shift is in the same location relative to the chain and cogs with a long cage as it is with a short cage.

Shimano road rear derailleurs are available only in short or long cage. Campy used to be available in short medium and long cage but their current listings is only short and medium. Some Shimano MTB derailleurs are available in short, medium and long also.

and road long is actually the same thing as MTB medium.

the one problem with going with a true long cage is that if you ride offroad, there's a chance of having the RD hit something protruding on the ground.

the one problem with going with a true long cage is that if you ride offroad, there's a chance of having the RD hit something protruding on the ground.
I suppose it's possible but in 130,000 miles of long cage road use, it's never happened to me.

The only rear derailleur damage I've ever had was due to a piece of wire being thrown up by the front wheel into the chain that went through the derailleur and wiped it out. The cage length had nothing to do with it.

Lance uses a short cage on his road bike, if that matters to you. ;)

But, to answer the question. a long cage is typically required whenever you have a triple crank, or if you have a wide enough range of gears with a double or compact double.

Based on guidance I received from my bike shop, I needed a long cage for my compact double (50/34) with an 11-28 cassette. But I wanted one anyway, as it means I can choose whatever gearing I want without needing to be concerned with swapping the derailleur. If I had been building a standard double (52-39 or 53-39), or gone with a narrower range cassette, I could have gotten by with a short cage.

For most people, I would suggest a long cage... other than the potential "teasing" and minimal weight issues mentioned above.

I have a 53-39 crankset with a 12-23 cassette. Any help?

you can get away with short.

I have a 53-39 crankset with a 12-23 cassette. Any help?

Short OK for that combo.
Long if you want to use a triple up front or anything larger than a 28 in back.

Road short, Road long, MTB medium, MTB long
Maximum sprocket: 27T, 27T, 34T, 34T
Total chain wrap capacity: 29T, 37T, 33T, 45T

but that's just the shimano tech docs.

I got a free bike from a neighbor. It evidently had a medium or short derailleur. When I shifted to the 32 cog in the rear, the derailleur stretched forward as far as possible, then jammed up in the gears. I had to remove the rear wheel to get it unstuck. I immediately got it - now I know what long-cage derailleurs are for.

I got a free bike from a neighbor. It evidently had a medium or short derailleur. When I shifted to the 32 cog in the rear, the derailleur stretched forward as far as possible, then jammed up in the gears. I had to remove the rear wheel to get it unstuck. I immediately got it - now I know what long-cage derailleurs are for.

No, actually for a 32 you need a mountain type derailleur. A long cage road derailleur is rated for a maximum of 27 teeth, although a road derailleur will usually work with up to 30 teeth on the largest cog.
A mountain derailleur will work with a 34. (see AEO's last post above).

No, actually for a 32 you need a mountain type derailleur. A long cage road derailleur is rated for a maximum of 27 teeth, although a road derailleur will usually work with up to 30 teeth on the largest cog.
A mountain derailleur will work with a 34. (see AEO's last post above).

That figures. This old Peugeot road bike is something of a franken-bike. The previous owner put a triple mt. crank (removing the inner ring) on a road bike (japanese on a french spindle so it wobbles!). And he put a big honking Shimano freewheel (12-32). He also replaced the derailleur with an old Shimano SIS, whereas should have installed the mountain variety.

At any rate, I finally got it to shift today, but had to replace the rear wheel (I had one with a smaller freewheel), and then had to replace the cable, then clean, lube and adjust the derailleur. It's all working fine now.

I got a free bike from a neighbor. It evidently had a medium or short derailleur. When I shifted to the 32 cog in the rear, the derailleur stretched forward as far as possible, then jammed up in the gears. I had to remove the rear wheel to get it unstuck. I immediately got it - now I know what long-cage derailleurs are for.

Actually, the bigger problem you have is that the chain is too short for the gear combo you tried to use. Were you using your largest chainring and the 32 in the back at the same time? If the chain was sized for a normal road cassette, it would likely have been too short to work with the 32 tooth cog and the original big ring. When using a rear cog too big for the derailler the jockey wheel will rub against the cog making an annoying noise and quickly wearing the jockey wheel down.

I'm running a DA 53-39 crank with a DA short cage derailleur and a SRAM Red 11-28 and it works fine althought it did take some work on the adjustments.

I'm running a DA 53-39 crank with a DA short cage derailleur and a SRAM Red 11-28 and it works fine althought it did take some work on the adjustments.
If you sized the chain long enough to allow the big-big combination (53-28), which for mechanical safety you really should, you probably have the chain go completely slack in 39-11 and maybe in 39-12. No big deal there.

....
Based on guidance I received from my bike shop, I needed a long cage for my compact double (50/34) with an 11-28 cassette. But I wanted one anyway, as it means I can choose whatever gearing I want without needing to be concerned with swapping the derailleur. ....

Funny, I have exactly that combo (50/34 and 11-28) and the short cage RD works fine for me. I specifically asked my mech if I needed a long derailer with the 11-28 and he said "no". He did look carefully at the chain length though, although he did not change it, as I was changing from 53/39, 12-25.

Funny, I have exactly that combo (50/34 and 11-28) and the short cage RD works fine for me. I specifically asked my mech if I needed a long derailer with the 11-28 and he said "no". He did look carefully at the chain length though, although he did not change it, as I was changing from 53/39, 12-25.

It is obviously a combination that you can tweak to work, and most people configure their compacts with short cage derailleurs regardless of which cassette they use... almost anything could be made to work (short of a triple) with a short cage as long as you are good at getting the chain length right... The worst that can happen is low tension on the small/small combo or extreme tension on the big/big. Both gear combinations that should see little or no usage anyway. I see the critical adjustment as avoiding being too tight in the big/big, to avoid breaking the chain or derailleur if you happen to shift into that combination and the chain is too short to work.

Assuming the earlier message that a 29 tooth chain wrap is the official maximum for a road short cage, and since the difference between 50x28 and 34x11 is 78-45=33 it is actually a bit outside of the spec. But, the specs are usually a bit conservative, both on chain wrap and on maximum cog size.

As in many things cycling, if it works for you, then there is obviously no problem. ;)

I see the critical adjustment as avoiding being too tight in the big/big, to avoid breaking the chain or derailleur if you happen to shift into that combination and the chain is too short to work.
+1 and that's why I and others recommend the chain ALWAYS be sized long enough to allow big-big.

I've heard the argument that "I never use that combination" and, it's correct that you shouldn't. However, everyone has occasional brief episodes of brain-fade so someday you will try.

...29 tooth chain wrap is the official maximum for a road short cage...

Is this really true? My 2008 Allez triple came stock with 52-42-30 (22 tooth difference) and an SRAM 12-26 (14 tooth difference). Total tooth difference would be 22+14 = 34. The specialized site specs a short cage Tiagra RD. Do you think that's a misprint? I'm looking to go to a road double soon.

Is this really true? My 2008 Allez triple came stock with 52-42-30 (22 tooth difference) and an SRAM 12-26 (14 tooth difference). Total tooth difference would be 22+14 = 34. The specialized site specs a short cage Tiagra RD. Do you think that's a misprint? I'm looking to go to a road double soon.
It's either a mis-print or the Allez comes with a double or triple crank and you looked at the specs for the wrong model. Any triple crank Shimano drivetrain is going to come OEM with a long cage rd.

No, actually for a 32 you need a mountain type derailleur. A long cage road derailleur is rated for a maximum of 27 teeth, although a road derailleur will usually work with up to 30 teeth on the largest cog.
I have a longcage road RD for a 32. A-OK. Original factory set-up.

The type of riding that you do most: hills, trails, street, all around. On my road bike I finally decided to put a Shimano XT rear derailleur on it for the sake of having a variety of cassette usage. Because I like the freedom having an 11-32 when I need it, or a 12-34, or just a 12-23, so the longer cage allow for the versitillity.

I have a longcage road RD for a 32. A-OK. Original factory set-up.

What model derailleur is it. Shimano's road derailleurs are spec'd for a 27t maximum sized cog, but can take at least a 28 and probably a 30.

What model derailleur is it. Shimano's road derailleurs are spec'd for a 27t maximum sized cog, but can take at least a 28 and probably a 30.
I saw this 27T number on Sheldon's website. He said Shimano gives 27 because that's the highest cog they made. Perhaps they thought no-one uses anything but Shimano gear?
The bike I have with this long cage has a (non-Shimano!) Suntour 7-GT with a 14-32 freewheel. I looked it up at velobase. Max chainwrap=36T, max rear difference=20T.