If you look at the list of parts on my XO-1, you'll notice that I use a standard cage Shimano 105 rear derailleur with a fairly wide range triple crank and a fairly wide range cassette. According to the specifications, this should not work, yet I and many others use setups like this. I also have a Shimano Ultegra rear derailleur on another bike with a "road" triple chainring setup.
You can make this work too, if you don't use cross-over gears! Cross-over gears are those that combine a big chainring with the largest cog (and sometimes one or two next to it) or the smallest chainring with the smallest cog(s). If you have no idea what gears you use, or don't want to be bothered by having to think about your gearing choices, then use a long rear derailleur and be safe about it. If you want the advantages of a short cage rear derailleur, and are willing to absolutely avoid cross-over gears, (i.e. are you "allergic" to shifting into these gears?) and won't sue me if you damage your bike or yourself*, then read on.
A short cage rear derailleur (often called "road," but bike parts don't have to follow marketing definitions) offers several advantages over a long cage rear derailleur:
Are there any disadvantages? Sure, you can jam the chain severely if you use the big-big combo and your derailleur can't handle the chain tension, causing all sorts of potential problems, like bent dropouts, damaged derailleurs, broken chains, etc. Using the small-small combo leaves plenty of slack in the chain, which can now freely slam into your chainstay, removing paint and gouging it.
- Faster shifting.
- Smaller target for flying debris.
- Better ground/rock/log/rut clearance.
- There might be a wee less chance of chainsuck, with the increased tension.
- If you already own a short cage rear derailleur, you don't have to buy a new one.
- It's just plain snazzy!
Both front and rear derailleurs have a listed capacity. For the front derailleur this is the difference between the number of teeth on the largest chainring and the smallest chainring, and is difficult to exceed by more than 2 teeth. For example, many "Racing Triple" front derailleurs have a listed capacity of 22t, which happens to be just enough for the popular 30-42-52 setup (52-30=22, ain't arithmetic grand!)
Rear derailleurs usually have both a largest cog (which is generally hard to exceed) and a capacity listed. What's capacity anyway? It's:
(biggest chainring - smallest chainring) + (biggest sprocket - smallest sprocket)A rear derailleur consists of a parallelogram and a cage. (Links are to Sheldon Brown's mighty fine bicycle terms glossary, where you should go if there are any other terms in this article you don't understand.) The parallelogram generally determines the maximum cog, while the cage length and parallelogram design together determine the capacity.
The Shimano 105 standard rear derailleur has a largest cog of 28t and a capacity of 28t. The longer cage "triple" version has a largest cog of 28t and a capacity of 36t.
So, for the above 30-42-52 example which is commonly paired with a 12-23 cassette, the capacity needed is (52-30)+(23-12)=33 teeth, which falls somewhat in between the capacities of each version of the 105 rear derailleur If paired with the 13-26 or 12-25 cassettes, the capacity needed becomes 35t, which pretty well fills out the long cage derailleur's 36t capacity. In practice, however, if you're avoiding the cross-over gears, capacity can be thought of as:
(biggest chainring - smallest chainring) + (biggest sprocket you'll use with the big chainring - smallest sprocket you'll use with the small chainring)Confused? Let's look again at our first road triple example (I'm assuming an 8-speed cassette of 12-13-14-15-17-19-21-23), and let's say you don't ever use the 52x23 or 30x12 combinations, the capacity needed becomes (52-30)+(21-13)=30 teeth. If you only use the 15 through 23 with the granny, which is how many people use a road triple, you get (52-30+(21-15)=28 teeth and you're all set to use the short cage version of the rear derailleur.
Here's where things get tricky and where you will spend a lot of time. Depending on your chainstay length, you might end up getting a setup that should work but doesn't. If you can find an old Suntour rear derailleur instruction sheet it will tell you to put the chain on the largest chainring and largest sprocket, thread it through the derailleur's jockey wheels, and then cut the chain (remove excess pairs of links and then re-assemble the chain, leave the bolt cutters in the tool box!) so that the derailleur cage is more or less perpendicular to the ground ("Normal Setup" in the diagram).
Current Shimano instruction sheets have an entirely different approach, detailed instructions and much more are found in Sheldon's Derailer Adjustment article (and he'll explain why he spells derailleur that way too!) If you have to choose between cutting the chain so that the lower jockey is a little more forward or a little more rearward of the upper jockey, then choose forward.
To set up a shorter-than-legal rear derailleur, put the chain on the largest chainring and the largest sprocket you intend to use with the big ring (in our above case, instead of setting it up on the 52-23, use the 52-21), thread it through the jockey wheels of the cage, and then adjust the chain so that the cage is perpendicular to the ground, just like the you had put it in the big-big combo ("Extract more capacity!" in the diagram). Now start shifting through the gears (with the bike in a stand, don't ride it yet!) to see if it shifts well, and, if needed, add or subtract pairs of links of your chain.
Feeling brave? With the bike still in the stand, put the chain on the big ring, and shift from the biggest sprocket that you intend to use to the next larger one. It will probably work, and the cage will be way the hell forward of perpendicular, but if there's any movement left in the cage (see if you can move it even more forward with your hand) this gear will work OK, but is one you should avoid anyway (it will probably make noises in this gear, consider it a warning to shift out of it, fast!) If it just refuses to move onto the sprocket, then it's a true illegal "damage your bike" combo and you should never try to shift into it. In fact, sane people would add links to the chain so that there are no "damage your bike" combinations, even if that means giving up on short cage rear derailleurs and buying a long one. If you accidentally shift into this gear while riding, you could crash, and hard, and get to spend some time with the nice people at the local E.R. while you contemplate how much it will cost to repair or replace your bike.
I use such a combination on my XO-1, but I literally never ever use cross-over gears, or even the gear or two next to them, and since I have bar-end shifters, I have tactile feedback as to where my rear derailleur is, which STI (road or mountain), Ergo and twist-shifters don't offer. Be forwarned, and read my disclaimer again.
If you use typical road triple setups, like in our example above, you shouldn't get such a gear combonation, this only happens with the wide range gearing typical of mountain bikes, touring bikes and tandems. Touring bikes and tandems have no need to use short cage rear derailleurs, although the above instructions should work fine if you need to slightly exceed the capacity of your long cage rear derailleur on a touring bike or tandem.
Now try some of the small-small combo's that you're not supposed to use, you'll just notice that the cage is as far back as it can go, and that the chain is hanging limp, and that if you rode the bike with these gears, you'd kiss your chainstay's paint job good-bye. If you don't like what small-small combo's are available to you, remove some links (and run the possibility of creating some "damage your bike" combo's) or get a long cage rear derailleur.
OK, well put down your over-priced beer and check out the setup of my XO-1 once again. You'll notice that I'm using a 24-36-46 set of chainrings, with a 7-speed 12-14-16-18-21-24-28 cassette. Originally, I had a 26-38-50 setup up front, but decided on lower gears. With the big ring, I only use the 12 through 21 sprockets, and with the granny, I only use the 14 through 28 sprockets, so I need (46-24)+(21-14)=29 teeth capacity, and due to a fortunate chainstay length, I get that extra tooth of capacity. In practice, and on a stand, the bike will shift from the 46x21 to the 46x24, but will refuse to shift to the 28. Shifting from the 36x28 to the 46x28 will cause the chain to jam in the front derailleur's cage, and I'll have to spend about 5 minutes carefully extracting it, and that's on a workstand where I put only gentle hand-pressure on the pedals. Shifting into the 46x28 from the 36x28 while riding would almost certainly cause a nasty crash. With the previous setup, I just used the 16 through 28 with the granny to get the same capacity.Return to my bike page.
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I have a page on converting a double chainring bike to triple chainrings too.
Disclaimer: The author assumes no liability for property or personal damage that could result from following these directions. If you do something stupid, it's your fault, and it's time you got a spine and owned up to that. Life's tough, and there shouldn't be monetary reward for stupidity. Nobody owes you anything, not me, not any big or small corporation, not the government, and not even your parents. Deal with it.