We here at Harris Cyclery, under the tutelage of the indefatigable Sheldon Brown, are constantly experimenting with bike components to extract the maximum enjoyment from the least $s. Hence, these thoughts on derailers.
Many people buy a standard "road" bicycle, ride it a while and find out the bike is overgeared. Which means "you shift to the small chainring in front and the large sprocket in back and you still can't pedal up the hill without having to stand." See Sheldon's glossary item on gearing.
Most "road" bikes sold in America in the last 15 years are "road doubles", which have front chainrings of 39-42 tooth smaller ring and 52-53 tooth outer ring. The rear cassettes are 8 or 9 (and now 10) speed. The cogs are what the manufacturer feels is appropriate for "road" cassettes, which run from 11-21 teeth to 12-27 teeth. This means that the lowest (easiest) gear combination one can get is 27 in the rear and 39 in the front (as an example--- your specific combination may vary).
If you are trying to tour with a load, if you are trying to get up steep hills, or if you have gotten older since you bought the bike...this lowest gear may not be appropriate for you!
The general question is: " I've got a (fill in your bike's brand here), with a Shimano 39/53 double and a 12-25 rear cassette. I'm having a hard time climbing with this setup. Can I put a larger cassette on the bike?"
The short answer is
The next question is usually"Will I need to replace my rear derailer with a 'mountain' derailer?"
The short answer to this question is
The long answers (the rest of this article) are that the success of this combination depends on both the rider's technique and on the components. In this article, we talk specifically about Shimano, and Shimano-compatible, cassettes and rear derailers.
Modern derailers use a slanted parallelogram to keep the jockey wheel at the same distance from each sprocket (thank you SunTour). This provides for consistent shifting characteristics. Because of the variety of cassettes Shimano makes, the 'slope' from the smallest sprocket to largest sprocket can vary. An 11-21 cassette has a much shallower slope than an 11-34 cassette.
See two examples here:
The road cassette on the left has less of a size difference between the smallest and largest cogs than the one on the right. This gives it a shallower 'slope'.
To accomodate these differences, and to provide for chain take-up on triple front chainrings, Shimano provides short, medium and long-cage derailers. The differences between the three models are in the length of the chain cage (the distance between the jockey pulley and the tension pulley) and in the length of the parallelogram.
Shimano's derailers are double pivot. They all have a spring that tensions the derailer body against the frame dropout's derailer stop. Shimano derailers have a spring-loaded "B" screw that allows precise adjustment of the position of the derailer body. This "B" screw allows for adjustment for variations in various cassettes' "slope".
For more info on derailers, and the details of their design, read Frank Berto's "The Dancing Chain". This book is a wonderful history of the development of the modern bicycle and the modern derailer.
The differences between short-,,medium- and long-cage derailers, while significant, are not so significant as to automatically preclude using your existing derailer. It's a difference between Shimano's Optimum-Shifting-Performance, and my "good-enough-for-regular-use" shifting performance.
By rotating a short-cage derailer back further than normal, its parallelogram angle approximates that of its long-cage brothers, allowing it to push the chain onto the largest cogs and maintain a reasonably consistent chain-gap.
Rotating the derailer backwards is done by screwing in the B screw on the back of the derailer.
First, check to see if your derailer has a B screw on the back of the body, like the two Shimano derailers in these pictures.
The derailer is spring-loaded to rotate forwards, so the B screw pushes against this spring to allow you to set the optimum position of the derailer's jockey (top) pulley vs the specific cassette being used. According to Frank Berto, the key consideration for rear derailer performance is the 'chain gap', the distance between the jockey pulley and the cog.
If your derailer has a B screw, then you install the new cassette, shift to the large cog, and adjust the "B" screw so that the jockey pulley doesn't hit the teeth on the large cog. See picture below for a fully screwed in B screw. Check Sheldon's page about derialer adjustment for more B screw info.
If, after screwing in the B screw completely, the pulley and sprocket still hit, then you need a longer screw. If you don't have a longer screw, then reverse the B screw in its mount. See picture below.
This usually suffices for current Shimano derailers and most manufacturer's frames. However, the design variables of your specific bike and cassette may dictate that you need an even longer screw. These are common 4 mm screws, available from us and others. Screw it in backwards so the head rests against the frame stop.
If your current rear derailer does not have a B screw, all is not lost. You can still achieve the same goal, but you will need to insert a shim between the derailer's tab and the frame's stop. Some older derailers are not spring-loaded, so you loosen the derailer bolt (the bolt that attaches the derailer to the frame), rotate the derailer backwards slightly and insert a shim (your choice of material and size) between the tab on the derailer and the stop on the frame. Then pull the derailer down onto the shim and tighten the bolt. You may have to repeat this a few times before you find the correct shim thickness.
If the derailer is spring-loaded, you need only pull the derailer backwards until its stop moves away from the frame's stop, then insert a shim. See Sheldon's page on rear derailers for more general info.
Most people don't want to spend money on a new cassette only to find it's not a good enough solution (they want still easier gearing). So I would recommend getting a cassette with the largest sprocket possible, the 34 tooth. These are available on standard Shimano 7, 8, and 9 speed cassettes. The 11-34 is the widest Shimano cassette currently available. If you are touring or commuting, then you might enjoy the versatility of these.
If you find that the derailer must be rotated so far back that the screw head doesn't rest on the frame stop, NOW it's time to get a long-cage derailer
These are available in every price range. These have a longer parallelogram, so will accomodate 34 Tooth cogs without having to screw in their B screw completely.
How much chain can a Shimano short cage derailer wrap? How much wood would a wood-chuck chuck?
If you have installed your larger cassette, then you will need to determine if your chain is long enough. Check your chain length. Shift to the large chainring while running in the middle of the cassette. Shift to larger cassette cogs and check the rear derailer's stretch. If you are approaching the picture below, you will need a longer chain. This picture is significantly beyond the limit of a derailer's stretch. Notice that the jockey pulleys are almost in a straight line. Don't go this tight.
With the chain sized as above, this Ultegra short cage derailer will wrap enough chain to run in the 3rd from the high gear (an 18) and the small ring (30). This means that this short cage derailer will wrap ( (32+52) - (18+30) = 36 teeth maximum. Shimano's specification for this derailer is 29 teeth total with a maximum 27 tooth cassette cog. If one pushes the derailer to it's limits, it is possible to exceed Shimano's recommendation by a few teeth, but not a lot.
Shimano specs its equipment for neophyte riders, who may push levers until something happens. They can get into, and ride in, the small-small or large-large combinations without being aware that those are not good combinations. Therefore, for experienced riders (who don't ride in the 'small-small' gear combination), it is certainly possible significantly to exceed Shimano's recommendations and get by with a short- or medium- cage derailer shifting a 'mountain' cassette.
SO, to go back to the original question, "Do I need to replace my road derailer when I put on a mountain cassette?" The answer is still Maybe, but hopefully you now understand the mechanics behind the components, and your options.