|This pair of derailleurs are very similar, except ...|
the one on the left has a short cage, and the one on the right has a long cage.
|Rear view of the same pair of basic derailleurs.|
Short cage on the left, long cage on the right
It is easy to see why a manufacturer, such as Shimano, makes different "grades" of derailleur, from the basic models shown above up to the Dura-Ace models used in racing.
The "basic" models tend to be heavier, and the bearings are cheaper. For Shimano products, you find them stamped with their country of manufacture, and the lower-end models ain't stamped "Japan".
Sometimes a model a couple down from the top is the best value, as it has decent build quality and bearings, while the top models can actually be less strong because there is more of an emphasis on weight reduction.
Bottom end derailleurs contain steel and moulded plastic. Top-end derailleurs contain (in the case of Campagnolo, at least) aluminium alloy and carbon fibre.
By why have different lengths in the same "grade" (like the pair shown above?)
well, think about the types of bikes there are:
a) some bikes have a limited gear range (like a 14-24 gear set), and one or two chainwheels at the front.
Say the front is a 50/34 pair (not uncommon). So if you add up the difference in the rear cogs (24-14 = 10) and the difference in the chainwheels (50-34 = 16), you get the "range" of the gearing (26 in this case - it is 10-16) Remember that the derailleur spring has to take up the slack so that the chain is not slopping about whether the chain is on the small cogs or the big ones at the back and front. Small/small will give maximum chain slack (34 and 14 in this case), while big/big will keep the chain tightest (50 and 24 in this case).
Bikes with limited gear ranges have the "short" type of derailleur.
|Left column : A typical low-end derailleur with a short cage (a Tourney model) with a "range" of just 28 teeth capacity,|
and it is designed for a 14t "top"/high gear. Even with a 14-28 at the back, this couldn't handle a 50/34 up front (limit on the front is just 13 teeth)
Right column: the "medium£ cage variant of the same model derailleur has a range of 34, but is still only designed for a 14-28 set of rear gears.. At least it would work with most front "double" rings!
Both are useless with an 11-32 rear cassette.
Pic is a screenshot from Shimano's technical pages
|That;s a bit more like it. This one is a Tourney SGS (long cage) derailleur. This one will handle an 11-32 alright, but isn't designed for the 22t difference I have at the front (well, I could get away with it, but shifting is likely to be slower and less reliable)|
Most triple chainsets come in sizes like 42/34/24 or 48/38/28, or even 50/40/30, so this would be fine.
Note that range (total capacity on the spec sheets) is 43t for the long cage, 20t for the medium cage, and 13t for the short cage models.
Again, this is screenshot from Shimano's technical info.
b) take a bike like mine. I have an 11-32 on the back, and a 48/34/26 triple on the front. What's the "range" the derailleur spring has to work with?
well it is 21 at the back, and 22 at the front, giving a total of 43.
That is VERY different from the range of 26 we found in our first example.
"Long" derailleurs are simply better at taking up the slack than short derailleurs.
Why have any short derailleurs then, since long derailleurs are so good?
a) length: short derailleurs are less likely to hit things on a trail, and in my case the long derailleur hits the bike rack at work, so I need to park my bike "front first".
b) weight: because they have shorter cages, it is easy to see why short derailleurs weigh less! The cage simply has less material in it
If you want a list of derailleur weights, then you could try WeightWeenies that covers most models.
In order of spec, I have 3 long cage deraileurs in working order (although one needs a clean):
Altus (cheapest), weight = 313g (I weighed it!)
Alivio M410 = 312g (from WeightWeenies)
Deore M591 (dearest) = 291g (I weighed it).
The reason I didn't weigh the M410 is that it is in an oily bag, and would need a serious clean up, otherwise I would just be weighing the oil!
The closeness in weight between the Altus and the Alivio model may be due to the Altus being a later model, and featuring more plastic. Plastic can be strong enough for some parts of a derailleur, and can bring the weight down.
(I was surprised to find that a sealed Bottom Bracket unit with a pair of plastic cups that came from a very modestly priced bike actually weighed a touch less that the much higher-spec aluminium, with two ally cups, model I compared it too!)
So a "road bike" would normally have a shorter derailleur, because it weighs less, and if you pay big money for a light bike, you don't want any weight you don't need (like the extra cage length)
Bikes with triple chainwheels are likely to have a "long" derailleur, because of the wider range of their gearing.
Of course, there will always be exceptions, and in some ranges, more than two lengths exist - there can be a third, medium, length as well, combining some of the advantages of each, but having some of the penalties of each, too. Less range than a "long", more weight than a "short".