GTWilliams said:
As I said, I'm not a mechanic but I understand the general concepts of gearing. What I do not understand is the relationship between the gearing and the derailleur. I thought the derailleur just moved the chain up and down. Higher quality ones shift smoother which is very important.
But why would I need to consider changing the derailleur if I change the gears?
Well, on first blush, the whole process of moving the chain from one cog to another OR from one chainring onto another might not seem to be a significant mechanical endeavor ...
But, it has taken decades to achieve the precision mechanism which most of us benefit from.
Without a boring historical recap, let me just note that there was & is a variability in the WIDTH of a chain's outer plates + a variability in the space between the front derailleur's cage plates ... 0.1mm of variance isn't a NASA tolerance, but it is enough to preclude the chain from being moved sufficiently OR too much ... this is often NOT evident in a DOUBLE chainring setup, but becomes very apparent with a TRIPLE crankset
(at least, IMO) + Shimano STI shifters (SRAM doesn't have a triple-capable shifter, AFAIK).
Further, the ANCHOR bolt on the parallelogram on the rear derailleur & the length of the rear derailleur's parallelogram can & does vary.
Back in the
Stone Age, the movement of the derailleur was controlled by a FRICTION shifter ... that is, the lever was moved/pulled a certain amount & that moved the cable which subsequently moved the derailleur AND "friction" held the lever in place. A spring 'returns' the derailleur when the lever was moved in the reverse direction ... at one time, a second cable was used to move 'return' the derailleur.
I believe that SunTour is generally credited with
creating indexed shifting for external derailleurs ... however, I suppose Sturmey-Archer (?) could be credited with creating indexed shifting for its 3-speed internal hubs. THAT was a long, long time ago.
Shimano is generally credited with "perfecting" the mechanism AND (especially) the marketing of indexed shifting.
BTW. For reasons that are ineffable to me, MANY bike shop mechanics, et al, hate Shimano & embrace SRAM (this is a MTB thing) ... I
love Shimano --
SHIMANO has GREAT Customer Service in North America ... and, they have well engineered components. SRAM (since I mentioned them) has INDIFFERENT Customer Service -- so, good luck to anyone if they need something from them.
Now, whether or not you need to change the rear derailleur depends on which TYPE of rear derailleur whatever bike you get already has ...
When Shimano created their 9-speed components, one of their engineers had the "brilliant" notion that they could reduce pulley friction by ~10% by using an 11t pulley wheel instead of a 10t pulley wheel because so-called
SERIOUS road riders usually didn't use a cog larger than 26t ... so, a 27t cog was generous in someone's mind.
Further, the parallelogram's cage was shortened, and apparently this means that the shifting is incrementally faster ...
Personally, I was miffed when I discovered (while I was reading the spec sheet) that Shimano had spec'd their Ultegra 6503 rear derailleur (as an example) at only 27t. Unfortunately, I mistakenly presumed that the Ultegra 6503 was a "touring" rear derailleur; but, sure enough, it was not capable of handling a cassette with touring cogs!
Tentatively, I took an XTR rear derailleur, and fortunately (despite suggestions by everyone I knew to the contrary), it indexed just fine with a set of Ultegra 6500 STI shifters.
The parallelogram on the XTR/XT/LX/etc. MTB rear derailleurs is slightly longer (same as the 8-speed ROAD Shimano rear derailleurs by my reckoning) than the parallelogram 9-speed rear derailleurs and can handle a cog up to 34t.
Eventually, I just needed to know how Shimano managed to cripple the capacity of their ROAD rear derailleurs -- so, I looked & looked at it until I deduced (this seems obvious in retrospect) that the 11t pulley wheel was the reason for the largest cog limitation. AND, by trial-and-error, I determined that simply changing the upper pulley wheel to a 10t will easily allow a 30t, and sometimes a 32t cog.
Does that matter? Well, yes, sometimes it does for some riders, particularly if they are using a 10-speed STI setup & want a larger rear cog (e.g., when using an IRD 10-speed cassette OR if one were to restack their 10-speed cassette with a larger cog cannibalized from another cassette).
MOST of the time, simply installing a cassette with a 32t cog will provide low enough gearing for MOST people on MOST inclined roadways ... for the DEATH VALLEY CENTURY ... or, other
occasional ride!
To use cassette stacked with a 34t cog, you will need a MTB rear derailleur.
Now, with regard to the various Shimano rear/front derailleurs ... MY impression is that ALL but the cheapest are as functional as the most expensive ... the
actual difference between an ACERA-grade & a Dura Ace (beyond the difference in parallelogram geometry) is in the finish, materials & weight ... and, BLING.
If the Dura Ace shifts better than the Acera it is because it is closer to being "blue printed" to the engineers specs than the Acera ... AND, the Acera (which is burdened with barely-finished steel bits) probably was not attached (i.e., the cables & housing were probably NOT lubed) nor adjusted with the same care. Installed & adjusted with the SAME level care & cables/housing and the Acera/Tiagra/whatever will be almost as good (most probably won't notice the difference) as the DA if it were atttached to DA shifters.
The bottom line is that Shimano, for the most part, makes GREAT components ... so if you do need to buy a MTB rear derailleur, you can opt for a lesser, DEORE (vs. DEORE XT) rear derailleur and anticipate the same level of shifting quality IF you ensure that the cables are lubed, etc.
BETTER components are generally
slightly-or-signficantly easier for the mechanic to work with ... and, since I do my own wrenching, sometimes it's worth it to me to pay a little more ...
let your budget be your guide.
