Component manufacturers like to sell you lots of new parts, even if you don't need them. This has led to much confusion as various parts are labeled as if they are incompatible with other parts even though they are actually usable with little or no problem. Also, design often is churned by spec hype, and "keeping up with the Joneses," as in more sprockets, lighter weight, higher-priced components must be better. "Jones" is also a slang term for a drug addiction!
In reality, the fancier parts aren't always the most suitable, in the same way that a Ferrari, while it is a great racecar, isn't at all as good for daily transportation as a Toyota -- there are practical issues of cost, reliability, serviceability and durability. With bicycle components, the performance gain with higher-end models is often minusucule. Remember, you account for 80 percent of the weight even if you are riding a rather heavy bicycle!
There are, on the other hand, some real compatibility issues.
Campagnolo/Shimano 8-speed cassettes have different spacing , so you can't generally get good indexing using a Campagnolo 8-speed wheel with a Shimano shift system or vice versa.
With 7-, 9- and 10-speed systems, the sprocket spacing between brands is close enough that it rarely causes any difficulty in practice.
See my "Spacing Cribsheet " for more details on this.
Narrow chains bring other problems, though. They are usually more expensive and -- with 10 or more speeds -- don't last as long -- even when used in the intended system.
The narrowest chains also are more trouble to maintain. A master link, the SRAM PowerLink, makes it easy to disconnect a chain for cleaning. The 7/8 speed SRAM PowerLink works with SRAM and Shimano chains, probably others too. The 9-speed PowerLink works reliably with SRAM chains, but it may lead to a Shimano chain's jumping forward. The 10-speed SRAM Powerlock (note different name) is good for one-time use only: You must install a new one every time you reconnect the chain -- but then, if the chain has worn significantly, it will cause a "clunk" every time it comes around, because this one link is shorter than the others! Wipperman makes a similar "Connex" link for 10-speed chains and it is re-usable. Shimano's 9- and 10-speed system is even more trouble: you must press in a special link pin, using a special tool, every time you reconnect the chain, and this, too will cause a "clunk".
Most SRAM derailers are fully interchangeable with Shimano systems.
SRAM "ESP" or "Exact Actuation" derailers and shifters (generally designated by a decimal model number: 7.0, 8.0, 9.0 etc) can only be used if the shifter and rear derailer are both ESP systems .All SRAM 10-speed derailers and shifters are of yet a third type. A Jtek ShiftMate pulley adaptor can make a SRAM shifter work with a Shimano derailer, or vice versa.
Front derailers are generally 2- or 3-chainring specific.
Different seat-tube angles and chainwheel sizes also may require different front derailers. See my Front Derailers Article and Derailer Adjustment Article for more detail. Also see "Road" vs. "Mountain" comments later on this page.
There's considerable interchangeability among hubs . If you're upgrading from a system with fewer than 8 rear sprockets to one with more, you may need to concern yourself with the frame spacing. See my Frame Spacing Article for details on this.
The freewheel threading on these older hubs is generally interchangeable except for some very old French units. If you go from a 5-speed freewheel to a 6- or 7-speed freewheel, you will usually need to add some spacers to the right end of the axle between the cone and the locknut. Once you have done this, you'll also need to re-dish the wheel to bring the rim back to the centerline. You may need to re-space the frame if you have added spacers to the axle. See my Frame Spacing Article for details on this.
Shimano 5-speed and 6-speed shifters are made to index with the 5.5 mm spacing between sprockets on older 5- speed and 6-speed freewheels (or even, some new freewheels made as replacements). Without indexing, and because chains with protruding rivets are no longer widely available, it is possible to shift partway between sprockets and for the chain to "skate" along one side of a sprocket and on top of the teeth of the next smaller sprocket. On the other hand, old SunTour "Ultra" and modern 7-speeds have 5 mm spacing and will index with Shimano or SRAM 7-speed shifters, or 8-speed shifters with alternate cable routing. On an older freewheel without Hyperglide (or similar) shaped sprocket teeth, shifting will not be as clean as on a modern freewheel or cassette, but the indexing will work.
Up through 9-speeds, all cassettes use very nearly the same wdith of sprocket teeth, and will work with 7/8 or 9-speed chains. Old Uniglide 6-speed cassettes have larger spacing between sprockets, and like older freewheels, have the same shifting issues.
10-speed sprocket teeth are narrower, to cram in one more sprocket. As a result, 10 speed sprockets do not wear as long. The structural strength of 10 speed sprockets also can be a concern; some have bent, a problem unheard of with other cassettes. Manufacturers have come up with clever ways to strengthen a cassette by riveting several sprockets together or machining them out of one piece of metal, but generally at the cost of increased expense, and of having to replace several sprockets at a time when only one is worn.
8-speed Campagnolo cassettes will not fit on newer "9-speed" and "10-speed" hubs.
9- and 10-speed Campagnolo cassettes will not fit on 8-speed hubs.
It is theoretically possible to upgrade Campagnolo 8-speed hubs with newer cassette bodies, but in practice the parts don't generally seem to be available.
These older hubs can usually be upgraded to work with modern Hyperglide cassettes by replacing the Freehub body. Click here for details.
These chainrings have the teeth slightly farther to the right than the older chainrings to work a little better with the narrower chains. There is no difference whatever in the crank spiders.
The manufacturers also are concerned about clueless users. The worst-case scenario is that you will be riding along with the bike in its highest gear (large front, small rear) and then for some bizarre reason shift down in front before downshifting in the back. (There is no shift pattern in which it is reasonable to shift in this sequence.) [Not with a 9- or 10-speed cassette, to be sure -- John Allen] If you do shift this way, there's a small chance that the chain might "skate" over the edges of the teeth for maybe half a turn.
In practice this "problem" almost never materializes. Many, many cyclists are using 9- and 10-speed chains with older cranksets and having no problems whatever.
Long-cage (SGS) derailers have greater takeup capacity, and work with all types of cassettes. Long-cage derailers are commonly called "mountain" derailers currently, though in the past, this style of derailer was known as a "touring" derailer. (The marketeers retired the use of "touring" as a buzzword in the late '80s when mountain bikes became the hot item.)
While rear indexing is the same for all recent shifters/derailers, Shimano fronts use a different amount of cable pull for drop-bar vs flat-bar controls.
An additional complication is that "road" front derailer cages are shaped to fit well with a 52-53 tooth big chainring, while most current "mountain" front cages are shaped to fit with a 42-44 tooth big ring.
This makes it difficult if you want to use a "road" crankset, with full-sized chainrings (52, 53 top) with straight handlebars, or if you want to use a "mountain" crankset (42-48 tooth top) with drop-bar STI shifters.
Although "Road" and "Mountain" hubs are no different as far as cassette fitting is concerned, they are different in terms of overall spacing. "Road" hubs generally use 130 mm spacing, while "mountain" hubs are 135 mm.
"Mountain" hubs will likely be slightly better sealed against dirt and mud than "road" hubs, but this is rarely an issue in practice. The wider 135 mm spacing will generally result in a slightly stronger wheel due to reduced dishing of the spokes.