Once upon a time, cyclists were free to mix and match transmission components but that all changed when Shimano introduced indexed gear shifting in 1984. The new system provided very accurate shifting, but it depended upon precise compatibility of the shifter with the derailleurs and drivetrain.
Indexed shifting works because the rear derailleur travels a precise distance in response to a pre-set amount of cable pull within the shift lever. Cable tension is critical for accuracy, but the geometry of the derailleur and the cog spacing must match the indexed cable pull, otherwise the derailleur will not align with each cog.
Shimano, Campagnolo, and SRAM all manufacture indexed shifting systems according their own specifications rather than sharing a common standard. For example, SRAM levers employ 1:1 actuation (1mm of cable pull moves the rear derailleur 1mm) while Shimano and Campagnolo use higher ratios (1.4 to 1.9:1).
As a consequence, there is little interchangeability between brands (though Shimano and SRAM share the same specifications for the chain and cassette).
The evolution of the transmission from six-speed to 11-speed created further incompatibilities. The introduction of eight-speed systems depended on an increase in rear hub width from 126mm to 130mm to accommodate the extra cog. After that, the width and spacing of the cogs was reduced to allow more cogs to be added. Chains were narrowed too, slimming down to 5.50-5.90mm where once they were 7.80mm.http://sheldonbrown.com/cribsheet-spacing.htmlFORWARDS OR BACKWARDS COMPATIBILITY?
As a rule, mixing and matching different transmissions from one brand will not work due to changes in cog spacing. Brands will likely not support mixing components either. There are some exceptions though such as Shimano’s earlier groups and its Di2 E-Tube system.CHAINS AND CASSETTES
According to Leonard Zinn’s testing, all 11-speed transmissions are compatible. A comparison of a Campagnolo 11-speed cassette with a Shimano equivalent demonstrates that the cog spacing is well matched. However, there is a measurable difference in the width of each company’s 11-speed chain (Shimano’s chain is 5.62mm wide, Campy’s is 5.50mm).http://velonews.competitor.com/2013/09/bikes-and-tech/drivetrain-compatibility-hidden-in-plain-sight_303199http://www.bikerumor.com/2012/12/12/interchange-shimano-campy-11-speed-cassettes-save-your-wheels/
In practice, the measurable differences account for little and a Shimano 11-speed cassette works well with a Campagnolo groupset, and vice versa. Neither combination is perfect though. The imperfections express themselves in the form of a little extra chain noise and/or occasional lazy upshifts for some, but not all, of the rear cogs. Overall, the compatibility is more than adequate in most circumstances, though not all users (or fussy mechanics) will be satisfied.
For 10-speed users, there is also reasonable compatibility between Shimano/SRAM and Campagnolo. While the chains are almost identical in width (Shimano’s chain is 5.88mm versus Campy’s 5.90mm), Campagnolo employs variable spacing that leads to some extra chain noise and lazy upshifts that is more obvious than that seen for 11-speed transmissions. Regardless, the compatibility is more than adequate in an emergency, but the mix is unlikely to satisfy all users.
Attempting to mix 10- and 11-speed cassettes will cause significant problems due to differences in cog spacing and/or the width of the cassette. For Shimano/SRAM, 11-speed cassettes are too wide to fit properly onto 10-speed freehubs, and 11-speed chains are a little narrower (5.62mm versus 5.88mm). In contrast, Campy’s 10- and 11-speed cassettes will fit onto the same freehub body, but the 11-speed chain is significantly narrower than the 10-speed chain (5.5mm versus 5.9mm).CHAINSETS
Owners of 10-speed chainsets fitted with power meters will be pleased to hear that there is no strict need to replace the chainrings if they upgrade to an 11-speed transmission, at least if they’re using Shimano or SRAM.http://velonews.competitor.com/2014/01/bikes-and-tech/technical-faq/technical-faq-mixing-matching-and-more_314177
I’ve seen many people use Shimano or SRAM chainsets with Campagnolo chain and cassette and vice versa. FSA recommends strictly matching its chainrings to the transmission, as does Campagnolo, though anecdotal evidence suggests there is little to worry about.HUBS AND WHEELS
As mentioned above, Shimano/SRAM users will need an 11-speed freehub body fitted to their hubs/wheels in order to upgrade to an 11-speed transmission. Owners of Mavic wheels needn’t worry though; their 10-speed freehub bodies are already 11-speed compatible.
Many manufacturers offer conversion kits for their wheels/hubs, although there are instances where there are none (e.g. Zipp wheels with silver hubs). In such instances, owners may be able to install a Campagnolo freehub to use an 11-speed cassette if they can’t afford to replace the hub or wheel.
A more subtle incompatibility exists between different brands of hubs, since they can differ in the positioning of the freehub body on the rear axle. Such differences arise due to variations in the spacing of the right-hand hub flange and/or the spacing of the right-hand axle locknut. As a consequence, the rear derailleur limit screws have to be reset when swapping wheels to ensure crisp shifting onto the smallest and largest cogs.
If you own more than one set of wheels with different brands of hubs, then it is worth fine-tuning the axle and/or cassette spacing for perfect interchangeability.SHIFTERS AND DERAILLEURS
As the brains behind the operation, each brand’s shifters must be matched with their own derailleurs, especially the rear derailleur, in order for the indexed system to function properly. As noted above, there is little or no backwards or forwards compatibility for Shimano, Campagnolo, or SRAM. It is possible to cheat with the front derailleur since there is more tolerance for variation in shifting but as the least expensive component in a groupset, there is little to be gained by leaving it out when upgrading your transmission.BRAKES
Shimano recently overhauled the cable pull and leverage of its brake levers so they are no longer compatible with earlier calipers (e.g. mixing Dura Ace 7900 levers with 7800 calipers results in less braking power). The re-design introduced more leverage to the calipers such that SRAM or Campagnolo levers pair poorly with the new caliper design (due to too much leverage).
The issue gets a little murkier with aftermarket brakes where there is very little information on their performance with different levers. At present, it appears Campagnolo and SRAM road brake levers enjoy greater compatibility with one another’s brake calipers and aftermarket brands than Shimano’s latest design.
In absolute terms, 11-speed Shimano/SRAM and Campagnolo chains and cassettes aren’t interchangeable, but I’m sure there are plenty of riders that will be satisfied with the performance offered by a mix. Similarly, 10- and 11-speed cranksets are practically interchangeable amongst brands, although brake calipers and lever/shifters may not be.