Industry rumours suggested they would introduce the extra gear on their top-end group, XTR. But instead they've chosen to debut it on updated versions of Deore XT and SLX.
Shimano are calling their new drivetrain technology Dyna-Sys, and insist they haven't simply added an extra cog to the cassette. In fact, they're calling it "a whole new way of shifting." Both groupsets will get new cranksets, chains, cassettes, derailleurs and 2-Way Release rear shifters.
Only a triple chainset will be available (42-32-24T) – an interesting decision, given that one of the main perceived advantages of a 10-speed cassette is that you can ditch a front chainring yet still retain an adequate number of gears.
The Big S say that having a wider gear range at the back, more closely spaced gears at the front, and improved rear mechs and shifters, creates smoother and more intuitive shifting, more efficient and powerful pedalling, and allows the rider to stay in the middle and largest chainrings for a higher proportion of the time.
"The larger the chainring size, the lower the chain tension, the lower the impact on rear suspension and ultimately the most efficient transfer of power," says Shimano. This helps reduce drivetrain-induced suspension movement on bikes suspension, of which the majority are designed to be most efficient in a 32-tooth middle chainring.
Shimano also reckon the Dyna-Sys setup with its smaller large chainring (42T rather than 44T) and larger small chainring (24T rather than 22T) will reduce the number of 'recovery shifts' needed at the rear when changing at the front, making it easier to maintain your pedalling cadence and keep forward momentum.
Cosmetically, the updated groupsets will look identical to the current XT and SLX, and brakes and front shifters will be unchanged. All Dyna-Sys components will be inter-compatible, so, for example, you'll be able to use an XT chain with an SLX cassette. However, Shimano say the new parts – which will be available from June 2010 – won't be compatible with their nine-speed mountain bike components or 10-speed road components.
Deore XT Dyna-Sys
Cassette (CS-M771-10): The XT cassette is available in three configurations: 11-32 (not available at launch), 11-34 or 11-36T. To save weight, two three-ring spiders are used, along with an alloy lock ring. RRP is £59.99.
Chain (CN-HG94): The new HG-X 10-speed chains are directional (the right-hand side is optimised for front shifting and the left for rear shifting), so make sure you fit them with the logos on the outside. The zinc alloy plated XT version costs £34.99.
Crankset: At present only a triple crankset is available (24/32/42T), with alloy inner and outer chainrings, a carbon fibre/steel composite middle ring and a choice of four crankarm lengths: 165, 170, 175 or 180mm. RRP for the Hollowtech II unit is £189.99.
Front mech (FD-M770/1): A key aim with Dyna-Sys was to offer riders a range of front mech mounting positions in order to get the best possible chainline and minimise chain rub. The XT derailleur is available in four versions: Top Swing (£34.99), Down Swing (£34.99), Direct Mount (£29.99) or E-Fit (£29.99). It features a hollow link pin to save weight.
Shadow rear mech (RD-M773): The low-profile rear mech is available in GS (medium cage) or SGS (long cage) versions for £64.99. Both pulley wheels roll on sealed bearings (on current nine-speed XT, the guide pulley runs on a ceramic bushing).
Rapidfire shifter (SL-M770-10R): The new 2-Way Release rear shifter with alloy main lever and removable optical gear display is available on its own for £49.99 or with a front shifter for £89.99.
Cassette (CS-HG81-10): The SLX cassette is only available in two sizes: 11-34 or 11-36T. It doesn't have its big brother's second spider or alloy lockring. RRP is £49.99.
Chain (CN-HG74): As with XT, the SLX chain is directional. RRP is £29.99, and it comes in a grey finish.
Crankset: The SLX Hollowtech II crankset gets a steel inner chainring and glass fibre/steel composite middle ring in place of the XT's alloy and carbon fibre/steel equivalents. It's available with 170 or 175mm arms, for £139.99.
Front mech (FD-M660/1): The front derailleur is available in the same versions as the XT unit: Top Swing (£29.99), Down Swing (£29.99), Direct Mount (£26.99) or E-Fit (£26.99).
Shadow rear mech (RD-M663): Available in an SGS (long cage) version only, the rear derailleur costs £49.99.
Rapidfire shifters (SL-M660-10R): The SLX shifter does without its bigger brother's alloy lever. Cost is £59.99 a pair or £29.99 for right-hand only.
A cheaper non-series Dyna-Sys crankset will also be available, the FC-M552, in black or silver. It has solid instead of Hollowtech arms. Pricing has not yet been set for this crankset, and it may be an OEM-only product (ie. supplied with complete bikes but not available aftermarket).