CALCULATING MAXIMUM DERAILLEUR CAPACITY
The following procedure allows you to calculate the derailleur capacity you will need given the chainrings and cogs:
Determine Maximum Chainring Difference by subracting the number of teeth in the smallest chainring from the number of teeth in the largest chainring [Rando's: 50T- 28T = 22T] [Possible new: 48-24 = 24T]
Determine Maximum Cassette Cog Difference by subtracting the number of teeth on the smallest cassette cog from the number of teeth on the largest cassette cog [Rando's: 30-13 = 17T] [Possible new: 34- 11 = 23T]
Determine Total Drivetrain Capacity (chain-wrap capacity*) by adding Maximum Chainring Difference to the Maximum Cassette Cog Difference [Rando's: 22 + 17 = 39T] [Possible new: 24 + 23 = 47T]
Record the Maximum Cassette Cog (the number of teeth on the largest Cassette Cog) [Rando's: 30T] [Possible new: 34]
* Chain-wrap capacity is a number that tells whether or not a derailleur will be able to do this on the drivetrain you plan to use it on. Calculate chain-wrap capacity with this formula: Chain-wrap capacity = (large chainring - small chainring) + (large rear cog - small rear cog). Example: 53/42 chainrings and a 12-23 cassette: (53 - 42 = 11) + (23 - 12 = 11) = 22-tooth chain-wrap capacity."
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For Shimano derailleurs:
SS - Short Cage Road Double - Maximum Cassette Cog is 27 and Total Capacity is 29
GS - Medium Cage MTB/Road Triple - Maximum Cassette Cog is 34(MTB)/27(Road) and Total Capacity is 33(MTB)/37(Road)
SGS - Long Cage MTB - Maximum Cassette Cog is 34 and Total Capacity is 45
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RANDO'S REAR DERAILLEUR
Deore DX (RD-MT650-GS) (long cage)
[VeloBase info]: Shifting systems: SIS, STI, friction. Originally designed for 7 speed.
[BikePro]: Can be used as both a 6, 7, and 8 speed changer. Weight = 318.5 grams. Early 1990s, and discontinued in 1994.
Maximum rear cog is 32T. Total capacity is 38T.
Rando's current total drivetrain range: Max Chainring Difference X Max Cassette Cog Difference [22 + 17 = 39T]
Current lowest GI (28 X 30 = 25); Possibilities with current derailleur (26 X 30 = 24) (26 X 32 = 22) (28 X 32 = 24)
Low GI possibilities with new derailleur (26 X 34 = 21) (24 X 36 = 18)
Current front derailleur's max ring difference: 26T
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MINTY'S REAR DERAILLEUR
Original: Shimano Light Action L525 SS (Short Cage): Disraeli Gears info: Maximum cog: 28 teeth (Sutherland’s 6th edition); Total capacity: 28 teeth (Sutherland’s 6th edition); Pulley centre to centre: 55mm; Index compatibility: 6 speed (Sutherland’s 6th edition says the SS version is SIS compatible); Chain width: 3/32”; Weight: 264g
Tuten's: Shimano Ultegra RD-6500 (Long Cage) [Shimano Tech Doc PDF] [Disraeli Gears] [Cambria]
Info on using it with 32T: You have to adjust what is called the "b" screw in order to rotate the body of the derailleur back enough that the top pulley doesn't ride on the larger cassette sprockets. It isn't the best shifting experience I've ever had, but it works well enough. )
The Shimano Ultegra Rear Derailleur (RD-6600 GS) can handle a maximum 27t cog as the largest size rear cog, at least as stated by Shimano, however I have seen a 28t cog work. The minimum rear cog size is 11t, FYI. Maximum chainwrap capacity is 38t. " An important part of a rear derailleur's job is keeping the chain taut as you change gears. Chain-wrap capacity is a number that tells whether or not a derailleur will be able to do this on the drivetrain you plan to use it on. Calculate chain-wrap capacity with this formula: Chain-wrap capacity = (large chainring - small chainring) + (large rear cog - small rear cog). Example: 53/42 chainrings and a 12-23 cassette: (53 - 42 = 11) + (23 - 12 = 11) = 22-tooth chain-wrap capacity."
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Harris Cyclery's favorite derailleur for touring bikes: http://harriscyclery.net/itemdetails.cfm?ID=950
Deore SGS RD-M591 Rear Derailleur SILVER - Model # (RD7592) $64
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Shimano Derailleurs
7 Speed
1993: Deore XT RD-M735 SGS (long cage), STI compatible 7speed SIS compatible
1993: Deore DX RD-MT650 SGS (long cage), 7speed STI compatible
8 Speed
1994-1995: Deore RD-M737 SGS (long cage), STI compatible 8 speed SIS compatible,
1996-1998: Deore XT RD-M739 SGS (long cage) , STI compatible 8 speed SIS compatible
1994: Deore XTR RD-M900, 8 speed STI compatible
9 Speed
1999-2005: Deore XT RD-M750 SGS (long cage) , STI compatible 9 speed SIS compatible
2005: Deore XTR RD-M960-SGS (long cage), 9 speed STI compatible
Deore LX RD-M571: 9 spd; Large 34, Small 11, Total 45, Front difference 22
Deore LX: RD-M581
Deore XT
Deore XT M735 1990 6/7-speed
Deore XT M737 1993 8-speed: Max 32T; Tot 33T
Deore XT M739 1995 8-speed: Max 32T; Tot 38T
Deore XT M750 1999 9-speed
Deore XT M760 2003 9-speed
Deore XT M770 2008 9/10-speed
Deore XT M780 2011 10-speed