Shimano Ultegra 6601/6604 Crankset

The Shimano Ultegra FC-6600 series cranksets are a classic and highly versatile component from the 10-speed era of road cycling. Here is a breakdown of the industry information and compatibility for the models you mentioned.

Industry Information

• Market Position: The Ultegra 6600 groupset, which was released in 2005, was a cornerstone of Shimano’s product line. It was positioned as the “workhorse” groupset, offering a remarkable blend of high performance, durability, and value that sat just below the flagship Dura-Ace line.
• Hollowtech II Technology: The FC-6600 series was the first Ultegra crankset to feature Shimano’s Hollowtech II technology. This design integrated the bottom bracket spindle into the right crank arm and used outboard bearings. This made the crankset lighter, stiffer, and easier to install than previous generations that used traditional square taper or Octalink bottom brackets. Hollowtech II became an industry standard that is still in use today.
• Variations: The numbers you provided (6601, 6604, 6650) indicate the different versions of the Ultegra 6600 crankset:
  • FC-6601: The standard double crankset. It came in a 53/39t configuration and was the most common version.
  • FC-6604: The triple crankset. It had a different spider and chainring configuration (e.g., 52/39/30t) and was aimed at touring, endurance, and recreational riders who needed a wider gear range.
  • FC-6650: The “compact” crankset. This was a hugely popular innovation, featuring smaller chainrings (50/34t) and a smaller bolt circle diameter (BCD). It allowed riders to get lower gears for climbing without having to use a heavier, more complex triple crankset.
• “Road/Cross/Gravel”: The crankset’s durable design and solid performance made it a popular choice for more than just road racing. Its compatibility with various bottom bracket standards (and its reliable shifting) made it a versatile component for cyclocross and early gravel bikes. The 53/39t configuration is a traditional road racing setup, and while not the best for gravel’s demanding terrain, the FC-6650 compact version was often used for those applications.

Compatibility

Compatibility for the FC-6600 series cranksets is generally excellent, but there are a few key points to consider.

• Drivetrain Speeds: The crankset is designed to work with Shimano 10-speed drivetrains. The chainring tooth spacing and ramp profiles are optimized for 10-speed chains. While it may work with a 9-speed chain, shifting performance will be compromised.
• Bottom Bracket: The crankset is only compatible with Shimano Hollowtech II bottom brackets. This is the standard for most modern Shimano cranksets. The bottom bracket itself must be compatible with the bike’s frame shell (e.g., BSA/English, Italian, or various press-fit standards).
• Front Derailleur: The crankset should be used with a 10-speed front derailleur and a 10-speed shifter. The FD-6600 front derailleur is the ideal match for the FC-6600 series cranksets. Using it with an 11-speed front derailleur may result in some chain rub or imprecise shifting due to the slightly narrower cage of the 11-speed derailleur.
• Frame Compatibility: The crankset’s compatibility with a specific frame depends on the frame’s bottom bracket standard and chainstay clearance. The Hollowtech II system and the 170mm crank arm length are standard, so they will fit most road, cyclocross, and gravel bikes from the late 2000s and beyond.
• Chainrings: The 53/39t chainrings are compatible with each other and are interchangeable. The BCD (Bolt Circle Diameter) of the FC-6601/6604 is 130mm, which is a standard for many older road cranksets. The FC-6650 compact crankset has a different BCD of 110mm. This is a critical distinction if you are planning on changing chainring sizes.

Crank Arm Length: The 172.5mm length is a very common size, suitable for a wide range of riders. Other lengths were also available (e.g., 170mm, 175mm).