FSA OS-99 CSI Stem

Priced: $153.00 - $179.99
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FSA OS-99 CSI Stem -

The 20-words-or-less summary is easy to do: The FSA OS-99 CSI stem is an FSA OS-99 stem wrapped in carbon fiber. But there are nuances to it that require elaboration: For starters, this is different than most other wrapped carbon fiber stems we've seen. It's much lighter than those stems. For the older wrapped stems, manufacturers were generally taking their heaviest stem, wrapping some cloth around it and clear coating the thing. But the carbon fiber wrap on the OS-99 CSI is actually doing something. It's structural.

The CSI acronym stands for Carbon Structural Integration. It is designed to stiffen up a really light stem (it adds 20g in weight in comparison to the standard OS-99). A stiffer stem is a more durable stem. This isn't to suggest that the OS-99 is weak. It isn't, and maybe some people won't notice the difference between the two stems, but if you're a bigger rider, or want some more bling on the bike, this is a great way to do it.

We suspect CSI is a technology FSA will use more and more of in the future. It's already being used in the aerospace industry. The basic idea seems to be going around the bike industry some. Shimano has an super-light clincher wheelset that bulks up an ultra-lean aluminum rim with carbon fiber.

The FSA OS-99 CSI Stem is a forged and CNC'd aluminum stem wrapped in carbon for extra stiffness. The aluminum base is AL7050. A gloss carbon is wrapped on top of the aluminum. There is a four-bolt sculpted face clamp. The clamp is anodized black. The four faceplate bolts and two steerer pinch bolts are made from titanium. The stack height of the stem is 41mm. The stem angle is plus or minus (+/-) 6 degrees aka 84/96-degrees. The stem comes in 80-130mm lengths in 10mm increments. The finish is gloss 3K weave. Weight is 118g.


  • Gloss carbon 3K weave carbon-wrapped & 3K weave carbon front plate


  • 4-bolt sculpted faceplate & Ti hardware
  • Titanium hardware
  • +/-6 degrees
  • Forged and CNC-machined from AL-7050
  • 41mm stack height
  • claimed 120g (110mm)
Jenson USA
FSA has taken their lightweight aluminum OS-99 and wrapped it in a beautiful carbon 3k weave. The resulting carbon stem is not all show with very little go. What FSA ends up with is the OS-99 CSI. A stem that is stiffer and more durable with only a slight weight penalty. Not to imply that the aluminum OS-99 is a noodle, but adding the wrap gives the stem a different character. This one works well for the powerful rider that wants a very stiff front end, or for that rider who appreciates the look of carbon in all the right places.

Mountains Plus Outdoor Gear
FSA OS-99 CSI Mountain/Road Stems are constructed of forged and CNC-machined 7050/T6 aluminum that is gloss carbon 3K weave carbon-wrapped and has a 3K weave carbon front plate.
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FSA OS-99 CSI Stem Review:

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