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Item # FSA0022

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Item # FSA0022


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 comes in either Gloss 3K Weave or Gloss UD. Weight is 118g.

Tech Specs

aluminum, carbon fiber
80 mm, 90 mm, 100 mm, 110 mm, 120 mm, 130 mm
+/- 6 deg
Clamp Diameter:
31.8 mm
Steer Tube Diameter:
1-1/8 in
Recommended Use:
road cycling, cyclocross

Community & Reviews

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FSA OS-99 CSI Stem

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Here's what others have to say...

Avg. ride time: 2h 32m per week
  • Average ride time is based on Strava activity over the last 3 months. Give your reviews credibility by connecting your account.
5 5

Looks good, super light

  • Familiarity: I've used it several times

Much lighter than some higher end stems, this carbon wrapped aluminum core gives me pride. I like the look of it and it's rigid and solid. Looking at other reasonably priced stems, it is very light and long enough for my gorilla reach.
My only complaint is that the torque requirement isn't printed prominantly on it. 4.5Nm is probably the magic number but always nice to have confirmation.

4 5

Perfect Fit and Good Looks

  • Familiarity: I've used it once or twice and have initial impressions

The weight is somewhat off on the advertise weight, but still very close to the much more expensive stems. I have 100mm stem at 124g, this is within the margin for error of 5% advertised weight of 118g. The stem is light, stiff and looks good. It just tied up the front end of my Orbea Orca Euskatel EUskadi edition. May need to get another one for my secondary bike that has UD carbon layup. Great product!!

5 5

Worth buying again

  • Familiarity: I've put it through the wringer

This is the third OS-99 stem that I have purchased and I now have one on all my road bikes. I also have an Aluminum, 3k, and now OD to match each of the bike builds; that flexibility with the same product is nice to have. The weight is competitive with much more expensive stems. I have attached a picture of the weight of my newest 100mm stem. At 126g, this is within the margin for advertised weight of 118g.

Worth buying again
Avg. ride time: 14h 8m per week
  • Average ride time is based on Strava activity over the last 3 months. Give your reviews credibility by connecting your account.
5 5


Has anyone noticed the alloy os99 to be weak or flexy? Im 140lbs and trying to decide bt these two and I like to sprint.

Avg. ride time: 15h 59m per week
  • Average ride time is based on Strava activity over the last 3 months. Give your reviews credibility by connecting your account.
5 5

it holds my handlebar

  • Familiarity: I've put it through the wringer

Light, stiff, looks good, and the bolts are high quality. The finish holds up well to multiple on/offs with the Garmin mount, too.

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5 5

Great FSA carbon wrap stem

Really light. Super stiff. Looks amazing! Perfect if you are one of those people (like me) where it has to be carbon for you to put it on your bike ;)

Great FSA carbon wrap stem