FSA manufacturers carbon fiber stems, but arguably its very best stem (though not the most exotic or the most expensive) is the OS-99. The OS-99 is the lightest stem in the FSA family and — believe it or not — it's made from 7050 aluminum alloy.
Don't let the extreme lightness of the OS-99 give you concerns about durability. FSA manufactures it using a proprietary 3D forging technique that uses slow forging speeds and extremely high pressures to eliminate surface deformation. It carefully controls the heat treatment process to ensure ideal crystal growth. The heat treatment is so critical to the finished product, in fact, that FSA documents and inspects every heat treatment session to ensure quality. It might be exceptionally lightweight, but it has the toughness of a heavyweight.
The forged faceplate has a serrated grip area to securely fix both alloy and carbon fiber handlebars. All sharp edges are radiused to eliminate potential stress risers — even the washers and counterbores for the Ti bolts are radiused for better stress flow. And the clamping area is diagonally cut to better distribute the clamping load across the steerer tube.
The OS-99 is available in six lengths, from 80mm to 130mm. It has an 84 degree rise (+/- 6 degrees), with a clamp stack height of 41mm. The clamp diameter is 31.8mm.
- 7050 aluminum body
- Titanium bolts
- Alloy and carbon faceplate with serrated grip surface
- 31.8mm clamp diameter
- 1 1/8in steerer tube diameter
- +/- 6 degree rise