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FSA's new KFX i25 is a narrow-yet-light wheelset

KFX i25 carbon wheels
FSA's KFX i25 carbon wheels are light and aero (Image credit: FSA)

FSA's KFX i25 is a new contender in the best mountain bike wheels space, targeted at riders who value lightweight bikes.

The KFX i25 is narrower in profile than most, with an internal rim width of only 25mm. Even XC mountain bikers have recognized the value of wider rims and tires of late, with industrial designers settling on 29-30mm as the ideal internal rim width.

FSA believes that there remains a technical argument for the 25mm rim, which might be better if XC riders run 2.1- or 2.2-inch tires, instead of the wider 2.3- to 2.4-inch casings.

These new wheels use an asymmetric carbon rim which is tubeless compatible, and riders have a choice of either 27.5- or 29-inch wheel sizes.

Hub standards are predictably 15x110mm up front and 12x148mm at the rear. The rear hub internals uses a combination of seven sealed cartridge bearings to spin, whilst a six-pawl system manages engagement.

Handbuilt quality 

All FSA’s KFX i25 wheelsets are assembled by hand, ensuring optimal spoke tension. An interesting aspect of these new mountain bike wheels is their aero properties, something that is often ignored by off-road riders.

If you are a dedicated XC or XCO mountain biker, you might appreciate the advantage that a more aerodynamic wheelset can deliver, when grinding out those flat and fast gravel road miles.

FSA uses aero spokes for its latest mountain bike wheelset and there are only 24 of them per wheel. That should reduce wake turbulence and lessen aerodynamic drag when you are laying down the watts on an open section off-road.

Drivetrain compatibility covers the full spectrum of Shimano 9 to 12-speed cassettes, including the Microspline standard. SRAM’s 9 to 12-speed systems are also covered, including the XD cassettes.

As a mass reference, the larger 29-inch specification KFX i25 wheelset weighs only 1,566g. Both the 27.5- and 29-inch FSA KFX i25 wheelsets retail for $1,417.50.