Carbon Rimmed FSA SL-K i25 MTB Wheels from FSA are XC Rigid and Racy [Review]

FSA SL-K wheels

My dedicated XC racing self of ten years ago would have been so stoked on these 1,600g SL-K Off Road i25 wheels from Full Speed Ahead that retail for a reasonable €/$1,260. They are lightweight and notably stiff, with far wider rims than we were riding back then. They’re also hand-built, and the builder’s attention to detail shows in the fact that they have been spinning true and free of issues through the damp fall and winter riding season.

I tested the SL-K wheels on my steel Cotic Solaris Max that is built and ridden a little rowdier than most XC rides. To start with the conclusion, these wheels feel super stiff and responsive, providing the accurate handling and power transfer that we all expect from carbon XC hoops. Their low weight is always appreciated on the climbs, though surprisingly they don’t feel fragile on rougher trails. When used for the XC, marathon, and “light trail” riding styles that they’re designed for this wheel set performs smashingly, and I would recommend them to anyone who runs 2.35″ wide (or narrower) tires and wants a featherweight spin up. They should also fit well with gravel tires in case you have two frames that need to share, and the brand’s AGX gravel wheels are actually slightly wider at 26.2mm internal. Now, on to the details.

The SL-K carbon rims have a 25mm internal measurement, limiting the tire width you will want to mount up to about 2.35″. Fortunately, there is a boatload of tires in that width, from low-profile XC treads like these Maxxis Rekon Race tires to heavier mud grippers like the Schwalbe Nobby Nic. I mounted up both of those treads on these wheels and the rim width provided a fairly round tire profile for riding fast which requires a little extra lean to find the shoulder knobs in flat turns. A lot of rims are moving to 30mm internal width measurements to square off tires a bit, and I would expect to see a similar update from FSA at some point.

The rim’s 30mm depth is likely where the bulk of its rigidity comes from, and it creates plenty of scalpel stiffness for precise handling. The 24 double-butted spokes laced two-cross may dial back stiffness a touch, but rigidity is clearly the name of the game with these wheels. Compliance is one element that keeps the SL-K hoops from being mounted on my longer-travel bikes, as they are a bit too stiff for legit party trails. For XC racing this rigidity is exactly what I would want, and it’s precisely what this wheel system is designed for. While I have ridden stiffer XC wheels, the SL-K i25s offer the kind of race rigidity that I can enjoy all day long on smoother trails without being so stiff that they feel nervous and difficult to manage.

Twenty-four spoke holes are drilled with a healthy offset to allow for spoke tension balance and a stronger build. The offset looks substantial enough that the spokes on either side might be close to the same length. We asked FSA about this and will update the exact spoke length info if they respond.

The rear hub uses the usual four bearings, with two spinning up front. Both hubs have a preload adjuster to fine-tune the end caps, and a pinch bolt to hold them in place on the axle once everything is dialed. The freehub body is still in great shape with very little indentation from the smaller cassette cogs. Most of what looks like denting in the freehub photo above is leftover grease. Six pawls grip 54 points of engagement inside the hub shell, providing 6.6° of engagement that feels plenty tight. These wheels spin up fast and are a pleasure to climb with, leaving me with no complaints about engagement.

I’ve ridden the SL-K Off Road i25 wheels on some inappropriately rough trails and through all forms of terrible weather and they remain true as the day I first mounted tires. The cartridge bearings feel smooth and clean, with plenty of life left for summer riding.

FSA SL-K wheels
A nail went through the tread and exited the tire bead, leaving the rim and tape unscathed.

Now for the whining part. This is a trite gripe, but it’s also a glaring one. Remember when 29ers first took over the industry, and bikes had their wheel size printed all over the frame and wheels? They made it grossly clear that you were not riding 26″ wheels anymore. These Boost wheels have “148” printed twice per side and once on each hub, almost as large as the main logo, front and rear. Yes, the 110mm spaced front wheel has five “148” stickers on it. It looks quite silly. If I purchased the wheels I know they are Boost, thanks.

Additionally, I would love to see these rims gain some width and maintain their weight if possible. Maybe FSA could shorten the overall depth for increased compliance and move that material outward? I’m no rim engineer, but I know from experience that a 30mm-wide internal rim measurement makes for fantastic tire profiles no matter what style of riding you’re into.

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