While it's billed as a traditional trail wheel, Mavic's XA Pro Carbon 27.5in Boost Wheelset is actually anything but traditional. Which is saying something, considering that we're discussing a brand that has become synonymous with tradition while North American wheel manufacturers blasted off into the stratosphere of wildly unchecked innovation. Instead, Mavic sticks with what works, and the XA Pro Carbon indicates that the brand is finally comfortable enough with carbon technology to trust a full carbon trail wheel for the kind of abuse that favors the latest breed of slack, 5in all-mountain bikes.
In typical Mavic fashion, the XA Pro Carbon wheels are shot-through with an expansive list of technologies whose ultimate aim is to increase drive stiffness without taking away from the rim's slightly compliant bump absorption— speed that privileges comfort. Or comfort that privileges speed, if you'd prefer. Ultimately, the wheels strike one of the best balances of efficiency and cushion in the trail market, and they do it while foregrounding Mavic's insistence on safety and reliability. Despite that staid approach, the XA Pro Carbon does represent a pretty drastic change to Mavic's mountain wheel philosophy, so there's a lot to cover; however, since the wheels are built with one of Mavic's first full carbon rims, that seems the most appropriate place to start.
The rims feature a lay-up that targets radial flex (read: squishy cushion for unruly runs) in order to balance the inherent lateral stiffness of carbon with the forgiving vertical compliance of low-profile alloy rims. We like stiff wheels while hammering, but wheels that are too stiff presents issues beyond just comfort. Cushion aside, the lay-up's real benefit to aggressive trail riders is that it helps maintain contact with lumpy terrain by absorbing impact through engineered flex, bouncing less, and keeping tires glued to the dirt. (The spoke material and lacing pattern helps with that, too, as we'll discuss later.) That focus on traction, stiffness, and comfort is obviously well-suited to a tubeless setup, and Mavic's UST design - first developed in the mid-90s— returns with some appropriately impressive developments. The most transformative of these is the lack of a bead hook.
Eliminating the bead hook improves impact resistance by eliminating the fragile shelf of a bead hook and ensuring that structural integrity isn't compromised by post-production machining. Instead of that delicate construction, the XA Pro Carbon features 3mm of impact-resistance that compares favorably to the usual 1.2mm thickness of alloy rim walls. Eliminating the hook also reduces production costs by either eliminating the machining step or, if manufacturers are molding the bead hooks, by reducing the number of factory rejects. Bead hooks are precise constructions, and it can be difficult for even manufacturers of Mavic's caliber to perfectly stuff that much material into such a claustrophobic mold and still hit the minute tolerances that we expect from the yellow label. We trust Mavic because the brand has consistently placed rider safety ahead of mindless pursuit of innovation trends. Adopting the hookless bead inverts that model, but for the same reason: safety first.
The hookless rim has a final, counter-intuitive benefit: In addition to handling bottom-outs with stoic aplomb, the XA Pro Carbon's hookless rim actually holds the tire more securely and reduces blow-offs (while you've got the wheels hooked up to a compressor) and burps (while you're getting sick). The rim owes these properties to two features not included in rims with bead hooks: a more pronounced central channel and a pair of bead locks, not hooks. These two additions more effectively divide the duties of traditional bead hooks. The central channel centers the tire and helps it seat while inflating, and the bead locks keep the tire bead in place so it doesn't unseat during hard cornering at low PSI. We should note here that tubeless-specific tires are, among other things, less prone to bead stretching, so we recommend sticking to them with these rims.
The rim changes extend beyond the hookless construction. Instead of being hidden behind an undrilled inner wall, which made truing a pain, the spokes now thread into conventional nipples that are exposed in the tire bed. This means rim tape, which we don't mind installing (and comes already installed). The holes are drilled asymmetrically, which improves the spoke bracing angle at the flange for more lateral stiffness that doesn't hamper the rim's engineered radial flex. The spokes are also stainless steel instead of Mavic's ultralight, ultra-stiff Zicral aluminum. And that's a good thing, because steel spokes encourage that forgiving radial compliance while Zicral—which is ideal for lightweight climbing wheels—all but erases it.
The rim's ability to damp bumps is further enhanced by the two-cross lacing spoke lacing pattern, which does sacrifice a small amount of lateral or drive stiffness compared to radial spokes, but it also increases that bump compliance, requires less baby-sitting without risking spoke failure, and (most importantly) better handles the torsional load transferred from hub to rim while stopping with disc brakes. And of course there're those beefy Boost hubs, which kick the flanges out by an additional 10 and 6mm in the front and rear, respectively, in order to further improve the spokes' bracing angle, adding to both lateral stiffness and allowing for a more balanced spoke tension in order to hit the rim's sweetest possible spot of radial cushion vs. lateral stiffness.
As with its road line, Mavic demonstrates some restraint with rim width, sticking to a tire-to-rim ratio that makes sense for practical applications rather than scoring on-paper points in the rim-width arms race. A bit of lateral fold in the tire actually benefits cornering, so the vertical side walls of the latest high-volume rims aren't necessarily the best solution for flowy handling. Instead of the 30mm+ rims we're seeing, Mavic opts for a more appropriate 26mm, which lets 2.4in tires strike a balance between the dome-like gumdrop shape of ultra-wide rims and the overly floppy lightbulb bulge off wide tires mounting on narrow rims. Those wider rims also risk migrating the tire's shoulder knobs onto the center rolling strip and exposing the side walls to the trail furniture that the shoulders are meant to protect the casing from. We think tires in the 2.4-2.5in range are shaping up to be the new all-mountain standard, and the 26mm internal width featured here suits those tires perfectly while also saving weight compared to the wider rims.
- A trail wheel that veers eagerly into all-mountain terrain
- Carbon rim with engineered flex for cushion and traction
- Rim width lets mid-sized tires sit perfectly for flowy handling
- Hookless bead burps less and makes tire installation easier
- Freehub with two pairs of offset pawls for 7.5-degree engagement
- Two-cross spoke lacing stiffens laterally but gives radially