Some days, we wonder why there aren't more super-light XC wheels on the market. Then we ponder the complexity of building wheels that are both tough enough for off-road use and still supremely light, and we're glad that a company that's as fixated on carbon as Reynolds is on the job. They're Mountain XC Carbon Wheelset takes advantage of their carbon-fiber know-how to do just that -- be really light and really tough at the same time.Since the rims are the most critical components of high performance wheels, this one is 19.5mm deep and uses high-modulus carbon-fiber to come in at a claimed 290g. And to better work with the realities of mountain biking, Reynolds uses traditional exposed alloy nipples. They're certainly easier to true on the trail. They come with a Reynolds tubeless-ready rim strip that allows you to choose between using tubed and tubeless tires.The rims are laced to Reynolds hubs. Both wheels have 24 straight-pull spokes. The front has cross-two on the left side and radial on the right, so it's strong enough to withstand heavy braking, but still saves some weight. The rear is cross-two on both sides to better resist the forces of heavy braking and hard accelerations when you mash on the pedals.The front hub has a 100g claimed weight, with a carbon-fiber center section affixed to two aluminum flanges. The hub works both as a traditional quick-release and as a 15mm thru-axle. All you have to do is change the end caps, both of which are supplied with the wheel.The all-aluminum rear hub is also a transformer. It comes setup as a 135mm quick-release hub. With a 17mm cone wrench, you can convert it to either a 12 x 135mm thru-axle or a 12 x 142mm thru-axle. All the axles are supplied with the wheels.While the wheels should be plenty tough, all Reynolds wheel owners can buy themselves some extra peace of mind by purchasing Reynolds' two-year damage protection plan. It's called RAP, short for Reynolds Assurance Plan. The insurance means you should have little fear that a crash will ruin your wheel investment. Find out more about it on the Reynolds Cycling site.