Four hours into a marathon race, your hands and lower back are doing as much work as your legs. On roots and rocky climbs, every impact the wheel passes on to you is energy you spend holding the bike on line. The 9th Wave Flex SL was designed around that problem: a light XC wheelset that gives a little where a stiff rim would not.
A low rim for vertical compliance
The Flex SL has an 18.5mm deep rim, far lower than a typical XC rim. A low rim gives a little in the vertical direction. That takes the sting out of small impacts and trail chatter, and it helps the tire stay in contact with the ground over roots and rocks instead of skipping off them. The gain on a long race is less accumulated fatigue in your arms and back, so you still have control and power in the last hour. Lateral stiffness comes from the build, not from the rim depth.
ART keeps the build even
A rear wheel has dish: because of the hub spacing, the spokes on one side carry more tension than on the other. The ART asymmetric rim design moves the spoke holes off-centre to bring the two sides closer together. An even tension keeps the wheel stiff when you stand on the pedals, and it lowers the chance of a spoke going slack under load. Every wheel is hand built in the Netherlands, so the tension is set by the builder and not accepted as it comes.
The specs, and what they mean on the trail
| Feature | Spec | What you notice |
|---|---|---|
| Wheelset weight | 1299 g | Less rotating mass to accelerate on climbs and out of corners. |
| Inside width | 30 mm | Supports the sidewall of 2.25 to 2.40 inch tires, so they corner with less squirm. |
| Rim depth | 18.5 mm | The low profile that gives the vertical compliance described above. |
| Rim type | Hookless, tubeless ready | Easy tubeless setup; stay within the recommended tire pressure. |
| Hubs | DT Swiss 240 EXP Boost, 6-bolt | A light hub with an instant-engagement freehub. |
Where it was raced
Henrique Avancini became Marathon World Champion in 2023 on the Flex SL Ninefold Ultimate, a special lightweight version of this wheel built with Berd spokes. Wheels with Berd spokes are no longer part of our range: today our Ninefold wheels use Sapim RC1 carbon spokes. Read how that race went in our article on Avancini’s world title.
Note: The Flex is no longer in production. We can still build it to order on request. Its successor is the Yarrow SL29.
