Large Marge sent me.
When it comes to hammering singletrack, manufacturers have come to realize that wider rims are better rims. Ibis is leading the wide-rim charge with its 741 Carbon Rim. The 741 adds width without adding a ton of weight, so you get more traction and stability without much more weight.
Wider internal rim widths (note: not simply a wider rim body) means the tire beads can sit farther apart, which means greater tire volume, which means that lower tire pressures can be run for greater traction and handling, because the tire is more free to deform and deflect around obstacles without drastically altering the path of the wheel's momentum. Ibis says you can run 4-6 less PSI of pressure, on average, with the 741s. Wider rims also translate into a wider tire footprint, again boosting traction, and increasing stability at the same time. A wider footprint also means that your tire fattens up relative to its labeled size; for example, a 2.2-inch tire can widen to the volume of a 2.3- or 2.4-inch tire, depending on the rims themselves, so you can run lighter tires without giving up capability or gnar-ability.
Now, take a look at the emerging world of 27.5-inch all-mountain-ready bicycle wheels, and you'll find internal rim widths from 21-26mm. Ibis takes that average and gives it an "F." Witness the 741: 35 millimeters. That is gigantic. That is tremendous traction and capability. That is stability for days. But they must weigh a mega-ton, right? False. The 741 wheelset's carbon rims keep weight down for these monsters, and coming from the company that raised legendary all-mountain machines like the Mojo and the Ripley, you know durability is a given.
Need numbers? Ibis tested the gravy out of the best competitors' rims for comparison and found its new 741s to be anywhere from 50% to 300% stronger in various tests. Part of what made this possible was the hookless bead construction of the rims, which optimizes tubeless tire compatibility while strengthening the bead by replacing traditional musical note-shaped beads with stout rectangles of carbon. Spoke holes are also reinforced by additional carbon to ensure strength, too.