Introduction The carbon fiber bicycle industry has been innovating and advancing all the time. Being lighter and stronger has always been the pursuit of the professional bicycle industry, especially for the mountain rims for XC (Cross Country) riding , where riders have extreme weight requirement. In the year of 2017, we first tried to use Toray T800 carbon fiber material and to improve the production process, reducing a weight of 60g for the XC mountain rim, which is a huge improvement. In the year of 2022, we upgraded two models of Ultralight Edition carbon fiber 29" mountain bicycle rims for cross country, the NXT29UML30 and NXT29UML34 with extrernal width 30mm and 34mm. We increased the rim lip thickness of these rims to 3.0mm, which provides stronger impact resistance. The rim height was reduced to 20mm and 22mm. The shallower height improves the toughness of the entire wheel and makes riding more comfortable. This new series is about 15-20g lighter than the previous generation. The weight of model NXT29UML34 is only 310g. The profile drawing of this rim is as below. This rim could be used for XC (Cross Country), Marathon and Trail riding. The tire size for this model of rim is recommended at 1.85" ~ 2.4". The tire pressure is recommended at 30 ~ 60 psi.