Before releasing the latest take on its SB chassis design and Switch Infinity suspension platform, Yeti put prototypes through a year of testing under pros like Graves and Rude. Though it was obviously seeing lots of sunlight, it didn't garner media attention because the frame's lines run similar to the SB5 and SB6. This allowed it to speak softly, going unnoticed until its official release. Now that we've gotten our hands on the SB4. 5 Carbon X01 ENVE Complete Mountain Bike, though, we understand that the bike's real MO is properly Rooseveltian: it speaks softly, but it bludgeons the hell out of everything from enduro courses to backcountry marathon trails. While the obvious bling of ENVE's 29er M60 Forty wheels and SRAM's big stick equivalent drivetrain, X01, would typically be all we need to know about a bike to identify its rough rider pedigree, the frame's 67. 4 degree head tube angle one of the slackest we've seen on a 29er and 4. 5in of 29er tuned travel are the real driving force behind the SB4. 5's aggressive swagger. These features combine for a bike that's every bit as rugged as a six inch monster but still displays a presidential level of reserved manners on climbs. The inclusion of a Boost 148mm rear axle is at least partially responsible for the bike's unflinching approach to going up steep grades, and coupled with the innate aggression of 29in wheels and the stiff, collet style hardware the Boosted rear triangle certainly feels like it may be, as Yeti claims, the most responsive pedaling platform in the brand's stable. In the end, the bike is a Yeti, so the suspension design inevitably closes out the docket. The SB4. 5's Switch Infinity suspension may be the most advanced design we've seen from the industry to date. Its linear travel hits the sweet spot of anti squat to keep pedaling sharp across rocky climbs while erasing the momentary harshness that its predecessor, the Switch Link design, would display when pushed to the absolute limit.