Yeti has produced a lot of big, scary, enduro sleds that gobble terrain with the abominable aggression of the company's namesake, but the latest monstrosity to mate the Switch Infinity suspension design with a slack head tube may be the most frightening of all. The all new SB4. 5 Carbon Mountain Bike Frame is driven by 29in wheels, led by a 67. 4 degree head tube, and enabled by 4. 5in of Yeti's signature suspension design. Just a few months ago, we would have told you this combination was unlikely, if not impossible yet here it is, in all its abominable glory. Yeti claims that the SB4. 5 has a stiffer pedaling platform than most of its other bikes. While we don't have numbers to verify this claim, it's definitely a responsive go getter when the going gets mashy, and we think the inclusion of a Boost 148mm rear axle is at least partially responsible for the bike's eagerness on climbs. Coupled with the innate aggression of 29in wheels and the stiff, collet style hardware, the Boost ed rear triangle certainly feels every bit as stiff as any of Yeti's other models. The frame's aggressive speed is not to be outdone by its slack front end and surprisingly capable Switch Infinity suspension, though. This is one of the slackest head tubes we've ever seen on a 29er, and it makes for a boost of confidence while picking lines across terrain that would make a more typical 70 71 degree head tube stumble. The Switch Infinity suspension is equally confidence inspiring, and its linear travel hits the sweet spot of anti squat to keep pedaling sharp across rocky climbs. It also erases the momentary harshness that its predecessor, the Switch Link design, would display when pushed to the absolute limit. Of course, most riders will never experience that dead spot on the Switch Link, but the SB4. 5 isn't for most riders. It's for the all mountain racer who demands that their machine flattens big hits with the same tenacity that it cleans long climbs a rare breed in the enduro s...