Schwalbe Thunder Burt 27.5 x 2.1For those seeking the most rugged terrain, there's no denying the magic of a meaty MTB tire on a drop bar gravel steed. As bikes gained he clearance, the Schwalbe Thunder Burt quickly stood out as an exceptional piece of rubber. While too minimal for most hardcore mountain bikers, applied to a gravel, the Thunder Burts seem to have all the right traits.