The key to the Exploro’s speed is the aero downtube that is wide enough to catch the airflow coming off the big front tire (a skinny downtube wouldn’t even touch the airflow). But at 50mm wide, we cannot use a full airfoil shape as the tube would be 150-200mm deep, causing too much surface drag. So we square off the tail, keeping most of the aero performance with a 50x75mm cross section that is also close to perfect for strength and stiffness; our Sqaero shape. The wide downtube also effectively shields the water bottles.
More details on the 3T Exploro Team frameset.