Designing a folding bicycle that is as stiff as a non-folding bicycle is a difficult task. While our bikes are generally considered among the best riding folding bikes in the industry, we're always looking for ways to improve them. Increasing torsional strength and stiffness of our frames is always a design goal. After countless tests and modifications, we finally settled on a custom tube shape we call Sonus. The tube is taller than previous designs for superior fore/aft strength. And the tube profile features a wide, flat base that is more resistant to twisting forces. Sonus tubing is over 35% stronger and stiffer than the oval tubing it replaces.