Different wheels sizes in one frame to reduce height, length to gain stability
HI-LO frames combine different diameters of wheels in the same frame but distributed in a way that all wheels touch the ground when skating. Smaller wheels accelerate a little easier while larger wheels provide a higher top speed. Hi-lo frames are usually a bit shorter than their 4-wheel pendants, which are equipped with 4 equal wheels of the same maximum diameter. Additionally, the smaller wheels reduce the weight of the frame and reduce the height of the frame for a lower center of gravity.