Topeak RX Beam Rack CB Carbon Fiber Bicycle Seat Post Rack Fits seatposts from 25.4-31.8mm (shims included) Features light molded construction; dual bolt, hinged seat post clamp Molded Carbon Fiber Beam with Aluminum Rack Integrated rubber bungee cord secures your cargo; comes with 2 cords 17.5 by 3.1 by 2.9 inches (L x W x H); carrying capacity is 15 pounds