Docooler&reg; MTB Bike Bicycle Carrier Rack Seat Post Rear Shelf Aluminum Alloy Quick Removal and Installation Fix tightly onto the seat post of your bicycle. Suitable for mountain bike and road bicycle whose seat post diameter is less than 31mm. Length can be adjusted by two inner hexagon screws. (38cm ~ 64.5cm) Adopt high-strength aluminum alloy material, durable and sturdy. Quick installation and removal.