A GO-BIG-OR-GO-HOME kind of pack. From Ortlieb:

All systems go for your next bikepacking adventure! Conveniently mount the ORTLIEB Seat-Pack to your seatpost with the help of two extra strong Velcro straps. In addition to its other duties, the seatpost bag serves as a fender in off-road terrain. The spacious, waterproof Seat-Pack keeps your extra clothes dry. Go ahead and dial in the pack‰۪s volume from 8 to 16.5 liters via the roll closure on the side to meet your stowage needs. Use the convenient compression valve to make your pack even more compact. And what should you do with your jacket on those uphill stretches? Simply stow it under the quick-access, X-link restraining cord on the top of the pack. And once you arrive at your destination, you can use the Seat-Pack as an ideal replacement backpack, no matter whether you‰۪re out with a cross bike, mountain bike, road bike or electric mountain bike.

Further Features:

also suitable for carbon seatposts
lightweight design
four 3M Scotchlite reflectors for various roll volumes

Note:

You‰۪ll need 15 cm of free space on your seatpost to mount the Seat-Pack.

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