Sierra Designs Flex Lumbar is One Massive Hip Pack [Review]

This oversized hip pack from Sierra Designs holds two water bottles and more gear than many hydration backpacks. We give it a test on the mountain bike trail.

Sierra Designs may not be a well known name in the mountain bike world, but the brand has been designing sleeping bags, tents, packs, and even apparel for outdoor enthusiasts for more than 50 years now. I’ve been testing the company’s Flex Lumbar 7-10 “hiking” hip pack and surprise! It works for mountain biking too.

While it may seem like hip packs are designed for carrying a small amount of gear, they actually come in all shapes and sizes just like hydration backpacks. The Sierra Designs Flex Lumbar is a large hip pack, capable of carrying up to 10L of cargo. As a point of comparison, the popular Camelbak Mule hydration pack offers slightly less — 9L — of carrying capacity.

On their website, Sierra Designs says the Flex Lumbar has “your bases covered for any day hike,” so the lack of bike tool organizers inside the massive main compartment isn’t surprising. However, inside there is a hook for a key ring, a small zippered pocket, and two pouches capable of holding nothing thicker than a couple credit cards. The rest is wide open space, ready to pack just about anything a mountain biker could need for an all day ride.

Looking inside the main storage compartment.

Personally, I’ve yet to completely fill the Flex Lumbar pack on a mountain bike ride. For example, a recent ride required a mini-pump, multi-tool, spare tube, rain jacket, tire plugging tool, can of Mountain Dew (with Koozie), phone, and a spare jersey. I would have had room to pack my down jacket if I wanted it.

When not completely packed, three straps on the outside of the Flex Lumbar can be cinched down to stabilize the load. This effectively reduces the storage space from 10L down to 7L. Remember my packing list above? That all fit in the first 7L.

The top map pouch.

Sierra Designs cleverly calls this expandable design the Flex Capacitor, and instead of growing out (which would cause the pack to become more unstable), it expands upward. The pack is made from ripstop Nylon-polyester material that also seems to be fairly water-resistant.

On top of the pack there’s a slim, zippered map pocket that’s a good size for a cell phone. For some reason I keep getting this compartment confused with the main one when I’m looking for gear, perhaps due to its prominent zipper placement.

There are two bottle holders on the pack, one on each side, and they are stretchy enough to hold most bottle shapes. Even a 32oz. Nalgene bottle, the type that seems to be favored by most hiker-types, fits no problem.

The waist strap offers medium firm padding around the hips and is wide enough to effectively transfer a decently heavy load. The cinch strap and plastic buckle are fairly standard — durable, but medium quality in terms of feel and appearance. Youth and small-waisted riders may find the waist is too large. My 32-inch waist leaves just an inch or so of adjustability. On the other end of the spectrum, Sierra Designs says the pack will fit waists up to 52-inches.

Finally, there are two pouches on the sides of the waist belt itself; one zippered, the other open. These are both a good size for a bar snack or a point and shoot camera.

photo: Leah Barber

Despite the ability to overload this pack with a lot of weight, it feels surprisingly stable on the trail. For myself and many other riders, getting weight low and off the back is important. Many hip packs require a compromise over backpacks in terms of carrying capacity, but the Sierra Designs Flex Lumbar does not.

At $74.95 (available at, the Flex Lumbar 7-10 is reasonably priced given its carrying capacity. Sierra Designs also markets a 3-6 version of this pack for $49.95. With the ability to carry up to 6L of gear plus two water bottles, even the smaller version represents a fairly large hip pack.

Thanks to Sierra Designs for providing the Flex Lumbar 7-10 for review.