Sierra Designs Flex Capacitor

8.5 score
[Editors rating (8.5)] = (TheGearHunt) score (8.5)/10

Editor rating: 8.5 / 10
User's rating: based on 0 user ratings
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Sierra Designs Flex Capacitor Review Facts

When looking for a good lightweight backpack for hiking, we have often given you the down-lows and overabundance on choice features and styles. There are so many to choose from, and like most things in life, companies do tend to design each pack for a specific gender. On a rare case, you may find alternate versions based on this one lone premise.

For the Sierra Designs Flex Capacitor, the design is around giving a lightweight advantage to those with a more masculine build, but that doesn’t mean that only the guys can be comfortable in it!

Of course, we want to provide you with straight facts and let you decide for yourself if the pack is ideal for you, or if you may want to sway yourself to something else in the Sierra lines that fits better. We just want to let you know right away that this pack had our heads tilted and our brows raised because it really appealed to the senses from the start. So onward to fulfill our endless curiosity and answer the question- is this the right one for me, perchance?

Editor's Pros & Cons

Extremely comfortable

Great suspension

Good sizing


Reputable company

Adjustable capacity

Good ventilation


Limited adjustability for wearing

Stiff back


The most notable feature of this pack is what gave it the name ‘Flex Capacitor.' While many of us will be nerdy enough to immediately think of our childhoods and laugh at the reference to ‘Back to the Future,’ this pack has a fantastic ability to go from a 25-liter pack to holding about 60 liters. The straps on the sides are more than just for visual appeal and support; they are adjustable for the center compartment to allow for more room.

For a lightweight pack, this is quite a feat to behold since it holds up where you might not expect it to in carrying equipment. Of course, just because there is room to hold something, as we all know, that doesn’t mean what you put in may not still be too much for the pack to carry. Leaving us happily shocked, the full weight carrying capacity is recommended at 40-50 pounds for longer trips, so it really can hold up under pressure in both size and weight.

Another significant feature that this pack sports is the fact that it has two sizes for torso builds, which means you can suit it up to be comfortable for you depends on what you are planning on taking with you. Unfortunately, however, if neither works in its design, the adjustability seems to stop there for fitting the user. While the other straps can be adjusted, the main sizing is in the torso fit, which leaves it at a little of a disadvantage to some other competing lines which offer full customization based on the customer.

This shouldn’t entirely be a deterrent, given that those who have still taken the opportunity to put the pack to use have found it does everything that it is touted to do, and does it well. The sizes may not have a lot of flex initially in the way it is set up to be, but few have found that they are disappointed by it, and it fits most individuals. The reason that you have to use one of the two given sizes is because of the fixed bar support in the back, which we will describe more of later.

It does offer four hip belt sizes as well, which means you are likely to find one that fits and balances appropriately for your build. This means you can assure that balance and comfort are maintained no matter how much you expand your pack. Combined with the adjustable arm straps, you may find that one of the two sizes offered actually works for you regardless, and that is a major plus.

So, what sets this product apart from the rest in how it expands? Well, in most cases, if your bag grows to contain more than the initial amount, it does so by extending laterally. This can set everything off balance and actually cause it to lose both comfort and stability rather quickly.

With this particular model, the pack expands horizontally, and out away from your back evenly, allowing everything to maintain weight and balance. This means there won’t be extra pull in a single direction that eventually leads to feeling like your being pulled back every time you look up. Those who have tried this pack have said that no matter what they have it set for, it remains perfectly comfortable through the entire resizing.


What does the Sierra Designs Flex Capacitor offer in the way of durability for materials? We have gone over how well it fits and the fact that it has a lot to offer in the way of fitting what you need inside of it, but fitting it and being able to handle those objects requires a durable make. Well, we can definitely say they use some of our favorites as far as reliable body materials go.

The entire body of the pack is made up of a durable 100 density count nylon polyester with the ripstop feature. What this means is that the weave of the fabric, combined with its strength and initial durability, prevents small tears from becoming larger. That is if you really place something in your pack both strong and sharp enough to actually rip a hole in it, to begin with, which most of us know what not to put in it.

The secondary fabric used, in the lining and straps, are made of 420 density nylon oxford materials. What is that, you might ask? It is a coated fabric used for its protection against the elements and water resistance. It keeps your pack protected as well as everything inside of it against damage caused by water and other natural elements that may cause your bag to wear out faster.

The support bar uses DAC Pressfit aluminum in order to help keep everything supported and balanced on you in your pack so that you don’t have to worry about overstressing your muscles or your pack. This is a sturdy and non-flexible metal bar that runs the length of your bag vertically and is the reason why you only have two choices for a torso fit. Since the bar has to be in place for the bags support system to work, and it doesn’t adjust, there isn’t a lot of room for extra customization.


All those who have purchased and tried this particular model have found that comfort is something the bag supplies. The support bar keeps you from feeling most of the weight, and many of the straps and belts provide the extra weight dispersal needed when you expand the bag to its full capability. The main draw is in its design, which extends horizontally rather than vertically, and this prevents the usual discomfort caused when the bag becomes off-balance with overpacking.

This is a great plus for anyone who finds that they don’t always need the extra room but do want it when it becomes necessary for bringing all the equipment they need for a trip. What most do not realize is when a bag extends vertically- and mostly upwards- this place extra strain on your neck, upper back, and shoulders no matter how much support is found elsewhere in the harness. This can be the cause of falls and severe muscle and back injuries if done continuously.

Not only is the design offering additional comfort, but safety as well. The comfort doesn’t stop there either, also providing a very breathable back panel, which also aids in preventing muscle exhaustion and strain.


The materials used are the kind one would expect for any bag that is worth its meddle when you are preparing for a long hike or camping trip. A high-density rip-stop fabric with water and weather resistance is what gives any backpack its durability. The stronger and better the material is in these regards, the longer it is bound to last.

Combined with its ability to expand, which means you don’t have to overfill your bag beyond its capacity, your Flex Capacitor is not likely to give you cause to go out seeking another bag for months, if not years to come.


As mentioned in much of the above sections, the capacity for this particular bag is what sets it apart from others in its category. Designed to expand, this product goes from being a 25 or 30 liter to being a capable 40-50 liter bag and can handle around 50 pounds worth of needed items for your journey.

This means you have both a lightweight bag and a medium to heavyweight carrying capacity all in one, which is definitely something most don’t expect. After all, it isn’t every day your hiking pack can nearly double, if not double, what it can hold for you.


This is not your typical hiking backpack by any stretch of the imagination. Saying it has a primary use would be a major downplay to what it can do for anyone no matter what the reason for consideration is. This backpack can easily be used as an overnight bag, but since it can change exactly how much it can hold, you could also double it up for week-long hiking or camping trip with ease.

If you also take into consideration just how durable it is and the fact that it will keep your items safe from outside elements, this model is perfect for just about any task you can toss at it for the great outdoors.


Where you may expect it to be lacking in physical appeal, we have found that all-around this design is appealing to every aspect that you can look at it. The actual look of the bag, beyond the color, is absolutely gorgeous. It doesn’t look like the workhorse that it actually is. The ability to expand usually means having a tangled look of bungees and straps along the outside that just drag the eye and make you want to wince in most cases.

Surprisingly, the straps being just off-center of this bag don’t have that effect. They look incredibly aesthetically pleasing and, honestly, made us want to have one just because it looked so good. There are two colors we could find easily for this model, grey and black, and while neither made is flinch, we have to admit that we fell for the black one right away.


The support system on this bag is amazing. While it isn’t what you get from a lot of other hiking or camping packs, it realistically and completely sets up support that will aide you from feeling all that weight on any single point. The Y bar on the back is padded, and it is what takes most of the weight for you and spreads it out evenly.

Of course, the straps and harness are what make up the rest of this support system and setting everything to perfection is cinch. What makes the design even better is you don’t have that odd ‘surrounded by a cage’ that you normally get when it is a bar support system since it is hidden away between you and the bag.


While this model doesn’t have a lot of pockets, it does offer just enough to keep your things in order. Some feel it could have done more, given the way most of its competitors have added a plethora of different side, strap and inside pockets. But when you consider the main calling of this one, you have to admit that it would be a complicated task to make a series of collapsible pockets as well.

Key Features

- Expands horizontally
- Comfortable
- Durable
- Limited Lifetime Warranty
- Great style
- Lightweight

Bottom Line

If you want a backpack that fits into every angle of your outdoor life, whether it be in hiking or camping, you definitely get that with the Flex Capacitor. This bag will go where you need it to go when you need it to go there. Find both comfort and dependability no matter what your goals and do so with confidence.