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Extending Pack Capacity (On The Side): Part 2: The Solution

Extending Pack Capacity - The Solution

Accentuate The Positives, Eliminate The Negatives


We were a little downbeat and negative in Part 1, so in Part 2 we'll try and provide a little positivity and hope, as we investigate a genuine solution to this surprisingly tricky problem. If you don't know what we're talking about please read Part 1: The Problem with Pouches, Pockets and Dry Bags.


The Scramble Tower 11L Full Zip Pack Extender



Introduction: So, What's Good?

In Part 1 we looked at three candidates for solving the problem of meaningfully extending a pack's capacity (i.e. substantially above 4L), namely: military style side pouches, heavy duty dry bags and tall side pockets (with only a few to choose from made by Bach of Ireland/Switzerland and Tatonka of Germany).

Let's first look at what we like about each of these:

  • We like the capacity, toughness and security (the robustness of the attachment system) of the military side pouches.
  • We like the weather-proofing and compatibility (simplicity) of the heavy duty roll-top dry bags.
  • We like the high centre of gravity, narrow profile and most of all ease of access of the tall side pockets (specifically the Bach XL).

At Scramble, we've tried and tested pretty much every option out there and after a while it became one of those issues that both fascinated and confounded: "This can't be that difficult to get right, yet no one has nailed it". We've described ourselves as reluctant manufacturers; we don't do constipated product development meetings trying to force out the latest tweak to an equation solved long ago. In fact, we generally assume that someone has already solved the problem and we just go shopping, like everyone else, looking for the best solution (to test and review). But sometimes it's just not there. This is one of those times.


All the Scramble team engage in long distance solo treks - that's our thing. I asked a simple question:

It's winter, you're out for 10 days straight and the temperature range is -10°C to -20°C. You're using your favourite 30 - 40L climbing pack. Ideally, what kit goes outside the main body of the pack?  (note: unanimously, 10 days food and the sleeping bag went in the main body).

We measured all the items listed.

A 15cm diameter turned out to provide the ideal depth; enough for many UL solo tents, hooped bivvies, crampons, cooking kits (pots like the MSR Titan and Alpkit's Myti 900, bowls like the 13.5cm diameter Snowpeak), large 500g gas canisters, waterproof trousers, jackets, light to medium-weight insulated jackets, tent floor protectors, sleeping mats etc...).

Personally, this is what I wanted in mine:

My thinking was that on the other side I'd have my ice axe and hooped bivvie, which should sufficiently balance the weight.

That's quite a bit of kit, so we need to maximise capacity and that means a square base not a circular one (as pointed out in Part 1). So we have a 15cm square-based cuboid, but how tall?

Zpacks and Bach both went for slimline extension with a high centre of gravity

There's a great deal of space on the side of a pack which, like the Z-Pack Dry Bag and the Tatonka and Bach side pockets (pictured right), allows us to keep the centre of gravity high, maintaining good pack balance and stability. A 50cm cuboid with a 15cm square base provides 11.25L of capacity, which is pretty much the average size of a military side pouch (which tend to be between 10 and 12.5L). It also allows for awkward items like tarp poles to sneak in too.

We contacted a small company in Scotland with decades of experience making high-end gear for the outdoor market and asked them to knock up a sample. They did, and all the gear in the lists fitted, multiple combinations, permutations, mixing and matching and all good.



So, what materials to use? Well this was easier, we're all pretty good when it comes to materials; simply a case of asking "what's the best stuff out there that's extremely strong, water resistant, abrasion resistant yet lightweight?". The answer to that was Dimensional Polyant's X-Pac VX21HS laminate ... but damn it's expensive!

So we came up with a range of high-end materials, all excellent but some just more extreme than others. We had a name for our product, the "Tower", now we had a range: The Hex, HiT, Dura, Neo and X-Pac.

Theme and Variations: A range of tough materials all with slightly different attributes

Material: Overview and Attributes

Firstly, here they are in digestible tabular form, which shows that the HIT and the X-PAC models are the best all-rounders. Note, the DURA-500 (available only in black) will ultimately replace the black HIT model as it's very similar performance-wise. The DURA-500 offers marginally better abrasion resistance but with a 15g penalty.

Scramble Tower 11L Attributes Table

That said, for our purposes the HEX and the HIT are ideal and we've done most of our testing with these two models; if they're tough enough, then the others absolutely are. Here's a little more detail for the more geeky among you:

  • HEX (Limited Edition)
    Material: 200gsm PU Coated 400D High Viz Nylon Hexagon Ripstop
    Attributes: lightweight, strong, abrasion resistant and highly weather resistant
  • NEO-LITE (Limited Edition)
    Material: 205gsm Neoprene Coated Nylon
    Attributes: lightweight, strong, abrasion resistant and extremely weather resistant
  • X-PAC (Core Edition)
    Material: 200gsm X-PAC VX21HS laminate
    Attributes: lightweight, extreme strength, abrasion and weather resistance - hardcore across the board (the best all-round performer without the added weight - but also the most pricey)
  • HIT (Core Edition)
    Material: 225gsm PU Coated 420D High Tenacity Nylon
    Attributes: light to mid-weight, strong, very abrasion resistant and highly weather resistant
  • DURA-500 (Core Edition)
    Material: 270gsm 500D PU Coated Cordura
    Attributes: mid-weight, very strong, highly abrasion and weather resistant
  • DURA-1000 / 850 (Core Edition)
    Material: 380gsm 1000D PU Coated Cordura / 390gsm 850D Ballistic Nylon with internal acrylic coating
    Attributes: heavy duty, extremely strong, extremely abrasion resistant and highly weather resistant



Compatibility and Attachment (Security)

Any guitarist that wants to sound like they know what they're talking about will tell you that "making great music is often about the notes you don't play". Well that's really been the case making the Tower. Some of the failings of the designs we've critiqued are due to over-thinking and prescribing how the product should be attached and used. Here we started from the most compatible of all the items, the humble dry bag, and added only what's necessary.

A dry bag uses the pack's compression straps to transfer the load into the body of the host pack. The good ones, like Ortlieb's PD350 have attachment points at the base and at the roll-top enclosure. These are useful for providing a little support at the base and stability at the top (i.e. to stop a tall dry bag from flopping away from the pack's body above the top compression strap). We basically copied this system having a top and bottom attachment loop.

However, we wanted attachment loops at the sides also, mainly as guides for the compression straps and though not intended to take the full weight of the Tower, these will take some of the load and so needed to be strong and heavily stitched. The important design choice here was to forego attachment loops for the lower compression strap. This is important as, a) it means we don't have to second guess the spread between the top and bottom compression strap and, b) some packs, like the Blue Ice Warthog 40 don't have a quick release clip on the lower compression strap meaning it can't be threaded through a loop.

The Tower has two loops on each side (four in total) for the top compression strap. This allows users to adjust the height it sits, i.e. using the lower loop will sit the Tower higher on the pack.

The Zig-Zag Compression Straps of the Crux AK47

Though having both side attachment loops threaded by the top compression strap is ideal, the Tower is designed assuming only one loop will be used. This is because we don't know where the strap buckle or clip may be positioned (i.e. it's awkward if the clip falls underneath the wide side loop). This also means, that the Tower will also work with packs that have z-compression straps like the Crux AK-47 (pictured right).

One of the benefits of not over-designing the Tower is that it's been surprisingly versatile. One of these can, for example attach to the back of much smaller packs (if they have the appropriate attachment options) like Scramble's favourite Dragonfly 18L, turning it into a super lightweight 30L pack weighing just 470g (great for summer treks).


The Tower features a full length vertical zip opening housed underneath a peaked "waterproof cap" overhang. We chose YKK Vislon zippers. These are superb non-snagging zips with an easy glide and provide a very good water-resistant seal being plastic moulded.  

The lack of quick and easy access to kit was at the root of our dissatisfaction with many of the current products on the market. Even the best of them in terms of access, the Bach XL Side Pocket is not perfect, due to its use of a storm flap. Remember "the notes you don't play", well this was one of those notes. We thought long and hard about zip storm protection. The problem with storm flaps is that although they look nice, tidy and protective, in practice they can, a) act as a gully directing water into the zip housing (since these things stand vertically), and b) when you're wearing winter mitts and it's dark, it can be a faff trying to locate the zip pull which has lodged under the storm flap. We chose to forego this approach and instead designed a rather novel "cap" for the Tower. This also acts as a "pull-tab" when opening the zip, but is much wider than most and extends further, acting as a brim, so run-off from the roof of the Tower is pushed away from the zip below.

Because we've made the Tower almost symmetrical, it's agnostic about which way it's facing. So for example, you can have the zip facing forward (where the user can potentially reach over their shoulder and access kit at the top) it can face out to the side (its default position) or face backward (useful if walking into a storm). However, what we've found most useful is the ability to turn the Tower zip facing the pack - obviously this completely negates access, but sometimes that's a good thing. For example when in transit or in urban environments where "pick-pockets operate in this area" it's sometimes helpful to make access as difficult as possible. Also, in extreme storm conditions, where the thought of taking a relaxing break is the last thing on your mind, this setup also provides the most extreme level of weather protection, making the Tower practically 100% water-proof.


The Tower is designed to hold all the kit you may want access to while on the move and to provide a tough protective layer between your gear and the sharp and pointy world outside. It's not a dry bag, but rather like most high-end packs, the Towers are all highly weather / water resistant (and nearly but not quite fully waterproof). The seams are stitched in such a way as to make them very water resistant and the stitching / seams on the attachment loops and the zip cover are all sealed (the top of the Tower is 100% waterproof). The YKK Vislon zippers are likewise very water-resistant.  

The beauty of the Tower is that being so tough, it allows the use of ultralight dry bags inside, so even if you take the heaviest bombproof model (the "DURA 1000" which uses 1000D PU coated Cordura) and include a large Sea To Summit Ultra-Sil Nano 8L dry bag (24g), the total weight is still a fraction less than an Ortlieb 13L (really 12L) dry bag. But with the Tower you get tougher protection, a narrower profile, a higher centre of gravity and much more convenient access to your gear (note: if you have items like down jackets, electronics etc. that you absolutely must keep dry, then a small ultralight (~ 20g) dry, zip-lock or plastic bag would be sensible, but things like cooking kit, water filters, waterproofs, tents, tarp poles etc. don't require that degree of protection, so dry bags can be used judiciously).

On the lighter side, if you choose the HEX or NEO-LITE models, the total weight (incl. the 8L dry bag mentioned above) is around 145g (substantially less than the Ortlieb PD350 13L or a Karrimor SF Sabre Side Pouch) and still lighter than even the much smaller capacity 6.5L Bach XL side pocket.


Where Are We At?

We need to conduct our final field tests of the finished models which are coming off the line very soon. After all our testing, we're extremely confident that these will perform as expected. Assuming all goes well we'll likely have the first models available around the end of November 2018.

We're very excited about this product. Our favourite packs are the Karrimor SF Predator 30 and the Blue Ice Warthog 40L (review coming). The Scramble Tower is the perfect companion for both of these and pretty much all 30L+ packs on the market. The only packs that won't play well with the Tower are those with permanent protruding side pockets or over-designed clutter on the side - but basically if your pack can hold a closed cell foam sleeping mat, it very likely will host a Tower.  

We looked at a number of 60L packs and noted their weights. If you add a couple of Towers to the Blue Ice Warthog for example, you end up with a very comfortable, agile, incredibly tough 60L pack that weighs just 1350g (much lighter than the vast majority of 60L packs out there). Since testing, the Scramble team have dumped their pouches, pockets and heavy duty dry bags. I have to admit I've done the same, it's basically solved all my load extension issues and has meant that as long as my 30/40L pack of choice can handle the load (without becoming uncomfortable) I can seriously extend its capacity and in so doing extend my range, especially in Winter when kit is always bulkier and heavier and at the same time reduce my overall pack weight.

Furthermore, it enables buyers of new packs to consider a different approach; being able to opt for a tough minimalist climbing pack knowing it can be configured and extended for touring / backpacking with the use of a Tower or two. The outdoor industry might not appreciate selling less packs, but at Scramble we've always had the view that one or two packs should really suffice (a small 20L pack and a 30/40L pack that can be extended to 60L). The Tower makes that ideal a genuine reality.

Addendum: A Sneak Preview

The final Towers have just landed so let's have a sneaky peak.

The Towers hold a fair amount of kit

You can even take photos with your spare hand

The Tower's Attachment Loops

The Tower's Minimal Feature Set

Last Words

Before we wrap up, let's revisit the scorecard from Part 1. We've attempted to be as objective as possible here. We used a table like the one below to guide whether we were genuinely solving the problems outlined in Part 1. Our approach was pretty straight forward, to eliminate all the negatives associated with current options and where possible accentuate the positives - after all, apparently, "that's what gets results". As you can see it's not perfect; but it is a genuine solution to what proved to be a more tricky problem than we first anticipated.

Lastly, we've been really lucky finding such a great company to partner with and their expertise and input has been invaluable. This is really the first time we've been able to control the entire design and manufacture process and consequently make the product as close to ideal as we're able.

Personally, I can't wait to test the final release models and get them out to the world.

Update: The final test run results are in, see: "Scramble's Tower 11L Full Zip Pack Extender".


Last Updated: 02/01/19 (updated attributes table)


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