Hi there! I've been writing bag reviews for a few months now, and I'm always looking for ways to improve. I'm currently considering adding a scoring system alongside my usual pros and cons, but I'm unsure how useful it would be for you as readers and would love to hear your feedback.

Here is the system: each category gets an integer score from 1 to 4 — meaning 1, 2, 3, or 4 only. You can think of them as "Bad", "Decent", "Good", and "Great". Scores are summed up and divided by the number of categories, and the result is the final score. I'll use the Alpaka Traverse as an example.

• Aesthetics: 4 / 4 — It's subjective, but so are my reviews. I like the crossover look of the Traverse a lot, so I'm giving it a 4 out of 4.
• Size & Weight: 3 / 4 — The dimensions work well as a carry-on, and even though the volume is less than advertised, it's still a practical size for EDC and minimal travel. The weight is on the heavier side for a travel backpack.
• Organization: 2 / 4 — There are good things going on in the main compartment, but the front compartment and side pocket are poorly thought-out, and the lack of a proper false bottom on such a sizable bag is disappointing.
• Build Quality: 3 / 4 — Generally really good, but not quite up to par with Aer or Able Carry.
• Comfort: 3 / 4 — Good weight distribution, thick well-ventilated back panel padding, wide shoulder straps, and practical adjustable sternum straps make for a comfortable bag. That said, there's a slight pullback effect, and the shoulder straps lack ventilation.
• Overall: 3 / 4

Draft of a score card

A couple of things to clarify:

• Why 1 through 4 with no partial scores? It forces me away from giving 2.5 to everything and to think more carefully about whether a score should be a 2 or a 3.
• No "value" category? Value is even more subjective than looks, and the MSRP often doesn't reflect reality outside the US. For example, The Alpaka Traverse is available here—in Thailand—on local marketplaces, often at a discount, for the equivalent of $150–170 USD. A competitor like the Able Carry Max EDC would cost me $450 USD imported — a far cry from the $199 vs. $280 listed on official websites.

There you have it. I'd love to read your thoughts on whether this scoring system would be a worthy addition to my reviews and bag reviews in general, or if it’s too reductive?

Hello again, fellow bag lovers! Last time, we explored nylon, and now it's time for X-Pac.

Introduction

You often hear that X-Pac comes from sailcloth. But what exactly is it? How did sailcloth end up on bags? What do cryptic ciphers like VX21 and X42 stand for? And what about the fearsome “delamination”? Let’s dive in and find out.

What is a laminate?

A laminate is two or more layers bonded together. Each layer can be a different material with its own properties, so by combining ones that offset each other's weaknesses, you get something more than the sum of its parts.

Simplified visualization of a car windshield structure (generated images for illustration purposes)

Think of a car windshield. It's two layers of glass with a thin plastic sheet sandwiched between them. The glass gives you structure and clarity. The plastic holds everything together if the glass shatters—which is why a cracked windshield stays in place instead of spraying into the car.

Shattered car windshield (Photo by Umberto on Unsplash)

X-Pac is a laminate. So what's actually in it?

Polyester

If you thought, “Wait, aren’t we talking about X-Pac?”, bear with me for a moment. We explored nylon in detail previously, but to better understand X-Pac, we need to touch on polyester as well.

Polyester, like nylon, is a synthetic polymer made from oil. The chains are linked together with a different type of bond, called an ester bond, instead of nylon’s amide bond. The chemistry is different, but the basic idea is the same: long-chain molecules are melted, pushed through tiny holes to form fibers, and woven into fabric.

The version of polyester used in bags is called PET—polyethylene terephthalate (yeah, don’t try to pronounce that). You have heard the term “PET bottles,” right? It is the same plastic, just spun into fibers.

So what’s the difference between polyester and nylon?

• Polyester absorbs almost no water. Nylon soaks up to 10% of its weight in moisture; polyester is closer to 0.4%. Wet polyester stays the same weight and dries faster.
• Polyester attracts more lint. Its strong resistance to absorbing water comes with poor electrical conductivity, which means polyester builds up static and attracts lint.
• Polyester doesn't stretch under load. Nylon stretches a bit when pulled, then springs back. Polyester is dimensionally stable—once it is woven into a shape, it stays in that shape.
• Polyester resists UV better. Sunlight degrades nylon faster than polyester. A polyester bag left in the sun for years will hold up better than a nylon one.
• Polyester is denser. 1.38 grams per cubic centimeter versus nylon’s 1.14. That means a polyester fabric of the same denier as a nylon fabric is thinner. Remember that denier is mass per length. If two yarns weigh the same per length but one is made of denser material, the denser material packs more mass into less volume. Less volume means a thinner yarn.
• Polyester is less abrasion-resistant and less tear-resistant. For a bag that is going to be dragged across surfaces and snagged on things, nylon is the better choice. For something that needs to hold its shape and not absorb water—you know, like a sail—polyester wins.

Now all the building blocks are in place, and we can finally get into X-Pac proper.

What is X-Pac?

There is a sailcloth manufacturer called Dimension-Polyant, founded in 1966, with operations in Germany and the US. In the mid-2000s, they noticed that the same attributes that make good sailcloth—waterproof, dimensionally stable, lightweight—would also make great bag fabric, and launched X-Pac, a purpose-built laminate designed specifically for bags. It's an important point to get across: this is not just repurposed sailcloth; it was designed from the ground up using the same technology.

Schema of X-Pac X3 structure (image from X-Pac official website)

X-Pac comes in two variants: 3-layer (X3) and 4-layer (X4) laminates. The common layers are:

1. Face fabric (usually nylon, sometimes polyester). Provides abrasion and tear resistance and is what you see and touch on the outside of the bag.
2. X-PLY reinforcing fibers. Those signature X-shapes that create the distinct diamond pattern. They provide structure, reduce stretch, and distribute load evenly across the fabric.
3. Polyester film (not fabric—actual film, like the material a plastic water bottle is made from). This is where the waterproof property comes from.

Sample of X-Pac X3 (image from X-Pac official website)

X4 adds a fourth layer: a thin polyester fabric backer on the inside. This protects the waterproof film from being scratched up by items inside the bag and gives the interior of the fabric something actually nice to the touch instead of slippery, plastic-bag-like bare film.

Schema of X-Pac X4 structure (image from X-Pac official website)

In practice, though, most bags have an additional nylon or polyester inner lining, so the choice of material usually comes down to weight, abrasion and tear resistance, and available colors.

Sample of X-Pac X4 (image from X-Pac official website)

With the basics covered, let's get into the technical weeds. Be warned, X-Pac naming convention is a bit of a mess.

Aer City Pack Pro 2: VX42 X-Pac exterior and orange ripstop nylon interior lining (image from Aer website)

X-Pac Specs

If you go to the X-Pac website and search for details about fabric options, you’ll see cryptic stuff like “Tear Strength: Warp 139.7 N / 31.4 LBS, Fill 105.4 N / 23.7 LBS,” “Abrasion ASTM 3884: 1700 cycles,” and “Waterproof: 13.8+ bar / 200+ psi.” It’s time to unpack these.

Specs for X-Pac X42 (screenshot from X-Pac official website)

Tear Strength

Tear strength measures how much force it takes to propagate an existing tear through the fabric. Note “propagate”—this isn't how hard it is to start a tear; it's how hard it is to keep one going once it's started.

The test: a small slit is cut into the fabric, then the fabric is pulled in a way that drives the tear through the material. The force required to keep the tear moving is the tear strength. The standard test for fabrics is ASTM D2261 (tongue tear) or ASTM D1424 (Elmendorf tear), and results are reported in newtons (N) or pounds (LBS).

Warp is the direction parallel to the long axis of the fabric roll (lengthwise). Fill (also called weft) is perpendicular to that (crosswise). Tear strength is almost always different in the two directions because the yarn count, yarn type, and weave structure can differ between warp and fill. For X-Pac specifically, the X-PLY scrim adds reinforcement on diagonals, which affects both directions but not equally.

Tear: warp vs. fill direction (generated image for illustration purposes)

So this fabric—which is X42—needs 140 N (about 14 kg of force) to propagate a tear lengthwise and 105 N (about 10.5 kg) to propagate a tear crosswise.

Abrasion

ASTM D3884 is the Taber abrasion test. The fabric sample is mounted on a rotating turntable, and two abrasive wheels are pressed against the surface as it rotates. Each rotation is one “cycle.” The test runs until the fabric fails—typically defined as a hole appearing.

Taber Abrasion Tester Machine

So 1,700 cycles means the fabric survived 1,700 rotations under the abrasive wheels before failing the test criteria.

"Waterproofness"

This is a test of how much water pressure the fabric can hold before water penetrates through. The test (ISO 811 or AATCC 127) involves clamping the fabric and gradually increasing water pressure on one side until water visibly pushes through.

ISO 811 Test Machine

X-Pac Variants Comparison

Tom Bihn Synik 30 in X-Pac RX-30 (image from Tom Bihn website)

Within Dimension-Polyant's own lineup, numbers are directly comparable since they all come from the same testing setup. Comparisons across different fabric brands (Cordura vs. X-Pac, etc.) are trickier because test conditions vary and not all numbers are available.

Fabric	Face	Weight	Tear (warp / fill)	Abrasion	Waterproof
X21 (X3)	210D Nylon	177 g/m²	71 / 53 N	900 cycles	Yes, up to 13.8+ bar
VX21 (X4)	210D Nylon	210 g/m²	109 / 77 N	500 cycles	Yes, up to 13.8+ bar
X42 (X3)	420D Nylon	260 g/m²	140 / 105 N	1700 cycles	Yes, up to 13.8+ bar
VX42 (X4)	420D Nylon	297 g/m²	238 / 169 N	1700 cycles	Yes, up to 13.8+ bar
RX30 (X3)	300D Recycled Polyester	262 g/m²	85 / 80 N	2000 cycles	Yes, up to 13.8+ bar

All variants are waterproof, which isn't a surprise considering that's the main feature and they all use the same polyester film. What's more interesting is weight, tear- resistance and abrasion-resistance.

• Even though RX30 has a thicker face (300D polyester) and weighs more, the lighter X21 (210D nylon) matches it in tear strength. Polyester really is less tear-resistant than nylon at comparable denier.
• The extra layer in X4 fabrics (VX21, VX42) adds significant tear resistance—45–70% more—for only 15–20% more weight compared to X3 versions with the same nylon face denier (X21, X42).
• RX30 actually scores higher on the Taber abrasion test (2,000 cycles) than X42 (1,700 cycles), despite the conventional wisdom that nylon should rain superior here. Possible reasons include the thicker 0.5 mil film (vs 0.25 mil in other variants) and the specific recycled polyester yarn used.

Able Carry Max EDC in X-Pac VX21 (image from Able Carry website)

Also, looking closely at the spec sheet helps us decode the naming convention: V means it has a backer (X4 construction), the X is for the X-PLY scrim, and 42 indicates a 420D face fabric.

All X-Pac variants share the same (possible) weakness, though: delamination. We'll get to that next.

Delamination

Remember that X-Pac is a laminate, with layers bonded together by adhesive. When that adhesive fails and layers separate, you get delamination. Visually, it shows as bubbling, wrinkling, or the face fabric lifting away from the underlying scrim and film.

There’s no guarantee you’ll experience delamination. The general consensus is that it’s rare but not uncommon, although I couldn’t find any concrete examples, so don’t let that turn you away from X-Pac right away.

A few practical tips to minimize chances of delamination: don't machine wash and dry your X-Pac bag, don't leave it in a hot car for days at a time, and don't store it wet for extended periods. Beyond that, just use it.

With that out of the way, let’s try to answer the question.

When to consider X-Pac

Consider X-Pac if you value structure and weatherproofing. X-Pac bags hold their shape, are highly water-resistant (the fabric itself is waterproof, but there are still seams and zippers), and look distinctively technical. Also, something that the spec sheet won't tell you is that X-Pac has a distinct, crinkly hand-feel that contrasts sharply with smooth-feeling, high-denier Cordura nylon.

Conclusion

Thank you for reading till the end. Leave comments below, and feel free to point out where I was wrong and share your experience with X-Pac bags. Next up: Ultra. Stay tuned.

Hello there, bag lovers. This post is going to be different from my usual reviews because I want to dive deeper into what our favorite bags are made of, starting with fabrics. Strap in, we are going down to the molecular level.

What is a polymer?

A polymer is a really long molecule, and a molecule is a collection of atoms. Most molecules are small. Water is three atoms, and sugar is a couple dozen. A polymer is hundreds or thousands of atoms strung together in a chain. That’s the basic concept.

There are multiple ways to link molecules into chains. The one that is interesting to us is a polyamide.

What is a polyamide?

A polyamide is a polymer in which molecules are connected by an amide bond. This bond is strong and stable. It does not come undone easily and also allows neighboring chains to grip tightly to each other. Basically, very sticky spaghetti.

Silk and wool are examples of natural polyamides. Nylon uses the same principle, but it is produced in a factory.

So what is nylon?

The raw material for nylon is oil. First, oil is broken down into small molecules that become the beads of chains. Then those beads are polymerized into chains. At that stage, nylon looks like small pellets.

Nylon Pellets (generated image for illustrative purposes)

Now, before we go further, you should know that there are two widely used types of nylon: Nylon 6 and Nylon 6,6. Both are used in bags but let's take Nylon 6,6 as an example. It absorbs less moisture, has better wear resistance, and has a higher melting point of 265°C. Why does that matter? Because to make fiber, nylon is melted.

After it’s melted, it gets pushed through a large metal plate with thousands of holes and cooled down to form fiber. It’s similar to how spiders make their web, actually.

Nylon Fibers (generated image for illustrative purposes)

The resulting fibers are twisted together to form a yarn, yarns are woven together into a fabric. That fabric is what we usually mean when we say “nylon”.

Nylon Yarn (generated image for illustrative purposes)

Weaves

There are many ways to put yarns together, those ways come in distinct patterns and are called weaves. Three weaves that matter in the context of bags: plain, basket and ripstop.

Plain weave

Plain Weave (generated image for illustrative purposes)

The simplest possible pattern. One yarn over the other, under the next, over the next and so on. A lot lightweight nylons are plain weave.

1000D Cordura Nylon in Plain Weave (image from Able Carry's website, AI upscaled)

Basket weave

Basket Weave (generated image for illustrative purposes)

A variation on plain weave where instead of going one-over-one, you go two-over-two, or three-over-three yarns at a time. Up close, this type of weave looks like a checkerboard.

The advantage of such weave is abrasion resistance. When something rubs against the fabric (image dragging your, eh, maybe someone else’s bag on the ground), it’s rubbing across multiple yarns at once, the loads spreads out, the fabric resists better. This is why ballistic nylon uses a basket weave. Originally developed during World War II as a flak jacket material (hence the name "ballistic"), it turned out to be too weak to stop bullets but excellent for military backpacks.

There is a tradeoff, though, the basket weave is less tear resistant, if a tear starts, it runs along multiple yarns grouped together instead of just one. To offset that weakness, ballistic nylon usually comes in higher denier, like 1050D or 1680D (we’ll talk about denier—the “D”—later, for now think: the higher the denier, the thicker the fabric).

1680D Cordura Ballistic Nylon (image from Able Carry's website, AI upscaled)

Ripstop weave

Ripstop Weave (generated image for illustrative purposes)

It’s a plain weave with reinforcement yarns woven in at regular intervals, usually every 5 to 8 millimeters. These reinforcement yarns are thicker than the base yarns, and they create the signature grid pattern.

The name tells you exactly what it does. When the fabric tears, the tear runs through the lightweight base yarns easily, but then it hits a reinforcement yarn and stops. The thicker yarn doesn't break, and the damage stays contained in a small grid square instead of running across the whole fabric.

Ripstop was developed for parachutes during World War II for exactly this reason. A small puncture in a parachute could otherwise tear into a catastrophic split mid-jump.

210D Cordura Nylon with Ripstop Weave (image from Able Carry's website, AI upscaled)

Denier

This is what “D” stands for in 500D or 210D, it’s a measure of mass per unit length of a yarn, or specifically grams per 9000 meters. Why 9000 meters? You are going to have to go to medieval France to find out. Anyway, so 500 grams per 9 kilometers is a 500D, 1000 grams per 9 kilometers is a 1000D.

Now, different materials have different densities. Nylon is 1.14 grams per cubic centimeter, polyester is 1.38 grams, Ultra — 0.97 grams. That means that the denier does not translate into thickness across fabrics but is a good mental model for denier of the same fabric.

Weight

Let's work out how heavy a square meter of fabric actually is. Take 500D nylon in a plain weave with 12 yarns per centimeter running one way and 12 yarns running the other. 500D means 9000 meters of yarn weigh 500 grams, so 1 meter of yarn weighs 0.056 grams.

In a 1 square meter of fabric, you've got 12 yarns per centimeter multiplied by a 100 centimeters, that’s 1200 yarns running in one direction, each 1 meter long. That's 1200 meters of yarn. Same in the other direction: another 1200 meters. Total: 2400 meters of yarn packed into one square meter of fabric. 2400 meters × 0.056 grams per meter works out to about 134 grams per square meter.

Real 500D Cordura nylon is about 220–250 grams per square meter, so where all the extra weight comes from?

First, yarns don't lie flat in a weave. They wave up and down as they cross over and under each other. So a 1 meter length of fabric actually contains about 10% more in a typical plain weave. That bumps our 134 grams up to around 147 grams.

Second, the back of the fabric is coated. Almost all bag nylons have a layer of polyurethane (PU) painted onto the back side. The PU coating seals the weave, makes the fabric water-resistant, and adds some structure. It also adds weight — typically 30 to 50 grams per square meter, depending on how thick the coating is. That's a huge chunk on top of a 134 gram base.

The face side of the fabric usually gets a treatment too, called DWR — Durable Water Repellent. It's a chemical finish that makes water bead up and roll off instead of soaking in. DWR is light, basically negligible weight-wise, but it's the reason a new bag sheds rain so well.

So the full picture of a typical premium 500D bag fabric: high-tenacity Nylon 6,6 yarn, plain weave at roughly 12-14 yarns per centimeter in each direction, PU coating on the back for water resistance and structure, DWR on the face for water beading. About 230 grams per square meter all in. That’s the real weight of the fabric used in bags.

Brands

You see “Cordura” a lot, including in the examples above, but what is it? Cordura isn’t a fabric. It’s a brand that makes fabrics, typically nylon-based, though there is some polyester as well. It was established in 1967 and has become the default source of fabrics for premium bag brands like Aer, Able Carry, and GORUCK. But Cordura is not the only one.

Creora Robic, or just Robic, is a line of nylon fabrics from the Korean manufacturer Hyosung, which has also been around since the 1960s. It’s less popular among boutique brands, but you can find it on Osprey or ULA packs.

UPDATE: Difference between Cordura and Robic, plus a note on bag brands’ proprietary fabrics.

Differences between Cordura and Robic:

• Robic uses Nylon 6. It is a little less wear-resistant and absorbs a bit more moisture than the Nylon 6,6 Cordura uses, but both are high-tenacity yarns and the practical difference for most bags is small.
• Robic is most commonly seen in lightweight and ultralight packs, while Cordura dominates the heavy-duty and tactical end. Both come in a range of deniers, but their market positioning differs.

Beyond off-the-shelf fabrics, some bag brands use their own proprietary nylon fabrics, most notably Evergoods and Alpaka:

• Evergoods uses a heavier 840D Nylon 6 and a lighter 420D Nylon 6,6.
• Alpaka has the Axoflux series: Axoflux 400D and 210D are nylon fabrics with a ripstop weave. Note that Axoflux 600D and 300D are polyester-based fabrics.

If you’ve made it this far, you deserve the Bag Nerd achievement. Let me know in the comments what you think about posts like this, where I was wrong (I almost certainly was wrong somewhere), and whether you’d like to see similar deep dives into X-Pac and Ultra.