When you're making or repairing outdoor gear, the fabric you choose often outlasts the stitching and the zip. Nylon and polyester are the two workhorses of the outdoor world. Both are synthetics, both are strong for their weight, and both can be coated or treated to handle rain and abrasion. But they are not interchangeable. I've learned this the hard way after watching a silnylon tarp sag into a puddle overnight while a polyester bike frame bag shrugged off three summers of UV without fading. The differences come down to four practical things: stretch, UV resistance, drying time, and cost. Understand those, and you'll pick the right fabric for every project.
Nylon is the more elastic of the two. It stretches under load and recovers well, making it ideal for hammocks and straps. It also has higher tensile strength and better abrasion resistance than polyester of the same weight. That makes nylon a go-to for backpack bodies, tent floors, and knee patches on cycling trousers. However, nylon's stretch becomes a liability when wet: it absorbs water, swells, and can sag, so a tarp pitched tight in the evening may be flapping by morning.
Polyester is stiffer and more dimensionally stable. It has lower elongation, so it holds its shape under load and resists stretching out over time. That stability suits bike frame bags and panniers. Its abrasion resistance is slightly behind nylon when wet. For heavy-duty wear, nylon still wins, but for UV-heavy and shape-critical jobs, polyester is the smarter choice.
Sunlight is the silent killer of outdoor fabrics. Ultraviolet radiation breaks down polymer chains, causing fading and weakness. Polyester resists UV far better than nylon. It can handle months of direct sun before serious degradation, so it's common in sunny climates. Nylon degrades faster under UV unless treated with UV inhibitors. Even then, it will yellow and weaken before polyester does.
For bikepacking and camping in exposed places, such as a summer tour, polyester is the safer bet for anything that lives outside your pack. Nylon is fine for gear that spends most of its life stuffed away, like a sleeping bag liner. But if you're making a tarp, a tent fly, or a saddlebag that will be strapped to your bike day after day, polyester's UV stability will reward you with a longer service life.
Water absorption is where the two fabrics diverge most dramatically. Nylon is hygroscopic: it absorbs moisture into its fibres, up to 4% of its weight. That absorbed water makes nylon swell, stretch, and dry slowly. Polyester is hydrophobic, absorbing less than 1% of its weight. It sheds water quickly, dries fast, and stays lightweight when wet.
A silnylon tarp will sag when wet because the nylon stretches as it absorbs water. A silpoly tarp (silicone-coated polyester) stays taut because the polyester doesn't stretch. For dry bags, polyester dries faster and resists musty smells. Nylon can be treated with DWR (durable water repellent) finishes, but the underlying fibre still absorbs water once the coating wears off.
Polyester is generally cheaper and more widely available. It comes in many weights. Recycled polyester is common and often costs little more than virgin. Nylon, especially high-tenacity, is pricier. For small projects it's affordable, but for large tarps or full bikepacking sets, the cost adds up.
Both fabrics sew similarly, but there are quirks. Nylon can be slippery and fray-prone, so use sharp needles (a 90/14 or 100/16 for heavier weights), polyester thread, and consider sealing cut edges with a hot knife or fabric glue. Polyester is slightly easier to handle because it's stiffer, but it can dull needles faster if it's heavily coated.
There's no single winner. The right choice depends on what you're making and where it will live. Here's a practical starting point:
My own rule of thumb: if the gear will be pitched, strapped, or parked in the sun, choose polyester. If it will be dragged over rock, stuffed into a pack, or needs to flex and recover, choose nylon. For repairs, match the original fabric. With a little thought, you'll get years of reliable service from either fibre.
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