Water Resistance

Water Resistance of Bike Bag Fabrics

Water Resistance is our #2 ranked bike bag fabric property in terms of our estimate of its importance to the majority of bikepackers and cycle tourists. While water resistance is not important in all use-cases of bike bags, it often is a critical consideration.

When reviewing a bike bag’s overall water resistance, it is important to note that its fabric’s water resistance is only one of the components at work. The bag’s construction method and closure system work together with the fabric to keep water out. How well these three components work in combination generate the bag’s overall water resistance efficacy.

In this guide we are focusing in on the water resistance of the fabric used in bike bags. A fabric’s ability to resist water penetration or absorption defines its water resistance. To help categorize and compare fabrics water resistance, we look at how fabric is tested against water penetration and absorption. We also consider the breathability of fabrics, important when there is wet gear transported inside of the bag.

Given that we are considering the water resistance of a bike bag’s fabrics. It is important to note that some bicycle bags are designed to rely on an external waterproof cover for protection during rainy weather. While these bags have some water resistance on their own, obviously the water resistance with the cover installed is what matters.

***Note water resistance’s connection to long-term moisture and microbial exposure.

Water Resistance Factors

The level of water resistance in fabrics comes together as a result of a variety of factors of the fabrics. A very important distinction in comparing the water resistance of different fabric types is between laminated and non-laminated fabrics.

Non-Laminated Fabrics + Base Layers of Laminated Fabrics

Non-laminated fabrics rely on the water resistant factors of the fabric itself. With laminated layers, while the waterproof membrane provides the primary water resistance, their base fabric layer provides a backup layer of protection from water penetration.

Fiber Type:
- Nylon: Inherently hydrophobic, which helps in repelling water. However, nylon fibers can absorb a small amount of water (around 4-5% of its weight), meaning it is less effective at maintaining water resistance compared to polyester and UHMWPE.
- Polyester: More hydrophobic than nylon fibers, repelling water more effectively. Polyester fibers absorbs significantly less water than nylon (around 0.4% of its weight), making it highly effective at maintaining water resistance and better suited for applications requiring consistent water repellency.
- UHMWPE: Extremely hydrophobic, offering the highest inherent water resistance among these three fiber types. They absorb virtually no water, making fabrics made from UHMWPE fibers exceptionally effective at repelling moisture and providing the highest level of water resistance.
Fabric Coatings: Enhance water resistance by creating hydrophobic barriers that prevent water from penetrating the material. Fabric coatings can vary in their ability to maintain breathability and flexibility while providing robust water repellency.
Durable Water Repellent (DWR):
Polyurethane (PU):
Silicone:
Acrylic:
Polyvinyl Chloride (PVC):
Fabric Construction: Fabrics with tight weave patterns such as, plain, twill and ripstop, significantly enhance a fabric’s water resistance with their densely packed yarn. Moderately tight weave patterns such as, basket, satin and oxford, offer less but still some water resistance. For any type of weave pattern, higher thread counts and thicker fibers increase the fabric’s water resistance by yielding increased density and reduced gap size.
Fiber or Yarn Chemical Treatments: A variety of chemical treatments are sometimes applied to the fiber or yarns of fabrics to increase their hydrophobic properties

***Notes on Condition, Breakdown of Fabrics and Coatings.

Waterproof Membranes (of Laminated Fabrics)

Laminated fabrics rely on their waterproof membrane layer for their primary water resistance.

Membrane Type:
Membrane Thickness:
Pore Size and Structure:
Membrane Coatings:

***Notes on Condition, Breakdown of Membrane Layer and Delamination

Water Penetration, Absorption & Breathability Testing

Bike bags are exposed to water through rain, tire spray and even submersion during river and creek crossings. How a fabric repels and absorbs water as well as breathes can be measured through a variety of tests. While the test data isn’t always available for every fabric used, it is sometimes available. Often times the available test data of a closely related version of a fabric can be used anecdotally to gain a better understanding of the expected performance of a specific fabric in a given bike bag.

While we are considering both water penetration and water absorption it should be noted that resistance to one is closely correlated to resistance to the other. Fabrics that resist water absorption typically have hydrophobic properties, meaning they repel water and prevent it from being absorbed into the fibers. Consequently, these fabrics are also less likely to allow water penetration through the material, as the water beads up and rolls off the surface rather than soaking in. In contrast, fabrics with high water absorption tend to allow water to penetrate more easily, leading to wetting out and potential leakage. Therefore, a fabric’s resistance to water absorption directly influences its resistance to water penetration.

Water Penetration Testing: Hydrostatic head tests are commonly used for testing water penetration in outdoor fabrics. These tests measure the fabric’s resistance to water penetration under pressure, simulating real-world conditions such as heavy rain or wet environments.
ASTM D751: This test also called the Standard Test Methods for Coated Fabrics, evaluates the resistance of coated fabrics to water penetration under pressure. This test method involves applying water to the fabric specimen while gradually increasing the pressure until water penetrates, providing a standardized assessment of the fabric’s ability to withstand water ingress.
AATCC Test Method 127: This test assesses the water resistance of fabrics by measuring the pressure required for water to penetrate through the material. During the test, water is gradually applied to the fabric specimen, and the pressure at which water starts to penetrate is recorded, providing a quantifiable measure of the fabric’s resistance to water penetration.
Water Absorption Testing: Evaluating how much moisture fabric can absorb when exposed to water, provides insight into its performance and suitability for outdoor activities. For bike bags, the two main concerns of water absorption are carrying extra weight while wet and contributing to increasing the potential for water penetration.
AATCC Test Method 79: This test evaluates the water resistance of fabrics by subjecting them to a standardized spray impact test. During this test, the fabric is exposed to a controlled spray of water, and its resistance to water penetration is assessed based on visual observations and standardized criteria.
AATCC Test Method 42: This test measures the resistance of fabrics to the penetration of water by impact. Fabric specimens are exposed to a standardized spray of water, and the extent of water penetration is evaluated visually or quantitatively, providing a measure of the fabric’s water resistance under conditions simulating rain or splashing.
Breathability Testing: While breathability is not nearly as important of a characteristic for bike bags as it is for clothing, shelters and sleeping bags, breathable fabrics can help prevent moisture buildup inside the bags, reducing the risk of mildew or odor and keeping the contents dry and well-ventilated during extended rides. This can be an especially important consideration on long tours and in hot, wet climates where it is difficult to keep everything dry.
ASTM E96: This test measures the water vapor transmission rate (WVTR) of materials, including fabrics. It evaluates the ability of a material to allow water vapor to pass through it under controlled conditions, providing a quantifiable measure of its breathability.
ASTM D737: This test determines the air permeability of textile fabrics. It measures the rate of airflow passing per unit area of fabric under a specified pressure difference, providing a standardized measure of the fabric’s breathability.

Maintaining Water Resistance