Structure

Structure of Bike Bag Fabrics

Structure is our #5 ranked bike bag fabric property in terms of our estimate of its importance to the majority of bikepackers and cycle tourists. While structure is a fabric property with defining importance for bike bag designs, it is generally taken for granted in discussions comparing the various properties of bike bags. When a fabric is too flexible or too rigid or too stretchy or not stretchy enough, the problem is glaringly obvious. Within the acceptable range of functional amount of structure, the variability in options is still wide. Within this range of structure choices, it will often come down to more of aesthetic preference of the bag maker though critical aspects of function can sometimes come into play as well.

In this guide we are focused on the functional aspects of a fabric’s structure. Consider how a typical bike pannier sits against a bicycle when unloaded. Now imagine if t-shirt fabric were used in place of the bag’s fabric. If this were the case, it would hardly hold any shape at all and would appear to be several layers of fabric draped against the bike rack. It would have major functional problems from being difficult to load and unload to not keeping the gear from sagging and hanging down into the spokes of the rear wheel. Now contrast this with the structurally stiff fabrics that are used with panniers. The shape the pannier holds is largely due to the structure of its fabric, though bag design certainly plays an important role in this structure as well.

Structure is also one of the four dimensions of appearance, where we examine the aesthetic aspects of a fabric’s structure.

Structure’s Effect on Specific Bike Bag Functions

Bike bag designers typically limit themselves to two or three fabric types per bag. Considering the different sections of bike bags and the variety of functions that they must handle, it would be impractical to attempt to find the perfect structure for each section of the bag. For the sake of consistency, bike bag designers will choose fabrics with a view towards the bag’s overall performance across sections and functions. Or sometimes they will optimize around one particular section and function that they consider to be most important for the bag.

While Riding

Positives of Rigid Structure: Increases the bags’s ability to limit shifting and sagging while loaded and riding, thereby helping to reduce the potential for interference with the bike’s balance and handling. Higher fabric rigidity also helps to prevent bike bags from interfering with spinning tires and wheels.
Positives of Flexible Structure: There are no real positives for more flexible fabric while riding. Though the effects may be minimal, lower fabric rigidity is less than ideal because it increase the bag’s potential for shifting and sagging. When cycling over rougher terrain, the downsides of flexible fabric are more pronounced.

Loading and Unloading Gear

Positives of Rigid Structure: A bag that can maintain its shape while it is being loaded and unloaded, will be easier to organize and access the gear.
Positives of Flexible Structure: The bag adapts more readily to odd gear shapes, making it convenient to load and unload items of different sizes. When overloading a bag, flexible fabric structure makes things more manageable.

Handling Mounting and Closures

Positives of Rigid Structure: For bags with solid mounting hardware, rigid fabric can aid with making them easier to mount. Additionally, a more consistent bag shape, makes operating zipper closures smoother to handle.
Positives of Flexible Structure: For bags that strap directly to the bicycle or rack, more flexible fabrics allows for easier maneuvering of the straps and buckles especially when the fit is not ideal. For bag’s with roll-top closures, flexible structure allows for easier closing.

Off the Bike Handling (On-Ground, In-Tent, Traveling)

Positives of Rigid Structure: The bag has a more stable base that is better for propping the bag up on the ground or table. This rigidity also makes it easier to organize and access gear inside the bag when brought into a tent or other confined spaces.
Positives of Flexible Structure: Allows for a bag to fit into tight spaces and adjust to the available room, making it convenient for organizing gear in confined areas.

Flexible to Rigid Structure Spectrum

A fabric’s structure can be considered along a spectrum from flexible to rigid. On the flexible extreme, there is a limit to how flexible a fabric can be before it is too flexible and doesn’t hold the bags design as would be the case with the t-shirt fabric example above. On the rigid extreme, the limit is reached for bags where some flexibility is required for proper functioning such as roll-top closures, liners and internal pockets.

Between the extremes of as-flexible-as-possible to as-rigid-as-possible, bicycle bag designers are optimizing their choice in fabric structure typically based on a perceived preference for optimal usability. Does greater rigidity or flexibility help with how the bag opens, how it sits on the bike or bike rack, how easy it is to stuff gear inside?

To measure and compare how fabrics react to bending forces, a measurable aspect of their structure, bike bag designers can utilize the following testing standard.
ASTM D4032 – Circular Bend Procedure: This test determines the stiffness of fabrics by measuring the resistance of a fabric sample to bending following a circular bend procedure. It’s useful for assessing how easily a fabric can be manipulated and its propensity to resist permanent deformation.
ASTM D1388: While not exclusively a test for bending, this test can give insights into a fabric’s bending behavior by measuring its stiffness. The stiffer the fabric, the higher the resistance to bending and creasing it may exhibit.

Flexible

Fabrics with a flexible structure are highly pliable and adaptable, increasing their ability to conform around loads and bend and curve as needed for complex bag designs. Flexible fabrics are often lightweight and can be easily compressed, bent and rolled without losing their integrity. In bike bags, flexible fabrics are most suitable for pockets both internal and external, expandable compartments and liners.

Flexible Fabric Examples
  • Ripstop Nylon: This fabric is lightweight, durable, and has a grid pattern that prevents tears, making it ideal for pockets and expandable compartments.
  • Nylon Taffeta: A smooth, lightweight fabric often used for linings and pockets due to its flexibility and ease of packing.
  • Silnylon (Silicone-coated Nylon): Both waterproof and highly flexible, this fabric is perfect for liners and areas that need to keep contents dry while being pliable.
  • Polyester Taffeta: Similar to nylon taffeta but made from polyester, this fabric is lightweight and flexible, suitable for internal and external pockets as well as liners.
  • Spandex/Elastane Blends: These fabrics, often blended with nylon or polyester, provide excellent flexibility and are suitable for expandable compartments and pockets that need to accommodate varying loads.
  • Lightweight Cordura: A durable yet pliable version of Cordura, which can be used for pockets and expandable compartments due to its balance of strength and flexibility.

Semi-Flexible

Semi-flexible fabrics offer a balance between flexibility and structure. They provide a little more support than highly flexible fabrics while still offering a decent degree of adaptability. A bicycle bag’s main body, reinforcement panels and roll-top closures are all good candidates for the utilization of semi-flexible fabrics.

Semi-Flexible Fabric Examples
  • Polyester: Offering a balance between flexibility and structure, polyester retains more shape than nylon while still providing some flexibility.
  • EcoPak: Made from recycled materials, EcoPak provides a nice balance of structure and flexibility.

Semi-Rigid

Semi-rigid fabrics provides a strong level of support and durability while retaining enough flexibility to be practical for a variety of bag functions. They are ideal for parts of the bag that need to hold shape under load but still require some adaptability for usability. A bicycle bag’s main body, reinforcement panels, back panels and internal frames are all good candidates for the utilization of semi-rigid fabrics.

Semi-Rigid Fabric Examples
  • X-Pac: The combination of layers and membrane leads to a balance of high structure and a minimal amount of flexibility. It is perfect for areas of a bicycle bag that require moderate rigidity.
  • Cordura: Offers a semi-rigid structure that provides good support and shape retention while allowing some flexibility.

Rigid

Rigid fabrics offer maximum support and maintain shape under heavy loads, making them ideal for areas of the bag where strong structural integrity is very helpful to the bag’s function. Rigidity can also play into a fabric’s durability contributing to the reinforcement of areas of the bike bag that experience high stress. A bicycle bag’s base, frame and high-impact zones are all good candidates for the utilization of semi-rigid fabrics.

Rigid Fabric Examples
  • Ballistic Nylon: Originally developed for military use, ballistic nylon is incredibly strong and durable while offering a noticeably more rigid structure than most fabrics.
  • UHMWPE: This relatively new type of fabric offers exceptionable strength to weight all within a very rigid structure, making it ideal for section of a bike bag requiring maximum support and protection. The noticeably high rigidity makes the fabric unappealing to the touch and not an ideal choice for bag sections where some flexibility is desirable.

Stretchable to Non-Stretchable Structure Spectrum

A fabric’s structure can also be considered along a spectrum from stretchable to non-stretchable. This spectrum generally correlates to the flexible to rigid spectrum, with flexible fabrics being more stretchable and rigid fabrics being more rigid.

The majority of fabrics used in bike bags either have minimal stretch or are non-stretchable. Nylon and polyester fabrics are minimal stretch fabrics. While still offering plenty of structure, their minimal amount of stretchability can allow for the bags to better conform around loads. UHMWPE, XPac and EcoPak are non-stretchable with this characteristic contributing to their structural rigidity.

Highly stretchable fabrics are generally only utilized for specific applications in bike bags, including expandable pockets, flexible liners, expansion panels and panels along zipper openings. The stretchable fabrics frequently used in bike bags are fabrics blends. Common blend include nylon and spandex, polyester and spandex and nylon, polyester and spandex.

To measure and compare the stretch and recovery of different fabrics, bike bag designers can utilize the following testing standard.
ASTM D3107: This test determines the stretch and recovery properties of fabrics. It measures how much a fabric can stretch under a specified tension and how well it returns to its original dimensions after the tension is released.

Manufacturing Process Effects on Fabric’s Structure

A fabric’s structure is a property that emerges from the combining contributions of its yarn, weaving processes and post-weaving processes. Here we examine how specifics of the fiber and manufacturing processes can alter a fabric’s structural levels.

Yarn

  • Fiber Composition: The basic structural properties of a fabric are determined by the type of fiber its made from. The synthetic fibers used in nylon, polyester and UHMWPE are engineered to be optimized for structural and other properties.
  • Yarn Twist: The fabrics in bike bags use multi-filament yarns. These yarns are manufactured by twisting multiple filaments together. The amount of twist in the yarn affects the fabric’s structural properties. High twist produces stronger, more durable yarns that leads to a more rigid fabric. Lower amounts of twist result in softer, more flexible yarns and fabric.
  • Yarn Texturing: Yarns can undergo a variety of texturing process to have an impact on the fabric’s structure.
  • Muti-Ply Yarn: Multiple yarns can also be twisted together to change the yarn and fabric’s structural characteristics.
  • Denier: A measurement of the thickness of the yarn and measured in weight (the mass in grams of 9,000 meters of yarn), denier results from the above described processes. A higher denier indicates thicker, stronger yarn, contributing to a more rigid and durable fabric. A lower denier indicates thinner yarn, resulting in a lighter, more flexible fabric. Higher denier fabrics tend to be heavier, offering more structure and durability, while lower denier fabrics are lighter and more flexible.

Weaving Processes

  • Weaving Techniques: The tightness and pattern of a fabric’s weave can significantly impact its flexibility or rigidity. Tighter weaves generally produce more rigid fabrics, while looser weaves can create more flexible materials. Patterns generally increase structure and rigidity. Orienting
  • Warp and Weft Orientation: Related to weaving techniques, a fabric will have different structural properties depending on how it is oriented. The fabrics weaving techniques can be adjusted to change the structural outcome of its fabrics in a specific orientation whether toward the warp or weft side or diagonally orientation. The direction of the fibers can affect the fabric’s strength and flexibility. The diagonal direction between the warp and weft, often offers the greatest flexibility and stretch.
  • Blended Yarns: Mixing different types of yarn made from different types of fiber is a weaving technique utilized to create a blend of properties. Amongst the fabric utilized in bike bags, yarn blending is not a common practice. It is most common with UHMWPE fabrics, given their specialized characteristics. Blending flexible fibers with more rigid ones, can create a fabric with tailored structural properties that balance flexibility and rigidity.

Post-Weaving Processes

  • Coating and Lamination: Applying coatings (such as polyurethane or silicone) or laminating multiple layers can increase a fabric’s rigidity by adding structural layers.
  • Heat Treatment: Processes such as heat setting can alter the molecular structure of the fibers, increasing rigidity and stability of the fabric.
  • Embossing and Calendaring: These processes involve passing the fabric through heated rollers to apply pressure and create a more rigid structure or adding texture that can increase the fabric’s flexibility.