How to Design Bicycle Rims in Line with ISO 5775-2:2021

Introduction

Learn the key ISO 5775-2:2021 requirements for bicycle rim design, including rim diameter, flange height, bead seat width, and well dimensions.

Details

ISO 5775-2 specifies the tyre-fitting dimensions and designations for bicycle rims. Together with ISO 5775-1, which covers tyre designations and dimensions, it provides an important reference for ensuring proper tyre and rim compatibility.

For rim manufacturers and wheel brands, understanding these requirements is an important part of developing new products.

ISO 5775-2:2021, the fourth edition published in March 2021, classifies bicycle rims into five types: Straight-Side (SS), Tubeless Straight-Side (TSS), Hooked Bead (HB), Crotchet (C), and Tubeless Crotchet (TC).

Many modern carbon rims use TSS or TC profiles. In this article, we'll focus on these two types and highlight some of the key dimensions and design considerations, combining the standard's requirements with our carbon rim engineering experience.

Key Dimensions: BSD, Flange Height, and Hook Width

Bead Seat Diameter (BSD) refers to the diameter of the rim at the point where the tire bead sits, rather than the outer edges of the rim. It defines the wheel's nominal diameter and is essential for proper tire fit.

Please note that the same nominal wheel diameter can correspond to different BSD values. To avoid confusion, it's best to specify the BSD alongside the wheel diameter or size when defining a product.

ISO 5775 includes a table listing the nominal diameter, specified diameter, and measured diameter (with tolerance). Here are some common sizes and their corresponding BSD values for reference.

Diameter	Nominal Diameter	Measured Diameter (±0.5)
26”	559	558.45
27.5”/650B	584	583.55
29”/700C	622	621.95

Flange Height (G) refers to the height of the rim flanges. Depending on whether the flange has an inward-facing protrusion, rims can be classified as hooked (TC) or hookless (TSS). Flange height is important for proper tire retention.

Rim Type	G Height (±0.5)
TC	5.5
TSS	6

For Tubeless Crotchet (TC) rims, ISO requires the inward protrusion width to be at least 0.5 mm.

Design Considerations: Well Depth and Bead Seat

The well and bead seat design affect tyre installation and inflation. ISO 5775-2 specifies several dimensions for these areas, while leaving room for manufacturers to make design decisions within the defined profile requirements.

The well is the recessed area in the rim bed where the tyre beads are positioned during installation. The well depth (H) affects how much room is available to move the tyre beads toward the centre of the rim, making tyre mounting and removal easier.

Rim Type	H (min)
TC	2.6
TSS	2.9

Bead seat width is suggested to be no less than 3.5mm. Too narrow design can lead to improper tire seat, while too wide can have unwanted tire movement.

Rim Development at NTP: Two Ways to Start

With experience in building a direct-to-consumer wheel brand, we understand that wheel brands have different team structures and resources. Some teams have riders and engineers, while others focus more on customer service, branding, and sales. What they have in common is a passion for cycling and a commitment to bringing quality products to riders.

At NTP, we offer customized rim development solutions to suit different needs.

If you already have detailed rim drawings, our in-house engineering team can review them and suggest adjustments based on our manufacturing experience and production capabilities.

If you only have a rough idea, we can also work with you to develop a rim design from scratch, from initial specifications to sample production and testing.

If you'd like to learn more about ISO 5775-2 requirements or discuss a new rim project, feel free to leave a comment below or email us at koko@ntpcomposites.com. We'll get back to you within one business day.

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