Rims 101

In the section, we hope to offer insights from our three decades-long adventure in rim design. While our opinions are not unique, they are uncommon. To begin, the function of rims firstly to be a stable harness for spokes and a connection to the tire.  Rims can also serve as an airfoil to improve aerodynamics. Now with the popularity of tubeless rims, rims must also act as a primary component in air retention.

From here forward we can quickly get into areas of highly varied theories. From the issue of the aerodynamics of 30mm vs 60mm or 80mm deep rims, or narrow rims or wide rims. Coincidental spoking, asymmetric spoke count, special spoke patterns, internal versus external nipples, so many choices, none of which are wrong in and of themselves. But performance does come from a well-selected mix of these features and how well they are assembled. We will leave this choice to you, the customers, to define and drive as part of your ideas and preferences for the optimal these blends of features.

Hook height

Hook heights (Marked in RED), according to international standards, start as small as 5mm and are as large as 8.5mm. Generally, the international standard outlines taller hooks as rims become wider.  The key to defining the hook height on a rim is driven by width and tire well depth and shape. The taller the hook, the deeper the well must be in order to fit tires. The term we use for this measure is Tire Mounting Dimension. This is a measure from the top of one hook to the nearest part of the tire well. This measure controls hook height and well depth.

Tire well depth and shape

The tire well or drop center (Marked in RED) is the feature that allows fitting of a tire onto the rim. The deeper the well the easier it is to mount the tire. As we make tubeless rims we start to have more strict limitations on the form of this tire well. The well needs to crowd the beads toward a narrow area to allow for hand pump inflation by offering a passive seal. For this reason, the well should have as small a flat bottom as practical.

Bead well geometry

Bead well (Marked in RED) is the most important part of the rim with regards to tire fit. This should be at the target diameter of the rim. So this should be Ø559 for MTB, Ø584 for 27.5 and so on. The shape can vary depending on clients needs, important features are width >3.2mm, hump size (0.5mm), and outer radii no larger than 1.7mm.

Hook or hookless

Crochet hooks are a relatively new development in rims. Originally developed for early foldable or high-pressure clinchers in the late 1970’s, these were intended to act as additional security in holding the bead in place. Hookless rims are neither a new idea nor absent from high-pressure tires. Both hookless and crochet hook rims are included in international standards for bicycle rims. We can build rims with or without hooks as per your needs. While we are not confident hooks perform any meaningful function, on rims using a proper bead shelf. Before the bead shelf design the hook served to capture the bead while inflation to be sure it did not escape the bead wall area of the rim. we build rims for clients and follow clients requests. For more information on hook function etc, click here.

Rim diameters

We are confident that the key diameter that needs explicit definition and therefore must be better controlled is bead well, also called bead seat diameter. Insufficient coordination between rim and tire makers combined with no explicit tire bead diameter specification makes for less confidence among rim makers in the determination of proper diameters.  Ultimately outside diameters (in form of hook height) can vary considerably and maintain good tire fit provided that the tire well is deep enough to allow an easy fit. Often it’s been claimed that rim outside diameters are not consistent, but this is simply not true. Rim designs vary considerably, and with these designs, differences come varied diameters, even from a single brand. But in a single model these differences are quite small, usually within 0.5mm effective diameter or 1.57mm in circumference, well within the international standards.

Spoke bed

We refer to the area where the spoke nipple passes through the rim as the spoke bed (shown in RED). Over the last decades, we have found a few nice methods of lowering stress concentrations in the spoke bed while maintaining a light weight profile. Even with these improvements, we would normally prefer not to make spoke beds <1.85mm thick, and even at this thickness we must always consider if we have single pitch spoke placement in the rim or grouped spokes, paired spokes etc. These things all have a large effect on localized stress at spoke holes.

Last but not least the geometry of the spoke bed had a significant impact on wheel stiffness. When a nipple can rock back and forth due to two point contact, the support offered by the spoke is limited. Each spoke nipple can act like a hinge. This is because a simple concave form to the spoke bed when rolled into a hoop form to make a rim becomes a messy “hyperbolic paraboloid”. This in a form most commonly seen as a pringle’s potato crisp. This form can not offer a firm foundation for a tapered cylinder that is a spoke nipple head, even the Polyax form does little to fix a hyperbolic paraboloid spoke bed. As can be seen in image below, left and right of nipple is in contact while fore and aft are gaps, this is the two point contact we speak of.

Minimum profile thicknesses

Most extruders have a lower limit of 0.8mm for extrusion thickness.  Felloe’s founders have worked to lower this limit through refined tooling and tuning chemistry of the alloys. We often extrude thinner than 0.8mm and commonly at 0.7mm, in special circumstances, we process aluminum as thin as 0.45mm. The key driver to such thin profiles is the total distribution of mass so these thin sections require balancing all of the profile thicknesses.

General form of profile.

Rim profiles must strike a balance between function and processability. The perfect rim might not be an extrudable profile, while the best profile to extrude is hardly well suited to high-performance rims. The image on here shows a CAD thickness analysis, this can also serve as an effective extrudability analysis.  During the design phase of any project we will work closely with you to find the best performance and maintain stable production.

White paper rim extrusion tolerances