Our handle-anatomy piece covered what's inside a handle — core, grip layer, attachment point, webbing. This piece isolates one specific dimension of that grip layer: diameter, and why it's chosen deliberately rather than defaulted to a single "standard" size.
The same physics as an anchor disc, applied differently. Our anchor-point piece explained why a door anchor uses a molded disc: spreading tension across a wider surface area lowers the force per unit of area at any given point, even though total tension hasn't changed. The identical principle governs handle diameter. For a given amount of grip force applied by a hand, a wider-diameter handle spreads that force across more contact surface area than a narrower one, lowering localized pressure at any single point of contact.
Designing around a range, not a single measurement. Hands vary considerably in size across a general population, which means a handle designed around one narrow measurement would fit some grips well and others poorly. Manufacturers typically target a diameter that functions reasonably across a broad range of hand sizes — a middle-ground dimension rather than a size optimized narrowly for one hand profile.
Foam density as a second variable. Diameter isn't the only lever available. Foam density — how compressible the grip material is — interacts with diameter to shape the overall contact-pressure profile. A denser foam under a slightly smaller-diameter core can produce a firmness and contact feel comparable to a softer foam under a larger-diameter core; the two variables are adjusted together rather than independently.
Why this is a manufacturing decision, not a default. None of this happens by accident. Selecting a target diameter range and a matched foam density requires a manufacturer to make an explicit design decision balancing broad-hand-size compatibility, material cost, and molding feasibility — the same category of decision covered elsewhere in this series for tube wall thickness, webbing denier, and anchor disc material.
The takeaway. Handle diameter is a deliberately engineered dimension, chosen using the same force-distribution logic covered in our anchor-point piece, just applied to a different point of contact. Recognizing that is useful context for understanding why handle designs vary — and why "diameter" is a real spec worth paying attention to, not an incidental design detail.