Rim Width Calculator — Optimal Rim for Your Tyres

Rim width affects tyre shape, contact patch, rolling resistance and handling. A rim that is too narrow causes a bulging tyre profile; too wide and the tyre becomes stretched and prone to burping.

The Rule of 105, coined by Josh Poertner (formerly Zipp, now Silca) from 2001 wind-tunnel work, is an aerodynamics rule: the external (widest) rim width should be at least 105% of the measured tyre width so airflow leaving the tyre re-attaches to the rim and is smoothed, lowering drag. For a 25mm-wide tyre you want a rim at least 26.25mm wide externally. It is about aero efficiency, not fit safety — that is a separate ETRTO compatibility question (tyre 1.45–2.0× the internal rim width).

Wider rims provide better tyre support, a rounder tyre profile, and improved cornering grip. They also allow lower tyre pressures without sidewall collapse. The trend is towards wider rims across all disciplines.

There are safety limits. A tyre too narrow for the rim is poorly supported and prone to burping, and the exposed rim can be damaged on impacts. Per ETRTO and Schwalbe compatibility charts, tyre width should be at least 1.45× the internal rim width (the safe envelope is 1.45–2.0×). So a 25mm internal rim wants a tyre of at least ~36mm, and a 19mm internal rim at least ~28mm.

Hookless rims require specific tyre compatibility (check manufacturer lists). They typically require slightly wider tyres relative to rim width and have specific pressure limits.