Hookless rims have a number of claimed advantages that we’ll get into in more detail below.
A hookless rim is simply one with a profile that lacks bead hooks, the protruding edges that help retain conventional clincher tyres under pressure.
Instead, a hookless rim has straight, vertical sides, and indeed in wheel maker circles the technology is often referred to as “tubeless straight side” or TSS (not to be confused with “Training Stress Score”, a performance metric developed by TrainingPeaks).
Hookless rims have been around for mountain bikes for years, but in the road world, where tyre pressures are typically much higher, they’re relatively new.
Hookless rims are usually tubeless-only, i.e. you must run a tubeless-specific tyre, which will have a stiffer bead than a conventional tubed clincher.
It is possible to run a tube (e.g. in the event of a puncture that sealant won’t fix) but only in that same tyre – it’s not usually considered safe to swap to a standard clincher tyre.
On the face of it, you might assume a hookless rim is less safe than a hooked one, but the reality appears to be much more nuanced.
The safety of a rim–tyre interface is heavily dependent on the tolerances of the two components and proponents of hookless claim it’s perfectly safe (or indeed safer than hooked) and offers real performance benefits.
Detractors suggest the move to hookless is driven primarily by manufacturing concerns and not for the benefit of riders.