Whilst the use of the word “hookless” in relation to wheel rim design isn’t new anymore in cycling, there is still a high degree of confusion around what exactly it means and, more importantly, the implications for a rider when it comes to choosing the right wheel design and right tyre to run. So, we wanted to set out a few details to help our riders make the right choice, as well as explaining our perspective at Parcours.
What is hookless?
A wheel rim would traditionally have had a “hooked” design at the outer edge, adding an additional layer of security to hold the tyre bead in place. This allowed tyre manufacturers to use a folding bead design, saving weight versus a rigid (or wired) bead and also making it slightly easier to fit.
However, with tighter tolerances on both tyre and rim, it is possible to remove the bead hook, leading to a “hookless” rim design. Here, the tyre is held onto the rim by the tyre bead pressure against the straight-edged sidewall of the rim.
Why is hookless used for rims?
There are a number of reasons quoted across the industry as to why hookless rims offer benefits versus hooked rim designs:
Reduced manufacturing cost: by removing the hook, you simplify the internal shape of the rim, meaning less complex tooling can be used in the manufacturing process. You also reduce the risk of defects and therefore QC issues and rejection rates.
Parcours perspective: whilst costs are reduced, it’s debatable as to whether this is always passed on to the customer. Our focus remains on delivering the best possible product for our riders so we would need to see additional benefits before offering a hookless rim design
Improved rim strength: a hooked design introduces a stress concentration at the rim edge reducing the failure resistance under impact. A hookless rim design removes this stress concentration making for a more robust rim design and improved impact resistance.
Parcours perspective: impact resistance is particularly important for off-road riding where lower tyre pressures and rougher surfaces make it more likely that you will “bottom out” and impact the rim. This makes a hookless rim very well-suited for off-road or gravel use
Improved aerodynamic performance at the tyre/rim interface: a rim hook will push the tyre sidewall inwards, creating a slightly more “lightbulb” profile for the tyre. Whereas removing the hook allows the tyre sidewall to sit directly against the rim edge.
Parcours perspective: whilst it is likely that some aero benefit can be derived from a hookless rim design, our own testing (see below) shows that when comparing two rim designs that are identical in all respects other than one being hooked & the other hookless, there is no material difference in aero performance (<0.1W @ 48kph)
Reduced rim weight: removal of the rim hooks will reduce the overall rim weight as less material is required
Parcours perspective: total weight saving is likely to be <10g/rim which in our opinion is not a meaningful amount when taken in the context of the overall wheel system
What about the downsides of a hookless rim?
There are restrictions on the type of tyre and tyre pressure that can be used on a hookless rim: if using a hookless rim, you must use a tyre that is hookless-compatible. Whilst it is not strictly required to be run as a tubeless setup, it is strongly recommended and in any case all hookless-compatible tyres will also be designed for tubeless running.
Furthermore, ETRTO guidelines set a maximum pressure of 5 bar (72.5psi) for any tyre fitted to a hookless rim, regardless of whether tubed or tubeless. This is to reduce the risk of a blow-off where the tyre bead is no longer able to hold the tyre in place against the edge of the rim wall.
Parcours perspective: a hookless rim therefore restricts a rider in choosing their preferred tyre. In addition, for a road setup with narrower tyres, the 72.5psi pressure limit may not be suitable especially for heavier riders. In our opinion, for road use and with narrower (<32mm) tyre widths, a hookless rim may not be suitable for all riders. However, for gravel use, where wider (>35mm) tyre width are widely used and where the majority of riders are already running a tubeless setup, there is less of a downside
A hookless rim will have wider internal rim diameter than an equivalent hooked rim: this has an impact on the tyre width that can be safely fitted to a rim. ETRTO guidelines specify the maximum internal rim width that is considered safe for a particular tyre width. As an example, recent updates to the guidelines state that a 28mm tyre should not be run on anything wider than a 23mm internal rim width.
Our #thinkwider rim profiles have a 22.5mm (hooked) internal rim width, making them compatible with a 28mm tyre. However, if the hooks were removed for a hookless design, the internal rim width would grow to ~24mm, meaning it would no longer be compatible with a 28mm tyre. Given the rim profile is aerodynamically optimised for a 28mm tyre, there would actually be an aero penalty to running the rim with the wider tyre required.
Parcours perspective: when testing hookless rim designs with our #thinkwider profile, we would have to recalibrate the inner vs. outer rim widths to optimise for a specific tyre size. Alternatively, the rim wall thickness would need to be increased to maintain compatibility with a specific tyre size, thereby increasing the rim weight
On balance, at Parcours we believe that hookless rim technology is currently best suited for gravel riding. Our Alta & Alta 650B gravel wheels have a hookless rim design, which allows us to use our IMPACT+ rim technology to boost impact resistance and improve overall strength. With wide tubeless tyres considered standard for gravel riding, a hookless rim doesn’t remove any real level of choice for our riders when looking at which tyre they would like to run.
For road riding however, we haven’t yet seen enough benefit to make the move away from a hooked design. Whilst there may be a performance benefit in future rim designs, at present the test data shows that it is just not there. The other design implications (e.g. internal vs. external width) will also need to be accounted for in any future design.
Meanwhile, a not-insignificant number of our riders continue to prefer either a tubed setup and/or tyre pressures of over 72.5psi so our hooked rim design will allow for this.
We have recently seen a number of instances of either confusion over ETRTO guidelines or issues with hookless rim setup that suggest the average rider is not yet ready to consider the transition to a hookless rim. Until the industry (ourselves included) is able to both deliver a significant performance benefit & also educate our riders to a sufficient degree, we would continue to offer our road wheels with a hooked rim design.