Challenge's response is even more detailed (and fairly savage, if true):
Consumer Product Safety Bulletin (technical clarification, following ENVE’s Consumer Safety Bulletin of March 6th, 2019) This bulletin is to reconfirm that Challenge Tires should not be used with any rims or wheels that do not comply with either the current 2019 ETRTO, past ETRTO or soon to be released ISO 5775 global standards for tire and wheel fit.
Challenge specifically clarifies herewith, that both old and new ETRTO and future ISO standards require a minimum 0.7mm rounded radius on all hooked rims.
It has come to Challenge’s attention that ENVE SES model carbon wheels do not comply with the above standards on the design of the rounded hooked rim radius. In fact, ENVE’s SES have two sharp (<0.2mm) radii in their hooks, that leave two parallel cuts about 0.5mm apart, around the entire radius of our (and other company’s) tires, at around 3-4mm above the bead.
Challenge has noted failures of its clincher tires, communicated by consumers and athletes, caused by the sharp edges on the ENVE SES hooks. This international design standard violation has been communicated back to the affected consumers, shops, endorsed teams, ENVE and their parent company, as Challenge has been made aware of problems. ENVE, however, continues to produce and promote usage of the said SES models, as part of their line.
Challenge has consequently been forced to list the specific ENVE SES rims as not compatible with any Challenge clincher tire models, on the website. Challenge asks that consumers do not mount ENVE SES wheels with any model of Challenge clincher tire whatsoever.
Failure to follow this warning could cause the ENVE SES wheel to cut Challenge’s quality casings resulting in explosive air loss from the tire and/or inner tube, potentially causing a crash and injury to the rider.
This bulletin also aims at providing the technical clarification, that only a precisely defined 622.0mm+-0.5mm bead seat diameter, specifically defined wall height and center channel, as described in the new above standards, will safely and easily allow proper tire and rim fit and function. Challenge has regularly been attending standards meetings and being involved for the past three years in confronting the market’s accelerated changes, such as the implementation of progressively wider rims, carbon fiber clinchers rims and tubeless and tubeless ready tire technologies. Challenge has invested time, knowledge and energy together with other elite bicycle industry companies to address and update tire & wheel fit issues with the ETRTO and ISO.
Challenge handmade tires have been produced for close to 20 years using the same handmade processes that have been proven over the past 50 years at the highest levels of racing without significant failures - if the rims and wheels comply with internationally accepted design standards. And this applies for both aluminum and carbon fiber rims.
Regarding carbon fiber rims, there had also been other periodic problems in the past with some carbon rims that had cut our (and other tire manufacturers’) tire casings just above the beads. These tire cuts were however normally caused by less than optimal QC at the factory, resulting in quality issues of improper finishing at the tire hook during production. Normally roughness at the hook was due to failure to completely remove resin flashings created while molding the rim. Proper finishing leaves the required 0.7mm min. radiused rim hook smooth, without roughness, thus eliminating a potential tire failure.
In the above cases, Challenge has routinely and successfully worked with the wheel suppliers (including ENVE) to fix the problems or replace these wheels to the consumer. However, specifically with ENVE’s SES model wheels, it is the first time that Challenge has known of a rim hook design that does not meet the above recognized design criteria, reason for which Challenge feels compelled to follow up with the release of this informative bulletin to protect its customers.