For bike techies some general and specific wheel information and why latest tech wheels are expensive. The video shows cool testing apparatus and will help explain why worn clincher rims are unsafe. The last five minutes is mostly Zipp marketing if you wish to skip. Clincher rims must structurally support weight, torsional forces, and the hundred and more air pressure pounds pushing the tire outwards along the rim sides. These forces increase with brake heat, cornering, and speed. By the way tubular rims do not have to structurally support tire pressures, because “tubs” tubular tires encapsulate the inner-tube and support themselves and best tubs cost $150 and up. That is also why tubular rims are lighter.
I learned that the best current tire kevlar beads fail at 375 psi, but Zipp was mum about critical temperature for their rim resin; although, they did claim it was 100 degrees higher than other manufactured wheels, and I remember a study that showed tandem wheel rims can reach 400 F degrees with rim brakes on long down hills. Couple other terms mentioned by Zipp’s tech boss and engineer are GT, glass transition referring to epoxy temperature. I had to look up this one.
The Glass Transition Temperature has to do with polymer changes from a hard, rigid or “glassy” state to a more pliable, compliant or “rubbery” state. The latter state is not good for a bicycle wheel. GT is a temperature range over which the mobility of the polymer chains increase significantly. That means heat lessens the structural strength, again not good for bike rims. GT is determined by the chemical structure of the epoxy resin, the type of hardener, and the degree of cure.
NACA (National Advisory Committee for Aeronautics) is the standard’s organization for aerodynamic forces, rim design are air-foil shapes. In planes the forces are called lift, on bicycles rims the forces are called drag; ergo, designers use computers to study rim drag curves. http://en.wikipedia.org/wiki/NACA_airfoil
And finally tororidial, the term for proprietary new rim designs and the video shows good cross section examples. Enjoy https://www.youtube.com/watch?v=vXrNfIwnk7M
Recent Comments