Alpine touring binding
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An alpine touring binding (also known as an AT binding or randonnée binding) is a specialised ski binding that allows the skier to have the heel of the ski boot free and toe of the ski boot in the binding when using Nordic skiing techniques for ski touring and to have both the heel and the toe of the ski boot in the binding when using alpine skiing techniques to descend the mountain. Most AT bindings have a DIN-rated safety release like an alpine binding. Specialized alpine touring ski boots are usually used with an AT binding. AT boots combine features of a downhill ski boot and a plastic climbing boot. They are light and flexible enough for comfortable walking and ski touring, yet sufficiently stiff to provide control when skiing down. The soles of the boots provide more traction than most Nordic and alpine ski boots and they can hold a crampon. Crampons are used when slopes become so steep that skiing up is no longer practical and the skier switches to hiking up with the skis carried on a backpack.
During ascent, the skis are fitted with self-adhesive plush climbing skins to prevent them from slipping backwards. The ski binding heels are released to allow a Nordic style of striding. The skins are very effective in preventing the skis slipping backwards, much more so than grip wax, and quite a steep hill can be ascended directly without resorting to side stepping or herringboning. The heels of the bindings are often fitted with adjustable steps to maintain the soles of the boots horizontal on a steep climb. Before descent, the skins are removed, the heel steps removed or lowered, and the binding heel piece is locked down. Descent is then by conventional alpine skiing technique.
Compared to Nordic boots and bindings, the alpine touring binding is less suitable for undulating terrain, but is more versatile on steeper and more difficult snow. Like telemark skiing gear, this equipment is popular with people from an alpine skiing background; but unlike telemarking, it requires no learning of a new type of downhill turn. Because the fixed heel provides additional support and lessens the force on the toe hinge in downhill mode, modern AT gear and telemark gear are comparable in weight, with both sacrificing control on the descent to reduce weight.
Most touring bindings are designed for ski boots falling under one of two ISO specifications:
- ISO 5355:2005, for traditional alpine boots. In this variation the pivot is located in the front of the binding.
- ISO 9523:2008, for boots in which the pivot is formed at the boot / binding interface.
The two setups are mutually incompatible: in the former, the boot lacks sockets to engage ISO 9523 compatible bindings, while the later boot toe and heel-piece dimensions are incompatible with ISO 5355 bindings.
The main manufacturers of AT bindings are Silvretta, Fritschi Diamir, Marker, Dynafit and Plum. Backcountry Access also produces a binding adapter to allow downhill alpine skis and bindings to undertake limited touring.