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I just emailed armada and they came back with this,
The reason we recommend-2.5 back is this. If you were to measure from contact point to contactpoint on the Halo and divide by two that line is -2.5 on the ski but centermounted in the sidecut. This gives the ski optimal performance.When youcenter mount a standard ski you are actually mounting it in front of thesidecut as well as the thickest part of the core, reducing performance.