ATRA: ( Adjustable Training Resistance Aluminum) The ATRA has a machined aluminum bearing retainer for better heat dissipation and greater strength than the original plastic ATR. The lever actuated system has a neutral and four resistance positions. The lever is now made from moly lubricated nylon for better durability. Fits 900 and 800 series skis. Most skis are sold with the optional ATRA speed control system.