Designed standard with an aluminum frame. Aluminium frames reduce weight, maintain structural integrity, and resist corrosion better than traditional steel frames. Narrow doorways are an ever present eality. This is why the Ultra 4 armrests wrap around the backpost. The wrap-around design provides the narrowest possible profile for a side-frarne mounted armrest. Along with the standard silver-texture frame- color, black, and forest green frame colors, the Ultra 4 features a new deeper shade of blue and a special Silver Polish, which simulates chrome.