The Radius body features an offset double cutaway shape with an aerofoil profile which is thicker at the bottom and thinner at the top. It has a flat areas in the center of the top and back flowing into gently rounded edges on the perimeter. The other innovation that was introduced with the Radius is the tilt neck joint, which would be used on the early RG models.
The 540R was the first Saber model, introduced in 1987. The 540S was offered as part of the Roadstar Pro series which supplanted the Pro Line series. The initial Roadstar Pro lineup consisted of five new original body shapes: the Radius, the Saber (or S), the Power, the Turbot and the Ballback. The Roadstar Pro designation was phased out between 1990 and 1992 in favor of separate designations for the S series, Power series and Radius; the Turbot and Ballback were abandoned after just one year.
The Radius shape was developed by a design team at Chesbro, the west coast United States distributor for Hoshino. The creative team was led by Mark Wittenberg. The parent company conducted an informal design competition pitting Chesbro against their east coast subsidiary, Hoshino USA. This competition resulted in the Radius shape from Chesbro and the Saber and Power shapes from Hoshino USA.
For 1987 the Radius, Saber and Power models had several commonalities including DiMarzio IBZ/USA pickups in an HSS configuration, Edge double locking tremolo bridges, and an unusual control layout which eschewed a pickup selector in favor of individual mini-toggle switches to control each pickup and a push/pull switch on the tone pot to coil split the humbucker.
Guitarist Joe Satriani was an early fan of the Radius shape. He suggested a few tweaks to the design such as the shape of the horn on the inside of the cutaway and this updated design became the basis for his first signature guitar, the JS1000. So although the Radius was discontinued after 1994 its shape lives on in the Joe Satriani series which it inspired.