RPM Soft tennis string. RPM Soft is the most durable of the soft strings, combining control, strength and spin in one string. RPM Soft tennis strings are designed to provide comfort during impact while providing 38%25 more strength than other soft strings. Main properties of the string: COMFORT. Polyamide material provides more flexibility than polyester or co-polyester for superior comfort. ENDURANCE. The string monofilament structure has a longer life cycle than the polyamide structure. The double aramid coating further increases the strength of the strings. THE ROTATION. The polyamide composition optimizes the movement possibilities of the string, which creates spin possibilities. In the set 12m - for one full racket.