A string tensioner is configured to apply a substantially constant tension to a string over an operational range even if such string stretches and contracts over time. Tension is provided by a spring. Flexers can attach the spring to a force modulation member and a frame. The flexers preferentially bend out-of-axis so that the spring does not bend out-of-axis when the force modulation member rotates. A dampening system can slow the force modulation member's response to vibrational forces. A flexible stop can prevent rotation of the force modulation member beyond a desired point, but flexes to remain in contact with the force modulation member over a small range of movement.