The contra spring or torque spring is responsible for ensuring that the V-belt is always taut by applying pressure to the rear pulley. If the V-belt slips, using a harder spring is advisable. This can happen when using heavier variator weights or of course when the engine has been considerably tuned. We have torque springs in OEM quality and a lot of sport and racing springs for Minarelli, Piaggio, Peugeot engines from manufacturers like Stage6 and Polini. Also available are spring seats like the Malossi Torsion Control.