Answer-RC 1.1mm 3 Shoe Clutch Spring Set (Black)
A lighter spring allows the clutch to engange earlier with more progeression. A thicker spring allows the engine to rev higher prior to engagement of the shoe coming in with more bite and more aggression. If driving on a low grip surface it is generally considered best to run a lighter spring making the car easier to drive out the corners.
Due the generic nature of 3-shoe clutches these springs should be suitable for the majority of 1/8 buggys and truggys on the market.