The MINI Cooper S uses a serpentine belt to drive the various ancillary components on the engine. This includes the alternator, A/C compressor and the supercharger/water pump. The serpentine belt is kept in place by a small dampener and a coil spring mounted to the end of an idler pulley. This is called a belt tensioner. MINI Cooper Belt Tensioner Replacement - Pelican Parts Is this answer helpful?Thanks!Give more feedbackThanks!How can it be improved?How can the answer be improved?Tell us how MINI Cooper Belt Tensioner Replacement - Pelican Parts wwwcanparts››MINI Cooper Technical ArticlesTiming Chain Guide Replacement > Friction Wheel Replacement > (R50 Cooper) Shown here is the new belt tensioner for the Non-S Cooper models. It will come with the locking pin installed. Comments: After following your detail on replacing the belt idler arm on my R50 Mini Cooper found I the following method, Mini Cooper Timing Belt Tensioner - Belt Tensioner - Dayco wwwsgeek››Mini Timing Belt Tensioner›Cooper PartsThe springs allow the timing belt tensioner to exert the proper degree of force on the timing belt. When the timing belt tensioner is defective, your Mini Cooper's timing belt may start to slip. You can buy a replacement timing belt tensioner for your Mini Cooper online at PartsGeek.