Once your chain starts stretching you will find the stock chain tensioner on the VMX/MM450 just doesnt work all that great. Sold at 2WheelHobbies.com is the Alleven RC Chain Tensioner sold in red and gold for the VMX & MM450 bikes (link is to red one). The nice thing about this setup versus the Atomik aftermarket version is you do not have to take the wheel off to adjust the tension, whereas the Atomik version you do.
NOTE: THIS WAS SPECIFICALLY DESIGNED TO WORK WITH THE ALLEVEN RC MICRO-DRIVE CHAIN/SPROCKET SET. SO BUY BOTH MICRO-DRIVE AND CHAIN TENSIONER TOGETHER AS A SET.