NO! DONT SHORTEN CHAIN ! What I see people do with the stock chain tensioner is they put a Large Nail or a bolt on top of the frame between the chain tensioner, that way when you tighten IT there pa more meat so to speak for it to grab on to.
So, on. Reapeat in-between the yo PC's of metal that clamp together, you need atleast a 16 framing nail or even better a screw with nuts on it so it won't bend as time goes by. Remember, on top of the frame in-between the two piece clamp. It gives it more to grab on to.
If you don't understand hit me back its actually very simple. I personally use an (ARCH IDLE ARM TENSIONER) They are king 2md This. None in my opinion. Mine has NEVER FAILED ME, LOSENED. THICK STEEL MADE IN U.S.A. mbrebel.com GET the proper length though, it attaches from top of rear wheel frame to bottom. You can slide it up or down as far or near as u need. Good luck.
As always ne safe and have fun.
W.C.B