Believe me it stops the flex. No matter if you're running a nitro engine, RC-Monster direct drive, or Tekno RC's clutch setup, that brace will really stop the gears from seperating. But you really need to upgrade to a 5mm shaft motor, the smaller ones can't take the stress a heavy vehicle puts on it anyway.
Also, if you really don't think that brace will stop it, there are two other options to try.
This brace and
this brace. Don't give up on it so soon!