Magnum, and all Arrow rides, are going to be rough in the turns by design. B&M rides shouldn't be this way. I rode two of the oldest B&M's in the states at SFGAm (Batman and Iron Wolf), and they were smooth as glass*. When you can feel the cars shifting under you on the lift, which is straight track, then there's a problem with the wheels.
Batman: Knight Flight's wheels are visibly in bad shape, especially the guide wheels (on the outside), which accounts for the extra roughness on the turns and screws. I'm fairly certain that there is a bit of spring adjustment as well, and that would appear to be in poor shape as well. Combine the two and the wheels can hunt and bounce off of turns like an Arrow looper, and that's just poor maintenance.
(* Smooth in their tracking motion, though the transitions tend to be very harsh. These quick transitions should not be confused with rough tracking.)
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Jeff
Webmaster/Admin - CoasterBuzz.com