Where did you get your info to shim so the string is above the track or commonly know as the rail? Only reason I ask is I have never heard of this being done intentionally. A few guyshave came in here and said they have had problems shooting and the string not being firmly on the track has been identified as one of them. Did you get this from someone with experience in your CB or just a idea that someone came up with trying to offer assistance? If the string is up, this can cause the string to jump over the arrow when shot.