lt300 holddowns

[MIsawyer]

I don't know if any of you are familiar with the wm lt300 but a little background on my problem, I saw rift and quarter lumber on a lt300 and the timbers are first squared and quartered on a headsaw and then I stand them on end and saw them down until I flip what is left before I get to the backstops, now what is left after I flip is a triangle peice were the final board or two are sawn down.
now my problem comes in here when sawing a longer triangle peice 13ft and longer there tends to be a bow in the triangle and im having a problem with keeping this flat on my deck as it wants to bow up. I have my timber turners that can hold it down in the center but these are nor reliable because I dont want to apply to much pressure as this to will cause miscut lumber.  Sorry if this is a little confusing.  Thanks for the help