The adjustable curve Thule saddles just keep flattening out on the car top no matter how tight you clamp down the knob if you haven't already broken the knob off trying to tighten it. Simple fix to make them behave: go to the hardware store and buy shims (wedges). These days they came in composite as well as pine. You want the composite. Look at the picture and slip the shim / wedge in the crack, allow a bit of extra, then cut off the thick end and tap the thin end into the crack as shown. A thin bit of scrap wood that you can tap with a hammer is helpful. That's it. You are done. The saddles stay in the shape you want them in. SEE THE PICTURE. The shim is the lighter colored horizontal piece sticking out just above the bar.