Why cant I adjust my roof rails on the RX450H and use all the fixing points?

 

Terminology:
Rails are the front to back fixed components, They have fixed anchor points in the roof.

Slats:
Members that go across and transfer weight to the rails while providing elevation to object placed on them.


If I understand the question right, your question is about location of slats on the rails.
Depending on which slats you have, there is a way to locate them on the rails. One should be able to remove them too [by removing the back plastic covers of the rails and sliding the slats off.]
That said there would be recommended locations (for slats provided by Lexus to minimize noise ... air flow over them can produce noise)

Salim

 

Spacers located fore and aft on the rails prevent members from exceeding approximately 21" in length. I need 28" to accommodate Thule Sidekick cargo carrier. Can spacers be removed?

 

Yes, I had to get a we bit more for this Thule. I had forgotten about that (and where I put them once I removed them) until you asked.