Instructions for round cross bars:
Using a 11mm. socket tighten the legs of the U bolts evenly. Once you can not pull the stacker to flat the bolts are tight enough. STOP TIGHTENING at this point. The U bolts can provide an extraordinary amount of pressure.
For square bars:
The stacker will work on square type cross bars but installation does require a little effort. The legs of the U bolt must be bent out slightly to go around the bar. This can be achieved by tapping the U bolt down on the cross bar then removing it (your plastic coating on your cross bar will be scratched slightly).
Then squeeze the U bolt back on the cross bar from the bottom with the legs facing up. As soon as the legs show through the mounting plate and washer, begin screwing the nuts into place evenly.
It is important not to push the U bolt all the way on to the cross bar until you place the legs of the U bolts through the holes of the mounting plate.
If you push them too far though, the legs will spread too far apart making them impossible to fit through the mounting plate.
If this happens you will have to remove the U bolt and squeeze the legs narrower then begin instillation again.
As soon as the legs show though the mounting plate just enough to place the washer and nut on stop.
Then tighten the legs evenly. The gap created by the curve of the bolt will not affect the strength, DO NOT OVERTIGHTEN.