How not to use the roof spreaders
 

In my owners manual it lists the roof rack cross bars as load control devices designed to keep the load from sliding forward or backward. Now I see why:

http://cgi.ebay.com/ebaymotors/Mercedes-Benz-Wagon-300TD-Roof-Rack-Stretchers-1970s_W0QQcmdZViewItemQQcategoryZ6763QQitemZ8017262738QQrdZ1

Look at the 2nd photo. These bars are cracked, and I suspect it is because someone put a load on top of them.

-Tad