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

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1995 G320
2003 C230 Kompressor Coupe
2005 ML500
2007 ML350