BMW 2006 328i Owner's Manual page 100

You should therefore always remember not to
exceed the approved roof load capacity, the
approved gross vehicle weight or the axle loads
when loading the rack.
You can find the applicable data under Weights
on page 140.
The roof load must be distributed uniformly and
should not be too large in area. Heavy items
should always be placed at the bottom. Be sure
that adequate clearance is maintained for rais-
ing the glass sunroof, and that objects do not
project into the opening path of the luggage
compartment lid.
Fasten roof-mounted cargo correctly and
securely to prevent it from shifting or falling off
during the trip.
Drive smoothly. Avoid sudden acceleration and
braking maneuvers. Take corners gently.
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