You will probably be fine.  I have a reluctance to using things for
purposes for which they were not designed, so I would not carry loads on
the pop top, period.  Plastic is just not designed to handle repeated
stresses over a long period of time (10+ years now).  Look at how many
of the luggage rack tie downs are pulled out on Westys.  That's just my
opinion of course.

I don't know of any horror stories, but I did do something stupid once
myself.  I drove into the airport parking garage (7' clearance)
forgetting I had the rack installed.  I made it all the way to a low
hanging sign before it caught the racks and ripped them off one side.
Slight bend in the rain rail on that side that was easy to hammer out
and no damage at all to the rack.  If I had had it on the pop top I
probably would have destroyed it.

We all have mental lapses, and remembering you have something on top can
be a challenge for some of us!
--
Stuart MacMillan
Manager, Case Program
800-909-8244 ext 208

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