rickc, you're describing what's known as "static" load and "dynamic" load. Dynamic load is what truck bed caps are rated for. So, whatever the manufacturer says is the "load rating", is dynamic. That means that is what they rate the cap to hold whilst driving down the road (with all the bumping and bouncing around). "Static" load is what the cap will hold just sitting still, not moving. To give you an idea, my SnugTop cap is rated at 200# dynamic load. That covers the 150# of my RTT. But guess what happens when to 2 people climb up in to it? Now we're talking 500# plus. My Thule rack is rated for 750#. After 3 years, there are no issues with my cap or RTT. But of course, I didn't cheap out in installing the rack system (I used ALL the mounting holes, not every-other one; and I use 3 cross bars - I have a Tepui Gran Sabana, one of the largest ones made = a Cal King sized bed).
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You'll be fine...