Hello,

I just traded in my vehicle and my new vehicle is only rated for 45 lbs on its factory roofrack/crossbars. The low rating is due to the moonroof.

I have a solo kevlar prism, I think it is like 37 lbs? Does the tension from the cam straps create downward pressure to make the total load greater than the weight of just the canoe? Seems obvious it would. I don't want to ruin my new ride, and I have read reports of roof leaks caused by overloading the rack.

-JC

No.

The only additional mass is the mass of the straps. The straps pull against themselves, Additional load cannot be achieved without adding mass or continually pulling on the straps (constant application of a force), once the straps are set they are in equilibrium; the downforce matched by the up force.

If loading the Prism with portage pads attached, ensure there's no pad contact with the roof before cinching down your cam straps. While our Old Town Columbia's portage pads cleared the roof of our Suby Outback, those of our Northstar Northwind do not, and must be removed before loading, otherwise the roof panel near the moonroof is clearly deflected as the straps are cinched.

Enjoy the new ride--may it take you to many lakes!

Banksiana: "No.


The only additional mass is the mass of the straps. The straps pull against themselves, Additional load cannot be achieved without adding mass or continually pulling on the straps (constant application of a force), once the straps are set they are in equilibrium; the downforce matched by the up force."

That makes sense, thanks!

Banksiana: "No.


The only additional mass is the mass of the straps. The straps pull against themselves, Additional load cannot be achieved without adding mass or continually pulling on the straps (constant application of a force), once the straps are set they are in equilibrium; the downforce matched by the up force."

But are you factoring in the air pressure? At highway speeds the front strap is most likely going to have negative downward force, where as the rear of the canoe will have increased force causing deflection in the roof...

nooneuno:
But are you factoring in the air pressure? At highway speeds the front strap is most likely going to have negative downward force, where as the rear of the canoe will have increased force causing deflection in the roof..."

At highway speeds the likely effect is lift rather than downward force; the inside of the canoe offers more resistance to the flow of air than the outside of the hull. Air flows over the hull faster than inside the hull, this imbalance creates lift, much like the concave underside of an airplane's wing vs the convex surface of the top of the wing. if you've ever witnessed strap failure at highway speed the notion of lift becomes quite obvious. I saw a Sundowner break free on I-35, at it's apex I would guess it was close to 25 feet above the highway before tumbling into the meridian.

I agree that the updraft would be more of the problem. But also cross winds I think would worry me more. Lots of twisting action. That is what caused my old town pathfinder to go. It tore the front of the rack off the vehicle while twisting the rest. Had I not reacted quicker the whole thing would have gone bye bye.

I would be leery not of the crossbars but how well the sidebars are secured to the car. What are those rated at?

Like Nctry I have watched our canoes lift on the Plains.

Yes once it lifted with when the tower broke.

I guess I won't give up on the extra fore and aft tiedowns.......

Hey JC, I agree that your roof straps won't add any load to your roof since they just clamp the boat to the crossbars but front and rear tie-downs will put some additional load on your roof so you may want to go easy on the tension. Also, I think the aero loads on the highway may also try rotate the boat a bit (if the front gets pushed up then the back may get pushed down) so if either bar is going to take some extra load it's probably your rear bar. So if your rear bar is further from your moonroof that's a good thing.