-- Last Updated: Jun-21-12 2:07 PM EST --
Let me preface this whole rant by saying this applies to the usual situation of clamp-on tower support feet, where the clamping part grips that edge within the door opening. If your rack mount bolts to the roof, of course you want to bolt through whatever reinforcing members are present, and not just through the exterior sheet metal. Still, the reinforced zone is "right there" and available along the full edge of the roof, and I bet Thule could design bolt-on feet to fit at better locations for bar spread while taking full advantage of that reinforcing if they really wanted to, though I bet they'd need a lot more variety available foot design.
Yeah, yeah, highly trained engineers and all that. Still, the crossbar mounting points are right along the edge of the roof, where there's already a tubular "wrap" of reinforcement. Is there really any benefit to making sure the mounts are located right where there's a cross piece of reinforcing too? Here's my thought, which applies to a friend's small car and canoe. You put this big ol' lever arm of a canoe out there in the wind on a pair of crossbars just 18 inches apart because that's what Thule says to do. You could put the racks 3.5 feet apart and drastically reduce the prying action of the canoe on the mounts caused by wind, AND reduce the strain on the boat by the same amount, and thereby stop most of that squirming action of the boat in the wind. Further, the crossbar mounts would be near pillar supports and/or front/rear edge-of-roof reinforcement, which are the strongest parts of the roof by far. Does Thule really think that those reinforced edges on each side the roof (especially at pillars or front and rear edges) are not strong enough to tolerate the much LOWER amount of wind-caused stress if the crossbars are far apart, but that greatly increasing that wind-caused stress by putting the crossbars extremely close together is okay just because there's a miniscule strip of reinforcing at that location which is only there to keep the sheet metal in the middle of the roof from flapping in the wind? That makes no sense to me since there's no way wind-on-boat stresses are going to overstress the reinforced edges of the roof. The boat will break before that happens, so why not reduce that stress in the first place by putting the crossbars farther apart? I suspect that what Thule is REALLY doing is ignoring wind loads and looking for every bit of extra strength that's available for carrying weight without running the risk of having crossbars too far apart to carry cargo boxes. In other words, I think they are looking for the strongest weight-carrying part of the roof within the center area, which is where people want their cargo box to be. Paddlers aren't carrying much weight and actually benefit by keeping bars out of the center-of-roof area. Until Thule justifies their instructions, or tailors the instructions for boats versus cargo boxes, who know what they are thinking? Until then, I'm convinced that consideration of wind loads is not part of the decision-making process.