Had my Prospeed rack for a few months now and finally got around to mounting my ski rack. It was both complicated and simple. I have a Yakima FatCat rack and they sell a mounting kit for t-slot racks, but it isn't the right size for the Prospeed/Prinsu/etc... side t-slot:
SmarT-Slot Kit 2 - https://yakima.com/products/smart-slot-kit-2. I didn't realize this til after I ordered this kit, but I still needed this kit to create space between the roof and the rack so that the ski bindings aren't rubbing the roof. You don't need this kit, you just need some spacers to provide clearance or if you only have 2 sets of skis, you can just have them sitting upright so that the bindings face up only. With the Yakima kit it does have plastic nubs on the bottom of the spacer that is supposed to fit into the trough. Since they were too wide, I simply cut them off.
To mount the ski rack to the crossbar, I ordered several of the locking drop in t-nuts:
(DB-010) Drop-In T-Nut w/spring-ball | TNUTZ | TNUTZ - https://www.tnutz.com/product/db-010/ along with some
Black Button Head Socket Cap Screw | TNUTZ | TNUTZ - https://www.tnutz.com/product/imperial-button-head/ that were 1 3/4" long and matching 1/4-20 thread. It turns out this was too long. I think 1 1/2" would be better, but it's not really a big deal.
So my first attempt I reused the washer that came with the rack, but the screw was way too long so the rack moved around. I dropped by Home Depot and picked up some galvanized nuts and additional washer. So I just screwed the nut in first, then the washers and into the t-nut. After tightening the screw to the t-nut, I used a socket wrench to tighten the nut to the washer so it was a solid mount.
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Compared to the OEM roof rack with the same ski rack, it's definitely lower profile, so the bindings are closer to the roof. Here it's not touching, but really really close. As long as I have our kids skis in first and my skis on the outside, there's sufficient clearance because the roof isn't flat and bulges in the middle.
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Here's the final shot (without skis) showing how low profile it stays:
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I'm thinking of mounting my recover boards on the other side since the ski rack doesn't take up all the space. However, do people just leave the recover boards on top year-round? I take my skis in, but it would be a hassle to take the boards in and out often.