Homemade J-rack

-- Last Updated: Aug-21-14 3:47 PM EST --

My clever brother just sent me photos of the J-rack he built of PVC parts and other Home Depot hardware to haul the Looksha IV that I brought up to him in Saratoga Springs last week. He says it was less than $30 worth of material -- he needed a rack that would clear their rooftop "bubble" cargo carrier.

The photos linked below show the construction if anybody is interesting in copying it -- he did agree with my suggestion to test it around town for a few days before heading out on the highway with it (and has promised to add bow and stern lines to pacify his fretful big Sis.)

https://www.flickr.com/photos/snapdoodlekid/sets/72157646292262558/

Very freightening
It appears to rely entirely on a small number of drywall or deck screws to transfer loads from the kayak to the vehicle.



Such screws are typical brittle.



That means when they fail they snap. The don’t bend. They don’t stretch. They snap.



Screwed into wood.



Please ask your brother to not use it.

Disagree

– Last Updated: Aug-21-14 7:02 PM EST –

All you can tell from that photo is that they are wood screws. I'd bet this week's wages that the sheer strength of that 5-screw connection is over 1,000 pounds. The "tear-apart" strength might be less, but still enormous compared to the forces that will actually be present. The connection of the U-clamps to the wood are a lot weaker, but probably still able to hold a few hundred pounds. Adding one more U-clamp per attachment would add extra security, or the screws could be replaced with bolts.

By the way, ring-shank nails are awfully brittle too, but have you ever heard of them failing??? Have you ever tried to take something apart that's connected with ring-shank nails? It simply can't be done. You need to splinter apart the wood, because the nails won't come out.

I think those PVC pipes would reach their limit long before those screw connections even start to show what they can do, but I'm not sure there's any reason to worry about the PVC either. The connections of the car's roof rack to the roof will be weaker too, and the factor cross bars will be weaker still, of that I'm sure. I'd call the PVC the weakest link in the home-made part of the setup, and I think your concern about the wood and screws is equivalent to worrying about breaking a logging chain that's attached to a vehicle with a few wraps of cheap rope.

One thing that DOES look like a weak spot is the way those U-bolts connect to the factory crossbars with a "wedging" or "ramping" action at the crossbar edges. A very small amount of bolt tension will really put a lot of pressure on those contact points due to the multiplication of force by ramping action. The plastic could cave-in at those contact points, but probably not from boat loads, only from the bolts being tight. An easy solution would be to make those U-bolts "barely tight", then double-nut them to keep that minimal tension from being an excuse for the nuts to get looser.

deck screws
I am pretty sure he used treated posidrive deck screws left over from a bridge he built over their backyard pond. But I emailed him to check on that.

backups
you’ll note i also insisted that he run the straps over the car’s integral rack bars, so the j-rack mainly positions the boat and the straps bear on the fixed rack. And he just emailed me a shot of his added bow and stern lines.



I have an extra set of Thule stackers I am sending him, but he wants to take it to the shore next week and he won’t get them in time.

Hi
Sorry to dredge up an old post. But I want to clear one thing that MintJulep said that Deck Screws are brittle. How.?

Having driven thousands of drywall screws (into both wood and metal) and deck screws as well as extracted hundreds of previously installed ones during demo, I am pretty confident that drywall screws are very brittle and deck screws are very stronger and more resilient, comparatively. It does depend on source as off-brand Chinese-made screws can be very poor quality no matter what type. I have never had a deck screw head strip off or had the head actually break completely off the screw shank when driving it with a high-torque drill motor, as can easily happen with drywall screws if they bind up in the material into which they are being fastened. Drywall screws do not need to have shear strength so they are made of cheaper metal, hard but brittle. Deeper explanation here:

https://www.woodbin.com/doc/drywall-screws-for-woodworking/

Screws don’t matter as much as strapping the boat to the factory rack and not the pvc wood one. You have better force structure holding the boat in place.

Drywall screws break because they are actually very brittle. Deck Screws are not, they bend more before breaking, and they also have thicker shanks compared to drywall screws.

Personally I have made many things with screws and witnessed many things put together with screws (even house framing, headers, etc) and haven’t witnessed any broken screws. I like the way they draw things together much better than that of nails. Deck screws are not brittle, they are used mostly in lumber frames.

As I explained.