Cartop Cargo (kayak) Carrier for a Sports Car

My daily driver is a small BMW Z3 roadster. I like to do projects and go kayaking and other stuff that requires carrying fairly large (but not heavy) loads. These two facts quite frequently cause me problems.

While my wife drives an SUV, it's always inconvenient and often impossible to shuffle our schedules and needs around so that I can use her car to do what I want to do, especially since she refuses to learn to drive a stick shift.

After a recent bout of struggling with this dilemma (a 1967 Cadillac DiLemma? Always wanted to say that...) I decided to find a way to carry loads on my car.

My design requirements, in order of priority, were:

No damage to anything on the car
No modifications to the car
Secure and stable support of loads such as my kayak, a few 2x4s, etc.
Inexpensive
Easy and quick install/de-install
Preferably fits in the trunk of the car

I looked at inflatable car top carriers, but they seem to me to be unsuitable for loads such as lumber, and also look like they could damage the roof of the car.

I also found a device requiring a trailer hitch ($350) and the device itself ($150) AND it would STILL require placing a significant portion of the loading on the roof itself - again, possibly damaging.

Since nothing commercially available seemed to meet my requirements, I (of course) decided to see what I could come up with. Following is the result of 2-3 days of sketching and beer, followed by an afternoon of building - with no beer. (NEVER drink and saw wood. There are better ways to get to meet your local ER medical staff.)

This design uses the roll hoops as a load-bearing structure and with minor modifications should work on any car with roll hoops or a roll bar - though I suspect few owners of such cars would deign to stoop to such plebeian depths as to use a home built device on their garage queen. :-)

Since I'm pretty comfortable with woodworking, I went with a wood-based design, though if you're a weldor (yes, the "o" is appropriate, look it up!) you could make this out of steel (which is how I started this design, and may go there yet).

I decided that working with the top down would make it much easier to design a carrier, and it would be an extremely rare occasion that I would want to carry a load in the rain AND not be able to use my wife's SUV.

Load distribution:
I decided to use a 36" 1x4 sitting atop and bridging the roll hoops to support the aft load and 1/8" luaun plywood skirts in order to keep the 1x4 centered and stable. I would also need some sort of padding to protect the vinyl covering on the roll hoops.

The windshield would easily support the forward part of the load, though I would obviously need to make sure there was padding to protect the weatherstripping, and spread the load somewhat so as not to create a point-load which could deform the weatherstripping and/or trim.

Also, I would like to carry my cheap 30"x120" plastic kayak (yay for Wal-Mart!) without damaging it, so I'd want a means of supporting as much of it's bottom as possible while still being able to fit everything in my trunk when not in use.

Design and Materials

First I needed some critical measurements:

Depth of roll hoops (~2.5")
Width between roll hoops (~30")
Distance between top of roll hoops and windshield (~36")
Maximum size of one piece that would fit into the car's trunk (40"x16")

Once I had the basic measurements I started by deciding to make the "flower box" that would fit over the tops of the roll hoops 36"x4". This would allow some extra space for internal padding. (As it turned out I should have made it 4.5", as things got a little too tight and forced me to do some field-expedient engineering).

"window Box" Construction

I called this the "window box" because when it's assembled it looks like one - duh.

"Before we use any power tools, let's take a moment to talk about shop safety. Be sure to read, understand, and follow all the safety rules that come with your power tools. Knowing how to use your power tools properly will greatly reduce the risk of personal injury. And remember this: there is no more important safety rule than to wear these...safety glasses." [Points at glasses.] "And also hearing protection when necessary."

I cut a 36" 1x4, then ripped some 1/4" plywood into 4" strips and made 2 36" strips and 2 4" squares.

Glued & screwed the pieces into a 36"x4"x4" box missing it's top.

Fore & Aft Support

Not much to this step - I cut a piece of 3/8" plywood to 16"x40".

Originally tried to use 1/8" lauan but it wasn't rigid enough.

Padding

Not having any suitable padding material handy, I rooted around in the basement and found a couple of old super-cheap-almost-useless kneepads and cut them into suitable shapes, then used contact cement to glue them in place.

Unfortunately I didn't have enough to cover all the places I needed, so I also cannibalized a blue pool noodle. One of these days I'll make another one and make it pretty, but this is what you get for now for "proof of concept" purposes.

Forward Load Distribution Member

I forgot to take a picture of actually attaching the forward piece to the window box, but trust me, 2 short drywall screws and it's there. I have a small screwdriver I carry in the glove box anyway, but my next iteration will use either wingnuts or some other quick-disconnect means to attach the two main structures for faster breakdown.

Forward Windshield Padding

For now I used a towel to protect the weatherstripping from the forward load but later I'll add some real padding attach to the underside. It'll be cool, trust me.

Testing

Here's the semi-final product in use. Works great!

Disassembly and Storage

To store just remove 2 screws holding forward load member to window box and place both pieces in the trunk. Done!

Sometime in the next week I plan to redo the padding and paint the whole thing gloss black.

UPDATE: 2016

Just realized I never posted the final version. Been using this since late 2012, no problems.

As you can see, I refined it a bit;

+ Glued foam from old kneepads in strategic locations.

+ Ripped the main fore & aft piece lengthwise and hinged it so it would fit in the trunk more easily.

+ Glued padding to the section that rests on the windshield.

+ Used bolts & wing nuts for assembly & disassembly, and they hold it all together for storage. (Also painted them yellow so less likely to lose at riverside.)

+ Rounded off all the edges & painted it black to go with the car.