Hang Glider Rack
The majority of my life has been occupied by the desire to fly. In 2003 (I was 17) I obtained my Private Pilot License in Corvallis, OR, funded by 2 summers of minimum wage manual labor. In 2007 I took up kiteboarding, as the ability to jump to impressive heights was very appealing to me. I also dabbled a little in sailplane flying in 2012 but determined that although it was a much more exciting way to fly, it was too impractical of a hobby to continue. In 2014 I wanted to try either paragliding or hang gliding, and eventually settled on hang gliding as it was likely to be more of a bird-like flying experience than any other sport. The only thing that really put me off of the sport was the transporting and storing of the hang glider, as it only packs down to a 16 ft long package because of the length of the leading edge frame tubes.
Since I didn't want to limit the flying experience based on the ground handling experience, I went for hang gliding, and figured I would find a way to make it work. As I started taking lessons, I began thinking about ways to build a rack on my car that could carry an 18 ft long package. Since hang gliding is not the most widespread sport, there aren't any companies that build hang glider racks (a.k.a. "glider riders") for existing vehicles; everybody ends up designing their own custom rack based on their vehicle's configuration (whether it has an existing roof or cargo rack, what type of supports can be affixed to the bumper, etc.).
Since I drive a 2007 Volkswagen Rabbit (in terms of the body its basically the same thing as a Volkswagen Golf), and didn't have an existing roof rack, I had to start from scratch. A common approach is to add an off-the-shelf car roof rack that is compatible with your car and strap a ladder to it. The ladder rungs then provide the support for the glider. However, since the nature of hang gliding usually requires at least one car at the launch and one at the landing zone (LZ), I wanted a rack that could carry more than one glider, and function as a multipurpose rack to carry surfboards or other things. This eliminated the ladder design, and I decided to build a custom rack to attach to an existing off-the-shelf Yakima roof rack, as this rack has simple round bars that are most easily adaptable to interface with a rack of my own design. Based on existing support requirements for gliders provided by the manufacturer, I needed at minimum three supports along a 13 ft length. Since my car was only about 14 ft long this basically meant a support above the driver, one at the front bumper, and one at the rear as shown below:
Off-the-shelf support vs. required support capability
Below is the design that was chosen based on the available roof rack. The rack is divided in two sections, a front and a rear. The rear portion is hinged to allow the hatch to fully open, and is held down by PVC clip on T's that clip onto a suction cup handle (I discovered this was possible by shear coincidence while in the hardware store with the suction cup handle). The front section is supported by a vertical support that is mounted to the bumper.
Final architecture chosen for glider rack. The use of the suction cups for support over the rear precludes the need to attach a support to the rear tow point, permitting the hatch to open with the rack installed. The forward sloped angle is due to the positions of the off-the-shelf supports when installed.
The Yakima roof rack comes with 1.125" diameter round bars. However, the framing materials I intended to use (Steel Clamp-On Framing Fittings available from McMaster-Carr) were designed for 1.070" diameter round tubes. For this reason I purchased the Yakima rack (Q-Towers specifically), without the crossbars, and instead purchased two 58" lengths of 3/4" Schedule 40 iron pipe at the Home Depot for $16 (buying the two 58" Yakima bars would have cost me $90), which has a diameter of 1.050", slightly less than the steel framing system I would build the rack from, but much stronger as it had a wall thickness of .150", as opposed to the framing system which has a much thinner wall. Since I wanted to be sure they would be strong enough to mount the entire structure to these bars, and to be confident they would be compatible with any future attachments, I elected to use this material.
The most difficult thing to get past was how to attach the front rack to a single offset tow point recessed far into the forward bumper. This hard point was designed to screw in the 1T0805615A Volkswagen OEM towing eye shown below. The car has one in the spare tire compartment, but I purchased a second one on ebay as I was not sure if I needed to modify it, and still wanted one to use for towing purposes should the need arise.
1T0805615A Volkswagen OEM towing eye
Fortunately the shank of the tow eye nearly fit inside the remnant of iron pipe I had left over after trimming it to size, so I cut off the eye, filed down the two circular collars visible in the picture, and slid the shank into the pipe. I drilled a 1/8" hole through both and locked them together using a cotter pin as shown below:
Forward rack attachment to screw into tow eye socket in the right front bumper. It is composed of a 3/4" Schedule 40 Iron pipe with the towing eye shank slid inside (eye removed).
The only bit of concern I have regarding this adaptation of the tow eye is the strength of the shank in bending, as the rack is supported on a roughly 8" lever arm from the hard point on the car chassis. This can put a significant amount of stress in the material, and depending on the alloy, result in a broken attachment should it get overloaded. I performed the following basic max stress calculation based on the narrowest diameter at the base of the shank just above the thread (.625" diameter), the lever arm (8 in), and a conservative estimate of the ultimate strength of the alloy used for the tow eye. Based on the texture and the parting lines, it was obvious that this part was cast, though it was very difficult to drill through, leading me to believe it is made from one of the stronger casting alloys with ultimate strengths above 60 ksi (though this is merely an educated guess). As shown in the below calculations, this assumption shows that the tow eye might fail in bending at 180 lbs of load.
I believe this load estimate to be very conservative however based on the fact that it is rated to tow a vehicle. In any case, I elected to be on the safe side, and would preload the shank in bending in the upward direction by pulling the rack back toward rear by about an inch. This would put the vertical support in bending as shown below.
Preloading of the tow eye shank in upward bending to reduce the maximum downward bending moment and increase the carrying capacity of the forward rack.
The next challenge was to prevent the rack from deforming greatly due to the torsion that is a result of supporting the rack in the front on only one side. This causes the rectangular frame to deform like a parallelogram, with the front left corner drooping as it hinges about the front right tow point. To combat this two diagonal braces were added to prevent this type of deformation. These frame members were oriented such that they would be in compression under the rack's static weight. This is favorable when considering the shape of the framing attachments: under compression there is no risk of the members sliding out of the attachments, further stiffening the frame.
Diagonal bracing added to increase the rigidity of the rack. This is especially necessary due to the single offset support on the front bumper.
This additional bracing was very effective. However, after a test drive, the front-left Yakima Q-Tower began to shift to the rear due to the force from the rack. Additional iron pipe (I ran out of the yellow tubing I had ordered) was added between the front and rear Yakima bars to prevent this sliding, as the geometry of the car roof only allowed the front towers to shift aft, and the aft towers to shift forward, this forcing them to stay apart locks in their position. The rack stood up to a 5 hour trip from Los Angeles to Big Sur and back with a glider on top without any problems. Below are some pictures of the completed rack. Polyethylene foam pipe insulation was slid over the horizontal cross members to function as the padded support for the glider(s).
Some close-ups showing the plastic coated steel pipes and steel attachments that make up the framing system. All pipes were cut to length from 8 ft stock lengths using a $20 pipe cutter from home depot, an extremely valuable tool for this project, it even managed to cut the .130 in thick wall iron pipe without much trouble.
Rear portion of rack is hinged so that the hatch can open all the way when there are no gliders loaded. This is possible because the iron crossbar has a slightly smaller diameter than the yellow frame system bars (1.050 in vs. 1.070 in) such that a thin layer of waterproof bandage tape in the attachment (thin white strip visible under attachment in 4th picture) functions as a bearing surface. Clip-on PVC pipe T's fit perfectly around the suction cup handle, a complete but welcome coincidence.
The rack is easily capable of hauling at least 3 gliders at once. I haven't had to try more yet, but I'm sure that day will come.
The view from inside the car is not terribly obstructed, and the rack doesn't violate any state or federal laws as far as I was able to determine. I painted the two front supports black to minimize the glare from the headlights at night.