Guying the Tower

I built a road-going “micro-tower“ for my compact contest rover. The complete setup weighs ~75 lbs (34 kg) and extends about 7 feet (2.1 m) over the roof of my VW GTI. The overall height is 11’6″ (3.5 m), which is plenty tall enough to hit tree limbs on urban roads with landscaping trees. Less likely, but still a concern, is the possibility of striking a large bird such as a turkey vulture as it’s fleeing a carcass in the road. My “Contest Module,” as I call it, is fastened to the roof rack with twelve 5/16″ (M8) carriage bolts. They’re more than strong enough to hold the load; however, the rack’s crossbars are held with just one M4 bolt at each corner. Even though each M4 bolt has a tensile strength of ~3500 lbs (1600 kg), I wanted additional retention options in case the worst was to happen.

I’ve been hassled a little about my decision to guy such a small tower on my car. I sort of understand the dispute. But I’ll ask this simple question: If you were to put a 6-ft tower with three Yagis on the roof of your house and KNEW that it would be subjected to routine earthquakes and strong gale force winds every day, that is winds that exceed 47 mph or 75 kph, would YOU guy that small tower? I think most would answer YES. This is a highway-driven vehicle. So, the tower is subjected to lateral forces and gale force winds every time it leaves the house; and hurricane force winds every time I drive in excess of 74 mph or 119 kph on the highway. Yes, guying this tower is prudent.

Guying the tower to the rack itself would defeat the intent of adding attachment points. So, I knew I wanted to attach to the car’s body, but did not want to add any new mounting holes. I have a friend who uses a suction cup based rack to transport her bicycles. The system is by Seasucker, which was founded to support the boating community. I figured I’d buy some of their rear wheel supports and modify them to suit my needs. Little did I know that they have full line of attachment options for marine, camping, and overlanding applications. Their 6-inch cup with D-ring attachment develops 210 lbs (95 kg) of holding force and is perfect for my needs. Even at the angle of pull shown above, each cup can still hold ~100 lbs.

Sure, 100 lbs of holding force doesn’t seem like much. But consider two things: First, in their capacity as guy lines, none of these four points will have 100 lbs pulling against it. Most of the load is supported and retained by the rack while the guys add a touch of anti-sway during acceleration, braking, and cornering. Next, I’m using these as secondary retention points. Should I hit something that is able to shock any of the other mounting parts into failing, I can only imagine that such an impact will mangle my roof as the impact energy is dissipated. BUT, if even one of these cups holds long enough for me to reach the roadside and stop, even if the tower is dragging down the road, then the retention system has served its purpose. I’d much rather have my car mangled than have anything wind up in someone else’s car with tragic results. Hopefully, that will never be a problem. Still, I like to be prepared!

Seasucker cups are not intended for long-term use. They’re perfect for my application since I mount the tower only for contests and exhibitions, not more than a week at a time. The only catch is that the paint should be perfectly clean for best adhesion results. Yes, they’ll hold if the paint is dirty, but the cups will require more inspections since they may not retain full suction for as long as I’d like. With clean paint, though, I can easily drive for days without seeing indications of suction loss. Installation, removal, and adjustments are fast and easy! See this video segment for more details about how I’m using the Seasucker cups to guy my tower.