Aero Tire Deflectors
If you’re air, the only thing nastier than a grille is a spinning tire. The MK2 bumper was supposed to push air out around the front tires, but by the time we were done screwing it to the front fenders, it didn’t stick out enough. Duct tape, some coroplast and a strip of aluminum tape solved this problem in about 5 minutes.
Ok, if there’s one thing aerodynamically nastier than a spinning tire and nastier than a wheel. It’s the entire underside of the car. If you’re less ambitious or much smarter than us, you can make a good-sized dent in undercar drag by lowering the car and using an air dam to reduce the amount of air that goes under the car. It takes a lot of energy to push the air dam through the air, but not as much as it takes to drag that air across the bottom of the car.
By far the better solution, though, is to cover all that nastiness with a big, smooth sheet of aluminum. Only one problem: Have you looked at aluminum prices lately?
Ok, a big, smooth sheet of plywood. Plus some urethane deck sealer, just in case we hit a puddle some day.?
Making a big sheet of wood stick to the lumpy underside of the car without falling off, snagging on something or catching on fire is surprisingly hard. Here’s how we did it:
Up front, we traced the shape of the bumper onto our sheet of wood, cut it out, and bolted the wood to the thin plastic lip around the bottom of the bumper. Lots of bolts…
The TDI oil pan hangs too low to cover with wood. Instead, we bought some rubber sheet, screwed it to the bumper wood, stretched it across the oil pan, the Eurosport lower control arm brace, and back to the underfloor tray. It’s not perfectly flat, but it’s a lot smoother than an exposed engine.
Under the floor, we had to space the tray down far enough to clear the exhaust system, brake lines, fuel lines and the floor’s stiffening ribs. Once it was down far enough, there was nothing to attach it to. To solve this problem, we made long, L-shaped steel brackets that screwed to the pinch rail, giving us a flat surface about 3-inches below the rail that we could screw the wood to. To hold the center up, we wedged wood blocks between the tray and the floor’s reinforcing rails, then screwed right through the wood and into the ribs.
Dropping the undertray low enough to clear the hangy down bits required us to sketch up some side rails that would screw to the pinch rails and hold the wood 3 inches below the rocker panel.
To make these, we simply went to a metal supply house that had an on-site shear and bending brake. It’s only a few more bucks to cut your metal to size and throw a bend in it.
You can see the spacers in action here, as Jared spends his midnight screwing to the wood spacer blocks we have scattered around the floorpan.
This still put wood perilously close to the exhaust system, so we sandwiched some pink attic insulation under a sheet of aluminum and screwed it to the top of the wood, just under the exhaust. Even doing powerslides in the desert, nothing caught fire, so we’re going to call this successful. Remember, though, the exhaust on gasoline engines is a few hundred degrees hotter, so if you try this on a gasser, don’t blame us when it burns to the ground.
We may be dumb, but we’re not stupid. That tube down the middle is where the hot stuff goes. Hot makes wood burn…
The hardware store version of undertray safety is an a aluminum heat shield (stainless steel works better to minimize heat transfer, but it cost too much) and some fiberglass insulation.
The end result actually worked (at least with our relatively-cool diesel exhaust) and the car has still failed to burst into flames.
At the back, John McNulty, our aerodynamic consultant advised us to bend the undertray upward at precisely 5 degrees. Unable to bend plywood, we splurged and bought aluminum for this part, letting the metal supply shop put the bend in for us. Since the thin aluminum bends more easily than the plywood, we attached some reinforcing ribs to the top of the sheet.
In a rare instance of measure twice, cut once, Jared cut this elaborate sheet to dodge all the suspension and tailpipe bits that simply couldn’t fit above the tray.
This may well have been the best fitting car part Jared has ever fabricated.
Aero Hood Vent
Brilliant as it is, the undertray poses a problem. It may be too good. In sealing off the bottom of the car, we’ve sealed air into the engine bay. If we let air come in through the radiator, but don’t let it out, the Sipster might inflate. Just to be safe, we cut an exit vent in the hood, just behind the radiator. This will upset airflow across the hood slightly, but that same air is about to be upset by the windshield anyway.
Its amazing to me that we so utterly failed to get a properly exposed, in-focus picture of our hood vents, but I think we cut these out literally minutes before our 80-mpg test run, and mere hours before Jared had to fly back to New York, so we were pretty frantic. Stand about 10 feet from your computer and squint and you might get the idea. Basically, we cut two H-shaped slits in the hood, grabbed onto the sheet metal with our bare fingers, and bent the back of the steel down into the engine bay, and the front of it up into a little Gurney flap that should ensure a low-pressure area right over the opening to vent engine heat.