Project Sipster part 8: All the Drag-Reducing Details



Our biggest challenge was to make the painfully square body slip through the air like a Prius. We had all kinds of good intentions to do some science here. Coastdown testing on a windless day could reveal some interesting truths about how much any of this nonsense did, but, as with so much else on this project, there simply wasn't time. Instead, with some consultation from John McNulty, our favorite aero professor, we did everything we could think of that could actually be executed by a couple of hacks working after hours with no budget and no skill.
Aero Wheels
One of the most aerodynamically nasty bits of any car is the wheels. They have a lumpy, complex shape full of holes, and they're spinning through the air stirring things up in a most unpleasant way. We first planned to solve this problem with spun-aluminum MOON discs, until we saw the price. Then we decided to try Pizza pans, but we found it surprisingly hard to find 15-inch pans to fit our wheels. Finally, we stumbled into a set of wheels from a 1984 Honda CRX. With the exception of a few vent holes, they're as flat and smooth as a MOON disc, and they're so horrifically ugly that they're free.
The Honda wheels bolt right up to a Rabbit, but only if you get longer lug bolts to account for the thicker aluminum wheel (H&R), and use bolts with a Japanese-style tapered seat, rather than the ball seat preferred by the Germans. Oh, and the Honda center bore is slightly undersized to clear the Volkswagen hub. Find someone who makes jewelry (Jared), hand them a die grinder, and tell them to make the hole a little bigger. After all that, they bolt right up…
Project Sipster Aero Mods


For the mileage run, we used aluminum tape to cover the vent holes and lug holes, making the wheels perfectly flat-faced. Same theory as the grille here. We'll pull the tape when chasing Porsches and need some brake cooling.

Aero Front Bumper
The air's first encounter with a Rabbit is the blunt aluminum bumper, hung 6 inches out in front of the car. Air smacks into the bumper, swirls around into the huge gap between the bumper and the car, before finally smacking into the car itself. To modernize the shape, we simply grabbed a front bumper from a MK2 golf and stuck it over the original. The word “simply” ignores the fact that we had to push the stock bumper back 4 inches to make it fit (we just drilled the bumper shocks to drain the oil, then literally drove the car into a brick wall to compress them), and had to invent mounting points with a drill, sheet metal screws and chunks of pine. Yes, the pine that grows on trees.
Project Sipster Aero Mods


The MK2 bumper presented new aerodynamic challenges. The MK1 rabbit takes its cooling air through the grille and through two air inlets curiously hidden behind the bumper. The MK2 Golf, though, took the majority of its cooling air from a large vent built into the lower half of the bumper. Stick a Rabbit behind that vent and suddenly its an aerodynamic dead end. 
To smoothly push air around our new bumper, we covered over the vent with Monokote, a plastic film used by dorks to build model airplanes. The thin, lightweight film has a heat-activated adhesive built in, so if you simply stretch it across a big gap, like our vent, and run an iron around the periphery, it sticks. If you heat it a little more, the plastic also shrinks, so a heat gun magically pulls out any wrinkles and makes a drum-tight, surprisingly rigid membrane. Since it still probably isn't strong enough to withstand a 70-mph pebble, we filled the airspace between the Monokote and the bumper with expanding aerosol foam insulation from the hardware store. The aerosol foam expands roughly 50 percent, then air dries to a reasonable level of rigidity, which should back up our new skin with a little muscle to shrug off small road hazards.
This really seemed like a brilliant idea for about 4 hours. We filled the back of the bumper's new plastic skin, spray painted it to match the car, and in less than an hour the Sipster had a smooth new face. We went to bed with the smug satisfaction of a job well done. 
The next morning, at the crack of noon, we awoke to find the Sipster glaring at us with a lumpy, wrinkly, still squishy face. The foam's need to air dry meant the backside of the foam hardened quickly, but the foam trapped up against the Monokote was sealed off, so it stayed wet and kept expanding. A 2-part epoxy foam that doesn't need air to cure would be a much better idea.

Aero Cardboard Grille
Project Sipster Aero Mods


Above the bumper, the rabbit is still a jagged, square mess. The grille is straight upright. Any air that goes through it is forced through a torture chamber of radiators and greasy engine bits, and the air that goes around has to change directions 3 or 4 times just to find its way to the hood. None of this is good.
Our plan was to build an air deflector that would sit in front of the grille and headlights, smoothly shoving air up onto the hood. The center would be shaped from a block of rigid foam and covered with Monokote. The headlight covers would be made from 2-liter bottles, de-labeled and unwrapped to match the radius of our foam bit. It was beautiful in our minds. 
The reality ended up being cardboard and duct tape, and the headlights just sat there exposed like little parachutes.
But isn't that radiator there for a reason? Indeed. That's why both grill blocks, the fantasy one and the cardboard one, were removable. Cruising around town or down the freeway, it takes very little power to move the Sipster (roughly 15 hp to cruise at 60 mph) so very little cooling air is required. Chasing down Porsches at the track or doing powerslides in the desert? Just toss the deflector in the back seat.
Even at cruise, though, the radiator and intercooler need some air. The most efficient way to feed them is from the very leading edge of the car, so whatever air enters the radiator can go there directly, without changing direction on its way there. Two holes, then, in the front of the bumper. The driver's side blows on the radiator, the passenger's side blows on the intercooler. (If we were smart, we could have built a small gap into the bottom of our cardboard deflector and skipped the bumper drilling, but as you can see, there wasn't much smart going on here.)




Leave a Reply

Your email address will not be published. Required fields are marked *