I have some questions concerning drag, and where better to ask than a place where people spend their lives trying to eliminate it? I am no aero expert, so I was hoping to get some opinions on these ideas. I am working on a pickup truck for my retirement years that I am building to be exactly what I want. I've worked out most of what I want to do. I'll be lowering the truck, doing a belly pan, and will be lightening it up quite a bit.
My questions are:
1. My research into aerodynamics, some of which was on this site, shows that trucks have a lot of low pressure zones which ruin efficiency. One of the big low pressure zones is the area behind the tailgate. Most folks here solve this by extending and narrowing the rear, but has anyone looked at just filling the area in with air so it is no longer a low pressure zone? The truck will be getting a one piece fiberglass bed cover. I am thinking about installing a small spoiler that catches air over the bed cover to direct it straight down into the low pressure area. It would be something like this spoiler on the back of this Blazer, but smaller and designed to follow the bed cover's shape.
2. The tailgate will be welded to the bed so that it no longer opens. I have a second beater truck that I use as a truck, and this one will be used as a car. Before anyone says it, I have zero interest in driving a car. The welded tailgate with the bed cover gives me a decent sized trunk and, as it's a compact truck, won't be a problem to reach into. Since the tailgate will no longer open, I can reroute the exhaust piping through the tailgate so that it exits the truck out into the low pressure area. The physical pipe routing is not a problem, I've already worked out how. I'm just wondering whether or not it would help with the low pressure area in the wake. At cruise RPM I expect the truck to emit about 130CFM of exhaust. I'm thinking that between the exhaust and the spoiler mentioned in item 1, the low pressure area behind the bed should be eliminated. Plus, dumping the exhaust into a low pressure zone should help with exhaust flow by helping scavenge the pipe.
3. I don't recall the make and model, but I saw a compact car on Top Gear that the factory mounts a wing underneath the rear bumper to act as downforce. Apparently it was pretty effective at providing downforce without getting up in the air over the vehicle. Air underneath the vehicle is already disturbed . Such a spoiler would also push air into the huge low pressure area behind the tailgate. The spoiler was mounted much like this one under a Subaru, but I envision more of an angle to catch the air and provide downforce.
Any comments or suggestions aside from this? I've figured out how to deal with quite a few of the drag components on the truck including the one off the back glass. I am mainly interested in small, effective changes, not huge and obvious ones. The most obvious one would be the tailgate exhaust but the spoilers should be unobtrusive. I still want this to look like a nice truck with a bedcover, and some of you boys do go to some extremes that I do not wish to emulate. My truck's cd is .47 stock, and supposedly just a hard tonneau reduces this to .405 according to testing performed by others on the internet. The goal is to get it down to .30, keeping in mind that a goal is to be strived for, but not necessarily reached.
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