Truck aero help please

[Telco]

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.

[freebeard]

Make and model?

1. The 'spoiler' you specify won't do much. An example that would get you the most ROI with a fiberglass cap might be the Golf GT-W12-650:

Generous ducts on three sides. Else something with better access?


2. A study on the effect of exhaust position on drag and lift (08-29-2024) -- Logic

3. This will benefit from a full bellypan. Start the divergent angle as far forward as possible. A proper diffuser has strakes and fences -- an extreme example.


Make and model?

[Logic]

Originally Posted by Telco

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.

This is heartening.
It shows you have read this thread? and thx to plain old logic and deduction, find it to be of potential worth vs a large, heavy, instability inducing, unparkable boat tail. Thx!?

Other aero threads you may be interested in are Dielectric Barrier Discharge as a simple to test/implement option.

If you are the type that swings spanners at engines; pgfpro's lean burn, high compression posts are eye opening!

[Telco]

Freebeard - 2nd gen Dakota. The spoiler posted was just closest to what I could find on the internet. Those passthroughs on the Golf are neat looking, but I don't know that they would be practical on a pickup. I will give it some thought though, they might be possible. It would require relocating the brake lights, something I've been considering anyway.

What I had envisioned is a small spoiler that follows the curve of the tonneau lip, mounted about an inch up, just to divert some overflow air into the low pressure zone. My current beater has a tonneau, I might just have to experiment a little with it. Anything I do put on, I want to be unobtrusive. This should still look like a truck, without any glaring add-ons even if it means not quite getting there.

On my number 3, what I was suggesting is not a diffuser. It would be a wing spoiler designed to generate downforce. That's what the auto maker was using it for, and it was pretty much a wing spoiler like you find on any other car except it was mounted underneath where a diffuser would go and was tilted back at about a 30 degree angle, rear higher than the front, if I remember correctly. The car in question did not have a diffuser, but the Top Gear tester (one of the new bunch, not Clarkson, Hammond and May) seemed to think it made a noticeable difference.

Logic - Yes, read that center exhaust thread amongst several others, including one where some college students moved it about 20MM up at a time and tested, and found that about 1/3 up was probably the best location for it. I'd rather do nothing at all than have a pickup that looks like a land speed record holding bicycle, no offense to those who don't mind it

[freebeard]

The example was extreme to make the point. Starting with a [fiberglass] cap, about half-way between the two examples, one might turn any side windows into external ducts and cut a similar external duct into the top rear -- giving chamfers that retain half the rear exposure. It might look like this:



With a long entrance [NACA} duct and a narrow exit the effect should be an annular jet.

Else eliminate the spoiler and connecting rails as in the second example. Pickup aerocaps have a small rear window above the tailgate similar to that design.

The most unobtrusive would be a half-tonneau. That gives a lockable trunk with an open bed forward of it.

The underbody wing would necessitate a proper diffuser, and probably not contribute any more than fences would