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Recon1342
Recon1342 SuperDork
9/7/23 1:44 p.m.

We all know there is a huge low pressure area behind the cab of a pickup truck. What aero solutions exist for smoothing out airflow over that bubble?

Look at the Craftsman Truck Series in NASCAR.  But, that said, they might have rules that restrict inovation.

stafford1500
stafford1500 GRM+ Memberand Dork
9/7/23 2:44 p.m.

Recon: the air flows over the cab and usually gets recirculated along the bottom of the bed floor (after encountering the tailigate), then moves forward to the back of the cab and up. This is effectively generating a rolling vortex in the bed that forms the lowest drag shape from the cab top to the tail gate top. There is also some in/out flow along the bed rails (in at the front, out toward the back). Mimicking the sloping/curving  shape from the cab top to tail gate top is the lowest drag, but the least useful truck bed storage shape. That basically removes the recirculation which is a big energy drain since it requires (effectively) horsepower to keep that vortex rolling.

Purple Frog: The trucks are required to run a flat bedtop cover and the "Romeo" windows that fill the space between the cab and cage downtubes. The general shape generates downforce in conjunction with the spoiler they are required to run. The Speedway versions of the trucks have the same requirements and the drag is reduced by other means, some of which just happen to help keep the trucks on the ground when they spin at high speed. I had the pleasure of helping test those devices at a wind tunnel in the Detroit area more than a decade ago. I would not look to the truck series as a source of inspiration for drag reduction...

stafford1500
stafford1500 GRM+ Memberand Dork
9/7/23 2:49 p.m.

As far as "better" flow in the bed, what is it you would like it to do? The answer to that will help determine the answer.

I am willing to help come up with suggestions if you like.

Recon1342
Recon1342 SuperDork
9/7/23 2:57 p.m.

In reply to stafford1500 :

I was contemplating a cab roof lip, similar to what you see on newer SUVs. Nothing crazy, just looking to improve aero a bit and maybe reduce some of the buffeting at the back of the cab. I'll post a pic later today.

Toyman!
Toyman! GRM+ Memberand MegaDork
9/7/23 3:02 p.m.

Actually, if you keep the tailgate closed, it creates a high-pressure area in the bed that directs the flow over the tailgate. The area of drag is moved behind the truck and reduced. This study says closing the tailgate is worth a 15% reduction in drag. 

https://xplrcreate.com/2019/08/08/aerodynamics-of-tailgate-closed-vs-open/ 

Closed

Aerodynamics of Tailgate Closed vs. Open - A Family Adventure Blog

Open 

Aerodynamics of Tailgate Closed vs. Open - A Family Adventure Blog

Toyman!
Toyman! GRM+ Memberand MegaDork
9/7/23 3:06 p.m.

This is from a GM study. https://media.gmc.com/media/us/en/gmc/news.detail.html/content/Pages/news/us/en/2013/May/0516-gmc-pickup-aero.html

It suggests that they saw a good bit of improvement from sealing the gap between the bed and the cab. They also suggest a soft tonneau cover. 

Expand image

stafford1500
stafford1500 GRM+ Memberand Dork
9/7/23 3:15 p.m.

The drag reduction closed vs open tail gate is about the right range. The drag isn't so much moved behind the truck as there is a mirroring effect between the cab and the tailgate with it closed, AND the flow over the top of the tail gate is generally moving downward behind the truck compared to flowing mostly straight back to meet the flow from under the truck.

Edit: above is shown in the pics you posted...

A sharp-ish edge at the back of the cab will break the flow more cleanly and provide a local turning vane to the bed recirculation. A similar type of edge on the B/C-post may be a useful test to make that area more clean. The cleaner the trip edge the less buffeting you should feel.

There is also a jet of air coming up from under the truck between the cab and bed that reinforces the recirculation. That can be sealed with foam, but not hard materials since most trucks are still body on frame and the twisting would chafe the painted surfaces.

Edit 2: again, as noted above

bobzilla
bobzilla MegaDork
9/7/23 3:37 p.m.

Tailgate open/closed also have to factor in cab length and bed length. IIRC a really short bed with long cab couldn't generate the vortex to create the pocket of air needed to improve the airflow. 

stafford1500
stafford1500 GRM+ Memberand Dork
9/7/23 3:50 p.m.

In reply to bobzilla :

Its not that it can't generate it, it is more difficult to keep it 'contained' and therefore you get more buffeting on the back of the cab. 

Recon1342
Recon1342 SuperDork
9/7/23 6:24 p.m.

Profile of the cab/bed junction. I suspect a lip spoiler that extends 2-3" past the cab roof would smooth things out considerably. 
 

bobzilla
bobzilla MegaDork
9/7/23 6:57 p.m.
stafford1500 said:

In reply to bobzilla :

Its not that it can't generate it, it is more difficult to keep it 'contained' and therefore you get more buffeting on the back of the cab. 

I knew there was something with bed and can design making things all difficult. 

stafford1500
stafford1500 GRM+ Memberand Dork
9/7/23 7:19 p.m.

In reply to Recon1342 :

You may be able to find a car decklid spoiler that can be fitted to the top back edge without too much effort. Make the transition from the roof to the extension as smooth as possible and keep it sealed as good as you can get it. If you can smooth the bottom of the extension to the back of the cab that will help with the recirculation energy (drag). As noted earlier, you may find additional benefit with the extension turning down the B-post and running toward the bed rails. It would not need to run the full height. The cab corners are one of the dirtiest parts because the air is really trying to turn in two directions at the same location (inboard/down). Pushing that noise to a more distant location (from the cab) should reduce the amount of local noise/buffeting on the rear window.

All of this can be mocked up with cardboard/tape/sheetmetal/plywood/plastic or whatever other sheet good you may have on hand. The effect should be pretty obvious if you spritz the back widow with water and note if it dances in one spot or gets pushed away.

The cab/bed gap is another area that may need some attention. See my notes a few posts up that agree with what Toyman found on GM's research. The front of the bed rails look like they are rolled in from the B-post a little. That might be an area to fair in and smooth the flow as it passes too.

Toyman also noted that a tonneau should help with drag, but it will not necessarily help with the buffeting issue you called out. Also it appears you use the bed for truck stuff and a tonneau usually gets in the way for that sort of thing.

For the record, I do not spend any time on production vehicles if I can help it. I prefer race cars, since the budget guys don't tell us we can't make the vehicle better, that is left up to the sanctioning bodies.

bobzilla
bobzilla MegaDork
9/7/23 8:13 p.m.

I just had an epiphany. I now know why trucks are getting taller and taller. Air thins out the higher you go so they are just taking advantage of the thin air way up there in tall truck land. 

stafford1500
stafford1500 GRM+ Memberand Dork
9/7/23 8:56 p.m.

In reply to bobzilla :

Just need really tall tires to get the floor/undercarriage into that thinner atmosphere

Recon1342
Recon1342 SuperDork
9/7/23 9:47 p.m.

In reply to stafford1500 :

Fab is the easy part. If need be, I'll build a mold and glass it myself. 
 

A tonneau cover would be nice, but availability is basically nil, because 1992 Ram 50. 
 

A final question- will leveling the truck help at all? It does not get used for heavy duty truck stuff, and there's about a 2" difference between the front and rear... rake, not squat. 

Recon1342
Recon1342 SuperDork
9/7/23 9:47 p.m.
bobzilla said:

I just had an epiphany. I now know why trucks are getting taller and taller. Air thins out the higher you go so they are just taking advantage of the thin air way up there in tall truck land. 

An apostrophe? 
 

Bless you. 

stafford1500
stafford1500 GRM+ Memberand Dork
9/8/23 6:46 a.m.
Recon1342 said:

In reply to stafford1500 :

Fab is the easy part. If need be, I'll build a mold and glass it myself. 
 

A tonneau cover would be nice, but availability is basically nil, because 1992 Ram 50. 
 

A final question- will leveling the truck help at all? It does not get used for heavy duty truck stuff, and there's about a 2" difference between the front and rear... rake, not squat. 

Rake will not significantly change the flow around the cab. A 2" difference is about 1 degree, and not enough to chase. The truck moves around more than that and influence from other vehicles is even bigger.

Recon1342
Recon1342 SuperDork
9/8/23 11:22 a.m.

Thanks for the help, Stafford!

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