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Old 06-18-2020, 02:34 PM   #1 (permalink)
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Box van rear end mod

Hello All,

I have a non-articulated box truck with front wind deflector.

The rear of the box ends with a typical 90 degree angle with cross section of about 5 metres / 54 sq ft, so huge opportunity to reduce drag at the back. There are several options including flow deflectors and vortex generating devices, the latter of which are very cheap and easy to fit.

However, although I've found a lot of research and articles on the benefits of the different methods, what I really need to know is in the case that I fit vortex generators anyway, how much more improvement would I get from changing the rear profile with some flow adjustment to start convergence of the airflow before the end of the box?

I don't have any particular flow adjustment in mind, but know there are quite a few products for freight trucks in the U.S. As I do quite a bit of very long distance motorway driving, any improvement effort woud be worthwhile.

Many thanks.

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Old 06-19-2020, 10:27 AM   #2 (permalink)
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box truck wake

Dr. Wolf-Heinrich Hucho ran Volkswagen's climatic wind tunnel for 10-years. He personally tested everything known in his day, and reported that the only two technologies that ever demonstrated meaningful drag reduction were either boat-tails, or box-cavities.
I know of no vehicle which ever benefitted from VGs unless there was 'more' body downstream of the VGs, at which point the flow could reattach to.
The original 'Air Tab' was for a Honda Accord notchback, and helped the separated flow reattach onto the boot (trunklid).
The Mitsubishi Lancer EVO had factory-engineered VGs to provide for reattachment onto the boot of a notchback car.
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There's nothing to reattach to behind your truck and any claim that VGs can help you are extremely dubious.
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There are some boneheads that have been driving through New Mexico with bi-wing foils extending beyond the rear sides of their 18-wheeler's trailer van with the expectation of a benefit. Hucho tested a monofoil version and this was also a fail.
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I would recommend you track down, online, NASA's 'shoebox' and 'bat-truck' project,from the 1970s, conducted at Edwards Air Force Base,California.
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Old 06-19-2020, 01:10 PM   #3 (permalink)
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I had an idea for rear truck aero improvement, back just after the first gas crisis for a device, that would self retract, hinge to one side when its in driving position but quick rear truck access is needed, would be kept inflated by trucks compressed air system, it would need to be huge to be effective, it would be out of rear sight lines of the truck's driver, it would be light, it could be internally lit so other drivers would be aware of its location at night/dusk, it would inconsequential IMO in most accidents, unless it became detached, it would be another surface for company logos/artwork, it likely would best bang for buck if properly contoured for rear aero drag reduction. I was going to call it a "Bee tail" , and add stripes for the initial run. I got so enamored with my custom front of the truck aero device, which caused gas mileage from 5.4 to approx 7.4mpg at 70mph, I never followed up on it. Oh well. I probaly have a pic somewhere, because it was so uncommon back then, I took a lot of ribbing for it.

To the OP, I think its a lot easier to gain improvement at the front first, before going to the back, which you may have already done.

PS I believe in the US in most states, rear aero devices are not included in truck length restrictions, not sure if still widely the law.
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Old 06-19-2020, 01:47 PM   #4 (permalink)
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rear length

Quote:
Originally Posted by j-c-c View Post
I had an idea for rear truck aero improvement, back just after the first gas crisis for a device, that would self retract, hinge to one side when its in driving position but quick rear truck access is needed, would be kept inflated by trucks compressed air system, it would need to be huge to be effective, it would be out of rear sight lines of the truck's driver, it would be light, it could be internally lit so other drivers would be aware of its location at night/dusk, it would inconsequential IMO in most accidents, unless it became detached, it would be another surface for company logos/artwork, it likely would best bang for buck if properly contoured for rear aero drag reduction. I was going to call it a "Bee tail" , and add stripes for the initial run. I got so enamored with my custom front of the truck aero device, which caused gas mileage from 5.4 to approx 7.4mpg at 70mph, I never followed up on it. Oh well. I probaly have a pic somewhere, because it was so uncommon back then, I took a lot of ribbing for it.

To the OP, I think its a lot easier to gain improvement at the front first, before going to the back, which you may have already done.

PS I believe in the US in most states, rear aero devices are not included in truck length restrictions, not sure if still widely the law.
As far as I know, the DOE/ DOT will allow up to 60-inches of rear elongation if it's for fuel economy.
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Old 06-19-2020, 02:01 PM   #5 (permalink)
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What kind of truck are you driving?

Trailer tails are good for about a 5 - 6% fuel economy improvement on tractor / trailers. They should work on a trucks too. The fancier ones automatically open and close based on speed. Basic ones are about $2,000

Side skirts are more popular because they are cheaper for the same 5 - 6% improvement. They are about $800 (That is US testing - I don't know how the side protection required in the EU effects aero)

Wheel covers are the cheapest but lowest gains (1%) They cost about $300 for a set of 4.

Combined you can expect about 12% improvement.

Trailer Tail:



Side Skirt:




Wheel Covers:

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Old 06-19-2020, 02:14 PM   #6 (permalink)
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kind of truck

I believe that he mentioned a box truck. NASA got their's down from Cd 0.88, to Cd 0.238 with a full suite of add-on devices by 1980.
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Old 06-19-2020, 03:18 PM   #7 (permalink)
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Quote:
Originally Posted by aerohead View Post
I believe that he mentioned a box truck. NASA got their's down from Cd 0.88, to Cd 0.238 with a full suite of add-on devices by 1980.
He did but they but box trucks come in all sizes. Since he is in Europe I assume it is a cab-over.

Class 4



Class 8

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Old 06-19-2020, 04:02 PM   #8 (permalink)
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Thanks very much to all for the responses. There are a lot of claims from different sources about the effectiveness of the potential mods, together with counterclaims which leave the field unresolved. At least one of the vortex device manufacturers states and continues to state that they "reduce the partial, drag causing vacuum that exists at the rear facing surfaces".

It's not for me to resolve the usefulness of the mods, other than by trying them out; that's very cheap and quick to do with a v.g.. The vehicle is anyway no fun to drive in high crosswinds. As most other additions beside a v.g. will increase the surface area from the side (and I'm not aware of claims regarding major stability improvements from such changes). There are also other benefits, so the fitting of v.g.s is for now the 'base case'.

I hope I state correctly that suitably placed v.g.s at the back of the box will delay mixing of the free/boundary layer air and the 'trapped' turbulent pocket at at the back of the box. It seems to me logical, that as there a lower pressure area there (causing the drag force), the vortex boundary should deflect inward a bit (and this is what some suppliers seem to suggest). That inward defection is what the flow defectors seem to be doing.

@aerohead: yes, I accept there is usually nothing to reattach to, though very occasionally there may be a trailer; I went through the NASA project you mentioned some while back.

@j-c-c / JSH: The vehicle chassis is a Mercedes Sprinter, so not that bad aerodynamically and the box body is the part to focus on -currently the rear is the bit to figure out; further the box should soon get removed and fitted on a newer chassis, hence no point in a focus on the old one.

I've seen the Trailer Tail, Rocket Tail, Aerofin, Airtabs, among others:

Youtube: Inflatable aerodynamic trucktail for cargo trucks

&

Latest truck aerodynamics made in Germany Trucksack .

However, as overall I don't do high mileage, I'll be going for a simple solution and there is a bit less than a foot / 30cm between the end of the box and end of the vehicle, hence I could avoid overhang. Plus if it's a commercial fit it will need to have an EU type approval and I never yet saw a truck with any recognisable aftermarket deflector anywhere in Europe, though I read some are approved.

Back to the original query, if the consensus here is that v.g.s won't improve CD, then presumably any of these rear deflection ideas would yeild useful improvements in combination?
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Old 06-19-2020, 04:08 PM   #9 (permalink)
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I think the CD is around .37 for my vehicle, versus .32 for the Sprinter panel van.

Ask uncle google for images of 'sprinter luton van' and you'll get the idea.
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Old 06-19-2020, 04:10 PM   #10 (permalink)
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all sizes

Quote:
Originally Posted by JSH View Post
He did but they but box trucks come in all sizes. Since he is in Europe I assume it is a cab-over.

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Yes. Specificity would really help.
Thirty years ago, BESCO Bodies in England had a Cd 0.38 box truck. I believe that NASA's was around Cd 0.34 without the tail.
In the USA, U-HAUL is one of the few companies to use the NASA technology in it's fleet purchasing.

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