# Truckers, what is this thing? **URL:** \ **Category:** Factual Questions\ **Created:** [June 10, 2014, 9:18pm UTC](https://boards.straightdope.com/t/truckers-what-is-this-thing/690505 "2014-06-10T21:18:09Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![GaryM](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/garym/32/241_2.png) [@GaryM](https://boards.straightdope.com/u/GaryM)\ **Post date:** [June 10, 2014, 9:18pm UTC](https://boards.straightdope.com/t/truckers-what-is-this-thing/690505/1 "2014-06-10T21:18:09Z")
I’ve seen these “scoops” hanging under trailers for sometime, but just today had the chance for a picture. [http://i210.photobucket.com/albums/bb256/GaryM\_photo/2014-06-10150436.jpg](http://s210.photobucket.com/user/GaryM_photo/media/2014-06-10150436.jpg.html) I’ve seen two colors, gray and blue. They don’t look like they move, but I can’t really see the mounting. What are they and what are they for? Thanks. ---
**Author:** ![ZenBeam](https://avatars.discourse-cdn.com/v4/letter/z/3ab097/32.png) [@ZenBeam](https://boards.straightdope.com/u/ZenBeam)\ **Post date:** [June 10, 2014, 9:44pm UTC](https://boards.straightdope.com/t/truckers-what-is-this-thing/690505/2 "2014-06-10T21:44:53Z")
It’s for improving aerodynamics. [Here’s what may be the manufacturer](http://smarttrucksystems.com/undertray.php), but it’s at least similar. ---
**Author:** ![GaryM](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/garym/32/241_2.png) [@GaryM](https://boards.straightdope.com/u/GaryM)\ **Post date:** [June 10, 2014, 10:45pm UTC](https://boards.straightdope.com/t/truckers-what-is-this-thing/690505/3 "2014-06-10T22:45:22Z")
OK, I can see that. It directs air below the rear axle assembly and reduces turbulence. The back of it being open made me think it might be some kind of delivery chute. I’d seen delivery chutes on milk trucks many years ago. ---
**Author:** ![carnut](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/carnut/32/3531_2.png) [@carnut](https://boards.straightdope.com/u/carnut)\ **Post date:** [June 10, 2014, 11:11pm UTC](https://boards.straightdope.com/t/truckers-what-is-this-thing/690505/4 "2014-06-10T23:11:41Z")
> [@ZenBeam](#): > > It’s for improving aerodynamics. [Here’s what may be the manufacturer](http://smarttrucksystems.com/undertray.php), but it’s at least similar. This. It has a terrific effect on fuel milage. A trucker I know says it helps handling too, but I can’t find anything to confirm that. ---
**Author:** ![carnut](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/carnut/32/3531_2.png) [@carnut](https://boards.straightdope.com/u/carnut)\ **Post date:** [June 10, 2014, 11:19pm UTC](https://boards.straightdope.com/t/truckers-what-is-this-thing/690505/5 "2014-06-10T23:19:58Z")
Edit to the above: With what I know about race cars I think he’s probably right. In car racing, undertrays and diffusers help the car hold to the road better by directing airflow under the car. Though these pieces on trailers are very different and much smaller relative to overall vehicle size, it would very likely have a much smaller, but similar effect. ---
**Author:** ![GaryM](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/garym/32/241_2.png) [@GaryM](https://boards.straightdope.com/u/GaryM)\ **Post date:** [June 11, 2014, 12:21am UTC](https://boards.straightdope.com/t/truckers-what-is-this-thing/690505/6 "2014-06-11T00:21:26Z")
I guess the deflector I posted is used instead of the side skirts I see installed. The ones running down the side below the trailers box. Maybe both are used but we can’t see them. ---
**Author:** ![rsa](https://avatars.discourse-cdn.com/v4/letter/r/839c29/32.png) [@rsa](https://boards.straightdope.com/u/rsa)\ **Post date:** [June 11, 2014, 12:43am UTC](https://boards.straightdope.com/t/truckers-what-is-this-thing/690505/7 "2014-06-11T00:43:48Z")
Looks like a good place to stow-away for a long distance lift. \ ---
**Author:** ![carnut](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/carnut/32/3531_2.png) [@carnut](https://boards.straightdope.com/u/carnut)\ **Post date:** [June 11, 2014, 4:38am UTC](https://boards.straightdope.com/t/truckers-what-is-this-thing/690505/8 "2014-06-11T04:38:46Z")
> [@rsa](#): > > Looks like a good place to stow-away for a long distance lift. \ So when the driver hits the brake suddenly, what will you use to prevent yourself from sliding out and under the rear wheels? ---
**Author:** ![Terr](https://avatars.discourse-cdn.com/v4/letter/t/839c29/32.png) [@Terr](https://boards.straightdope.com/u/Terr)\ **Post date:** [June 11, 2014, 4:41am UTC](https://boards.straightdope.com/t/truckers-what-is-this-thing/690505/9 "2014-06-11T04:41:08Z")
> [@carnut](#): > > So when the driver hits the brake suddenly, what will you use to prevent yourself from sliding out and under the rear wheels? You mean if the driver accelerates suddenly. ---
**Author:** ![Shmendrik](https://avatars.discourse-cdn.com/v4/letter/s/ee7513/32.png) [@Shmendrik](https://boards.straightdope.com/u/Shmendrik)\ **Post date:** [June 11, 2014, 4:44am UTC](https://boards.straightdope.com/t/truckers-what-is-this-thing/690505/10 "2014-06-11T04:44:23Z")
> [@carnut](#): > > So when the driver hits the brake suddenly, what will you use to prevent yourself from sliding out and under the rear wheels? Newton’s first law, presumably. ---
**Author:** ![carnut](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/carnut/32/3531_2.png) [@carnut](https://boards.straightdope.com/u/carnut)\ **Post date:** [June 11, 2014, 4:48am UTC](https://boards.straightdope.com/t/truckers-what-is-this-thing/690505/11 "2014-06-11T04:48:20Z")
No. What happens when you brake hard? Think about it. When the driver hits the brake, your unrestrained body will slide forward and \*then \*back down that little slide. Truck drivers are not generally known for accelerating suddenly when there’s a trailer attached to the truck. But careless car drivers are known for cutting in front of a truck and finding that they have to brake fast. Causing the truck driver to brake fast. ---
**Author:** ![johnpost](https://avatars.discourse-cdn.com/v4/letter/j/f17d59/32.png) [@johnpost](https://boards.straightdope.com/u/johnpost)\ **Post date:** [June 11, 2014, 5:00am UTC](https://boards.straightdope.com/t/truckers-what-is-this-thing/690505/12 "2014-06-11T05:00:08Z")
maybe some smoky links and hash browns will cause the truck driver to brake fast. ---
**Author:** ![bob\_2](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bob_2/32/3341_2.png) [@bob\_2](https://boards.straightdope.com/u/bob_2)\ **Post date:** [June 11, 2014, 10:05am UTC](https://boards.straightdope.com/t/truckers-what-is-this-thing/690505/13 "2014-06-11T10:05:16Z")
Any gadget that offers a 10% improvement in fuel consumption should be treated with grave suspicion. I was involved in an assessment of fuel saving devices a few years ago (this wan no one of them) and we found that the most cost-effective thing to do, was to reduce the space (and therefore the turbulence) between the tractor and trailer to the bare minimum. Over her most tractor units have spring loaded wings at the rear sides, to close up the gap as much as they can. ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [June 11, 2014, 12:09pm UTC](https://boards.straightdope.com/t/truckers-what-is-this-thing/690505/14 "2014-06-11T12:09:52Z")
> [@bob\_2](#): > > Any gadget that offers a 10% improvement in fuel consumption should be treated with grave suspicion. 10% is a pretty big gain. It’s worth noting that the manufacturer - SmartTruck - appears NOT to be listed in [the EPA’s offical SmartWay program,](http://www.epa.gov/smartway/forpartners/technology.htm) which lists numerous other manufacturers of MPG-improving hardware for trucks. The fact that they chose “SmartTruck” as a name suggests they are trying to take advantage of potentail confusion by customers. For reference, some of the aerodynamic items on the SmartWay page offer the following estimated gains in fuel economy: Trailer gap reducer: 1% Trailer boat tails: 1% Trailer side skirts: 4% Advanced trailer end fairing: 5% Advanced trailer end fairing: 5% the 10% improvement claimed for the SmartTruck device seems unlikely. ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [June 11, 2014, 12:18pm UTC](https://boards.straightdope.com/t/truckers-what-is-this-thing/690505/15 "2014-06-11T12:18:46Z")
:smack: GAH! I need more coffee. It seems the SmartTruck devices **ARE** listed on the [SmartWay page (under “Advanced Trailer End Fairing”).](http://epa.gov/smartway/forpartners/technology.htm#tabs-2) I also found [this page,](http://web.ornl.gov/info/ornlreview/v44_3_11/article06.shtml) detailing how the SmartTruck folks utilized Oak Ridge National Lab’s supercomputer for simulating the aerodynamics of the device. Near the bottom of that page, I found this paragraph: > [@ORNL](#): > > The EPA SmartWays program has certified the UnderTray system components with a 6.8 percent fuel savings, designating them as CARB-compliant for use in California. This goal was reached with a minimum package; a full set of UnderTray components provides nearly a 12 percent savings. If all of the 1.3 million Class 8 trucks in the country were configured with just the minimum package of new components, the United States could annually save almost 1.5 billion gallons of diesel fuel; reduce CO2 by 32.7 billion pounds (16.4 million tons); and save $4.42 billion in fuel costs. So depending on which components are installed, they seem to be pretty earnest about claiming a large fuel economy improvement. Unlike most car owners, truckers and fleet owners track their fuel expenses pretty rigorously, so if the claimed MPG improvements were bullshit, I would expect them to be found out pretty quickly. ---
**Author:** ![gotpasswords](https://avatars.discourse-cdn.com/v4/letter/g/c57346/32.png) [@gotpasswords](https://boards.straightdope.com/u/gotpasswords)\ **Post date:** [June 11, 2014, 8:30pm UTC](https://boards.straightdope.com/t/truckers-what-is-this-thing/690505/16 "2014-06-11T20:30:51Z")
I haven’t seen one of these in real life yet, but aside from the aerodynamic effects, it looks like it could also hold a spare tire. ---
**Author:** ![carnut](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/carnut/32/3531_2.png) [@carnut](https://boards.straightdope.com/u/carnut)\ **Post date:** [June 11, 2014, 10:45pm UTC](https://boards.straightdope.com/t/truckers-what-is-this-thing/690505/17 "2014-06-11T22:45:40Z")
> [@johnpost](#): > > maybe some smoky links and hash browns will cause the truck driver to brake fast. 🙂 Eggs over easy, and coffee. ---
**Author:** ![Green\_Bean](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/green_bean/32/1760_2.png) [@Green\_Bean](https://boards.straightdope.com/u/Green_Bean)\ **Post date:** [June 11, 2014, 11:14pm UTC](https://boards.straightdope.com/t/truckers-what-is-this-thing/690505/18 "2014-06-11T23:14:10Z")
How much do these things cost installed? Assuming a conservative 5% savings, an average 18-wheeler, and current fuel prices, how many miles would it take to pay for itself? (Sounds like a math quiz!) ---
**Author:** ![Terr](https://avatars.discourse-cdn.com/v4/letter/t/839c29/32.png) [@Terr](https://boards.straightdope.com/u/Terr)\ **Post date:** [June 11, 2014, 11:44pm UTC](https://boards.straightdope.com/t/truckers-what-is-this-thing/690505/19 "2014-06-11T23:44:13Z")
> [@Green\_Bean](#): > > How much do these things cost installed? > > Assuming a conservative 5% savings, an average 18-wheeler, and current fuel prices, how many miles would it take to pay for itself? > > (Sounds like a math quiz!) [$2600](http://www.primetrailerleasing.com/product/ut-6-plus-undertray-system/). Let’s say 6mpg. Savings of 5% = 0.09g per mile. Fuel - let’s say $3.5/g. So - you get your money back in about 8300 miles. Feel free to check my calc if I am mistaken. ---
**Author:** ![Terr](https://avatars.discourse-cdn.com/v4/letter/t/839c29/32.png) [@Terr](https://boards.straightdope.com/u/Terr)\ **Post date:** [June 12, 2014, 12:06am UTC](https://boards.straightdope.com/t/truckers-what-is-this-thing/690505/20 "2014-06-12T00:06:33Z")
> [@Terr](#): > > [$2600](http://www.primetrailerleasing.com/product/ut-6-plus-undertray-system/). Let’s say 6mpg. Savings of 5% = 0.09g per mile. Fuel - let’s say $3.5/g. So - you get your money back in about 8300 miles. Feel free to check my calc if I am mistaken. I see I did make a mistake. 6mpg. rounding up - 0.17 gpm. 5% of 0.17 is 0.009. times $3.5/g = $0.032 per mile. 2600/0.032 = 81,250 miles. eek. I guess it is great to save 5% on fuel, but with ROI of 81,250 miles, I wonder if it is worth it. [Next page](https://boards.straightdope.com/t/truckers-what-is-this-thing/690505.md?page=2)