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Discussion Starter · #1 ·
Has anyone noticed anything odd when towing cargo trailers? Here's mine.

We drive on pretty flat urbanized terrains in southern California with our 2004 T'Reg. Nothing special. Removed the top rack, and drive normally without any additional payload, aside the family of 4 people (2 adults, 2 children), and an occasional 4x8 utility trailer to run errands with.

We hauled the 4x8 trailer without any modding to the trailer for the longest time. Max onboard report MPG was 25.3 for a 75 miles trip, travelling at a constant 65mph.

Trailer without cargo was 350lbs, dry weight.

We then have the need to convert the open utility trailer to a cargo trailer design. I visited the EcoModder forum, and came up with some solutions to minimize the gap between the front of the trailer and the rear of the T'Reg. Total gap is no more than 46in with an incomplete cargo box trailer. Top of the cargo box trailer is 2ft lower than the top of the T'Reg. Added weight using pine, fir, and some hardwood, and hardware raised the trailer's total weight to 480lbs.

We had an errand to run with partially completed cargo trailer for the 69 miles trip, travelling on avg at 65mph most of the way, with the slowest speed at 55mph. I got a new value of 31.3 MPG. We're turning to each other and scratched our heads.

- Same slow acceleration utilizing the hypermiler method.
- Maintain constant speed using cruise control.
- No change in fuel type or formulation (Costco gas).
- Tire pressure has always been constant for both vehicle (45psi) and trailer (100psi) on the trips.

Has anyone noticed something odd like this?

I'm finishing up the trailer soon, with the upper design to reduce the total grap between vehicle and trailer to 30in, just enough to allow the trunk door to be opened when the trailer is in-tow, with wedges in promoting aerodynamic at 35° angles at the front, possibly with side skirts at the bottom of the trailer later just for the fun of it. I'm still a bit baffled, and excited at the same time.

We'll provide more info later about the MPG for the long trips of 2,700 and 7,900 miles, with a fully loaded trailer at 850lbs.
 

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2015 (2016 MY) Touareg SE 3.0 V6 TDi 262PS
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What were the wind speeds/directions for the two runs - with a large frontal area trailer the difference between headwind and tailwind on fuel consumption can be big. Even the effect of the prevailing wind can make a noticeable difference.

Your trailer aerodynamics would be much better with rounded corners, larger the radius the better - and consider making the undersides smooth.
 

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Discussion Starter · #3 ·
Wind direction was NW to SE, as I was travelling west. Since the hours between 3PM to 8:30PM during the late winter period, the late solar wind was relatively constant between 4mph, at max at 8mph during this time period. Same general direction, same wind patter, and pretty much the same time frame, within the same month period.

As for the trailer, yes, you're right about the round corner. I'm still trying to understand why such a big different between unmod vs mod. Unfortunately, any more substantial mod for the body curvature design will affect the storage area greatly, since I only have half of the garage to store the trailer. No additional storage area can be allocated. Otherwise, I would have gone crazy with the modding. Underside is definitely smooth from front to back of the trailer, aside from he accented rise near the hitch coupling, due to the tongue jack (with wheel).
 

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This is drag created by the shape of the trailer/Touareg combo. One is "cleaner" than the other. But also, there is a BIG difference in fuel economy from 55mph and 65mph when towing!! Trailer shape, and where it is inside of the vacuum created by the Touareg plays a large part in fuel economy.

When I tow a small (3000 pound) travel trailer, 7 feet wide, 10 1/2 feet tall with A/C, at 55mph the MFD reports 21 mpg on the flat, but if I drive at 65mph on the flat, it drops to 17mpg. That's the cost of trying to go faster through the wind with a large wind resistance. If there was a way to not let the air passing over the Touareg to fall into the gap between trailer and instead allowing it to flow up and around the TT smoothly, economy would improve.

There are devices sold that claim to help smooth out airflow on trailers, or for the SUV's, but they all cost a small fortune, so no real improvements in economy would be gained for a long long time. No payback. Only if it vastly improved safety and handling should anyone bother with these add-ons.
 

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I'm finishing up the trailer soon, with the upper design to reduce the total grap between vehicle and trailer to 30in, just enough to allow the trunk door to be opened when the trailer is in-tow, with wedges in promoting aerodynamic at 35° angles at the front, possibly with side skirts at the bottom of the trailer later just for the fun of it. I'm still a bit baffled, and excited at the same time.
Try also round out the rear corners if possible, changing the shape a bit to be similar to a fish, dolphin , or whale to get air to flow back together behind the trailer instead of being turbulent in a square vacuum. It would improve economy and handling.

Trailers aren't designed like this because it reduces storage capacity and increases the cost of the trailer. The crossover point between function and fuel economy is hard to justify increasing trailer cost to save a little bit of fuel. Most people will take utility over fuel economy.

I should also note that if you tow those two different shapes in mountainous terrain, you will get nearly IDENTICAL fuel economy. Once wind is no longer a factor, the weight is the driving force of fuel economy when climbing hills at lower speeds. Heavier trailers costs more energy to pull uphill. At 5-pmg the economy would be the same for a given weight. But on the flats at the same speed , the more aerodynamic trailer wins.
 

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You're hauling a box behind a box.

Two trips don't provide enough data to draw any conclusions.

Seems to me that you're over-thinking things: Report back after twenty trips and I guarantee you'll have a more reliable average to think about...and that by then you won't even care! When trailering, I use the MPG display for entertainment purposes only.

Be safe out there.
 

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Discussion Starter · #8 ·
This is drag created by the shape of the trailer/Touareg combo. One is "cleaner" than the other. But also, there is a BIG difference in fuel economy from 55mph and 65mph when towing!! Trailer shape, and where it is inside of the vacuum created by the Touareg plays a large part in fuel economy.
This is indeed true. I've driven between 55 vs 60 vs 65 with the open utility trailer. The gain between 55mph vs 65mph is about 6.8 MPG, and 55 vs 60 is about 3.5 MPG empty on flat road. Since we hardly add loads beyond the lids of the modded cargo trailer now, we saw a massive differences in stability and smoother ride, on the contrary to those who had reported in having a fully enclosed trailers caused sway due to side turbulence. Still trying to understand more about trailer's aerodynamics in the scheme of turbulence.

When I tow a small (3000 pound) travel trailer, 7 feet wide, 10 1/2 feet tall with A/C, at 55mph the MFD reports 21 mpg on the flat, but if I drive at 65mph on the flat, it drops to 17mpg. That's the cost of trying to go faster through the wind with a large wind resistance. If there was a way to not let the air passing over the Touareg to fall into the gap between trailer and instead allowing it to flow up and around the TT smoothly, economy would improve.
This is why I try to extend the upper part of the trailer to reduce the gap between the T'Reg and the trailer as much as I can , without having to add a shroud over the rear view mirror visible zone to maximize flow. I rather sacrifice a bit of fuel and still see the rear.

There are devices sold that claim to help smooth out airflow on trailers, or for the SUV's, but they all cost a small fortune, so no real improvements in economy would be gained for a long long time. No payback. Only if it vastly improved safety and handling should anyone bother with these add-ons.
Are you talking the AirTabs and similar products? I rather not spend the extra fortune to slightly increase the MPG. Wife don't like the vehicle looks like an odd experiment. She had me purchased the T'Reg because she wanted the raw power looking body in a slick and stud-muffin appearance. She turned down the Cayenne in favor of the T'Reg. *phew* Easier on my pocket book.

Try also round out the rear corners if possible, changing the shape a bit to be similar to a fish, dolphin , or whale to get air to flow back together behind the trailer instead of being turbulent in a square vacuum. It would improve economy and handling.
Planning on creating something similar to the fleet's trailer tail. A project in the making.


Anyone see a difference in MPG in relationship with ambient temperature, (worse when colder)?
Nope. We go up the mountain to have snowboarding day, and all the way through TX when temp was high, with days of high humidity. MPG remains constant. I was thinking that it would with cooler air in the 60°F to 70°F as optimum vs temps between 90°F all the way to 110°F. This T'Reg and 2011 VW Routan minivan changed that mindset. However, our old 2001 Hyundai Santa Fe did benefit from temperature range between 60°F to 70°F. We get between 7 to 12 MPG extra on flats, and 2 to 5 through the hills or mountains.
 

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Discussion Starter · #9 ·
You're hauling a box behind a box.

Two trips don't provide enough data to draw any conclusions.

Seems to me that you're over-thinking things: Report back after twenty trips and I guarantee you'll have a more reliable average to think about...and that by then you won't even care! When trailering, I use the MPG display for entertainment purposes only.
MFD display is what I used to gauge for odd anomalies, and of course, creating track records. We have logs of the T'Reg hauling the open utility trailer, fully loaded with the kayaks and camping gears in the past for thousands of miles. We'll have a few more chances soon for trips to WA and to upstate NY from CA. Plenty of chances to record data to iron out this anomaly :grin2:
 

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Anyone see a difference in MPG in relationship with ambient temperature, (worse when colder)?

Winter, Yes
1. Anti gel in fuel cuts mileage.
2. Colder air is denser and is harder to 'chop' with the turbo.

Altitude:
1. The higher you go, thinner the air is (again, easier to chop, so better mileage), offset by colder air.
My best mileage was over Eisenhower Tunnel/pass 11,158 ft (3,401 m)

If you really want to know your mileage, track it with fuelly.com

Lot of towing mileage: 14 T-Reg (Volkswagen Touareg) | Fuelly

#7k
#3k

Super low mileage from 100 MPH cross wind (literally).

I do have the drive video that I may post. 8 hours in 100MPH cross winds towing #3k

>:))
 

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Discussion Starter · #11 ·
Winter, Yes
1. Anti gel in fuel cuts mileage.
2. Colder air is denser and is harder to 'chop' with the turbo.

Altitude:
1. The higher you go, thinner the air is (again, easier to chop, so better mileage), offset by colder air.
My best mileage was over Eisenhower Tunnel/pass 11,158 ft (3,401 m)
Hm, that's interesting between TDI with diesel over the standard 91 octane gas. Good to know.

Super low mileage from 100 MPH cross wind (literally).

I do have the drive video that I may post. 8 hours in 100MPH cross winds towing #3k

>:))
Did you go through a tornado alley? Wow.
 

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Air pressure. All about the air pressure. Any turbo charged vehicle will have better power at higher elevations. This is simply air pressure.

At 20°C temperature (which does change from sea level to high elevation changes)
At sea level ( zero feet) about 14.7 psia.
At 4000 feet 12.73 psia
At 9000 feet 10.57 psia.
At 11000 feet 9.79 psia.

If you can make 240HP/406TQ at sea level with boost from the turbo, then at higher altitudes, the turbo boost will give SOME compensation. The air pressure is a BIG deal.. This is why engine output gets derated at higher elevations.

With a normally aspirated vehicle (no supercharging, no turbo) the engine must make do with incoming air pressure, and this results in a different fuel/air mixture, making less power. The air is slightly "thinner" at high elevations (less oxygen) but the reality it has very little effect on the engine. Air pressure has MUCH more effect. And COLD air robs power in the winter because it has to be heated. This is not a large amount, but a small fraction.
 
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