It's the wind, not the load!

[jsimpson]

Loaded or unloade with a 38' fiver or a tall 16 foot motorcycle trailer, I get about 121 to 12 mpg. Empty, I get about 17. It's the wind resistance. I'm wondering how much a front air dam would help clean up air flow under my 4x4???

 

[EEngel]

Air Dam

I think I saw a post once on this that said the air flow under the front is semi-critical in cooling. I'm not positive, but you may want to look into it.

 

[CChase]

I found something similar. Towing a 7K excavator on a trailer that weighs probably 1k, I got 13 (hand calc'd) towing from Oregon to Washington. Towing my 16' boat that is tall but light ~2,500 lbs with trailer, I got just slightly better mileage (13. 9), hmmmm... ... .

 

[RustyJC]

Well, if it's tall and heavy you really have a problem! Towing our 13'2" tall, 13,500 lb 5th wheel, we get about 9. 5 MPG at 70 MPH or so - it would get 10 to 10. 5 MPG before the DD2's went in. Of course, the previous V-10 3500 got 6. 5 MPG at the same speed over the same route. :{



Rusty

 

[RKerner]

wind/velocity relationships

For you engineering types that like to play with aerodynamics, heres a little insight into mileage, speed, and basic profile drag. Drag force due to profile and skin friction is proportional to the velocity of the vehicle squared.



Power to drive the vehicle due to the drag force is proportional to the velocity of the vehicle cubed.



Using some realistic numbers like a Cd of about . 6 (yes, they are that poor for pickups and semis), to increase your speed from 60mph to 75mph, the drag force increases about 56% and the power required to overcome the drag force is just about double.



Note that this doesn't include other power requirements like drivetrain losses, tire contact friction (kinetic friction), work lost in the cooling system, air conditioning, dogs hanging out the window, etc.



Hope this helps shed a little light on the mileage/speed issue.

 

[hasselbach]

Memo to self: Keep the damn dog inside the cab next time I tow, or get a cat.

 

[CoastyAV8R]

Down here in Corpus, Dodge does a lot of vehicle testing because of the heat and humidity. I see quite a few of the new Dodge vehicles, trucks and vans, pulling a trailer that has nothing more than a 10 foot tall flat face on it. The trailer weighs maybe 1500 lbs but the aerodynamic load is huge because of the front of the trailer. I talked to a driver once when they were getting gas and he said they drive them around and gather data on how the vehicle was doing towing and he reiterated how much of load that trailer puts on the truck, especially at speed. I personally get about the same mileage towing a 26 foot toy hauler travel trailer than our previous 34 5th wheel that weighed 3000 lbs more. My theory is the 5th wheel used some of the trucks aerodynamics while the TT just sits back there in it's own wind tunnel breaking it's own air.

 

[JStraw]

CoastyAV8R, remind me to not drive behind you when your TT is breaking air. Whew!!





I get 15 mpg towing with my 93 W250 with PODS and other performance tweeks. If the 3500 will only get 13 then there's a problem. It's lower to the ground and more aerodynamic than the W250 brick. Both are equal in HP and Torque. Same 3. 73's also.



Either way something isn't sitting right as the W250 is getting me 18. 5 at 75 mph, the 3500... 16 at best.

 

[jsimpson]

The air under the grille in the little openings above the bumper definitely plays a role in cooling. (The air under a Vette is almost the only air that goes through the radiator. ) But the Dodge has PLENTY of rdaitor and grille area without needing anything from underneath the vehicle. I may experiment with air dams. But then again I may never find the spare time... .

 

[Fireman]

Read "It's a drag" by Kevin Cameron in the back of issue 43.

Fireman

 

[jsimpson]

I think I was inactive when issue 43 was published.

 

[klenger]

There is a website somewhere that has an on-line calculator to determine HP needed given the truck and trailer sizes, weights, speed, and grade. If someone knows the URL, I'd love to post it on my website.