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Old 01-25-2015, 08:38 PM   #1
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Default Towing Aerodynamic Loads?

I just bought a new trailer. I towed it home empty. On the way home I noticed the following: When I'm pulling the trailer in the city, say 50 mph and below, my truck has no issues whatsoever. It accelerates, brakes, handles well and never complains. When I'm pulling my trailer on the highway, say at about 55-60 mph, my truck seems to be struggling. It does not shift into overdrive, but instead remains in 3rd gear. The engine rpm's stay quite high, say 2500-3000 rpm, it never shifts into overdrive, even when going downhill. I have to keep the gas pedal down pretty far to maintain a constant speed, even when not going uphill.

So here is my question: Is it normal for the truck to stay in 3rd all the time when towing? If I keep running it like this, is it going to cause any abnormal wear or damage? Has anyone else experienced this?

More details:
Truck - 1997 Ford F-150 Lariat Std. Cab 2WD 5.4L 3.55 LSD
Trailer - Bravo Scout SC8520TA2 20' long 8.5' wide 8.5' tall, 120 degree V nose, curb weight 2900 lbs. The tires were full at 45 psig cold. The trailer was completely and utterly empty as I had just bought it.

The whole trip took me 1.5 hours, during which time I traveled 65 miles, and consumed about 6 gallons of gas. Despite the high revving, that comes out to about 11 mpg or so.

Please keep in mind that I am not asking about overdrive, the overdrive button, or how all that works. That issue has already been covered elsewhere.

I'm assuming this is caused by the aerodynamic loads from the trailer itself, which, from the front, is much larger than the truck (see details below). I also assume that the truck stays in 3rd gear simply for the simple reason that it needs to. The aerodynamic load is simply just too great to maintain high speed in overdrive. I'm just wondering if this load is something this truck can handle or should I be looking for a larger truck capable of making into overdrive despite the trailer's aerodynamic load.
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Old 01-26-2015, 09:47 AM   #2
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Originally Posted by fishchoke View Post
...
I'm assuming this is caused by the aerodynamic loads from the trailer itself, which, from the front, is much larger than the truck (see details below). I also assume that the truck stays in 3rd gear simply for the simple reason that it needs to. The aerodynamic load is simply just too great to maintain high speed in overdrive. I'm just wondering if this load is something this truck can handle or should I be looking for a larger truck capable of making into overdrive despite the trailer's aerodynamic load.
That's one reason why I keep my speeds at 60mph. The air resistance increases exponentially (?) over maybe 50 mph and becomes much more important than the trailer's weight. And if you have a headwind, it adds to the resistance as well.

As to whether your engine can handle it, a lot depends upon how much/far you tow and whether the 3rd gear engine roar bothers you too much. It's the main reason I sold the old Tacoma and bought my EcoBoost with all its HP and torque at low rpms.

EDIT: By the way, welcome to the Forum.
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Last edited by brulaz; 01-26-2015 at 12:46 PM.
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Old 01-26-2015, 12:42 PM   #3
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I have '99 f250Ld 5.4 3.73. I pull 24' trailer. I always run 3rd gear towing so my rpm's are higher than yours. I put on about 50k miles like this and doesn't hurt a thing. You get used to it after a few hours. '99 has a few more horses than '97.

I pull at 63 mph. Air drag seems the same between my '24 rounded edge trailer and my son's 19' hybrid.
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Old 01-26-2015, 01:43 PM   #4
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Quote:
Originally Posted by fishchoke View Post
More details:
Truck - 1997 Ford F-150 Lariat Std. Cab 2WD 5.4L 3.55 LSD
Trailer - Bravo Scout SC8520TA2 20' long 8.5' wide 8.5' tall,

'97 Ford RV and Trailer Towing Guide says that if your truck has the Class III Trailer Towing Group, your frontal area is limited to 60 sq. ft. Your trailer has a frontal area of 72.25 sq.ft. So yess, you are exceeding the aerodynamic capability of your tow vehicle.


'97 Supercab 4x2 with 5.4L engine and 3.55 axle has a tow rating around 8,000 pounds if nothing in the truck but a skinny driver. So trailer weight or tongue weight should not be a problem.


I had a 2003 F-150 SuperCrew 4x2 with 4.6L engine and 3.55 axle. I had the same experience as you when I drug an enclosed cargo trailer with about 60 sq.ft. frontal area from Midland to Phoenix and back. Even with an empty trailer on the return leg, the truck seemed overloaded and would not stay in overdrive. I soon traded that dog for my current EcoBoost, and now my power/torque problems are no more.
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Old 01-26-2015, 01:53 PM   #5
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'97 Ford RV and Trailer Towing Guide says that if your truck has the Class III Trailer Towing Group, your frontal area is limited to 60 sq. ft. Your trailer has a frontal area of 72.25 sq.ft. So yess, you are exceeding the aerodynamic capability of your tow vehicle.


.

Frontal area does make a big difference...agreed. You are about 21% over the guidelines in sq. ft. but that doesn't mean you can't tow that trailer. Effectively, the extra frontal area reduces your GCVR by some factor
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Old 01-26-2015, 06:54 PM   #6
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Originally Posted by DieselDawg View Post
Frontal area does make a big difference...agreed. You are about 21% over the guidelines in sq. ft. but that doesn't mean you can't tow that trailer. Effectively, the extra frontal area reduces your GCVR by some factor
Have often wondered about the 60 sq ft decision. With trailers pushing 8.5' wide and 10' tall where did the 60 sq ft standard come from and if exceeded is there a formula to decrease GCWR? Nowadays the new engines are great for towing and when you look back the old 460's, 360's 302's and early Triton motors seem anemic.
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Old 01-26-2015, 08:03 PM   #7
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Have often wondered about the 60 sq ft decision. With trailers pushing 8.5' wide and 10' tall where did the 60 sq ft standard come from and if exceeded is there a formula to decrease GCWR?
...
The Ford 2015 Tow guide says you can go up to 75 sq ft with a fifth wheel or goose neck, so that should cover most of the big trailers.

My TT is about 8' from bottom of frame to top, and 8' wide so am a bit over the 60 sq ft rating for TTs. And it's a pretty tall trailer. Not too bad ...

For trucks with less than 7700# tow rating, they say only 55 sq ft.
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Old Yesterday, 02:58 PM   #8
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Actual frontal area can/might be adjusted a bit by the "shadow" of the truck itself...the lower half of your trailer will not feel the full force of the "wind" IMO.
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Old Yesterday, 03:07 PM   #9
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Actual frontal area can/might be adjusted a bit by the "shadow" of the truck itself...the lower half of your trailer will not feel the full force of the "wind" IMO.
I think Ford takes that "shadowing" into effect when they come up with their numbers. Their wording is "total area ... that a moving vehicle and trailer exposes to air resistance."
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Old Yesterday, 09:27 PM   #10
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Thank you guys so much.
That is exactly the kind of information I was looking for.
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