Tow Ratings and Frontal Area - Fiberglass RV


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Old 06-28-2016, 02:46 PM   #1
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Tow Ratings and Frontal Area

When I was doing some research for another thread, I found out something about the new SAE J2807 towing standards that seems like it has ramifications for owners of small egg trailers.

It appears the trailers used to test tow ratings under the new standards are cargo trailers, and the frontal area may be much smaller than the typical egg trailer of the same weight. For example, in the 1000-2000 pound class, which is where may 13'ers fall, the J2807 standards specify a minimum frontal area of only 20 sq. ft., defined as the entire front of the trailer down to the ground plane. Using that definition, I'd estimate the frontal area of a Scamp 13 at about 45 sq. ft. In the 2000-3500 pound class, encompassing many 16-17' trailers, the standards specify a minimum frontal area of only 30 sq. ft.

Here in an excerpt and a link to my source:
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SAE J2807 Towing Standards

Frontal area makes a significant difference at highway speeds. Air resistance increases as the square of speed. A 1600 pound Scamp with a frontal area of 45 sq. ft. is going to put a lot of additional demand on the tow vehicle's drivetrain compared to a 1600 pound, low-profile cargo trailer with a frontal area of 20 sq. ft. (even factoring in the Scamp's rounded profile).

The takeaways for me:
  • The new towing standards don't really fit the realities of molded fiberglass eggs too well. The ratings assume dense, compact loads. Our eggs are mostly air!
  • It is prudent to allow a significant margin in the tow rating when selecting a tow vehicle for an egg-type trailer due to the additional demand of the higher-than-tested frontal area.
  • Manufacturers of tow vehicles and trailers need to provide more information to consumers. Few owner's manuals I have looked at give any guidance as to how tow ratings are affected by frontal area.

I was keenly aware of how much wind resistance a lightweight trailer can generate on our trip to CA this summer. Climbing a moderate (5-6% or so) grade west of Palm Springs against a stiff headwind (estimated at 35 mph sustained with higher gusts), we were reduced to 35-40 mph in 2nd gear. Our Scamp only weighs about half the 3500# rating of our tow vehicle, but it felt like we were dragging an anchor.

Thoughts?
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Old 06-28-2016, 03:52 PM   #2
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I would be careful in how you interpret this section of the standard.

"This section defines representative ‘standard’ trailers, connections and ballast conditions for test purposes. This is not intended to define ‘worst-case’ trailer loading or specifications, or to establish limits in trailer types that can be towed."

The specification references Appendix A which gives examples of representative trailers that meet the spec.

The tests are intended to determine the weight towing and stopping capabilities of the vehicles and I believe the intention of these standardized trailers is to ensure that other factors don't enter into the results. At the end of Appendix A, there are even suggestions as to the type of equipment that may be considered standard.

Note that while a specific stretch of road is referenced for the test, that stretch doesn't actually have to be used:

"Highway Gradeability Test is accomplished by running Arizona SR 68 (Davis Dam Grade) or simulated dynamic grade in
a climatic wind tunnel."

This is all about leveling the playing field, apples to apples and all that.

Notice that no consideration is given for the draft effect of the TV on the trailer. Did you ever see the 'Myth Busters' episode where they tested gas mileage on a pickup with the tailgate up or down.
Mileage differences were not significant. Wind tunnel tests showed that the slipstream passed mostly over the tailgate when it was up.

As has been discussed in other threads about TT frontal area, consideration needs to be given to the draft effect of TV to TT. As regards frontal area wind resistance, I think we need to look at the TV/TT combination as a whole. Some would be better than others. Though in the "Myth Busters" episode the slipstream cleared the tailgate, it would have hit dead in the middle of most of our trailers. Likely a pickup with a bed cap or an SUV would fare better that a plain pickup.

You are best guided by your own experience and your owners manual. My Outback is rated at 2700 lbs with brakes, but if I come across a long uphill grade continuous for more than 5 miles with an outside temperature 104* F or above, the rating drops to 1350 lbs. I plan my trips around that. It might even be worthwhile to check the seasonal prevailing winds in the areas we plan to travel.
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Old 06-28-2016, 03:54 PM   #3
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Jon,

I just remembered that you are in Arizona. Ever thought of giving that Davis Dam grade a run??
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Old 06-28-2016, 04:10 PM   #4
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Quote:
Originally Posted by The Minimalist View Post
Jon,
I just remembered that you are in Arizona. Ever thought of giving that Davis Dam grade a run??
Unlikely I will ever be in that part of the state with the trailer unless we are headed someplace else! (And usually anxious to get there… )
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Old 06-28-2016, 05:02 PM   #5
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I have never seen a purpose built cargo trailer which didn't push more air than a Scamp.
There is a world of difference between a flat 45 square foot frontal area and the front of a BT fiberglass trailer.
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Old 06-28-2016, 06:18 PM   #6
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Originally Posted by floyd View Post
I have never seen a purpose built cargo trailer which didn't push more air than a Scamp.
There is a world of difference between a flat 45 square foot frontal area and the front of a BT fiberglass trailer.
I don't think that's the point here, Floyd. We're not comparing a 45 sq. ft. egg to a 45 sq. ft. box trailer. We're comparing a 45 sq. ft. rounded egg to a 20 sq. ft. box trailer. That's a very small cargo trailer, something like 4'Wx6'Lx4'H. A Scamp will surely push more air than that!
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Old 06-28-2016, 08:16 PM   #7
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Our 2009 Ford Ranger had a restriction in the owner's manual that for its tow limit, the towed object needed to be no bigger than that truck's own dimensions in width and height. So any least some vehicles are being sold with restrictions alluding to this...
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Old 06-28-2016, 08:33 PM   #8
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Our 2009 Ford Ranger had a restriction in the owner's manual that for its tow limit, the towed object needed to be no bigger than that truck's own dimensions in width and height. So any least some vehicles are being sold with restrictions alluding to this...
Just what are the consequences for using your own good judgement?
Do you get charged with truck abuse and Ford takes your truck away to an automotive shelter...ASPCT?
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Old 06-28-2016, 08:38 PM   #9
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For an example The Ford Escape towing guide limits the frontal area to 24 sq ft for the 4 cylinder Escape and 30 Sq ft for the 6 cylinder Escape. Ford F250 lists that 60 Sq ft is the frontal area limit. This is just another academic discussion that will in reality be totally ignored in practice.
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Old 06-28-2016, 08:38 PM   #10
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Just what are the consequences for using your own good judgement?
Do you get charged with truck abuse and Ford takes your truck away to an automotive shelter...ASPCT?
I can see the poor overworked and abused rangers finally getting to frolic unloaded...

Fwiw, we pulled a small canned ham clearly over the height and width....but well under the maximum weight...it seemed a bit odd to say a pickup couldn't tow a travel trailer!
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Old 06-28-2016, 08:46 PM   #11
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Here's the owner's manual from a 2009 ranger...this isn't advertised as the limits, it was a shock to us, honestly, after researching! Replaced it with a Sienna that tows more with no restrictions listed on frontal area.
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image.png  
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Old 06-28-2016, 08:52 PM   #12
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I just carry a couple pontoon boats on the RAV roof rack, to block the wind.
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Old 06-28-2016, 08:56 PM   #13
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Good one Glenn😜

Sent from my VS985 4G using Fiberglass RV mobile app
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Old 06-28-2016, 08:57 PM   #14
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I just carry a couple pontoon boats on the RAV roof rack, to block the wind.
We bought a new 20 ft pontoon boat last week . We pick it up right after the 4th . I will have to try your method when we head to the UP in late July.
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