wind drag/weight conversion

BTW, if you want drag, try pulling a 7x12 flat deck trailer with a 5' high, full-width expanded metal ramp raised up vertical.

That little 900lb single axle trailer pulls harder with the rear ramp up than my 2500lb 7x14 enclosed trailer.

Thanks for all the input and calculations. I will tell my friend, his theory is busted.

We expounded quite a bit on the effects of wind resistance and weight. In response to your original question: No, you cannot equate the additional trailer height to an added trailer weight with any accuracy, even as sloppy as a rule of thumb. There's just too many variables.

mkirsch wrote:
They don't call them rules of thumb for no reason.

This pretty well says what the OP was looking for, a general rule of thumb. THis is no different than the WB/Trailer length ROT that folsk throw around. Or the max trailer is 2xGRAWR that I learned back in the early 80s before manufacture GCWR ratings became prevalent. WIth this, the more HP, lower gearing you have, the better the performance/speed, generally speaking.

As noted by many, including myself. the actual how much more or less HP is needed for a given frontal area vs another style, can vary from a low of say 100, to a high of 160-170 hp for the same wt load, depending upon the shape etc of the given load. Assuming the same speed, and a level grade.

Going up hills, addition hp is based on weight only. For 15K, it is about 40HP per 1% additional grade you go up. A 4% grade would be about 160 hp plus the 105 or 135 hp amounts I quoted earlier, or in the 260-280 hp range. plus or minus depending upon the aerodynamics of the rig.

Marty

Horsepower requirements, weight vs wind, depend very much on speed and road grade.

Rolling resistance is a close to a constant, value depending on weight, tire type and pressure, road surface type, and a few other things that make it difficult to estimate. Because it is a constant, HP required is linear with speed.

Wind resistance is frontal area x a shape factor, both constant, but rises with the square of speed. Thus the hp required goes up with the cube of speed.

That's on level ground, at constant speed. To climb a grade, you need extra power, directly in proportion to weight (which is mass x acceleration of gravity). To accelerate, you need extra power in proportion to mass and rate of acceleration.

Depending on weight/mass (our culture tends to treat them as equivalent), frontal area, and the constants for shape and rolling resistance, the power required to pull the weight on wheels is at first dominant, but at some speed the power required to overcome aerodynamic drag rises to match it and from there becomes much more important.

Then there is the power required to run accessories, especially A/C and mechanical hydraulic pumps, which are load dependent, and others like cooling fans that are RPM dependent, and when you are sitting still these are dominant. Most calculators I've seen fudge these into rolling resistance, or ignore them, but a subcompact car at city traffic speeds might use more power to run the A/C than to move at 10-20 mph, but will use most accelerating in stop and go.

There are calculators out there to put these things together, and some built into spreadsheets will make pretty multi-curve graphs of the power needed but you are just guessing at some of the factors in the rolling resistance constant (which might only try to consider the tire part of that), and almost always leave you guessing at the aerodynamic drag coefficient which can range from 0.3 to more than 0.6 for typical motor vehicles and go beyond that range for exceptional ones.

Hi Doug,

Add that speed changes also applies to directional changes

The greater the weight, the greater the force required to change it's direction

This is where folks get lost in reference to their TV's limits...AKA ratings

Refer to that a the ability to manhandle the setup and even that is seemingly
lost in the discussion of ratings/limits to performance. Of which many don't
consider braking a performance metric...

Ron Gratz X2. I'm a retired Mechanical Engineer. There's no definitive relationship between aerodynamic drag and weight. Weight is a factor in elevation changes (work to raise a mass), speed changes (change in inertia or kinetic energy), and lastly rolling resistance of the tire (almost negligible compared to the other factors). So the effect of weight will vary depending on whether you're in hilly or flat country, in stop and go traffic rather than cruising, and how much you're able to maintain a constant speed. Thus the effect on mpg can vary widely.

They don't call them rules of thumb for no reason.

I recently traded a 19 ft travel trailer that had a fairly flat front for a 27 ft travel trailer with a rounded fiberglass front cap. The new trailer weighed almost twice what the old one did.
When I towed to old one (empty) to the dealer I got 9.1 MPG and then towing the new one home I got 9.9 MPG. It was a no wind day over very flat terrain of about 80 miles. Short test, but the 10% increase in MPG has been fairly consistent in the 5000 miles since I purchased the new tt.

The difference between my 8500# TT and my 2500# v-nose snowmobile trailer. Both same height is 1-1.5 mpg with 3 different trucks. Even with the snowmobile trailer being a v-nose. Thats 3X the weight with hardly any difference in mpg's. It's funny because it accelerates so much faster and uses less rpm's to move the lighter trailer but gets almost the same mpg's.