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Old 07-13-2010, 07:21 AM   #1
Jas
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Towing on a grade calculation

I have yet to see something on this in the forum yet. I see a lot of questions concerning wheter or not this vehicle can pull that trailer and a lot of answers from the weight police that would have you believe that the biggest semi in the world couldn't pull a popup safely. I thought I would bridge the gap a little bit with some basic/tribal knowledge on the subject.

on flat land all you have to worry about is the physical weight & wind loading on the trailer when selecting your vehicle. The weight will affect the vehicle handling and roll but if the drive line is "rated for it" you could physically move the trailer. The wind however can add the sway into the equation which is something to concider, if your vehicle isn't heavy enough or doesn't have enough traction your going to move some at some given road speed/wind load even with a mytical perfect weight/sway control hitch setup.

Now the fun begins, what happens on a hill? As you start up a hill the weight of the trailer starts pulling you down the grade which increases the effective weight on the hitch. It acts like a drag chute which is why you have to lean further into the pedal to keep speed, your effectivly pulling more weight. Now you may ask how much extra load am I pulling? here is where the calc comes in. For the sake of sanity and simplicity, lets just assume that wind and sway are non-factors as we found the majic setup. this is an excersise in pure physics.

The grade of the road is a percentage based on a give change in elevation over a straight line "flat land" distance". The down and dirty method is to multiply the grade by 100 to get a slope and multiply the slope by your trailer weight. You should forget the dry weight and either weigh the rig or use the Gross Weight (GW) of the trailer for this calc to project a worst case scenario.

Ex:

my surveyor is a SV291, 30' at a GW of approximatly 7500 lbs ( I like round numbers)

on flat ground or "0%" grade" I am only trying to move 7,500 lbs

on a grade of 1%, the trailer pulls like it's 75 lbs heavier (1%*100*7500)

on a 10% grade it pulls like its 750 lbs heavier (10%*100*7500)

on a 20% grade it pulls like its 1,500 lbs heavier (effectivly making the trailer pull like its 9,000 lbs on flat land) and so on

Now a side note is that the weight on your hitch also decreases on the grade making your weight distribution hitch and sway control mechanisms behave differently.

So in summary, don't just look at the GW of the trailer and the max capacity of the TV as the terrain you travel will definatly make the trailer pull heavier. Wind load will also wreak havoc on you if your not setup but that is a discussion that has been beaten to death on several other threads. Besides, it's been 10 years since I had to worry about calculating Reynolds numbers and drag coefficents and effective areas , etc.
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Old 07-13-2010, 07:59 AM   #2
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Good info, thanks for the post..
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Old 07-14-2010, 11:31 AM   #3
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Quote:
Originally Posted by Jas View Post
I have yet to see something on this in the forum yet. I see a lot of questions concerning wheter or not this vehicle can pull that trailer and a lot of answers from the weight police that would have you believe that the biggest semi in the world couldn't pull a popup safely. I thought I would bridge the gap a little bit with some basic/tribal knowledge on the subject.

on flat land all you have to worry about is the physical weight & wind loading on the trailer when selecting your vehicle. The weight will affect the vehicle handling and roll but if the drive line is "rated for it" you could physically move the trailer. The wind however can add the sway into the equation which is something to concider, if your vehicle isn't heavy enough or doesn't have enough traction your going to move some at some given road speed/wind load even with a mytical perfect weight/sway control hitch setup.

Now the fun begins, what happens on a hill? As you start up a hill the weight of the trailer starts pulling you down the grade which increases the effective weight on the hitch. It acts like a drag chute which is why you have to lean further into the pedal to keep speed, your effectivly pulling more weight. Now you may ask how much extra load am I pulling? here is where the calc comes in. For the sake of sanity and simplicity, lets just assume that wind and sway are non-factors as we found the majic setup. this is an excersise in pure physics.

The grade of the road is a percentage based on a give change in elevation over a straight line "flat land" distance". The down and dirty method is to multiply the grade by 100 to get a slope and multiply the slope by your trailer weight. You should forget the dry weight and either weigh the rig or use the Gross Weight (GW) of the trailer for this calc to project a worst case scenario.

Ex:

my surveyor is a SV291, 30' at a GW of approximatly 7500 lbs ( I like round numbers)

on flat ground or "0%" grade" I am only trying to move 7,500 lbs

on a grade of 1%, the trailer pulls like it's 75 lbs heavier (1%*100*7500)

on a 10% grade it pulls like its 750 lbs heavier (10%*100*7500)

on a 20% grade it pulls like its 1,500 lbs heavier (effectivly making the trailer pull like its 9,000 lbs on flat land) and so on

Now a side note is that the weight on your hitch also decreases on the grade making your weight distribution hitch and sway control mechanisms behave differently.

So in summary, don't just look at the GW of the trailer and the max capacity of the TV as the terrain you travel will definatly make the trailer pull heavier. Wind load will also wreak havoc on you if your not setup but that is a discussion that has been beaten to death on several other threads. Besides, it's been 10 years since I had to worry about calculating Reynolds numbers and drag coefficents and effective areas , etc.
I have to agree. There are WAY more factors that determine how well a vehicle will tow. There is the wheelbase, the rear overhang length, transmission gearing ( 4spd. vs. 5,6,7 spd. etc.), final drive ratio, tire size and quality ( LT tires vs. passenger tires for trucks, higher speed rated tires for smaller vehicles ), vehicle center of gravity and the handling characteristics of the vehicle itself. Hitches are often overlooked and could do a better job of weight transfer with some specialty shop reinforcing, trucks included.

Then there is the trailer; The frontal area, the shape of the front and back ( aerodynamics (or lack of) ), trailer layout, water/sewer tank locations, type of suspension, how or where the suspension is mounted, trailer center of gravity etc.

Then there is what people do; load everything in the back of the trailer, load everything in the front of the trailer, load everything in the back of the tow vehicle, skip on weight distribution ( from my experiences, trucks seem to be the worst for this ), skip on sway control, skip on pulling the trailer level, excessive speeding...

Towing a trailer is somewhat of a science. Since most people do not have the time or money ( or energy ) to research different setups with their respective tow vehicle I think that the importance of finding a dealer that does all this is often overlooked. The better dealerships will set you up and let you test tow BEFORE you buy. If it feels bad the first time it's probably because it is...

Camping is a great family activity, I hope I am able to do it the rest of my life. I know my kids have lots of fond memories already, one is 5 and the other is 7!

cheers,

shineysideup
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Old 07-14-2010, 11:40 AM   #4
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Originally Posted by shineysideup View Post
Towing a trailer is somewhat of a science. Since most people do not have the time or money ( or energy ) to research different setups with their respective tow vehicle I think that the importance of finding a dealer that does all this is often overlooked. The better dealerships will set you up and let you test tow BEFORE you buy. If it feels bad the first time it's probably because it is...
And this is why I found this forum, and the people on it, so helpful. When I had my pop-up, towing was a no-brainer....although there were still things that had to be thought out in advance. In moving up to a TT, I really wanted to know what I was getting in to before I actually got into it This forum has been a great help and I made the decision to continue forward with the purchase of our TT because of it. Even if I think that I know the answer to a question that I have about towing, I'm going to ask it here so that you all can either confirm or contradict my answer. That is the only way I can learn whether what I am doing is right or wrong.
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Old 07-14-2010, 11:57 AM   #5
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interesting read, your common sense approach makes it easy to understand just one of the many variables we should think about as we look for a tow vehicle and then the load it will be towing around.
my rig is 32k so at a 10% grade i would add 3200lbs,
i climbed a 6% grade in nm and it felt like we had a second rv pulling us back, i would add density altitude into the mix for hp de-rating. the de-rate does not affect weight on a hitch.
the 3200 would be 10% grade at sea level the entire climb, then add in the power de-rating for the altitude and air density, i would surmise the "feel" weight would climb pretty fast in the new load

25 years i never left florida with our rv, so it wasn't an issue, now i find myself scooting up non flat land.

cat has a very in depth formula on their site to figure load, speed, gearing, engine and even drag.

when i ordered a 650 a few years back, we sat down and had to know towed load, expected speed i wanted to travel, expected grades, weight of 650, etc.

the computer picked what seemed like an odd engine, tranny, rear gear combo, but it did deliver the performance i wanted......but mpg was not one of them

thanks again for your time to write this up.
it does make this site a great one with free exchange of positive ideas,
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