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Old 02-26-2014, 11:15 AM   #41
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Quote:
Originally Posted by padlin View Post
Brian, out of curiosity, is there a formula for hp loss on grades? I found one for altitude loss but not grade.
Altitude causes a power loss (due to air density decrease), but grade causes a power requirement (extra work needed to overcome the grade).

The general idea is to work out how much power would required to lift the rig in in elevator (so that's the vertical component of speed multiplied by the weight), or the power which would be required to pull the weight up a ramp (so that's the vertical component of weight multiplied by the speed along the road). Either way the numbers are the same, and amount to multiplying together:
  • the weight,
  • a factor for the grade (for small slopes, just the grade as % is a reasonable approximation), and
  • the road speed.
You need units of measure which work together; I use newtons and metres per second to get watts, but you could also use pounds and feet per second (and divide by the magic number of 550 lb-ft/s per hp) to get horsepower.

The Escape 17' GVWR of 3500 pounds (which I converted to kilograms and multiplied by 9.81 m/s2 of gravity to get newtons), 8%, and 110 km/h (which I divided by 3.6 to get m/s) multiplied together is my 33 kW or 44 hp value. I could have the number wrong, but it seems about the right magnitude.

Is that helpful, or just confusing? I can more clearly illustrate this, and explain the small-angle thing, but that will need to be this evening.
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Old 02-26-2014, 11:27 AM   #42
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Quote:
Originally Posted by Brian B-P View Post
I think Kip has some great points about vehicle suitability, but one comment concerns me:


What are trailer owners expecting? Do they really expect to maintain their flat-ground cruising speed while climbing the steepest grades they will encounter on a typical mountain highway? Just to elevate the weight of a fully loaded Escape 17 up an 8% grade at the 110km/h (68 mph) speed limit of BC's Coquihalla highway takes an additional 33 kW (44 hp) beyond the requirement to keep it moving against mechanical and aerodynamic drag... and a greater amount to elevate the tug's greater weight. You can do that, but you'll need that 300 hp engine and should expect that high fuel consumption.

When towing the trailer I maintain a reasonable speed up grades, enough to pass the big trucks when there is a lane to do so, but often less than my flat-ground cruising speed. I need higher engine speed than typical cruising, but I'm still not using all of the available engine speed or power. The tug easily meets my expectations.

There seems to be a tendency in this sort of discussion to confuse performance and safety. It is not unsafe or unreasonable to slow down somewhat while climbing hills. When climbing those grades I've noticed how many of the big commercial trucks maintain the speed limit, even when running empty: absolutely none. They would need - for my example grade and speed and a modest 30-ton load - about 1500 hp... nothing on the road comes close to that, 1/3 of that is great, and many make do with less.

Reliability and stability are important; every owner needs to decide whether or not the rig is expected to compete in hillclimb races.
Brian,
"What are trailer owners expecting? Do they really expect to maintain their flat-ground cruising speed while climbing the steepest grades they will encounter on a typical mountain highway? "

I expect to be able to not be a road block on hills. Resulting in folks getting impatient and passing under unsafe conditions.

I didn't say anything about maintaining my flat ground speeds on steepest grades I might encounter. I did say POSTED SPEED !

Most or all of the posted speeds I've encountered on steep mountain grades is 55 or less. The worse I've been on is an 8% and 3+/- miles in length. My Ridgeline will do that, when towing the Casita 17'er. And yeah, there is still some reserve in case it is needed. If it would not, I would purchase something with more horsepower.

Several times, I've been in situations where something was creeping up a grade and a frustrated driver tried to pass and nearly caused a wreck.
In one of those situations, I was the guy causing the problem. My TV pulled the trailer fine on most roads, but a steep mountain road brought everything to a near stop. Low gear was barely getting the job done.

I intend to not let that happen again, if possible. I've been towing campers, up to 30 feet long for over 40 years. Might say I've been there and done that.

Kip
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Old 02-26-2014, 11:35 AM   #43
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We tow out 19 with a Yukon xl , it works very well for us , everyone including the dog has there own seat, with the size of the truck and power I always feel in control. I have had trailers in the past that pushed us around and don't wish that every again . We do have enough power to maintain sensible speeds on our mountain passes with lots left if I need to pass something . I suspect a lot of us have watched someone pass a very slow moving vehicle in a un-safe manner . I don't want to be the
Cause of that but I also don't expect to go over the coquhalla at 120km/hr
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Old 02-26-2014, 02:28 PM   #44
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Originally Posted by cpaharley2008 View Post
Kip, there will be at least 8 Escapes here in April 7th Annual Eggscursion April 24-27, 2014 Townsend, TN
if you can wait that long?
Until the wife gets convinced, the wait is on. I have to be careful that wait doesn't become a big NO!
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Old 02-26-2014, 04:13 PM   #45
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Quote:
Originally Posted by Brian B-P View Post
Is that helpful, or just confusing? I can more clearly illustrate this, and explain the small-angle thing, but that will need to be this evening.
I get the gist if not the math, but thanks for the write-up.
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Old 02-26-2014, 04:49 PM   #46
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Originally Posted by Kip in Ga. View Post
I didn't say anything about maintaining my flat ground speeds on steepest grades I might encounter. I did say POSTED SPEED !
Okay, the term was "posted speed" rather than cruising speed, but in many cases the posted speed is even higher than the normal cruising speed. On the Coquihalla which I used as an example, the posted speed is 110 km/h, and few RV trailers run that fast even in flat sections - how many Escape owners cruise on even flat land at 110 km/h (68 mph)? I note - with the hope that this does not launch an unproductive argument about tires - that the tires supplied on an Escape are only rated for sustained speeds of 65 mph under normal loading and inflation conditions... so I'm guessing the posting speed on typical Canadian freeways (often 110 km/h) and U.S. Interstate highways (typically 65 mph, I hear) are higher than most of our towing speeds.

Quote:
Originally Posted by Kip in Ga. View Post
Most or all of the posted speeds I've encountered on steep mountain grades is 55 or less. The worse I've been on is an 8% and 3+/- miles in length.
I certainly have not driven everywhere - not even in every province and state - but I've never seen the posted speed limit of a highway reduced for hills; for turns yes, for grades no. If the highway is 100 km/h along the flat part, it's still 100 km/h when the road tilts up. There are lots of over-55mph highways in mountains. Since Escapes come from B.C., B.C. highways 5 (the Coquihalla section) and 97C are good examples of undiminished speed limits in areas of steep sustained grades. 3 miles of 6% to 8% grades is common on the Coquihalla and occurs in the Rocky Mountain passes of B.C. and Alberta.

Of course, steep roads are often twisty roads and usually not divided freeways; accordingly - due to the turns, intersections, and lack of a median - they often are not the highest-speed roads.

That posted number is the legal maximum speed, not the minimum required to facilitate the flow of traffic. On the 8% grades of the Coquihalla and similar western mountain freeways (which have two or more lanes for traffic in each direction) and other major highways (which commonly have passing lanes in grade climbs), you're not slowing traffic down at even 2/3 of that speed limit on sustained grades... assuming you are driving in the right (meaning both right-hand and correct) lane.

Quote:
Originally Posted by Kip in Ga. View Post
In one of those situations, I was the guy causing the problem. My TV pulled the trailer fine on most roads, but a steep mountain road brought everything to a near stop. Low gear was barely getting the job done.
What speed could that have been - 50 km/h or 30 mph with the low gearing of first? Even less because it was a "near stop"? I agree that we shouldn't be roadblocks, but there's a big difference between grinding along in first gear this way and letting the speed drop moderately from the legal limit. Now we're back to the need to understand what performance is expected. I don't pretend to know what others expect, or to tell them what they should expect, but I don't think that someone new to towing a travel trailer should be misled to think that typical travel trailer tugs can maintain any desired legal speed on any grade at will. Personally, I find that even 55 mph up an 8% grade is not necessary (although it is under the speed limit on some highways, and my rig will do it anyway), but again that's a personal preference... with costs.

It seems to be that in the decades before 300 hp engines became typical in light pickup trucks and family station wagons (which we currently call "SUVs"), people didn't expect to pull a ton or two (or three) of RV up sustained steep grades at such high speeds. Maybe they did, but if they did, I don't know how they did it, since their engines couldn't do it.
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Old 02-26-2014, 07:17 PM   #47
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Quote:
Originally Posted by padlin View Post
I get the gist if not the math...
This is a relatively common applied physics or mechanical engineering problem, so I expected to find lots of clear and illustrated explanations online. Well, I found three that might help people understand the power requirement of grade climbing, but none are great. I think the important part is to appreciate that hauling anything heavy up a hill takes power, doing it faster takes more power, and expecting to haul a couple tons up a mountain road grade at mile a minute should be expected to take a heap of power.

Climbing a Hill shows exactly what we're talking about, but mostly in geometry and algebra, rather than grahics. Please ignore the earlier and later sections on the page if you want to avoid information overload!
This also sort of explains my "small angle" comment: you need to use the sine of the angle of the slope, but that angle is the arctangent (inverse tangent) of the grade; since sine and tangent are nearly the same value for small angles, the two trig functions cancel out and you can just use the grade (where 8% is 0.08 ) for an approximation. Their example finds 70 hp for about twice the mass going up a steeper slope at half the speed.

There's a drawing in the Example - A car climbing a hill section of a Boston University prof's notes: Power. He talks about the slope only in angle from the horizontal, but of course the highway signs use grade, not angle. The arctangent (or inverse tangent) of the grade (as a fraction) is the angle; arctan ( 0.08 ) = 4.57° His example is a bit lighter, very steep (36%), and slow compared to our discussion, and he comes up with 79 hp in that case, including a small allowance for drag.

The most amusing - to my warped mind - is an online calculator for cyclists: Climbing Power Calculation. I put the numbers for a 3500 pound rider on a weightless bicycle climbing a hill at 110 km/h and gaining 8800 meteres in an hour (8% grade) - just like my earlier example - and it spits out 38 kW for the climbing, similar to my calculation. It also calculates drag, but I don't think bicycle tires or the aerodynamics of 3500 lb bike riders are very applicable to Escape trailers... or that I can find a cyclist who can put out 38 kW. The cool thing about this is that you can plug in numbers and play, without doing the math or geometry... as long as you can put in the grade as the distance climbed in any selected time. An image of my run is attached...
Attached Images
File Type: jpg 3500lbBike.JPG (42.1 KB, 12 views)
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Old 02-27-2014, 06:50 AM   #48
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Don't ya just love these fireside chats?

Brian, you wrote:

"What speed could that have been - 50 km/h or 30 mph with the low gearing of first? Even less because it was a "near stop"?

"Near stop" in my example was closer to 20 mph, by the time we reached a point of a lesser grade. Had the steep grade continued much farther, I feel we would have come to a complete stop, as RPM was dropping and resulting in a drop in torque and horsepower. My TV pulled that trailer fine on moderate hills and flats, but I didn't have the margin of safety I needed when that 8% grade was encountered.

"I don't think that someone new to towing a travel trailer should be misled to think that typical travel trailer tugs can maintain any desired legal speed on any grade at will."

By the same token, they should not be misled to thinking an "adequate" TV on flat ground, with little to no margin of extra power, will be safe for unexpected situations.

Considering these grades we are discussing. Would you recommend to a friend or loved one that adequate is fine? Do you believe a TV, with a 3000# rating, plenty good enough for a camping trailer weighing 2500# loaded?
I personally DO NOT!

" On the 8% grades of the Coquihalla and similar western mountain freeways (which have two or more lanes for traffic in each direction) and other major highways (which commonly have passing lanes in grade climbs), you're not slowing traffic down at even 2/3 of that speed limit on sustained grades... assuming you are driving in the right (meaning both right-hand and correct) lane."

So what is a marginal rig supposed to do when they find themselves on a busy 2 lane curvy road with no passing lanes and it becomes 6-8%?

"I agree that we shouldn't be roadblocks, but there's a big difference between grinding along in first gear this way and letting the speed drop moderately from the legal limit."

There we agree! Having enough TV to allow us to make the decisions is hugely better than the TV making the decisions for us.

I want to personally be able to decide the speed up the grade.
Just as important, or more so, I want to decide the downhill speed as well. With enough "Engine Braking" and wheel brakes and chassis to handle the trailer, even if it's brakes fail!

Thanks,
Kip
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Old 02-27-2014, 09:55 AM   #49
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Much of the decisions regarding "correctness" of a person's choice of tow vehicle really comes down to a couple of "musts" and various "personal preference" items. For example:

  • tow vehicle must have a tow rating/specifications that will accommodate the combined weight of the tow vehicle, it's passengers and cargo, and the trailer with it's contents. There is no legal or manufacturer specified reason that the tow rating must exceed this rating by some safety margin.
  • tow vehicle must have hitch that is sized appropriately for trailer and loads being towed.
  • if law states that due to it's size or weight, the trailer is required to have brakes, then the tow vehicle must be equipped with an appropriate means of actuating the trailer brakes.
As far as I can tell, those are the "musts", maybe there are more that others can list. Now, my thoughts are that the "personal preference" items for a tow vehicle should include items such as:


  • The amount that you tow on relatively flat terrain versus steep mountain grades may influence the amount of excess towing capacity you want. Maybe you are comfortable and happy with a tow vehicle that does not have the horsepower necessary to tow on the steeper mountain grades at posted highway speeds, and are ok with slowing down to speeds comparable to those of the big trucks in these areas. On the other hand, maybe you tow a lot on mountain grades and find the extra power from an oversized tow vehicle a necessity.
  • Whether or not your tow vehicle is your daily driver, and what kind of vehicle you want for your daily driver is a large factor. Many people don't want a large fuel intensive truck as their daily driver, and thus will look for a tow vehicle that suits their daily driving requirements first, and their towing requirements secondly. If you want or need a large or powerful vehicle as your daily driver, it would likely have a tow capacity that far exceeds what is absolutely necessary for towing a small fiberglass egg.
  • If your tow vehicle is purchased specifically to tow your egg, and is used for little else, your towing requirements probably played a greater role in the selection of that vehicle.
  • Personal preference for vehicle style plays a role. Some people are not comfortable in a smaller sized vehicle, others are not comfortable in a large vehicle. These preferences and physical comfort in your vehicle all influence your choice.


There are many more factors governing choice of tow vehicle. My personal opinion is that it is not a requirement (by law or otherwise) to have a tow capacity that far exceeds the actual loads you will be towing. However, in some instances, the extra power is really appreciated.
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Old 02-27-2014, 10:12 AM   #50
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My two cents worth is.......well, what Dave said.

Be safe, and be happy.
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