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Old 02-05-2017, 03:58 PM   #1
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Linear relationship?

I have a slightly different question regarding the age-old problem of what to tow or can it be towed.

Our 32' Southwind (Triton v10) quite happily pulls a 2012 Escape (~3300 lb). I'd like to pull a pickup instead, so as to also be able to carry an ATV.

If I drive the m/h up my favourite hill with and without the toad, and measure the difference in time, can that difference be extrapolated against the greater weight of the p/u and its load? I.E. if the p/u, loaded, weighs twice as much as the Escape, will the extra time be twice the current increment?

The simple answer would appear to be yes, since hp is just a measure of work done and more work will take more time. Once transmissions get into the equation do things become more complicated? Differences in wind resistance will be trivial.

For this discussion let's presume that braking systems, gcwr etc. are all within acceptable limits.

TIA for your collective wisdom.
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Old 02-05-2017, 09:22 PM   #2
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It is more of a MH chassis problem and not the motor. Depending on the chassis, you are probably limited to a max of 5,000 tow.
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Old 02-06-2017, 10:08 AM   #3
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Quote:
Originally Posted by dons2346 View Post
It is more of a MH chassis problem and not the motor. Depending on the chassis, you are probably limited to a max of 5,000 tow.
Yeah, I'd be surprised if a 32' Southwind had a 10K hitch which would be required to pull a pick up with an ATV in the back. I have an aluminum F150 Supercrew 4X4 and it weighs 4800 empty. Throw in some gas and the ATV and you are looking at well north of 5000 lbs which is the weight limit for the hitch on most smaller coaches have.

On edit: Can you get an ATV into a smaller pickup like the Colorado? A 4X4 Colorado extended cab weighs about 4300 lbs empty. It's not much lighter than the aluminum bodied F150 which is why I ended up with a Ford instead of Chevy, but it might be less than 5000 lbs with the ATV.
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Old 02-06-2017, 11:34 AM   #4
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You've missed the question. I used the numbers as examples, with the qualifier that we should assume gcwr etc are within acceptable parameters.
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Old 02-06-2017, 11:54 AM   #5
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No, it's not going to be linear. It'll be an increasing curve. At some point, you won't be able to pull the hill at all and the time goes to infinite. So double the towing weight will more than double the incremental time.
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Old 02-06-2017, 03:48 PM   #6
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FlyingDiver is correct, of course, but over a relatively modest range the hill-climbing performance is probably close to linear. If your rig is 20k lbs + a 4k toad, and you increase that to a 5k toad, you increased the direct load on the engine by 1k/24k or about 4%. That has little meaning on the straight & level, but when climbing the hill every bit of that extra has to be carried to the top. The problem is, what does that 4% do to the overall performance?

Toad vs no-toad changes more than just weight - it adds wind resistance and tire friction. However, increasing the weight of the toad probably doesn't change the wind resistance, but a different shape toad may well do that. And a heavier toad has only a tiny effect of tire rolling resistance, but a bigger toad probably has bigger tires and THAT almost surely changes things a bit. And so on... So if you added 1000# of bricks to the existing toad, the results are perhaps predictable, but changing toads to a heavier vehicle type throws everything up for grabs.
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Old 02-06-2017, 06:31 PM   #7
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Oliver II-

Test, and let us know the results.

<edit>I'll be interested in how you control for initial conditions.</edit>
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Old 02-06-2017, 06:34 PM   #8
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IMO the relationship is not linear but will be on a slight curve. As the weight increases the performance will expodentally decrease.
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Old 02-06-2017, 07:03 PM   #9
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Personally, I think it will be an exponential shape curve. Every vehicle has a max speed, all the various types of resistance add up and quickly as speed increases, few vehicles can get much over 100MPH. I suppose you could make some inferences by monitoring your mileage on flat and on hill both with and without your toad. Try to get an idea of the rate of mileage decline as the hill angle increases. Then make the curve appropriately steeper for the heavier load.
Also where the hill is located will make a big difference. Going up the Ike in the Rockies will probably not be an enjoyable ride!
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Old 02-06-2017, 07:18 PM   #10
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I am qualified to answer this...because I stayed at a Holiday Inn last night

The problem I see is the transmission will be shifting differently.

Look at it this way, if you kept your speed at 10mph, both the light and heavy vehicle would take the same time to climb the hill...but the engine would be doing different amounts of work coupled with the transmission shift points changing.

At 55 mph, with wind, rolling resistance, transmission shifts, limited RPM I don't see a simple prediction....I would not risk buying a new vehicle based on this guess.

It would be interesting to try a hill climb with empty Black & Gray tanks, then same test with full tanks. If you added 100 gal of water, that would be 834 lbs. extra. Could even include diesel fuel to vary the weight (7lb per gal)

But it is a very interesting question....

Dan
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Old 02-07-2017, 09:19 AM   #11
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No, it's not going to be linear. It'll be an increasing curve. At some point, you won't be able to pull the hill at all and the time goes to infinite. So double the towing weight will more than double the incremental time.
Agreed. No engine or driveline has an unlimited capacity to move a large weight.

My experience with towing various loads behind a car or SUV has been that the vehicle is generally good at pulling 1/2 the rated load and beyond that there is usually a big reduction in performance - handling, fuel economy, etc. Is the m/h likely to show the same characteristic if I increase the load significantly beyond that of my beloved Escape?
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