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P-pod for dry camping? |
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lostagain 
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Joined: 06 Sep 2016 Location: Quaker Hill, CT Online Status: Offline Posts: 2587 |
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 Topic: P-pod for dry camping?Posted: 07 Apr 2021 at 8:56am |
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No, Mark, you are engaging in sophistry misstating what I have said.
I didn't say that if you don't know all the variables then it's ok to push the manufacturers' limits. I said just the opposite. There are many variables that we don't know that make it judicious to take the limits of the manufacturer seriously and not push them. I suggested that many of those variables reduce the safe load capacity and that one should, as the engineers on this board have suggested, try to have a margin of safety below the manufacturers' limits. Your suggestion that the logical conclusion of my statements is that manufacturers are hiding the true load capacity and that it is really much lower, couldn't be more wrong. Again, you are engaging in sophistry by misstating what I said. I take the manufacturers' load limits seriously and unlike some, believe that they (and the government regulators and even lawyers), have my best interests in mind, though it may not be for the altruistic motivations I'd like. My point is that we should accept those load limits and add to the margin of safety other considerations that effectively reduce load limits such as a driver's skill, reaction time, experience and so on, not to mention the condition of the vehicle, road conditions, etc., all of which mitigate in favor of a wider margin of safety. OG and GlueGuy have both stated that we should have a wider margin of safety than less. OG has been nice enough to have given us the math that supports his advice. Both are engineers and I, as a lay person, am going to take their advice seriously and keep my towing loads below what Ford tells me is the limit.
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Never leave footprints behind.
Fred & Maria Kearney Sonoma 167RB Our Pod 172 2019 Ford F-150 4x4 2.7 EcoBoost |
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MarkW
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Joined: 25 Jan 2021 Location: Michigan Online Status: Offline Posts: 54 |
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Posted: 07 Apr 2021 at 7:03am |
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You logic seems to be that if you don't know all the variables, then it's just fine to push the manufacturer's limits.
And your logic (without any evidence that I'm aware of) is that manufacturers are lying and that safe tow loads are actually much lower than the published specs. But why would they do that? After all, few of their customers are engineers who understand the distinction. Certainly for pickup trucks (where -- unlike crossovers and minivans) max towing capacities are a major marketing issue, they claim their trucks can tow up to xxxxx lbs! and not 75% of xxxx lbs (even in the fine print). They'd face MAJOR lawsuits if they were misleading customers in this way. But when you look at the actual tests, it's pretty clear that this not what they're doing. The SAE J2807 tow tests specifically include extreme conditions (towing the max rated load up a long steep 11 mile grade at 100 degrees and with the A/C on full blast): To simulate some of the toughest conditions a truck and trailer combination will face, the J2807 “Highway Gradeability” tests take place on a well-known stretch of Arizona highway, the 11.4-mile-long Davis Dam Grade. If trucks are not tested on this specific stretch of steep road, they can be run in a simulation using a “climactic” wind tunnel. Ambient temperature plays a significant role in this test with a minimum temperature of 100 degrees required at the base of the grade. In addition to a hot climate outside the truck, the test requires the air conditioning system to be set at maximum cold, with outside air selected (not recirculating) and the fan running at full blower speed. http://www.trucktrend.com/how-to/towing/1502-sae-j2807-tow-tests-the-standard |
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lostagain
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Joined: 06 Sep 2016 Location: Quaker Hill, CT Online Status: Offline Posts: 2587 |
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Posted: 06 Apr 2021 at 7:09pm |
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The point is that it is not a good idea to exceed the working load, indeed, it's probably safer to stay a bit below it. There is more that goes into the equation than simply a working load. You also have to anticipate other factors that may affect the bolt's capacity, such as oxidation, metal fatigue, the climate where the bolt or other breakable thing is being used, it's age, etc. etc.
You logic seems to be that if you don't know all the variables, then it's just fine to push the manufacturer's limits. No, you don't know all the data, but more than likely the things you don't know are not things that will enhance safety. Many people who drive trailers with marginal TV's are not always the best drivers and don't realize the limits of their skills or their vehicle. Driver fatigue, wear and tear on the hitch and chasis, poorly balanced loads, poor brakes, bad electrical connections in the brake system, etc. etc. etc. are all things that can justify erring on the side of caution. We don't have all the real world data, but we do have some pretty smart engineers on this board who have wisely cautioned us, based on their engineering experience, that it is good idea have an adequate margin of safety. It's is unfortunate that NTSB doesn't offer a database that specifically addresses travel trailer towing accidents, but that does not mean we can use math and science to calculate the risks of towing. |
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Never leave footprints behind.
Fred & Maria Kearney Sonoma 167RB Our Pod 172 2019 Ford F-150 4x4 2.7 EcoBoost |
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MarkW
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Joined: 25 Jan 2021 Location: Michigan Online Status: Offline Posts: 54 |
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Posted: 06 Apr 2021 at 6:40pm |
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If a bolt has a tensile failure point of 2000 pounds, typically you do not see it used in applications where it would be subjected to 2000 pounds under normal working loads. The working load would be a percentage of the failure point.
Sure, but we really don't know if bolt strength ratings are a good analogy to vehicle tow ratings or not -- we can't just assume that's the case (and that's certainly not the way that manufacturers intend their tow or other weight ratings to be interpreted by customers). But the big problem with this discussion is a lack of real-world data. What percentage of those towing bumper-hitch travel trailers ever experience uncontrolled sway that causes a crash? For those that do, what were the characteristics of the TV, trailer, weight-and-balance, sway control, speed, winds, etc at the time of the accident? Again, we just don't know. There's no NTSB investigating every travel trailer crash as there is with aviation. Maybe somebody has a good collection of TT crash data, but I haven't been able to find it online.
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lostagain
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Joined: 06 Sep 2016 Location: Quaker Hill, CT Online Status: Offline Posts: 2587 |
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Posted: 06 Apr 2021 at 2:03pm |
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MarkW, when I stated that there would be no margin of safety, my reference was to the traditional concept of maintaining a desired percentage below the stated capacity as a margin of error.
If you take what I said too literally, using "absolute" capacity, then it would be more analogous to the median failure point as being the "absolute" capacity. When you reach that point, things start to break. If a bolt has a tensile failure point of 2000 pounds, typically you do not see it used in applications where it would be subjected to 2000 pounds under normal working loads. The working load would be a percentage of the failure point. Our engineers can discuss this safety margin far better than I, but from a lay point of view, it is safe to say that sh*** begins to happen when you exceed the working load limit. Certainly, the TV manufactures' engineers are not recommending working loads that are equal to the failure point, but we don't know how wide the margin is and when it comes to safety, it's better to err on the side of being a little overly cautious. Human nature is such that people make all kinds of excuses why the rules of Nature don't apply to them. Whether it's denying the existence of contagious pathogens or the laws of physics, when you test Mother Nature, she gets the last laugh.
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Never leave footprints behind.
Fred & Maria Kearney Sonoma 167RB Our Pod 172 2019 Ford F-150 4x4 2.7 EcoBoost |
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offgrid
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Joined: 23 Jul 2018 Online Status: Offline Posts: 5290 |
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Posted: 06 Apr 2021 at 11:57am |
Good to have some real world numbers. Does your vehicle list axle weights and MCGVWR? Should be on the driver's door sticker. If you have those then run through the scale with your wdh untensioned and get the weights on all 3 axles. You don't need to worry about max cargo weight of the tow vehicle. Specs are intended to be taken literally and you don't have much actual cargo weight. What matters are axle weights and max combined weight, because that is what the drivetrain and brakes have to deal with. The hitch and trailer weights can be gotten by going through the scale a second time without the trailer. Leave the wdh in the reciever, it is considered to be part of the TV not the trailer. If you really are within 130 lbs then you can't take any gear to speak of, but maybe using MCGVWR will help you. 350 ÷ 3225 is about 11% and I'd recommend keeping your tongue weight there if you can, around 10% the trailer can exhibit marginal stability. |
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1994 Chinook Concourse
1995 RV6A Experimental Aircraft 2015 Rpod 179 - sold |
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MarkW
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Joined: 25 Jan 2021 Location: Michigan Online Status: Offline Posts: 54 |
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Posted: 06 Apr 2021 at 10:58am |
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When people tow weights at or near the absolute capacity of their TV, they have no margin of safety.
That's not a proven fact (that there's a quantifiable 'safety margin' that goes to zero right at the rated weight). It's not like, say, fuel in an aircraft where you know what your fuel reserve is for the trip and where there is a sudden catastrophic failure when you exhaust the reserves (and, yes, sadly, running out of fuel is one of the most common causes of general aviation accidents). If somebody new to towing said that they felt their trailer was swaying, and were worried they might lose control, would your first advice be to buy a bigger TV? Or to make sure that there was sufficient tongue weight, add an sway-control system, and consider reducing speeds? Mine would be the latter. I'd also probably tell them that if they couldn't resolve instability problems by adjusting the load and installing anti-sway, they probably shouldn't be towing that trailer with anything. The reason to go bigger, in my view, would be if they couldn't get enough weight on the tongue to reduce the sway without exceeding the TV's max tongue weight rating.
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pedwards2932
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Joined: 21 Sep 2020 Online Status: Offline Posts: 338 |
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Posted: 06 Apr 2021 at 10:20am |
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When I weighed I had about 140 lbs (3 - 5 gallon buckets of water} that I didn't know was in there, an extra full propane gas tank, and the heavy table that comes with the rp189. At that time the trailer weight was 3400. I have 829lb capacity for the car which with the 350 tongue and my wife and I in the car gives us about 130 lbs of leeway. When I get a chance I will reweigh at CAT scales but I did weigh everything that I took out of the trailer which would put me at 3225 and all I have to do at that point is weigh what I add and weigh my tongue weight and balance the trailer for 350 lbs which is at least the 10% of total weight. I always empty the water out before we travel.
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lostagain
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Joined: 06 Sep 2016 Location: Quaker Hill, CT Online Status: Offline Posts: 2587 |
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Posted: 06 Apr 2021 at 10:16am |
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TV capacity has been an ongoing debate on this forum for years and it's never really going to be resolved. There are folks who have invested a lot of money in their travel trailers and don't really want to, or can't, incur the expense of buying a TV that has towing capacity higher than the trailer salesperson told them they needed to tow their 2200 lb. dry weight trailer. The result is they engage in a form of denial of the reality of the physics and engineering principles.
Good engineering practice is to build into any structure a safety margin above the anticipated use to reduce the probability of failure. When people tow weights at or near the absolute capacity of their TV, they have no margin of safety. The will experience premature failures, whether it's the in the drivetrain or in control of the rig. Something will inevitably and prematurely go wrong. The greater the percentage of over capacity in the TV, the lower the lower the probability of failure will be. Yes, one can tow 3500 pound travel trailer with a TV having a 3500 lb. towing limit, but it is certain that something will sooner than later break resulting in costly repairs. More importantly, there is the issue of vehicle control and the risk of an accident. Driving on a flat road at low speeds may be relatively safe. On the other hand, trying to navigate a curvy mountain road with steep ascents and descents or high speed freeway conditions, the risk of an accident is high. As I said before, if only the driver and his/her vehicle was involved, it would be simply a matter of personal choice. But when it puts others at risk of harm, then one should be more cautious in testing the limits of one's TV.
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Never leave footprints behind.
Fred & Maria Kearney Sonoma 167RB Our Pod 172 2019 Ford F-150 4x4 2.7 EcoBoost |
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offgrid
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Joined: 23 Jul 2018 Online Status: Offline Posts: 5290 |
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Posted: 06 Apr 2021 at 10:02am |
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Are you within your MCGWVR? All axle weights? Fully loaded? Some actual numbers would help folks here know what they can and can't do.
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1994 Chinook Concourse
1995 RV6A Experimental Aircraft 2015 Rpod 179 - sold |
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