Are They Meaningless? Tow Limits, Payload and Axle Load

[BayouBiker]

Originally Posted by ITSNO60

Subscribing as this is of great interest to me, especially after reading this article by Andy Thompson in which he dispels everything I thought I knew about Payload. (scroll down to Payload) https://rvlifemag.com/towing-half-to...e-quarter-ton/

I find the article generally correct. When towing, axle loads, combined vehicle limit, and tongue limit are far more relevant than payload primarily because of what was described but also because payload prescribed while not towing is to address suspension travel and perceived passenger comfort. So in the case of payload, among those who understand the basis, there is general agreement, that payload is more of a guideline.

[BayouBiker]

Originally Posted by jaybauman

We should also keep in mind that manufactures' limits are in-part determined by the weakest link in the chain.

(snip...)

In summary, there is likely additional towing bandwidth available as long as you're not at the extreme operating conditions. But I would like to hear from real automotive design engineers on this topic.

I completely agree. Though less so with heavy trucks because the suspension designs impose additional handling performance limits that present just north of expected highway conditions say 80+ mph or just over .5 g cornering. But many tow vehicles including lighter trucks and especially performance SUV's have significant towing performance bandwidth by nearly all measures except oversteer and sway stability. thus they tow beautifully as long as you stay away from the conditions that promote these two instabilities. If you know how they present, you can make adaptations to further limit their occurrence and you can construct no end of clever scenarios that specifically avoid them. Yet they remain the weakest link.

[tbashin]

Bigger is better. Can’t go wrong with this rule.

[Seanlb]

No more meaningless than high cholesterol numbers.

[DAVSCH]

We've all seen the clip, a bicyclist pulls an Airstream. Bravo. The real problem is the legalities. Get in a accident while towing anything and the police are going to take down data plate numbers. If you're exceeding limits on the plates, you're in a world of hurt. It doesn't matter if you've added everything under the sun to the tow vehicle to beef up it's capacities, the data plate remains the same unless you have the auto manufacturer evaluate your vehicle and issue a new data plate. Personally, I will NOT give the ambulance chasers that avenue to strip me of everything I own while I defend myself from them. Do as you see fit to loose.

[ROBERT CROSS]

Originally Posted by BayouBiker

I find the article generally correct. When towing, axle loads, combined vehicle limit, and tongue limit are far more relevant than payload primarily because of what was described but also because payload prescribed while not towing is to address suspension travel and perceived passenger comfort. So in the case of payload, among those who understand the basis, there is general agreement, that payload is more of a guideline.

Can I get an Amen from the Congregation...😇

Bob
🇺🇸

[dznf0g]

Originally Posted by BayouBiker

It is common in the towing community to hear that towing and hauling load limits from the manufacturers are all over the place with little or no relevance to vehicle ability to pull a trailer down the road. You often hear the numbers are meaningless and you hear this from novices, those who are seasoned and experienced and some who are considered experts in the field of towing.

One group notably absent from this group are the engineers and technicians employed by vehicle manufacturers who directly determine, test and establish tow and haul ratings.

How can this be that there is so much disagreement outside of the pool of those who establish these limits?

The claims seem to get started by the large group of people who don't understand how the limits are established nor how to interpret the limits. Many who do have some understanding seem to prefer to perpetuate the rumors and myths for some reason or another. The rare Engineering expert (Collyn for example) comes along from time to time and tries to shed some light on the topic.

So this thread will once again attempt to discuss and explain the meaning of these numbers and why they seem to be all over the place with no obvious basis used to establish or apply them.

I tried several times, but gave up a number of years ago. I am now retired and don't want to argue with the backyard experts who have driven all their lives.
They dont want mfr professional explanations.

[dznf0g]

Originally Posted by Jerrylax

Toyota is the only manufacture who files their J standards as I understand.

OMG, Toyota was about a year ahead of others. All mfrs utilize the j standard and have for quite a few years.

[dznf0g]

Originally Posted by Profxd

A combination of the following in no particular order. No manufacturer has to explain to the consumer why or how they made a determination for a specific weight limit.

1. To meet FMVSS
2. SAE Towing standard
3. Manufacturers internal durability requirements
4. To target a specific vehicle class limit.

True, but a bit simplistic. A "30,000" foot level statement.

[Prettygood]

I don’t know why we almost never hear someone bringing up the manufacturer’s lawyers. There absolutely has to be margin built in to all the numbers published to make sure that the vehicle doesn’t suffer premature wear, and that failures resulting in use ‘at the limits’ don’t get attributed to them.

I’m not suggesting the margins are massive, but they have to be there. I would never run a business with a client guarantee that was right on the edge of reality.

[dznf0g]

Originally Posted by Prettygood

I don’t know why we almost never hear someone bringing up the manufacturer’s lawyers. There absolutely has to be margin built in to all the numbers published to make sure that the vehicle doesn’t suffer premature wear, and that failures resulting in use ‘at the limits’ don’t get attributed to them.

I’m not suggesting the margins are massive, but they have to be there. I would never run a business with a client guarantee that was right on the edge of reality.

There is a margin, no question. But it's not the lawyers. Much more depth to it than that. No engineering group wants a high failure rate. No marketing group wants customers to walk away due to premature failures.........etc.
I pressed an engineer for the "built in margin". It was a long discussion. They start the vehicle line design with a desired overall number. As design and development progresses, component and part constraints start to encroach on their desired number. It can be due to many complex factors when all the bits and pieces come together to the end product.
The margins also take into account a particular duty cycle formula over the intended life of the vehicle.
In my case, my truck is pretty much a dedicated tow vehicle. It operates at about 90% of gvwr, about 80% of its miles. I am sure that exceeds the anticipated duty cycle formula.

[jcl]

Originally Posted by dznf0g

There is a margin, no question. But it's not the lawyers. Much more depth to it than that. No engineering group wants a high failure rate. No marketing group wants customers to walk away due to premature failures.........etc.
I pressed an engineer for the "built in margin". It was a long discussion. They start the vehicle line design with a desired overall number. As design and development progresses, component and part constraints start to encroach on their desired number. It can be due to many complex factors when all the bits and pieces come together to the end product.
The margins also take into account a particular duty cycle formula over the intended life of the vehicle.

That closely matches my experience.

[jcl]

Originally Posted by BayouBiker

It is common in the towing community to hear that towing and hauling load limits from the manufacturers are all over the place with little or no relevance to vehicle ability to pull a trailer down the road. You often hear the numbers are meaningless and you hear this from novices, those who are seasoned and experienced and some who are considered experts in the field of towing.

I think it would be worth separating out the different types of ratings, limits, guideance, etc.

I don't come across many here saying that axle and tire ratings are irrelevant. I think that they are very important, and I suspect that most agree.

Payload as stated on the payload label can be very relevant for hauling, but not necessarily for towing, for the reasons you have articulated above, namely that "When towing, axle loads, combined vehicle limit, and tongue limit are far more relevant than payload..."

The GCVWR has applicability and legal implications for commercial carriers, but for non commercial carriers who don't have to worry about the same specifics related to vehicle licensing, vehicles taxes, etc, it is less applicable. If we are concerned about powertrain durability, as one example, an easy response is to reduce the load factor or duty cycle, which means things like slowing down a little on steep climbs.

The vehicle tow rating is often due to a manufacturer decision, but we hear about "you can't change the rating, it is all designed to work together" as if all the limits of stability, performance, braking, etc are somehow reached at the same time. If that limit was simply due to a low capacity hitch being supplied, then sure, the capability can be increased by addressing the weakness in the hitch. Then you hit the next thing in the chain. Or not, if the supplied receiver was of such a low capacity that the vehicle can handle more than the actual load now being applied.

I think the bigger question is whether trailer weight should be considered the primary factor in what can be reasonable towed. It can be a factor, but there are many others that are not considered to the same degree. Trailer frontal area, side area, centre of gravity, suspension, profile, etc. Sure, these are often very similar for different models of Airstreams, and this is an Airstream forum, but we don't see qualifiers being expressed about a common tow rating applied to widely different trailers.

[jcl]

Originally Posted by Prettygood

I don’t know why we almost never hear someone bringing up the manufacturer’s lawyers. There absolutely has to be margin built in to all the numbers published ...

My practice isn't to use up the safety margins used by others, as I don't know what assumptions were built in to them.

That said, I think a valid question is whether the weight rating (since this is a weight thread) is the most relevant thing to towing performance.

[Profxd]

Originally Posted by dznf0g

True, but a bit simplistic. A "30,000" foot level statement.

Stated simply yes so here are some details to consider.
The most important FMVSS 105 braking test here https://www.nhtsa.gov/sites/nhtsa.do...105-03_tag.pdf

SAE j2807 here
https://fifthwheelst.com/documents/t...ds-2016-02.pdf

To say that payload (GVWR) is just a suggestion is just wrong, but hey it comes from those that think they are experts not real engineers.

[Bruce B]

I often wonder about design duty cycle and margins when I read these towing discussions. There are an awful lot of variables in the real world. No small one being the driver.
In my business we maintained European autos. Even within a single brand/model/year, we encountered huge variation on life cycle. How could that be? Given good maintenance, I typically attributed different failure rates to the driving habits of the owner.
These were very expensive, highly engineered vehicles that were not subjected to towing or even the Autobahn speeds they were designed for but rather, were used for transportation in and around New England.
One person could drive 100,000 miles on a set of brake pads and rotors and another could not get 20,000 miles to a set. Some suspensions felt tight at 100,000 miles and 5 years, some were awful in 2 years and 40,000 miles.
It seemed that the vehicles that were driven well had very few problems in general. Those that suffered a rough driver had all kinds of “premature” service needs.
Manufacturers who build vehicles designed for towing must be aware of the vast spread of ability different drivers bring to the table.
I suspect than if you were able to closely observe the driving habits of say a random group of 100 drivers, you would be appalled at some of what you learned.
I am no engineer but I have to believe that manufacturers assign numbers like payload to account for the vast range of possibilities given a very large pool of variables.
My educated guess is that if you choose to exceed a number of any kind, that you better be ready to trade “some thing” off. It might be brake wear, differential life, handling under duress or something you never thought about. Perhaps you will be fine if you are one of the truly gifted, smooth and capable drivers.
Perhaps you will find that reason for the number the manufacturer assigned.
My problem is that I have limited tools to bring to my assessment of a tow vehicles driving dynamics. I have studied math, physics, chemistry and more but I have zero experience applying it to the very dynamic models of towing.
I do know that the best feeling tow vehicle I have ever driven was the heaviest truck I owned. My seat of the pants tells me that bigger is better.
My Audi Q7 was a lovely, lovely vehicle to drive. I preferred towing with my RAM 2500...
My experience maintaining autos leaves no doubt in my mind that subjecting a smaller vehicle to towing will shorten its life. That life may still be long, but it will not be as long...
I do love reading these threads when people keep their cool and remain polite.

[BayouBiker]

Originally Posted by tbashin

Bigger is better. Can’t go wrong with this rule.

Bigger is better in towing unless you want to limit your towing to puttering down the highway at any speed, or if you are into slalom cones, racetrack or 80mph plus, then big heavy clunky trucks are not so good. Just stay away from conditions leading to sway and oversteer. Those who sing the praises of performance SUVs won't dare speak about such things.

[BayouBiker]

Originally Posted by Prettygood

I don’t know why we almost never hear someone bringing up the manufacturer’s lawyers. There absolutely has to be margin built in to all the numbers published to make sure that the vehicle doesn’t suffer premature wear, and that failures resulting in use ‘at the limits’ don’t get attributed to them.

I’m not suggesting the margins are massive, but they have to be there. I would never run a business with a client guarantee that was right on the edge of reality.

Sure there are margins but its important to understand the purpose. They are intended to address unusual situations and the normal statistical variation in forces. They are not intended to allow risk takers to push the limits. It is unfortunate some people misapply the margins.

[tjdonahoe]

Originally Posted by switz

Looking at manufactures web sites, one looks at their suggested tow ratings and wonders if the profit motive for upselling drives the number. As an example, my 2012 Ram has a GVW of 9,600 pounds. After adding just one leaf spring on the rear it suddenly has a 11,000 pound tow rating. No other parts are changed.

But a 3500 model sells for more $$$

Yep but you don’t need that extra spring...my new 2500 has coils...can’t see much difference

[Prettygood]

Originally Posted by BayouBiker

Sure there are margins but its important to understand the purpose. They are intended to address unusual situations and the normal statistical variation in forces. They are not intended to allow risk takers to push the limits. It is unfortunate some people misapply the margins.

I am not suggesting exceeding stated limits. What I am suggesting is that those posted number still have a margin built in on actual capabilities. If you stay at the states numbers, the vehicle should be able to handle it well. Exceeding them places you in an unknown and dangerous spot.

There are so many people that say ‘only use 50%, only use 80% of published numbers’. I’m suggesting that isn’t really required to still be safe.