Tongue Weight with Weight Distributing Hitches

[out of sight]

Trailer sway stability is primarily a function of two things: the distance of the trailer center of gravity to the hitch point (commonly expressed as % tongue weight), and speed. No matter how much tongue weight you have the trailer will become unstable if you go fast enough. So, for example, if you have 10% tongue weight you may be able to go 150 mph before instability arises, while at 8% tongue weight you can only go 120 mph. If you have zero or negative tongue weight you may only be able to go 40 mph.

[jcl]

Originally Posted by out of sight

Trailer sway stability is primarily a function of two things: the distance of the trailer center of gravity to the hitch point (commonly expressed as % tongue weight), and speed. No matter how much tongue weight you have the trailer will become unstable if you go fast enough. So, for example, if you have 10% tongue weight you may be able to go 150 mph before instability arises, while at 8% tongue weight you can only go 120 mph. If you have zero or negative tongue weight you may only be able to go 40 mph.

The numbers are off, but let's go with your example of 150 mph and 120 mph. You are ignoring that other trailer design factors (including the moment of inertia of the trailer) are variables. You may be able to go 150 mph (!) with 10% before instability arises, but with a lower moment of inertia you may be able to go 180 (!) mph with 5% tongue weight. Just to use your numbers, mind you. The problem is that you are ignoring all the other variables when you say it is just distance to hitch point and speed.

[DavidNJ]

Originally Posted by Profxd

So I gather from your post that didn’t show up that you believe that % of tongue weight has nothing to do with sway? Sway is caused by polar moments or is it trailer axle position?
Richard H Klein has done a study back in the 70’s on finding the optimal hitch weight. The outcome is that you need to minimize hitch weight yet keep enough TW to maintain sway stability for a given speed as well as minimize the use of WD.

Thanks for this lead! Klein and his go-author Henry T. Szostak had a variety of papers with extensive experimentation. They used cars rather pickups and SUVs (which then meant a Suburban), load leveling meant 25% rather than 80%-100% restoration of front wheel load, and they didn't have today's instrumentation. Those of you who remember capturing sensor output directly to a strip recorder will appreciate that.

I haven't waded though everything yet but the net is: it isn't about the trailer it is about the tow vehicle and its response to the trailer. Too high was as bad as too low. With weight transfer to the front wheels the desired hitch weight (his term) went lower.

Originally Posted by jcl

There doesn’t appear to be an acknowledgement here that 10% isn’t magic, it is simply a rule of thumb for typical North American trailer designs, with their typical moments of inertia. Euro trailers typically have masses such as appliances and tanks very close to the trailer axle centreline, because they are designed for 5% tongue weight.

Boat trailers similarly work well with 5%, because the mass of the engine(s) is very close to the axle centreline, in addition to the relatively long trailer axle to hitch distance.

This was the key point. I got into this research because an fully optioned 1/2 ton pickup had only a 1100 lb payload, too small to handle passengers, cargo, a 200lb 4-bar linkage Hensley/ProPride hitch and 10% of a large trailer (although still below GCVW). In various forums I've been challenging the GVWR number that determines payload, and the fixed nature of the GAWR which isn't re-evaluated for changes in spring, tire and wheel capacity. I've learned a lot more.

The European standard is car focused. There is a maximum hitch weight on the car, the hitch, and the trailer. They pretty much go for the minimum of the three, irrespective of the tongue weight (noseweight in the UK). WD hitches are rare and 4-bar linkage hitches (Hensley/ProPride) are non-existent.

Klein, Strozek would indicate keeping the front/rear weight balance in the tow car is the target. But under the covers, a 4-bar linkage hitch is more important than anything with a tag-along trailer. My guess, which I can't back fully, yet, is that 4% with a 4-bar linkage hitch would tow better than 10% with a conventional WD (which would be 7.5%-8% after adjustment). The problem is that the RVs aren't designed to provide that, and for car haulers that is a long tail behind the axles that is close to the ground.

The Tuson trailer ESC, less than $500, seems like a no-brainer. My guess is a lack of NHTSA regulation around trailers and lack of knowledge of the device keep it out of view. A tow vehicle with trailer aware ESC would also be important. My guess is LT tires would help because of their stiffer sidewalls and higher load capacities.

This is a post from yesterday that was blocked. Note the video at the bottom.

The paper: An experimental investigation of car-trailer high-speed stability

This is from the University of Bath. It is on point but didn't test with a WDH. They looked at tongue weight (they call it nose mass), trailer mass, moment of inertia, tire pressure and axle position. These are the folks who made some of the 'model car on a rolling roadway' videos.

Interestingly, the percentage tongue weight they tested for was -0.8% to 10.5%. They found "an increased nose mass improved the system stability, although the improvement becomes less significant when the nose mass rises above 6–7 percent of the total weight."

Another finding: "when the trailer inertia increases, the damping of the combined car–trailer system decreases dramatically. The inertia effect suggests that, when a driver is loading a caravan, the mass should be placed as close to the centre of gravity as possible in order to minimize the resulting increase in inertia."

Their conclusions:

CONCLUSIONS

Very little work has been published on the experimental measurement of high-speed car–trailer stability. In this study, extensive experimental measurements were carried out on a combined car–adjustable-trailer system. By adjusting the trailer settings, the effect of different trailer parameters on the system stability was examined. It was found that the dominant factors affecting stability were the trailer yaw inertia, nose mass (load distribution), and trailer axle position. The tyre pressure also affects the stability, although the effect is less significant. It is interesting to see that the trailer mass alone does not dramatically affect the stability; however, as a heavier trailer normally has a larger yaw inertia, a limit should be placed on the relative car–trailer masses.

A friction stabilizer is shown to be helpful in improving the system stability, although in these tests the stability was not increased hugely. In addition, high-speed towing tests were carried out on cars fitted with an ESP which automatically brakes individual wheels and controls the engine throttle position should the vehicle dynamic response differ from that expected. These tests demonstrated that, if the dynamic response ‘error’ exceeded a preset threshold level, the ESP operated and the highspeed stability was improved by controlling the car yaw oscillation associated with trailer instability.

This begged the question: what is the noseweight standard in the UK? Answer: 5%-to-7%. And they don't seem to be picky. They basically say it is the 5%-to-7% or constrained to the lowest of what the tow vehicle, hitch, or trailer is rated for.

These are two articles and a video:

How to measure and adjust your caravan’s noseweight

Caravan Club Leaflet: Noseweight

https://www.youtube.com/watch?v=PsgHZsJWXdw

[Profxd]

“It is interesting to see that the trailer mass alone does not dramatically affect the stability; however, as a heavier trailer normally has a larger yaw inertia, a limit should be placed on the relative car–trailer masses.“

The comment above notes the importance of TV to trailer weight ratio. This where the US gets in trouble. What was the weight ratio of the TV and trailer they used for testing? Most RV trailers made here have a large amount of yaw inertia and most ignore a 1 to 1 weight ratio for TV and trailer. Also note stability is directly related to speed which the US has higher speed limits then Europe.

[out of sight]

The National Association of Trailer Manufacturers has addressed the tow vehicle mass to trailer mass issue, coming up with the following guidelines:

[out of sight]

A further simplification of the issue is to say that your tow vehicle should weigh more than your trailer. Note that most Airstream owners don't follow this advice.

[DavidNJ]

Originally Posted by out of sight

The National Association of Trailer Manufacturers has addressed the tow vehicle mass to trailer mass issue, coming up with the following guidelines:

And if they had any research to back it up it would be interesting.

I am still wading through several hundred pages from Klein and Strozek. I'm also looking for a mid-1960s paper by Reese (yes, that Reese). K&S say they thought forward wheels on the trailer would be a solution but say Reese's paper dissuaded them. So...I'm looking for Reese's paper.

Fun reading them. The text is typed, the charts hand-drawn and annotated, and apparently setup for reproduction.

Note that 5th wheel/gooseneck trailers can be 2+x the weight of the tow vehicle. 18-wheels are all heavier than the tow vehicle, but those cabs are heavy and 50% of the trailer weight is on the hitch with the trailer wheels at the extreme rear.

The issue appears to be how the tow vehicle handles forces applied to the hitch. With a 5th wheel/gooseneck, the forces are applied directly over the rear axle with no moment around the rear axle. With a 4-bar linkage (Hensley/ProPride) it is about 12"-18" behind the rear axle with effectively the same result.

Quadruple that distance and there is a problem. But this is about horizontal forces. In vertical forces the 5th wheel/gooseneck also have no moment around the rear axle. Here the 4-bar has a similar or greater moment than a conventional hitch.

A 2500 gas engine truck is maybe 1300lb heavier than a 1500 (6700 vs 5400). The diesel, although available, kills so much of the payload to, IMHO, make it useless in a 10k GVWR 2500. In a 3500, the DRW diesel is 8400 and cas 7300. But that still isn't heavy when they are CC DRW diesel is rated for a 34k trailer.

The keys, keep the tongue weight within the tow vehicle capability. For practical purposes, using supplemental air springs, wheels rated at 2650 lb (there are only a few) and high enough load capacity LT tires could increase that slightly. Use a 4-bar linkage hitch. Maybe add the Tuson trailer ESC. Adjust the WDH to get the weight back on the front wheels using WD bars of appropriate stiffness for the weight of the trailer. If all that still isn't enough, lower front tire pressures or add a thicker front anti-roll bar in order to increase the native understeer of the tow vehicle (used as a criterion in the K&S papers). Having a tow vehicle with trailer compensating ESC also helps.

In the end, I wouldn't exceed the J2807 GCWR with one exception. Because of some of the crazy pieces of the standard, two identical trucks separated only by 20% in rear axle ratio may have wildly different GCWRs. In the Ram 1500 it is 13.9k vs 17.1k. The 13.9k could probably be breached without a problem. The original Cummins diesel Ram had 160hp and a 4-speed automatic. The 395hp V8 with 8-speed trans probably can handle it even with a 3.21 rear gear.

But using an arbitrary 10% is not recommended by any research...but virtually all of the above are.

[uncle_bob]

Originally Posted by out of sight

A further simplification of the issue is to say that your tow vehicle should weigh more than your trailer. Note that most Airstream owners don't follow this advice.

Hi

If you head out to farm country, you will find that roughly 99.9% of all farmers /ranchers also don't follow this advice. Head over to a car show and the folks with cars on trailers also don't seem to have ever heard of this one either. You might occasionally notice a semi on the road. They most certainly do not have more weight up front than in back ....

Bob

[lilboomer]

If anyone needs a 'new' Equal-i-zer WD hitch rated for 10,000 lbs, let me know. I have one I don't need. -Roy

[2MileHi]

OMG! Got three engineers in the family, and that's four too many. This thread could could be, in their terminology, infinitely long!

[mikapen]

@DavidNJ, your line of inquiry can never be satisfied until you acknowledge the first rule of Physics: It's Relative.

You are all over the place with your arguments. You need to decide what your question is, and define it in regards to "Relative to what?"
Relative to the TV, relative to the hitch weight, relative to what?

Once you decide what you are trying to find out, responses will be scattered and no answer can satisfy this discussion.

Perhaps you should re-read this thread and put the responses into "relative" categories - what category is each one referring to.


Your initial question is unanswerable. Now that you have lots of answers, maybe you could rephrase your question into one or more coherent questions.

[bhayden]

I've always set up my WD hitch with a tape measure by putting just enough tension on the bars to return the distance from the ground to the wheel well rim to equal (as it is with the van unloaded). My take is that evening out the load on the TV is the major benefit of a WD hitch. My trailer for example has been towed at freeway speed w/o the WD hitch by a 1T diesel PU with no sway issues what so ever. But without the WD hitch on my GMC Safari van it's a scary experience.


I hadn't considered the fact that the WD hitch also increases the load on the trailer tires. Next time I hook up I'll have to check the ground to wheel well distance on the trailer (tandem axle Argosy 24"). It will be interesting to see if the WD hitch also levels out the ground to trim distance of the trailer. Although I'm not sure if that translates to even weight on the trailer tires or not? IIRC, the bubble level on the trailer looks to be pretty much flat when towing.

[out of sight]

Originally Posted by uncle_bob

Hi

If you head out to farm country, you will find that roughly 99.9% of all farmers /ranchers also don't follow this advice. Head over to a car show and the folks with cars on trailers also don't seem to have ever heard of this one either. You might occasionally notice a semi on the road. They most certainly do not have more weight up front than in back ....

Bob

I think it's good advice for Airstreamers, many of whom are probably towing for the first time. Yes of course you can tow with a small vehicle but a large vehicle is safer especially at highway speeds. If you go slow enough you can get away with it, but then you'd be holding up traffic and causing road rage incidents.

Note that a semi is not a bumper pull trailer. It has completely different dynamics.

[mrfixit51]

Can anyone enlighten me on the same issue. Will the weight shown of a trailer, alone on the scale, with the wdh attached , change it's actual weight shown? thks

[Bill M.]

If I understand your question, the answer is yes. If you put just the trailer wheels on the scale the axle weight will be more when the WD is engaged than when it is not. WD hitches transfer part of the weight from the rear wheels of the TV to the front wheels of the TV and to the trailer wheels. The total weight of the TV and trailer is not changed. But where the weight is carried is.

[Halford1]

I have been towing for over 30 years without serious issues. I try not to complicate whatever is simple. I just load my pop up and AS 60% front and 40% back. I do NOT use sway control nor WDH at all. Use an appropriate TV to tow and can handle 10% Tongue Weight.

I came to this threat. Ummmm no comment. So complicated. Must be fun for y’all to talk about this. Guess engineers here here so are simple people.

[mrfixit51]

Originally Posted by Bill M.

If I understand your question, the answer is yes. If you put just the trailer wheels on the scale the axle weight will be more when the WD is engaged than when it is not. WD hitches transfer part of the weight from the rear wheels of the TV to the front wheels of the TV and to the trailer wheels. The total weight of the TV and trailer is not changed. But where the weight is carried is.

Thanks for the reply. I just rebuilt a vintage Canadian Travelux,23' tandem, which had a factory dry weight of 3500 lbs. With the wd active and with just the trailer on the scale,( the scale was not long enough for both pieces) it weighed 3980. Because it was a complete rebuild with heavier steel for the frame repair and plywood for all wood inside, i thought that it would be heavier than factory spec. Interesting to know thks!!

[smithcreek]

Originally Posted by mrfixit51

Thanks for the reply. I just rebuilt a vintage Canadian Travelux,23' tandem, which had a factory dry weight of 3500 lbs. With the wd active and with just the trailer on the scale,( the scale was not long enough for both pieces) it weighed 3980. Because it was a complete rebuild with heavier steel for the frame repair and plywood for all wood inside, i thought that it would be heavier than factory spec. Interesting to know thks!!

But, the way that you weighed the trailer, with it hitched to the tow vehicle, still understates the trailer's GVW since the tongue weight being carried by the tow vehicle is larger than the weight added to the axle when you engage WD. So, for example, if your tongue weight is 500 lbs. and you are putting 150 lbs. back on the trailer axle with WD, the trailer would weight 350 lbs. more than the scale reading.

[uncle_bob]

Originally Posted by out of sight

I think it's good advice for Airstreamers, many of whom are probably towing for the first time. Yes of course you can tow with a small vehicle but a large vehicle is safer especially at highway speeds. If you go slow enough you can get away with it, but then you'd be holding up traffic and causing road rage incidents.

.......
.

Hi

Hang on a second here. You are talking about trailers that weigh in between 5,000 and 10,000 pounds. By the "1.5X heavier rule" that gets you to 7,500 to 15,000 pounds for the tow vehicle. At that level, an F450 would not be "enough" to tow a 20' Flying Cloud.

Bob

[out of sight]

Originally Posted by uncle_bob

Hi

Hang on a second here. You are talking about trailers that weigh in between 5,000 and 10,000 pounds. By the "1.5X heavier rule" that gets you to 7,500 to 15,000 pounds for the tow vehicle. At that level, an F450 would not be "enough" to tow a 20' Flying Cloud.

Bob

I was talking 1:1, not 1.5:1. That's an F250.