What Is the Basis of the Recommended Tongue Weight Percentages?

[DavidNJ]

I'm not a trusting soul. I found what I believed to be computational errors in all the online towing calculators i found. Errors that could be proven by just looking at actual CAT scale weights. So I did my own calculations...something a sophomore in mechanical engineering should have no problem with.

For a conventional non-weight distributing hitch...nothing different.

However, a weight distributing hitch has some interesting effects. First, the more tension applied to the bars or longer the bars are (at a fixed tension) the more weight is placed on the trailer axle and a lower weight added to the truck. That is, adding tension not only transfers weight to the truck's front wheels but all reduces the weight added to the truck and increases the weight added to the trailer.

The second interesting observation and the reason for this post is that the weight on the ball increases. And it increases a lot. In an example with a 9500# trailer and nominal 10% TW, restoring front axle weight to a net loss of 100# moved 180# from the truck to the trailer axle and increased the force through the ball from 950# to 1970#. This is because most of the tension in the WD bars comes back though the ball, the remainder added to the trailer axle.

Reading various documents it seems that 10% to 15% tongue weight is recommended before the WDH bars are tensioned. One site said a maximum of 10%. Where did this come from? The same folks who decided on a 10%-12% safety margin between combined axle weight ratings and the GVWR?

If the tongue weight is determined to be enough that the ball remains firmly seated under all circumstances, is the real criteria 10% after the WDH bars are connected? That would require 6% TW with half the tension in the WD bars. (In my calcs with a 6600# GVW and 9500# trailer, at 5.5% static TW the force on the ball was 10% of the trailer and bar tension went from 1200# to 500#, which should ride and handle better.)

Why is this important? The simple transfer of weight to the trailer axle increases the de facto towing capacity before exceeding GVWR. On well-optioned Ram 1500s, and nearly Limited models with have the air suspension and power steps standard and are usually panoramic sunroof equipped, the payload is the limiting factor. By transferring less weight from the trailer to the truck, that capacity is increased.

If someone here knows the answer on how the tongue weight recommendation is determined, that would be great. If not, some recommendations on where it came from would be helpful.

Thanks,

David

 

[DavidNJ]

I finally got an answer that was meaningful on the 10% tongue weight. I extrapolated on stability.

Sway is a dynamic situation. Tongue weight (as normally defined) is a static measurement. Basic physics dictates that one is *not* the same as the other. ​

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The reason you see ~ 10% tongue weights is a practical one. You don't want the trailer dropping on it's back bumper when unhitched. Practical experience with people loading trailers over the years suggests that 10% is "enough" to keep this from happening. Indeed if you dig into boat trailers you *will* find cases of backwards tip ....​

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Axle ratings are not the same as receiver ratings. Moving weight between the axles still puts strain ( = load) on the receiver. It still can / will be damaged if overloaded. The numbers that truck factories guess at when putting on receivers are in no way a guarantee that everything will be fine. You do need to check *all* the numbers. ​

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If you want to dig further into the physics of sway, a good place to start is with the higher order moments of inertia and how they are calculated. Control systems theory would be the next step in the study course. Ultimately you will come up with a chart of phase margin (or gain peaking) vs speed vs weight location for a specific rig. ​

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Bob​

Thanks for the insights. I've thought the 10% may have occurred to have a number too high to cause the tow vehicle rear axle to be unloaded. If that were an issue it would be discussed. It isn't mentioned anywhere.

Do you know the approximate year the WDH was invented and marketed and who was the original company?

Polar moment is a bigger factor than MOI. Take the ball which can only transmit axial force. towing a 25ft box trailer, 10ft behind the centerline of the axle. A 5ft tongue gives a total length of 30ft.

A 100lb lateral force at the front of the box creates 500 lb-ft moment around the ball which is resisted by a 25lb lateral force on the trailer axle (and 75lb on the ball). That same lateral force on the rear edge of the trailer creates a 3000 lb-ft moment that is resisted by 150lb lateral force on the trailer axle (and 50 lb force on the ball in the opposite direction).

The typical towing ball is 5.5ft from the tow vehicle axle, more on some. those forces create 410 lb-ft and 275 lb-ft moments around the tow vehicle rear axle. With a 12ft wheelbase (typical of a 1/2ton short-bed pickup) that would be a 34lb and 23lb lateral force on the front tires

Those forces are a function of the placement of the trailer axle rather than the placement of static weight although, like a pendulum, static weight's polar moment counts. Note that on an 18-wheel tractor-trailer, the wheels are placed at the extreme rear of the trailer. On 5th-wheel trailers for pickups, that is limited by the payload and rear axle capacity of the pickup.

A Hensley or Propride hitch would have moved the virtual pivot point to maybe 16" behind the tow vehicle's rear axle. That would have made the moments about the rear axle 8 lb-ft and 6 lb-ft.

Obviously, is sway is a problem the owner should have a Hensley or ProPride hitch. To tow a $50k antique car in a $20k aluminum trailer, or on this forum a $50+k RV on an $800 WDH when $2800 4-bar linkage WDH hitch behind a $70k tow vehicle would solve the problem is probably not a sound risk analysis.

Which leads to the other problem. That $70k tow vehicle has a 10klb or 12lk pound receiver from the factory, and the advertisements talk about 1800lb-2000lb payloads, but the reality is different. The payload numbers are for stripped work trucks with heavy-duty springs that would yield an unacceptable unloaded ride. Loaded with options at the "Limited" level, these vehicles often have door stickers with 1100lb-1300lb payloads...or less. That makes most towing problematic with 4 people and some luggage in the tow vehicle.

If a lower disconnected tongue weight and weight transfer from the WDH are put in play, the net additional load for a 10k trailer could be 500lb, for an 8k trailer 400lb...or even slightly less.

Further, less tension in the WD bars allows more movement between the tow vehicle and the trailer which should result in a better ride and better handling.

The $64k question is: what is the real limiting factor on minimum tongue weight when towing heavy.

 

[Zeronet]

I understand where you’re going with this but my experience is with travel trailers. In the travel trailer world, most all trailers will start well above 10% tongue weight dry and have generally more storage space forward of the trailer axles. So in the real world, 10% tongue weight or less would be pretty rare and/or difficult to obtain. Plus it varies from trip to trip depending on what gear is loaded and where.

Now if we’re talking about car trailers, where you can easily shift the weight of the vehicle forward or back to achieve the desired tongue weight, then sure 10% or less is doable. Then it’s just a matter of stability while towing. I know 10-15% is recommended and have learned from much research that a trailer that is too light in the front or has too much weight behind the trailer axles makes it less stable.

There are some videos on YouTube showing examples of this with model cars and trailers on a treadmill. Shifting weight forward and back in the trailer has a drastic effect on towing stability.

Maybe 10-15% tongue is just dogma, but I doubt it.

 

[DavidNJ]

I understand where you’re going with this but my experience is with travel trailers. In the travel trailer world, most all trailers will start well above 10% tongue weight dry and have generally more storage space forward of the trailer axles. So in the real world, 10% tongue weight or less would be pretty rare and/or difficult to obtain. Plus it varies from trip to trip depending on what gear is loaded and where.

Now if we’re talking about car trailers, where you can easily shift the weight of the vehicle forward or back to achieve the desired tongue weight, then sure 10% or less is doable. Then it’s just a matter of stability while towing. I know 10-15% is recommended and have learned from much research that a trailer that is too light in the front or has too much weight behind the trailer axles makes it less stable.

There are some videos on YouTube showing examples of this with model cars and trailers on a treadmill. Shifting weight forward and back in the trailer has a drastic effect on towing stability.

Maybe 10-15% tongue is just dogma, but I doubt it.

That video is the one above. But it was about the location of the disturbance, not the tongue weight.

Yes, it is hard to shift 10%...but not impossible. It is just a matter of moving stuff from the front to over the axle. It is only a little easier in a car trailer, since there are tie downs in the trailer at specific locations and if the rear wheels are too far forward, more if the tail is too long, it has trouble turning on to inclines without scrapping. In my early days, I was actually stuck in that position for an hour until we worked it off. Our trailer then grew rollers at the tail of the trailer to handle the eventuality. If ordering a car hauler, it is sometimes possible to get the builder to reposition the axles.

Sway isn't an issue since it can be virtually eliminated with a 4-bar linkage hitch: Hensley or ProPride. I'm guessing a pitching moment toward the rear of the trailer unloading the tow vehicles rear wheels could be an issue. But then, why is 10% magic and 7.5%, where WDH hitches end up, not?

This is an issue because 1/2 ton pickups serve double duty as trucks and cars. 3/4 ton are generally too big for car duty. For example, if driven to Manhattan in NYC, it is impossible to park. As long as some of us want Limiteds with all of the options, the reduced payload makes them fulfilling their roles as trucks impossible unless the tongue weight can be reduced.

 

[DavidNJ]

These are actual CAT scale numbers posted on another forum:

My WDH is the Blue OX Sway Pro. With 1k bars. I put it up to the 9th Chain link as called for in the book, and the Tech who installed the the Blue Ox​

First, my TV is a 2014 F150 XLT 4x4, Supercab, 8 ft box, with Max Tow Package, and the HDPP. The FGAWR = 4050#, RGAWR = 4800#, GVWR = 8200#, CCC = 2286#, and the GCWR = 17,100#. The truck was rigged for towing, meaning all the normal tools, generators, gasoline, compressor, etc were on board as well as a full tank of gasoline.​

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My travel trailer is a new Jayco Jay Flight 23RB. It's a "stick and tin" built trailer and no slide. GAWR = 3500# each, GVWR = 6500#, Factory Shipped weight = 4758#, CCC = 1742#.​

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These are my numbers from my weigh tickets. Truck with no trailer, Steer Axle 3660#, Drive Axle = 3340#, Gross Weight = 7000#​

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Truck with trailer, but WDH disengaged. SA = 3340#, DA = 4640#, Trailer Axle = 5320#, Combined Gross Weight = 13,300#​

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Truck with trailer, but with WDH engaged. SA = 3520#, DA = 4280#, TA = 5460, GCWR = 13,200.​

 

[Zeronet]

Yeh, those Hensley hitches May eliminate sway but they also weigh ~200 pounds and that is tongue weight as well (and any cargo behind the rear axle). Further reducing available payload.

At least with a travel trailer, in my experience, less than 10% tongue weight is just not attainable. I have done everything I can think of in loading cargo, relocating batteries, changing to lighter lithium batteries. Still I have an 840 tongue weight with the trailer loaded to 6400 lbs. Add 80 for the hitch and it’s over 900.

We took a cross country trip earlier this year and the trailer weighed 6700 with tongue weight of 900. Add the hitch weight and it’s close to 1000.

Sure the weight distribution puts some of that back on the trailer axles. But with the wife, me, the dog, a generator and another 70 pounds of cargo we were very close to the truck GVWR crossing the scales with the WD hitch engaged.

Just the anatomy of a travel trailer, and most trailers. 2/3 of the length is in front of the axles, 1/3 behind. They are built with > 10% dry tongue and most don’t include the weight of batteries and propane. None include the hitch weight. ~3/4 of the GVWR of the trailer is already baked-in to the dry weight and already at 10% or more tongue. The remaining 1/4 is your cargo capacity that you can move front or back. Tough to get below 10% when loaded for travel.

So while the idea of towing below 10% tongue sounds like it could really increase your capability, it’s all moot if you can’t load the trailer to attain that < 10% goal.

If you’re planning on towing something other than a travel trailer please disregard.

 

[DavidNJ]

Yeh, those Hensley hitches May eliminate sway but they also weigh ~200 pounds and that is tongue weight as well (and any cargo behind the rear axle). Further reducing available payload.

At least with a travel trailer, in my experience, less than 10% tongue weight is just not attainable. I have done everything I can think of in loading cargo, relocating batteries, changing to lighter lithium batteries. Still I have an 840 tongue weight with the trailer loaded to 6400 lbs. Add 80 for the hitch and it’s over 900.

We took a cross country trip earlier this year and the trailer weighed 6700 with tongue weight of 900. Add the hitch weight and it’s close to 1000.

Sure the weight distribution puts some of that back on the trailer axles. But with the wife, me, the dog, a generator and another 70 pounds of cargo we were very close to the truck GVWR crossing the scales with the WD hitch engaged.

Just the anatomy of a travel trailer, and most trailers. 2/3 of the length is in front of the axles, 1/3 behind. They are built with > 10% dry tongue and most don’t include the weight of batteries and propane. None include the hitch weight. ~3/4 of the GVWR of the trailer is already baked-in to the dry weight and already at 10% or more tongue. The remaining 1/4 is your cargo capacity that you can move front or back. Tough to get below 10% when loaded for travel.

So while the idea of towing below 10% tongue sounds like it could really increase your capability, it’s all moot if you can’t load the trailer to attain that < 10% goal.

If you’re planning on towing something other than a travel trailer please disregard.

What was the trailer axle weight? DId you way the truck without the trailer and with the WD disconnected? What is your door sticker payload? Do you have dimensions for the ball-to-axle dimensions on the trailer and the rear overhang?

 

[Zeronet]

I weigh my rig at the beginning of most long trips including the cross country trip earlier this year. That trip was with my previous truck (2015 Ram 1500). Trailer axles were 5980 on that trip, that’s an all-time high but also the longest trip we’ve done. Door sticker truck payload was 1675 and truck weighs 5400 full of fuel and driver only. I had all the other measurements you asked about and used them in some tow calculators at some point in the past. But I’m not that motivated anymore since I have half a dozen weight tickets over a couple years.

When I bought the 2019 I immediately took it to the scales to get a base weight on the truck. The door sticker says 1479 which isn’t too bad for a nicely equipped Longhorn in my opinion. Scale said 5640 with full fuel only and just some misc stuff in the glove box and console. Haven’t weighed the new truck/trailer combination but it has done very well for the 1200 miles I’ve towed with it so far.

 

[DavidNJ]

I've been pointed at some DOT studies from the late 1970s. I haven't been able to get a copy of this report yet, but the summary is interesting. One of the cases was using a 1970s station wagon to tow a 31ft RV. That Wagon would have specs between a Tahoe and Suburban: 228" length, 125" wheelbase, 5000lb curb weight.

The results may be relevant to towing with Ram 1500 DT.


EFFECTS OF WEIGHT DISTRIBUTING HITCH TORQUE ON CAR-TRAILER DIRECTIONAL CONTROL AND BRAKING

This report presents the results of a full-scale test program, with supporting computer simulation analysis, aimed at determining the effects of weight-distributing hitch forces (or torque) on combination-vehicle (C-V) handling and braking. Two C-V configurations were used; a full-sized station wagon plus 31 ft travel trailer and a compact-sized sedan plus 18 ft utility trailer. Test procedures included a step steer (for tow vehicle understeer changes and transient response), a pulse steer (for trailer swing mode damping), straight-line braking (for stopping distance), and braking in a turn (for transient understeer changes). Over 800 test runs were performed in which load-leveling torque, hitch load, and tire pressure were the primary independent variables. Results show that increasing hitch load and/or load leveling torque degrade the tow vehicle understeer and reduce the speed for incipient jackknifing. On the other hand, trailer damping is somewhat improved with load leveling. Front/rear tire pressure differentials (front lower than the rear) have a significant beneficial influence when the hitch load is high.