Tongue Weight with Weight Distributing Hitches

[DavidNJ]

This is talked about frequently and seems to have rather conflicting answers with no real analysis behind them.

For example, it is common to see 10%-15% of trailer weight as the tongue weight. Yet, most receivers on the tow vehicles are spec'd at a MAXIMUM tongue weight of 10% of the maximum tow weight.

Another analysis or actual scale weighing of the axles would show that restoring the front to level can reduce the tongue weight by 18-25+% depending on the distance to the trailer axles. That is 10% become 7.5%-8%.

There are minimal (really no) papers on trailer stability from tongue weight. Even the papers published, most of which concentrate on using electronic controls to increase stability, talk about 10%-15% as customary with no citations.

Restoring the front ride height to level with the WD bars results in 150% to 180% of the disconnected tongue weight being transferred through the ball.

To put the numbers together, a 10k trailer with 1000# disconnected tongue weight and 200" ball to axles would, at level front, move 250lb from the tongue to the axles and have 2500lb force on the ball.

Meanwhile, that is all in the vertical plane and has at best an indirect influence on trailer instability ('sway'). The very stiff WDH bars can create their own havoc on the springing of the tow vehicle. Most likely only 60%-80% of the total front-to-rear weight transfer would be compensated for. Of course, that means the front wheels have less weight on them than before the trailer was connected.

Some WDH hitches attempt to control sway with added friction to turning. The Reese Dual Cam stiffens the WD bars in turning. Hensley and ProPride have 4-bar linkages which create a virtual pivot point about 52" ahead of the ball, about 12"-18" behind the rear axle. That virtually eliminates sway. However, this thread is about tongue weight not sway, unless someone has an analysis or explanation that connects them.

There is a (rather recent) SAE standard for GCVW. There is also one for the ball and the coupler. However, there don't appear to be any standards for GAWR and GVWR. For most, the GAWR seems to reflect wheel load capacity, tire load capacity, and spring rates.

I'm wondering if 10% tongue weight, at least for a WDH hitch, is as one trailer manufacturer told me 'an old wives' tail'. If so, would 5.5%-6% be a better number? For that hypothetical 10k lb trailer, it would still end up with 1000+ lb weight on the ball and over 600# net tongue weight. With much less tension on the WD bars, wouldn't the tow vehicle ride and handle better?

[overlander63]

Welcome to the forum.
The less tongue weight, the less stable the tow.
Tongue weight is NOT reduced by weight distribution.

[JamuJoe]

Originally Posted by overlander63

Welcome to the forum.
The less tongue weight, the less stable the tow.
Tongue weight is NOT reduced by weight distribution.

The OP is seeking data and analysis to support statements such as yours.

[PKI]

The information you seek is in studies and analysis that have been presented prior. The answer above is correct, except that "Tongue Weight" does get shifted between the coach and the tow vehicle, but the "Tongue Mass" does not. The "Mass" and associated moment arm forces which result from the dynamic movement of the rig are what provides the coach body with directional stability up to a critical velocity.

Have fun with your investigation. It will improve your understanding of the forces. Note - any analysis makes assumptions. Validate them for your application or adjust to accommodate as needed. Note - many measurements are static and the problem is dynamic. Pat

[mkcurtiss]

Here we go.......

[HowieE]

Trailers are designed to have a tongue weight between 10 and 15 % of trailer weight. That weight is the safe range at which a trailer can be towed, without a WD hitch, without producing excess sway.

WD hitches were first designed, back when common tow vehicles were softly sprung cars, to reduce the effect of heavy tongue weights on the tow vehicle. These combinations tended to go beyond what simple design limits would provide safe towing.

There are 2 functions to consider, now days, when using a WD hitch.

First is to reduce the effect of the heavy load placed at the rear of the tow vehicle.

Second sway control.

Failure to address the first will have the tow vehicle steering geometry out of specs and reduce steering tires road contact weight. That is why when setting up a WD hitch one attempts to load the front axle close to the unloaded weight. There is no concrete rule of thumb here because of all the variables involved depending on tow vehicle and trailer weight.

Most modern WD hitches have some form of sway control. They use friction to reduce potential oscillation between the tow vehicle and trailer caused by wind or a larger vehicle passing.

The trailer tongue weight will not change depending the use of a WD hitch. The effect of that weight is changed by transferring some of that load off the real axle and placing it on the front axle and trailer axles.

[uncle_bob]

Hi

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.

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 ....

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.

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.

Bob

[DavidNJ]

Originally Posted by PKI

The information you seek is in studies and analysis that have been presented prior. The answer above is correct, except that "Tongue Weight" does get shifted between the coach and the tow vehicle, but the "Tongue Mass" does not. The "Mass" and associated moment arm forces which result from the dynamic movement of the rig are what provides the coach body with directional stability up to a critical velocity.

Have fun with your investigation. It will improve your understanding of the forces. Note - any analysis makes assumptions. Validate them for your application or adjust to accommodate as needed. Note - many measurements are static and the problem is dynamic. Pat

Originally Posted by mkcurtiss

Here we go.......

It has been discussed but wrong or no conclusion. If you know of a meaningful technical paper on the rational for the 10% tongue weight please reference it.

Here we go again...only this time with physics and engineering.

[DavidNJ]

Originally Posted by uncle_bob

Hi

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.

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 ....

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.

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.

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.

https://www.youtube.com/watch?v=6mW_gzdh6to

[Profxd]

Why 10% TW? The purpose of setting any percentage of TW is because it sets the position of the trailer center of gravity. Let’s say effective tongue length is 200 in. which is measured from coupler to the axle centerline. 10% TW would position the trailer center of gravity at 20 in. forward or the axle centerline, at 15% it will be 30 in. forward of the axle centerline.
So the trailer center of gravity is point at which the trailer wants to rotate if some lateral force pushes on it. The closer the center of gravity is to the trailer axle centerline the easier it is to rotate it causing instability. If the trailer center of gravity is at or behind the axle centerline it drastically reduces the speed that instability occurs.

The equations for COG positioning can be found in this paper. https://pdfs.semanticscholar.org/4e8...336eef1e34.pdf
There are graphs in it for stability at various trailer COG positions

[PKI]

David - this is your thread. I'll just follow along and learn. Pat

[HowieE]

Originally Posted by Profxd

So the trailer center of gravity is point at which the trailer wants to rotate if some lateral force pushes on it. The closer the center of gravity is to the trailer axle centerline the easier it is to rotate it causing instability. If the trailer center of gravity is at or behind the axle centerline it drastically reduces the speed that instability occurs.

The equations for COG positioning can be found in this paper. https://pdfs.semanticscholar.org/4e8...336eef1e34.pdf
There are graphs in it for stability at various trailer COG positions

You analysis is correct for a trailer without a WD hitch. However a WD hitch transfers additional weight towards the trailer axles thus changing the COG of the trailer.

[Profxd]

No the trailer cog does not change with WDH. No mass is moved with a WDH.

[HowieE]

Are you saying the movement of mass does not change the COG?

[Profxd]

No . The tension of the wdh moves no mass so the cog does not change positions. It only changes the down force(weight) at the axles.

[DavidNJ]

First...for some reason...my last post is waiting for the moderators. It was the same time as my second post...when it appears.

Now...there is a misconception about weight distribution with WDH. The WDH bars are an extension of the truck. Think of the truck as a seesaw with the pivot around the rear axle. On one side, 12' feet from the pivot, is a 3000# load. On the other side, 5.5' from the pivot, is a 1000# load. WDH adds two wheelbarrow handles to the 5.5' side sticking out another ~28". They are held by a person standing on a platform suspended between the 5.5ft side of the seesaw and another pivot 20' behind the seesaw. Unlike the tongue weight, in the vertical plane they are rigid with the seesaw. Their load is shared between the seesaw and the extra pivot.

Now...continuing with the math, the load on those wheelbarrow arms, a proxy for the WD bars, will be 1.75x the initial weight on the short side of the seesaw. All of it carried by either the seesaw or the rear wheels. Shorter trailer distance ball to axle, more on the axle. The weight transferred to the rear axle reduces the weight through the ball. But the weight though the ball is still over 1.5x what it was without the WD bars.

If someone has CAT scale or equivalent numbers of the toe vehicle without trailer, with trailer without WD bars, and trailer with WDH connected it will be obvious.

[DavidNJ]

This is my missing post.

Originally Posted by uncle_bob

Hi

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.

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 ....

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.

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.

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.

https://youtu.be/6mW_gzdh6to

[overlander63]

Originally Posted by JamuJoe

The OP is seeking data and analysis to support statements such as yours.

And your statement is why I have very little to do with this forum any more.
Good luck.

[DavidNJ]

These are CAT scale numbers from 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.​
​
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#.​
​
These are my numbers from my weigh tickets. Truck with no trailer, Steer Axle 3660#, Drive Axle = 3340#, Gross Weight = 7000#​
​
Truck with trailer, but WDH disengaged. SA = 3340#, DA = 4640#, Trailer Axle = 5320#, Combined Gross Weight = 13,300#​
​
Truck with trailer, but with WDH engaged. SA = 3520#, DA = 4280#, TA = 5460, GCWR = 13,200.

[Ephraim]

The COG does not change with a WD hitch. The only thing that changes is that in addition to the weight introduced (by the mass of the trailer AND the COG with its relation to the both the tongue and the trailer wheels - which is a combination of linear and rotational forces) you ADD a rotational force at the hitch.

The added rotational force uses the front wheels of the TV and the trailer axles as it's center while the force is upward with respect to the hitch. This unloads some weight from the rear TV axle to the front axle and some to the trailer axle. The only way the COG would move is if you physically move objects on the TV and the trailer.