How much hitch weight gets calculated into payload when using a WD system

[dznf0g]

Originally Posted by Merlintiaga

Weight distribution is hard to envision b/c we tend to think in terms of forces not torque. We know that a force in one direction causes an equal and opposite force in the other. The same is true of torque. So in the case of weight distributing hitches, the WD hitch places a large torque at the receiver that results in taking weight off the TV rear axle and moving it to the front axle. An equal and opposite torque is applied at the hitch on the trailer. This results in the increased load on the trailer’s axle.

On net, as Uncle Bob’s data shows, front axle load goes up, rear axle load goes down, trailer axle load goes up.

In the attached figure, I sketched what is called a “free body diagram” in engineering. The torque labeled M subscript wd is the net torque from the WD hitch. The takeaway of this figure is the direction of this torque. Note it “lifts” the TV taking weight off the rear axle.

That is almost exactly what I have been saying....except, you have the torque to the trailer occurring at the coupler in your top drawing. It isn't happening there, but about 25" aft of the ball, at the jacks (or snap up bracket).

Torque and forces cannot be separated. Torque does its work through force vectors.

Upon further thought, I have another disagreement point. It is a linear vertical pull that affects trailer axle weight increase, whereas the front of the bar creates twist (torque) through the 90* turn in the bar front end design. You are correct, though, that the torque vector force at the front of the bar must equal the rear bar end force, in the vertical plane.

[Warpath]

I found this video helpful and answers a lot of common questions.

https://youtu.be/kCgRiVNaXFc

[WhatNext]

The free body diagram provided in post #60 is correct.

(Don't want to get into engineering 101 here, but the vertical force on the WD bars mentioned in #61 is what generates the torque at the coupler. So it is accounted for and the free body diagram shouldn’t show a vertical force at the coupler)

[dznf0g]

Originally Posted by WhatNext

The free body diagram provided in post #60 is correct.

(Not gonna get into engineering 101 here, but to give you a hint, the vertical force on the WD bars mentioned in #61 is what generates the torque at the coupler)

Of course, and that torque affects the TV only, except for the coupler having to "go along for the ride" with the effective lifting of the rear of the TV.

If I were really strong, and disconnected the spring bar, stood on the ground next to the jack, and lifted the rear spring bar end to impart the same torque at the hitch head, there would be zero increase in trailer axle weight.

If, however, I stood on the tongue and did the same thing, there would be an increase of weight on the trailer axles (not counting my body weight).

So, as I said above, there are two separate outcome effects to the system, generated by the same preloaded spring set.

The torque isn't generated at the coupler, but rather at the point where the front of the spring bar acts upon the head, commonly even with the ball and on either side of it.

[gypsydad]

Originally Posted by WellSaid11

You said "full tanks" and I assume that means fresh water as well. My TW was with full LP and empty F/G/B tanks.

Full FW tank: an additional 307 lbs. Would then be 1240 or so.

Some people have been saying 1100-1200 with no full F/G/B tanks, so....

We always travel with resh water full, as recommended in AS manual; we carry 3-4 gal in black tank, but gray was empty when I weighed last. Still, most 25's and larger scale in around 1000+lbs on the tongue, from my observations here, so your comments make sense to me now.

[dznf0g]

Originally Posted by dznf0g

Of course, and that torque affects the TV only, except for the coupler having to "go along for the ride" with the effective lifting of the rear of the TV.

If I were really strong, and disconnected the spring bar, stood on the ground next to the jack, and lifted the rear spring bar end to impart the same torque at the hitch head, there would be zero increase in trailer axle weight.

If, however, I stood on the tongue and did the same thing, there would be an increase of weight on the trailer axles (not counting my body weight).

So, as I said above, there are two separate outcome effects to the system, generated by the same preloaded spring set.

The torque isn't generated at the coupler, but rather at the point where the front of the spring bar acts upon the head, commonly even with the ball and on either side of it.

Ok, I need to publicly revise my position on one point upthread.

The opposing forces at the coupler (Clamping forces) are NOT a net sum zero game, leaving only the original coupler TW as a force on the ball. The net sum zero game is the offset to the bar end force at the jack end.

So:
Known TW including hitch hardware attached to tongue = 905#
calculated rear bar end linear force = 1050# (combined bars)
From scale ticket, TT axle wd increase = 120#

1050-120 = 930# spring bar end force applied to the coupler
930 + 905 = 1835# exerted to ball by coupler(TW+end force)
In order to have 120# go to the trailer, the opposing vertical force upward at the ball must be 1955#, or 120# more than the downward pressure exerted on the ball by the coupler.

Total "clamping force" between ball and coupler = 3780#

So, all that is derived by backing in from scale outcomes. I really would like to cross check with "forward-in" calculation utilizing spring bar input data to calculate torque and its resultant effect at the ball and through the truck beam. But I have a real mental block with torque calculations. Can one of you professional engineers help? Maybe take it offline, as "ain't nobody got time for dat!"

[WellSaid11]

Originally Posted by gypsydad

We always travel with resh water full, as recommended in AS manual; we carry 3-4 gal in black tank, but gray was empty when I weighed last. Still, most 25's and larger scale in around 1000+lbs on the tongue, from my observations here, so your comments make sense to me now.

Therein lies the issue (or at least one of them)!

1. The AS manual(s) recommends that you travel with FW full, but the spec sheet says the hitch weight is X with "LP and batteries" only.

2. Actual and weighed (Sherline) HW/TW is close to +/-900 lbs WITHOUT FW tank full (like the published 900 lbs. AS spec says).

3. Add 37 gallons to FW and now you have +/- 1200 lbs.

So......are people seeing their "published" hitch/tongue weight from AS and wondering why it is more because the AS spec is WITHOUT fresh water being added?

Seems like this could be one explanation for all of the "AS understates the TW/HW on all trailers" comments.

[Tom_T]

Originally Posted by Dick tracy

I would suggest transferring weight BACK to the TV front axle IS a major component of a properly setup WD system. getting the same weight on the front axle with the trailer attached as without is vital for the proper function of the steering and brakes. Mine is within 40lbs and moved about 400lbs to the front axle.

Well Dick -

I thought that is exactly what I said was the #1 function of WD in my part that you'd quoted:

"1.) To level the TV in order to maintain Steering Control & Braking effectiveness, since in most TVs 100% of Steering & 70-80% of braking is on the Front Wheels/Axle."

PS - Another of my key points was that all of this philosophical discussion is meaningless, & folks should just go weight their rigs for whatever is actually happening with their own specific TVs & TTs. Robert Cross & a few others have provided their specific TV/TT rigs' actual CAT Scale results to show what HW is actually transferred from the TV's axles & hitch back to their own TT's axle(s). Those are actual indisputable facts.

So the OP & any others who are debating, pondering, wondering, enlisting Cray supercomputers to run theoretical calculations, etc. - just spend a few bucks & get your properly set-up level WD rig & without WD weighed & then use those real world actual WD transfer numbers for your TV payload calculations, or use the actual weighed total HW if you want to be conservative. HW can also be weighed by either a Sherline HW Scale, or with the Jack on one CAT Scale plate & the Trailer wheels on another. Just Git 'er Dun!


To others questioning the CAT Scale readings with WD engaged or not &/or of the WD transfer of HW from the TV axles to the TT axle(s) - since the torsion bars are attached at one end to the Hitch Head & Coupler/Tow Bar or Stinger - AND their other end is bolted to to the TT's A-frame - it has to transfer some hitch/tongue weight back to the trailer axle(s) when the torsion bars are put into torsional flex by either the chains or jacks.

When no tension is put on the torsion bars & they're not functioning as WD - & then they're just dead weight hanging off the A-Frame & Hitch assembly - ergo there is in fact a difference at the CAT Scales between tensioned & untensioned WD torsion bars that you will see in the 2 weight readings of the two states.

Without getting into all of the moment arm lengths, suspension stiffness differences of TVs, etc. structural engineering details of which I'm well aware as an Architect -

Using a more common sense approach I also pointed out in simple terms that when I use little WD tension force on my WDs for a 3/4 ton pick-up vs. a lot of WD force on a smaller & lighter duty mid-sized SUV (Pathfinder) - the latter will naturally transfer more to the TT axle as well as the TV front axle, than to the less WD tension cranked in for the 3/4 ton pick-ups. The difference on my Hensley Cub jacks between the 2 TV types is about 1" - 1.5" more tension.

That explains for the most part why some folks are getting different TT axle(s) HW transfers on these posts, aside from any differences in the distance from the hitch to the TV's front & rear axles & from the rear torsion bar mount on the A-frame to the TT axle(s) (i.e.: different moment arm lengths that some are posting about).

I may stop by the Flying J CAT Scales on my way to &/or from the Pismo Vintage Trailer Rally next weekend to get current weight readings for rig axles weights with & without WD, & the overall gross trailer weight as currently wet/loaded.

The original 1960 Avion Sales Brochure listed our single axle T20 "Tourist" model at 2680# empty trailer wt. & 275# HW, but that was probably for a no tanks "Park Model" with no options (see attached). Ours has fresh & black tanks (no grey currently, but will be adding in future), 2x 30# LP tanks, & several options added or subtracted over the years, & my prior wet/loaded/optioned running GTW was 3000-3500# depending on what we bring (plus about 200# more with bikes on the rear bumper mounted rack). This 3000-3500# wet/loaded/options GTW would imply a 10%-15% HW of 300#-525# HW.

However, my Sherline weighed HW comes in heavier at 542# due to the 160# Hensley Cub, +/-20# for Tekonsha RF/steel mounting plate & hardware, +/1 80# for the 2 larger 30# LP Tanks - all on the typical 1960's relatively short 30" long A-frame; & the 20 gal Aluminum Pressure Fresh Water Tank & air pump replaced with a larger 27 gal poly tank & pump, AGM Battery & electric components (charger, inverter, surge protector, etc.) - all at the front wall.

So for the Cub/Tekonsha/LP that's 160# + 20# + 80# = 260#added to the "basic" 275# HW = 535# accounting for most of the increased HW, & a bit more than 15% (but Hensley & ProPride 3P hitches don't need a particular 10-15% HW due to their solidly connected "Stinger" Tow Bars & Hitch Head Struts &/or Yoke bolted to the A-Frame - in addition to the bolted WD Tension Bar mounts). The larger fresh water tank is probably mostly balanced out by the larger black tank at the rear bath (both by the PO's resto before we got it in 2012).

Whenever I can get current CAT Scale readings, that HW info can be compared to where the WD moves that total HW to various TV & TT axles.

Also note that I typically just use the total 542# actual total HW for TV payload calculations for a bit of a safety margin, as well as to keep things simple for my half-zymers brain at 69!

Cheers!
Tom
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[dznf0g]

"3. Add 37 gallons to FW and now you have +/- 1200 lbs."

What??? Added to TW? Where is your tank? In between the axle tubes (whose center of mass is 5" forward of the spindles center)?

If that's the case, then using a 30'er as an example.

37 gallons weighs 307, of which only 9# goes to the tongue and 298# goes to the axles.

[WellSaid11]

I know!!! I wasn’t saying that was possible!

I was merely commenting that it may be some kind of explanation for more weight and not the full explanation. not too many tanks up front of course.

I think the real issue is how people are getting such higher TWs on a “dry” trailer weight vs. the published AS spec - FW and whatever else.

[Dick tracy]

Originally Posted by dznf0g

Not unless the wheelbase of the vehicle = the distance from the rear axle to the receiver pin + the distance from the receiver pin to the trailer axles.

That would be a very odd truck/trailer combo.

I've actually weighed my rig and can tell you the weight is moving off the TV rear axle to the front and the trailer, odd looking or not, it works.
300 lbs to the TV front axle, 200lbs to the trailer

[Dick tracy]

Originally Posted by dznf0g

"3. Add 37 gallons to FW and now you have +/- 1200 lbs."

What??? Added to TW? Where is your tank? In between the axle tubes (whose center of mass is 5" forward of the spindles center)?

If that's the case, then using a 30'er as an example.

37 gallons weighs 307, of which only 9# goes to the tongue and 298# goes to the axles.

EXACTLY - when I fill my FW tank it lowers my TV rear axle weight by 80lbs

[Dick tracy]

Originally Posted by Tom_T

Well Dick -

I thought that is exactly what I said was the #1 function of WD in my part that you'd quoted:

"1.) To level the TV in order to maintain Steering Control & Braking effectiveness, since in most TVs 100% of Steering & 70-80% of braking is on the Front Wheels/Axle."

PS - Another of my key points was that all of this philosophical discussion is meaningless, & folks should just go weight their rigs for whatever is actually happening with their own specific TVs & TTs. Robert Cross & a few others have provided their specific TV/TT rigs' actual CAT Scale results to show what HW is actually transferred from the TV's axles & hitch back to their own TT's axle(s). Those are actual indisputable facts.

So the OP & any others who are debating, pondering, wondering, enlisting Cray supercomputers to run theoretical calculations, etc. - just spend a few bucks & get your properly set-up level WD rig & without WD weighed & then use those real world actual WD transfer numbers for your TV payload calculations, or use the actual weighed total HW if you want to be conservative. HW can also be weighed by either a Sherline HW Scale, or with the Jack on one CAT Scale plate & the Trailer wheels on another. Just Git 'er Dun!

Cheers!
Tom
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I was focusing on your statement at the end of your "dissertation" .



"Some responses are listing very low WD weights transferred to their TV's Front Axles - which should not be the main point of setting up your WD Hitch."


I was suggesting it was a component of the main point, to even out weight distribution between TV front/rear axle to allow the truck to perform within limits, stopping and steering as intended.

[dznf0g]

Originally Posted by Dick tracy

I was focusing on your statement at the end of your "dissertation" .



"Some responses are listing very low WD weights transferred to their TV's Front Axles - which should not be the main point of setting up your WD Hitch."


I was suggesting it was a component of the main point, to even out weight distribution between TV front/rear axle to allow the truck to perform within limits, stopping and steering as intended.

I. perhaps am one of those. My focus is staying closer to 10% coupler weight and minimizing WD end bar pressure due to the A-frame flex and front wall issue. I am still staying in all the numbers.
One of the beauties about the Maxtow on my truck is the larger rear axle and rear spring secondary leaf (commonly known as overload spring...but that is a misnomer). So, on my truck the front axle is exactly the same as a non-maxtow truck. The rear suspension has 350# more capacity than a non-maxtow. What that tells you is the truck is designed to be loaded more heavily biased to the rear. I don't think I would want to use my WD setting on a non-maxtow.

Because of that, loaded with supplies in truck and trailer for 3 months away, with a 50% return to front axle I am removing only 270# from the rear axle, restoring 180# to the front axle and placing 90# to the TT axles.

That represents 3506 on front axle with capacity of 3950
4087 on rear axle with a 4300 capacity. and
a whopping 7 pounds under payload and GVWR.

That's the way I roll and it has very good road manners.

[SYC2Vette]

Originally Posted by dznf0g

One size does not fit all. If you TV manufacturer says to return 100% of weight or height, then you are correct (usually SUVs and passenger vehicles). If your manufacturer says return 50%, don't go much more than that. Follow your manual. With (dare I say all) pickups which are born light in the arse and designed for concentrated loads aft, if you return 100%, the steering will feel "dead", you will spin rears at a green light (especially on wet roads surface), and most likely set up potential handling problems, when you need handling the most.

Aha, thanks that actually makes sense. I was curious to know why my owners manual says approximately 50% but others say 100% returned at the front.

Only time I have "tested" my settings was when someone ran a stop sign and I had to swerve to the other (empty) lane at about 50 mph. The trailer tires skidded right then left to clear but my truck did not over or understeer with no tire breakaway.

[Wazbro]

Originally Posted by labans

What is a "Burb?"

Burb is short for Suburban, a big Chevy SUV (Sport Utility Vehicle).

[ROBERT CROSS]

Originally Posted by labans

What is a "Burb?"

This is a "Burb"...our '06 2500.

Bob
🇺🇦🇺🇸🇺🇦

[Tom_T]

Originally Posted by Dick tracy

I was focusing on your statement at the end of your "dissertation" .



"Some responses are listing very low WD weights transferred to their TV's Front Axles - which should not be the main point of setting up your WD Hitch."


I was suggesting it was a component of the main point, to even out weight distribution between TV front/rear axle to allow the truck to perform within limits, stopping and steering as intended.

Okay Dick, I see what you're talking about.

What I'm saying there is that the the main point is to get the TV back to its Level State without anything hitched using the WD, & the weight transferred to the front axle is a byproduct of whatever WD tension that it takes to get the TV level.

In other words - a specific weight transferred to the front axle isn't the primary Goal #1 - but what results once you get it level.


Where axle weight transfer will come into play is when you need to move a specific amount of weight off of the rear axle in order to stay within your TV's RAWR limit.

That's not a problem with our 3000-3500# GTW & 542# HW Avion, but some folks posting here are listing 1000-1200# HW & they may have that additional issue as say goal 1-B.


And for those commenting about the AS recco to run with the FW tank full - if the FW tank is in the rear - then that's probably recco'd in order to balance out the trailer & keep the HW to 1000# or whatever are your trailer's & TV's limits.

You can also do this with the balanced loading of gear etc. inside the trailer that I mentioned.

Cheers!
Tom
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