What Does a Weight-Distributing Hitch Do?

[Brian B-P]

This is essentially a response to a comment (quoted below) from [b]Towing with a Jeep CJ, YJ, TJ, LC FJ-40, LR SIII 80?, Share your tales here regarding what a weight-distributing hitch does. At Roger's request, I have not posted this to the original topic, since it is not specific to the short-wheelbase vehicles which are the subject of that topic.

I would also like to make one thing clear up front: I do not have personal experience towing with a weight-distribution (WD) system, nor am I suggesting that they are not effective. The question here is exactly what they do.

...If properly installed and setup, a WDH never transfers the TV weight to the RV axle. It only redistributes the hitch weight more evenly over the axles of the combined TV/RV...

I have heard variations of this statement before, and it does not make physical sense to me. Does anyone have their own opinion, reliable reference, or explanation to offer?

[Brian B-P]

Here's my first shot at an explanation:

The basis of a WD system is little more than a pair of spring bars. They are inserted into sockets in the hitch, and tension is applied between their trailer ends and the trailer frame. They pry the rear end of the tug up, and use the trailer frame as their support.

Imagine that the tug/trailer combination is a long board. It's sawn in two somewhere near the middle, and splinted back together with couple of springy bars (the WD spring bars). There are stands supporting the board near the splint (the rear axle), further out at one end (the front axle), and back near the other end (the trailer axle). There are chunks of concrete scattered along the board (the mass of the trailer and tug).




If that middle stand (rear axle) is carrying too much weight, the splint can be tightened up or the end stands (front and trailer axle) lifted... either way, some load will be lifted from the middle stand (rear axle). You can't just lift of one end without the other getting more load, and whichever end stand (front or trailer axle) is closer to the middle stand (rear axle) will take more of the increase.

[Brian B-P]

The following is from the Weight Distributing Info page found under the Information section of the Reese Products website.

Originally posted by Reese - Weight Distributing Info
Rather than merely supporting the trailer tongue weight (TW), weight distributing hitches apply leverage between the towing vehicle and trailer causing the TW to be carried by all axles of the tow vehicle and trailer.

This clearly indicates to me that the WD system changes the weight carried by the trailer axle.

[Curtis F.]

This is a good basic description. Then, the differences of the different brands still use the same principles. They just use different mechanical designs. RV.Net Towing Forum

WDH is all about creating leverage at the ball to transfer weight. At first look, it makes no sense that it would work. Increased pressure (not weight) on the top of the ball and the angle of the ball change the distribution of weight between the TV and TT axels.

Curt

[Brian B-P]

A relevant previous discussion, to provide some context and perhaps reduce repetition...

New Smaller tow vehicle choices??[i] (starting with post #36)

[Brian B-P]

This is a good basic description. Then, the differences of the different brands still use the same principles. They just use different mechanical designs. RV.Net Towing Forum...

Yes, a classic thread. The technical description is very good, but Kenneth's description is my personal favourite.

I will caution members here that the linked discussion is very long (I've never had the time or patience to read the whole thing), and it can get hard to follow as people (well, mostly one person) throw in junk for Ron to correct. Whoever this Ron is, he has the patience of a saint and knows what he is talking about.

[CD Smith]

I prepared the following as an explanation of what a WDH does on my Escalade.


WDH_Complete_Weight_Analysis.pdf


I hope it helps your understanding.

[Brian B-P]

I prepared the following as an explanation of what a WDH does on my Escalade.

Attachment 6540

I hope it helps your understanding.

Thanks CD.

That's the document from the other topic which I linked (which I why I provided the link). It's a great start, but the axle weights are calculations (not real measurements). Also, in this analysis it is assumed that the WDH does transfer load from the Escalade's rear axle to the Casita's axle (400 lb taken off of the Escalade rear, 250 lb added to the Escalade front, and 150 lb added to the Casita). Those values are likely close, but not for the reason assumed.

Also, I read the diagram in this analysis which indicates something quite different from the values in the table... making part of my comments in that original topic incorrect. I have posted a correction in that topic, as post #44.

So, it seems we agree that the WD system does transfer load (weight) from the tug to the trailer. Back to the original quote, but I've added emphasis this time...

Originally posted by CD Smith
[b]..[b]If properly installed and setup, a WDH never transfers the TV weight to the RV axle. It only redistributes the hitch weight more evenly over the axles of the combined TV/RV...

So maybe the point here is to not overdo it; [b]don't crank the WD spring bars so much that the tug is carrying even less weight than without the trailer. Is that your point, CD? If so, I certainly agree.

[Steve L.]

I apologize in advance for my laziness is not reading through all the links...

I've always thought of the WDH as inducing camber in the whole trailer/tug assembly.

Using Brian's drawing as a reference, imagine the whole assembly in weightless outer space ("TTTUUUUGGGGSSS In Space! starring our intrepid campers Miss Piggy and Kermit). Without the spring bar in place the whole assembly would be a straight bar. Or at least could be a straight line. Connecting the spring bar on the bottom would cause a downward curvature with the trailer end and the tug drooping.

Gently lowering the assembly to earth would cause the whole assembly to straighten out but with more of the assembly's tongue weight being carried on the tug's front axle and on the trailer axle thus lightening the load on the tug's rear axle.

Inferred by this is a problem for the trailer axle if it is already near it's maximum. A WDH would worsen it's load.

The WDH does not magically transfer trailer axle weight to the Tug. It only transfers some tongue weight from the Tug's rear axle to the Tug's front axle.

Without the WDH the tongue loads up the Tug's rear axle and lightens the load on the Tug's front axle.

I'm just offering another way to look at the system. No new physics.

[Roger H]

(CD Smith @ Feb 24 2007, 08:23 AM)

<div class='quotetop'>QUOTE

(Roger H @ Feb 23 2007, 08:43 PM) *
That's true! And it's not always necessary or beneficial. Towing my 17' behind the Excursion it would be neither necessary nor beneficial to use a WDH, and may potentially overload my trailer axle and/or bend the frame since the Excursion weighs 7,000 lbs and a WDH would merely distribute the vehicle's weight to the trailer. It experiences no rear-end sag with the trailer added. I still use a friction sway control bar with that setup though.

I tried not to post a comment to these statements, but could nor restrain myself. Others may be misled by what you say.

What you say about your Excursion and WDH indicates a total lack of understanding about how and what WDH's are designed to do.

I tow with an Escalade and it has an auto leveling system. The rear of the Escalade never drops when the Casita is attached. However, because of the hitch weight of the Casita the weights on the front and rear axles do change. They change enough to affect both steering stability and tire wear.

If properly installed and setup, a WDH never transfers the TV weight to the RV axle. It only redistributes the hitch weight more evenly over the axles of the combined TV/RV. The mistake many WDH users make is to not level their TV, either added springs, air shocks, or air bags before hitching the RV. A WDH is not designed to lift the rear of a TV that has been overloded and made unlevel by that load.
[/quote]

CD, since my comments started this, I thought I ought to comment a little further.

Your situation with the load-leveling system on your Escalade is unusual. Most vehicles, of course, don't have load leveling, my Excursion included.

My comment above was general; and I believe that under extreme circumstances, the weight and stiffness of the suspension of my 7000 lb Excursion could, in fact, transfer substantial weight from the rear axle of the truck to the trailer axle. With a relatively light capacity axle under these trailers and the relatively small net margin of load capacity that they have, I believe that a WDH could cause the trailer to exceed it's GVWR, particularly as Brian pointed out, if I had overstressed the WDH. Further, re-distributing two thirds of the hitch weight from being solely on the rear axle of the tow vehicle to the other two axles in the system will, in most cases, cause leveling of the entire rig. The desired effect is for all three axles to deflect equally under that load.

A WDH system is dynamic on the road and the static weights you measure at rest will change; sometimes dramatically with braking, acceleration, and evasive maneuvers.

The suspension on my Excursion has sufficient capacity that the 350 lb tongue weight of the trailer is hardly noticeable against a 7000 lb curb weight. It's a significantly higher percentage of the 4500 lb curb weight of my Tundra, and does, in fact, affect the stability and handling of the Tundra.

As I said in my earlier posts, I'm NOT an engineer, and the math makes my eyes glaze over. I do have over thirty years and lots and lots of miles of practical trailer towing experience with and without WDHs with over a dozen different trailers and a similar number of tow vehicles. I will confess, thought, that all of my WDH experience is with Reese products ONLY. I've never used any of the other products available.

Roger

[Darwin Maring]

When in doubt, listen to Roger and follow his advice.

Absolutely nothing can replace years of experience and that includes degrees and slide rules.

[Gina D.]

don't crank the WD spring bars so much that the tug is carrying even less weight than without the trailer.

Does tha mean you'll get way better milage than when you are NOT towing??? (Off to my room)

Yes, I agree with Darwin. Everyone I know that tows on a regualr basis has told me that a WDH is a necessity for my new rig. I respect thier opinions and experience. So.. I bought one. (It's in the mail!)

I have not towed the trailer since I brought it home nearly a month ago. It towed great, and with less effort than my Element/13 combo.. BUT...I am going to be very cautious when I DO tow it next.. to the shop that is going to install the WDH, among other things. Most of those other things are related to safety.

It's a less than $300 investment. My investment in my Jeep and the value of the trailer just about equals half of what I paid for my house. (Yeah, I bought at a good time )

Thinking of it in those terms makes it a no brainer for me.

The gentleman that bought my 13 is going to be astounded at all the "Unnecesary" safety stuff I put on it. But I bet he'll get over it once he uses it. You can't skimp on that stuff when it comes to things like.. death and destruction.

[Per Walthinsen]

It is funny, puzzling, and sometimes reassuring that people come to different conclusions about some of these issues. The recent lines of thinking about WDH have had me trying to sift through the arguments to see how they apply to my situation.

Maybe I'm too focused on preventing any tendency for the trailer to "fishtail" since it has never happend to me even though I'v been in many situations where you'd think it would happen. In fact, I think that you will find that the 17' Burro may give you a feeling of utter stability (apart from the variable of the different tow vehicle). I predict you will enjoy it for that characteristic.

But I digress. As I attempt to understand the forces involved it has become more evident to me that since my tongue weight is well within limits, etc. etc., a WDH is the wrong approach for my combination and would adversely affect stability and introduce unwanted stresses. I think we could agree to disagree. OK?

[Roger H]

Well, Per... I'm not sure in your case whether a WDH would be useful or not, but from seeing how a Casita 17 drops the rear of an Odyssey, I would think that you could benefit from a WDH (unless Honda says specifically not to use one). And recognize that a weight distributing hitch and the function of sway control are completely separate functions, sometimes combined into the same piece of equipment (Reese Dual-Cam) and sometimes two (or three on large trailers) pieces, the weight distributing hitch with load bars and the friction sway control devices.

A weight distributing hitch won't control sway by itself, but it will improve handling in many cases. I know that a WDH was a necessity when I towed my Burro with the '94 extended cab 4WD Toyota compact truck. It's not AS necessary with my Tundra, I can tow without it, but the handling is much improved using it. I have friction sway control in use either way.

Roger

[CD Smith]

All I can say is that experience with 5 different vehicles and three different trailers with and without WDH has led me to the conclusion that, "Its all in the weights."

I am sure that your 7,000 lb. Excursion tows differently than my 6,000 lb. Escalade. Just as my 1750 lb. travel trailer I towed with a 2,300 lb. Covair Coupe.

However, as an engineer and a mathmatician I have a great deal more faith in actuals than I do in expert witness testimony. In other words, "Get it weighed!" It is the only way to know for sure.

[Per Walthinsen]

Roger:

I think your point about the 17' Casita may illustrate the issue pretty well. Whenever I've read the tongue weight of that particular trailer it seem to be in the 400 to 450 lb range (correct me if I'm wrong). I suppose it is possible that they had to put the axle a few inches further back than would be the average for similar trailers.

That would definitely change my thinking, since the Honda's tow weight limit is 350 lbs. Therefore my airbags would not do the job and only a WDH would. At 290 lbs tongue weight for the Burro I want to keep it right there over the rear wheels for stability. To quote: "It's all in the weights."

[Darwin Maring]

It needs to be mentioned that there are different levels of equalizer bars.
You should not need 1 thousand pound bars.

Roger: Is it possible to use just one bar?

Concerning sway: Consider a friction sway control.

[Pete Dumbleton]

I don't know how the auto-leveling system on the Escalade works, but if all it does is jack up the rear end (like air bags or helper springs do), it is NOT transferring any appreciable weight from the rear axle to the front axle -- Prove it by putting a load in the back of it, get to a set of scales with front/rear split, note the readings with system on, turn it off and note the new reading on the front axle... The leveling system is leveling the body, not the suspension.

That said, however, I believe the system should be OFF when setting up the WDH because proper setup calls for measuring the front/rear distance to the ground and keeping that ratio when the hitch is adjusted (in other words, the weight transferred to the TV should be spread evenly between the axles to make it 'sink' on an even keel) -- Hard to do if the system is constantly adjusting itself.

Any WDH system WILL transfer some of the weight from the rear of the tow vehicle to the trailer axle, and how much goes to front axle vs trailer axle will depend on the ratio of the ball-to-axle distances... Picture the extreme of the adjustment being set so high that the rear wheels are off the ground (like the old advertisement) -- The trailer axle is clearly supporting more than the trailer!

Easiest illustration I can think of is a conventional longbow with the ends on a table and the handgrip horizontal to the table but up in the air; the shorter the string, the higher the center will be off the table. The spring bars and chains are the string and the hitch ball is the handle, with the front and trailer axles becoming the bow ends.

[Roger H]

An interesting point is that most hitches that are rated at 1,000 lb tongue weight and 10,000 lbs towing are only rated that highly using WDH systems; otherwise they're limited to 500 lbs and 5,000 lbs dead weight.

Per, as CD says, it's all in the weights... if you have noticeable sag in the rear end of your tow vehicle, even though the tongue weight is within the capacity of the vehicle, then you'd probably benefit from weight distribution.

Darwin, Reese (and perhaps others now) make a single-bar WDH system just for trailers with a lighter tongue weight. Gina just ordered one. In the Reese world, the load bars come in a wide array of weights, and you need to have the appropriate set of bars for your application.

Roger

[Pete Dumbleton]

That would definitely change my thinking, since the Honda's tow weight limit is 350 lbs. Therefore my airbags would not do the job and only a WDH would. At 290 lbs tongue weight for the Burro I want to keep it right there over the rear wheels for stability. To quote: "It's all in the weights."

Per, if you read the literature carefully, or call the various tech supports, you will find that adding air bags (really air springs) or adding helper/overload springs or jamming 2x4s between the rear axle and the body WILL NOT change the tongue weight load restriction of the tow vehicle.

Also included in that trailer TW load restriction may be any inside or roof weight that touches to the rear of the axle -- Read your owner's manual carefully about this.