What Does a Weight-Distributing Hitch Do?

[Brian B-P]

Pete, you're right that the tongue weight restriction is not changed by spring changes, such as air bags. That, presumably, is exactly why Per said that he would need to go to a WD system if the trailer's tongue weight were higher. I think Per meant to indicate that the Odyssey's weight carrying limit is 350 lb, but the limit is higher with WD (that is the case for my Sienna, with the same 350 lb limit value).

This is an important point: adding springs helps the rear axle handle the load it is carrying, but does not change the factory limit, or the load carried by each axle. A WD system does change the loads carried by all three axles, reducing the rear axle load while increasing the others.

By the way, I call the Firestone Coil-Rite units (like the similar AirLifts) air "bags" because in our case they are just bags added to the coils; they are not standalone air springs. They do function as air springs in parallel to the coil springs.

[Brian B-P]

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...

Yes, I agree that this is true. With the WD system in effect, the rear-most attachment points of the hitch receiver frame to the vehicle structure are not under nearly as much load (they may actually be pushing up instead of pulling down on the vehicle), so structurally it is not surprising that the hitch and/or the vehicle will have a higher WD than weight-carrying limit.

I don't think this has anything to do with vehicle control, just structure, as indicated by the fact that these are limits established by the hitch manufacturer.

[Brian B-P]

Steve, an interesting and valuable perspective, as always.

...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...

... and some load from the Tug's rear axle to the Trailer's axle!

I know that may throw more fuel on the fire here, but consider this: the WD system is used because of the effect of tongue weight on the tow vehicle, but the way it works has nothing to do with tongue weight. You can crank up those spring bars enough to counteract the nose-up tilt of the tug, or less, or more (yes, CD Smith, we know this is bad), or whatever you want until something breaks or the rear wheels come off the ground - even if there was zero tongue weight to start with. The amount of load it is able to transfer is not limited or affected by the trailer's actual weight distribution.

The WD system changes the distribution of [b]load, but doesn't move any [b]mass anywhere, and therefore it can't (nor can any technology known to science) make the weight happen anywhere other than where the mass is. The tongue weight stays where it always was, on the hitch ball. It's just a matter of which tires support the whole mess.

[Brian B-P]

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?

Ack! No! (But Roger should answer, too...)
The single-bar design runs the bar down the middle. If you used only one bar of a two-bar WD system on it's original bracket, you would always be pulling the trailer over to one side, and applying a significant upward force and torque to the hitch off centre. This has to be bad.

I wholeheartedly agree that "thousand pound" bars are completely unsuitable. The idea of the differently rated bars is just that the higher rated ones are stiffer, and can be used to apply greater force and thus transfer more load. A bar which is too stiff means that as the combination goes over humps or through dips and changes angle, the force will change too much, to potentially damaging levels.

One of the reasons to use a suitably-sized WD hitch is to avoid excessive weight of metal to haul around... but the other would be to get softer spring bars, to better suit the purpose. Bigger is certainly not always better.

[Brian B-P]

I think all of us who have been active in this discussion and who have adjustable suspensions (CD's self-leveling Escalade, Per and I with air bags) understand that suspension adjustment does not change how much load each axle carries.

...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....

If you are using [b]ride height to adjust a WD hitch, then certainly the self-leveling needs to be off.

I think the problem is that this method is fundamentally flawed. Making the tug sink evenly does not ensure that the load is spread evenly among the axles (if that were even desirable, but that's another subject), because the front and rear spring rates are normally different. If it sinks one inch on each end, that might be twice as much increase in front axle load as rear... or half.

Is the target is really a specific preference for load distribution? That is what I think CD, Roger, and Pete are all saying... and I agree.

Then the only way to get there is to use a scale. You can scale the whole rig (with self-leveling turned on or airbags adjusted) at various WD tension settings until the balance is right, or you can do the math and then use a tension scale on the WD spring bar chain (confirming afterwards by measuring the axle loads), but in the end, ride height won't tell you what your WD system really did, unless you do something extreme (and bad) like putting a rear axle with no spring adjustment right back where it was with no trailer.

[Brian B-P]

... 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.

Ideally, this might be the case. The single bar design (from any of the Cequent companies, such as Reese) gets the closest, and I'm not saying that's necessarily good...

All of the two-bar designs (which is every other design on the market) have some additional effects, beyond the load transfer from rear axle to the other axles. Since they have a bar on each side, interesting things happen when the coupling pivots in any direction:

If the trailer [b]rolls a different amount from the tug, one side pulls down harder than the other on the trailer frame, tending to make the trailer follow the tug. This means badly controlled trailer roll is somewhat controlled by the tug, maybe reducing sway.... and if you don't want this coupling, you've got it anyway.

If the trailer [b]pitches relative to the tug, the bars tighten (nose-down pitch) or loosen, tending to keep the two aligned even though it means changing axle load distribution. That's how a too-stiff WD system between a massive tug and delicate trailer could be destructive.

If the trailer [b]yaws (either a normal turn, or wanders off-line when the tug is going straight) the spring bars are moved forward on one side and back on the other side (relative to the trailer frame), so the chains are no longer vertical and tend to pull the trailer towards straight. Since undesired "sway" is usually yaw, this might be good... but it also resists turning when you want (or need) to.

There is also friction (at the spring bar trunnions or other pivoting mounts) resisting any movement in the yaw direction.

While a friction-type link "bar" is a separate device, all of the other "sway control" features are accidents of the two-bar design, or deliberate exaggerations of them.

[Gary Lynch]

"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 checked the specs on an 2007, Odyssey. While it states it can tow 3500 lbs, it only has a tongue weight rating of 350 lbs. If you have a 17' SD fully loaded, the tongue weight is more than that. So, the Odyssey would be exceeding it's capabilities. If the manufacturer does not offer a towing package which would extend it capabilities, then I suspect exceeding it's listed specifications is not warranted by the manufacturer.

"It's not AS necessary with my Tundra, I can tow without it, but the handling is much improved using it."

It's interesting that a full size Toyota Tundra (assuming it's one of the V8 engines) that has a towing capacity between 8,500 - 10,000 lbs. would have any engineering problems towing a 17' fiberglass trailer weighing one third to one half of it's towing capacity.

[Frederick L. Simson]

(unless Honda says specifically not to use one).

Quite the contrary!
<div align="center">ODYSSEY
2003 Owner's Manual</div>

[b]Towing Equipment and Accessories
Weight Distributing Hitch
If the total trailer weight is more than 1,850 Lbs (840 Kg), you must also use a weight distributing hitch. This device transfers weight from the vehicle's rear wheels to the front wheels, and to the trailer's wheels. Carefully follow the hitch maker's instructions for proper installation and adjustment.

[b]Sway Control
If the total trailer weight exceeds 2,000 Lbs (900 Kg), you should install a sway control device to minimize swaying that can occur in crosswinds and in normal and emergency driving maneuvers. Your trailer maker can tell you what kind of sway control you need and how to install it.

[b]Trailer Brakes
Honda recommends that any trailer having a total weight of 1,000 Lbs (450 Kg) or more be equipped with its own electric or surge-type brakes.

I interpreted the above to mean that the towing capacity of the Honda Odyssey is:
1,000 Lbs (450 Kg) without Trailer Brakes
1,850 Lbs (840 Kg) without Weight Distributing Hitch
2,000 Lbs (900 Kg) without Sway Control
3,500 Lbs (1,580 Kg) with [b]all of the above but only [b]2 occupants.

[Roger H]

"
It's interesting that a full size Toyota Tundra (assuming it's one of the V8 engines) that has a towing capacity between 8,500 - 10,000 lbs. would have any engineering problems towing a 17' fiberglass trailer weighing one third to one half of it's towing capacity.

Gary, MY Tundra is an '02 with a 3.4L V6, 4WD, and the Access Cab 4dr. It has a towing capacity of 4800 lbs as equipped. It doesn't have engineering "problems" but 350 lbs leveraged off the hitch does drop the suspension about 3" in the rear. The truck is designed for that kind of load, no problem, but it handles better towing the Bigfoot 17' with a WDH.

Roger

[Gary Lynch]

Roger,

I was confused. You have a Bigfoot which is a heavier trailer than the 17' SD. A real nice 17' trailer I may add, with 15" tires. Interesting that it's hitch weight is only 315 lbs dry. Maybe this also helps with towing?

I do not know about your truck but the new Toyotas have a tow option which includes a transmission oil cooler and engine oil cooler, larger alternator and battery and a few other goodies.

One thing I was wondering about regarding the WDH. If the trailer tongue weight exceeds the manufacturers recommended specs, then is the WDH making the situation safe? I noticed with the Odyssey, they talk about towing with a WDH but do not mention original tongue weight after they state a 350 lbs max. So my question still is; If the tongue weight was exceeding the specs in the beginning, does the WDH make it ok/acceptable ? Less weight on the tongue?

[CD Smith]

Most Class III hitches, the part that mounts on the tow vehicle, are rated for a greater tongue weight when used with a WDH.

There should be a placcard (label) on the hitch which defines its ratings. Take a look and see if you can find one on your hitch.

[Roger H]

Gary, my Tundra has the towing package. The same truck, however, with the V8 is rated at 6300 lbs. It's only been the last couple of years in the Tundra that the tow ratings have risen pretty dramatically, probably in response to the tow rating claims of the Nissan Titan. The Tundra has grown from a nice around-town errand truck to a monster half-ton in '07. The curb weight on my '02 is about 4300 lbs. The curb weight on the '07 has risen to nearly 5500 lbs. The differentials under the '07 are huge. It doesn't "feel" a whole lot different inside than sitting in my Excursion.

And the 500lb-1,000lb hitch weight phenomenon is interesting. As CD says, trucks that have a Class III with that kind of tow rating have that weight rating stamped on the hitch. I don't know how Class IV hitches are stamped or rated, as I've never had one.

Roger

[Roger H]

The WD system changes the distribution of [b]load, but doesn't move any [b]mass anywhere, and therefore it can't make the weight happen anywhere other than where the mass is. The tongue weight stays where it always was, on the hitch ball. It's just a matter of which tires support the whole mess.

Brian, I think this is an excellent explanation. I couldn't have come up with it, but it explains my understanding of what a WDH does very well.

Roger

[Brian B-P]

...I don't know how Class IV hitches are stamped or rated, as I've never had one.

I've never had one either, but the authoritative definition of Class IV - meaning the one in SAE standard J684 - is a capacity for a trailer up to 10,000 lbs gross weight. The SAE standard does not specify tongue weight limits, and makes no reference to weight-distribution systems, so any limits related to the tongue weight or WD use are added by the manufacturer of the hitch as they see fit.

I suspect that the same relationship exists, with a higher rating in WD mode, for the same reasons as in Class III.

[Brian B-P]

Great info, Frederick.

The Honda requirement for WD over 1850 lb is a bit surprising to me; the very similar Sienna has no such requirement. Instead, the WD requirement is based on hitch weight, which I think is more directly relevant. Does the Honda manual have a similar section regarding [b]acceptable hitch loads?

Also, with 3500 lb of trailer the Sienna still has most of it's payload capacity available before reaching GVWR or GCWR, if the cargo/passenger load is distributed properly (not a trivial task). I could certainly have four passengers, some cargo, and the 3000 lb Boler (but we have two people and too much cargo instead...). This is off-topic, but it is relevant that both vans do need WD to get to their towing limits.

[b]Weight Distributing Hitch
This device transfers weight from the vehicle's rear wheels to the front wheels, and to the trailer's wheels.

I note no mention of hitch weight here...

[Pete Dumbleton]

Brian, I believe the point of a WDH is to put the tow vehicle back into its normal road position (front to back leveling) by weight distribution, not necessarily evenly spreading the weights around, just putting the front of the TV down to where it was so that the suspension components and wheel alignments, etc., are working as designed -- The TV doesn't really care how many pounds of the TW are on the trailer axle as long as the TV is restored to proper orientation.

PDF for a Draw-Tite WDH suitable for small A-frame trailers

Gary, the XXX lb TW limit on a vehicle is basically a restriction to keep the TV from getting into trouble because the rear becomes too heavy and the front becomes too light -- The WDH kind of throws that restriction out the window because it restores 'level' to the TV; the weight is still there, but its effects on the TV are different (and diminished) with a WDH, which is the whole point of using a WDH. The new limits with a properly adjusted WDH are now based on the combined weights of the TV, TT and their loads.

Within the limits of 10-15% of total trailer weight, the WDH doesn't care what TW is used to start -- That is strictly a function of what the TV manf thinks is the limit.

It should be noted by all that setting up a WDH properly may involve several trips to the scales because the original TW influences the WDH, then you take it to the scales and undo the bars to weigh the TW and trailer axle. If stuf has to be moved to get the right weight, then the WDH may have to be readjusted to get it back to the scales to check the TW...

[Brian B-P]

That's my point, Pete: I don't think the attitude is very important compared to the axle loads, at least for the degree or two we're talking about. It's true that the TV doesn't care about the trailer axle load, but it sure cares about it's own!

The attitude of the empty tug doesn't seem to me like an indication of any design intention for the loaded vehicle. Toyota set my Sienna slightly raked nose-down when empty: I don't think they wanted it to look racy, or thought it worked better that way - they were just allowing for the fact that the rear would go down more than the front when load was added.

Thanks, Pete, for this nugget in that "fit guide":

Originally posted by Towing Products
The purpose of a weight-distributing hitch is to remove excessive weight from the rear axle of the tow vehicle and distribute it to the front wheels and the trailer wheels.

Note: Towing Products is one of the company names in the Cequent family, which includes Reese and Draw-Tite. The product shown is the single-bar WDH which has come up in recent discussions.

In my Sienna, there is still a hitch weight limit, even with a WD hitch. There is a lot of force being applied by the hitch, and lateral force to control the trailer is not diminished by the WD system's action to unload the rear axle, so a limit is still appropriate (it's something like 500 lb for the Sienna).

I certainly agree that "the WDH doesn't care what TW is used to start". The weight of the trailer and the portion of that weight on the tongue doesn't change, and doesn't matter to the WD system.

[Brian B-P]

From the same Draw-Tite / Towing Products document:

A weight distributing system is properly set up and coupled when the tow vehicle has settled with the front wheel opening “F” at the original uncoupled dimension measured and slightly lower in the rear ”R”.

They are saying not to maintain the original attitude. They are using ride height, I suppose because they know most people won't use scales. Their other points:

• "This will assure the front wheel load remains unchanged. This results in good handling and the desired load on the rear axle and trailer axle."
  • They are suggesting only to counteract the transfer of load from front axle to rear by the hitch weight, not to have the front axle take any share of the added load of the trailer.
    
• "The front of the vehicle should never settle more than the rear. See Figure 11. If necessary increase the number of chain links between lift unit and spring bar."
  • This is a caution against overdoing the WD effect. I think we all agree on this.
    
• "Mini-vans and small sport utility vehicles will typically settle with the front wheel opening “F” at the original dimension to 1/2” higher than original. This is will still allow acceptable front wheel loads, good handling and the desired load on the rear axle and trailer axle."
  • This looks to me like a clear statement that some transfer off the front axle - and the attendant slight suspension alignment change - is okay.
    
• "If the rear suspension sags too much, additional leveling is required. The front wheel opening “F”may ONLY be settled lower than the original dimension IF the rear wheel opening “R” has settled by a greater amount (At least 1”)."
  • This is only for the most overloaded rear axles. Even then...
    
  • The greater "settling" in the rear seems to be intended to ensure that the net load increase (due to trailer weight and WD action) is greater on the rear axle than the front.

Relatively good stuff. My opinion of Cequent / Towing Products has gone up a notch.

[Frederick L. Simson]

Does the Honda manual have a similar section regarding [b]acceptable hitch loads?

Honda bases their numbers on the Total Trailer Weight. I think they feel that most people are [b]not going to weigh their rig more than once, and wouldn't bother to figure out their tongue/hitch weight.

<div align="center">HONDA ODYSSEY
2003 Owner's Manual</div>

[b]Total trailer weight:
The maximum weight you can tow depends on several factors. See chart below for limits for your towing situation. Towing a load that is too heavy can seriously affect your handling and performance.
[b]Tongue Load:
The weight that the tongue of a fully loaded trailer puts on the hitch should be approximately 10 percent of the trailer weight. Too little tongue load can make the trailer unstable and cause it to sway. Too much tongue load reduces front-tire traction and steering control.
[b]Maximum Total Trailer Weight:
Number of Occupants-----------------------------------Equipped with transmission cooler
Including Driver*-----------------------------------------and power steering fluid cooler
-----2-------------------------------------------------------------3,500 Lbs (1,580 Kg)
-----3-------------------------------------------------------------3,350 Lbs (1,520 Kg)
-----4-------------------------------------------------------------3,200 Lbs (1,450 Kg)
-----5-------------------------------------------------------------3,050 Lbs (1,380 Kg)
-----6-------------------------------------------------------------2,900 Lbs (1,310 Kg)
-----7---------------------------------------------------------------650 Lbs (295 Kg)**
*Based on 150 Lbs (70 Kg) per occupant
**Weight limited to avoid exceeding rear GAWR

[b]Gross Vehicle Weight Rating (GVWR)
The maximum allowable weight of the vehicle, all occupants, all cargo, and the tongue load is:
5,665 Lbs (2,570 Kg)

[b]Gross Axle Weight Rating (GAWR)
The maximum allowable weights on the vehicle axles are:
2,833 Lbs (1,285 Kg) on the front axle, and
2,845 Lbs (1,290 Kg) on the rear axle.

[b]Gross Combined Weight Rating (GCWR)
The maximum allowable weight of the fully loaded vehicle and trailer is:
8,265 Lbs (3,750 Kg) with the proper hitch and fluid coolers.

The above statement is the only reference to [b]Tongue Load. I guess they figure that if you're required to use WDH to meet the 3,500 Lb limit, and they can refer you to the hitch maker "for proper installation and adjustment" then their Lawyers are happy.

[Frederick L. Simson]

Most Class III hitches, the part that mounts on the tow vehicle, are rated for a greater tongue weight when used with a WDH.

There should be a placcard (label) on the hitch which defines its ratings. Take a look and see if you can find one on your hitch.

Do not exceed the lower of: Towing Vehicle's Manufacturer's Ratings or:
Hitch Type----------------Max Gross Trailer Wgt------------Max Tongue Wgt
Weight Distributing----------5,000 Lbs---------------------------500 Lbs
Weight Carrying--------------3,500 Lbs---------------------------350 Lbs

I interpret the disclaimer to use the Towing Vehicle's Manufacturer"s Rating if it is lower in my case to mean that I should use the Weight Carrying numbers as if they were the Weight Distributing numbers. I have friends who are confused, and tell me that if the label on the hitch says I can tow a 5,000 pound trailer, then I should be able to tow a 5,000 pound trailer!