Are Weight Distribution hitches and sway control sometimes a bad idea?

[thoer]

I am puzzled about this discussion of WDHs and maybe someone can explain the physics (that I have forgotten) to me.

How are the forces a WDH applies to vehicle any different than the forces that vehicle encounters towing without one? Every time a vehicle/trailer combination goes over a sharp hump or quick dip in the road, multiples of forces are applied thru the hitch. If a vehicle/hitch can withstand those forces of a trailer at its rated capacity then how can it then not withstand the WDH forces?

While I can see that a poorly adjusted WDH that applies too much weight transfer to the front wheels could cause problems, it would seem the same would apply to all the weight applied only to the rear wheels. In my mind, a vehicle/trailer combination with a properly adjusted WDH should more closely match an unloaded vehicle than that same combination with all the weight only on the rear hitch.

[David H]

Originally Posted by paulk

Before starting this thread I didn't realize that some tow vehicle manuals give specific guidance with respect to weight distribution hitches. It seems to me that to transfer weight to the front tires of the tow vehicle, a large amount of torque must be applied to the hitch in a direction that isn't shared with non-WD hitches. If a hitch isn't designed and tested to withstand that torque, then it would be hard to say whether it was safe or not.

My XC90 manual does not mention weight distribution hitches at all, so my original thought was that as long as I kept the total tongue weight below 500lb, that I could choose a weight distribution hitch or not. On further reflection though, I think the safer assumption is that when a manual does not mention weight distribution hitches, it would be best to assume the hitch is not capable of withstanding the torque safely. I would think that for tow vehicles which mention weight distribution hitches and which allow them, they would give specific guidance about allowed tongue weight in either scenario.

Volvo like most vehicle manufacturers has a technical group who you can contact with questions WRT your vehicle. My suggestion would be to contact them posing your question in the last paragraph above. Then hopefully you will have a definitive answer.

[Brian B-P]

Originally Posted by thoer

How are the forces a WDH applies to vehicle any different than the forces that vehicle encounters towing without one? Every time a vehicle/trailer combination goes over a sharp hump or quick dip in the road, multiples of forces are applied thru the hitch. If a vehicle/hitch can withstand those forces of a trailer at its rated capacity then how can it then not withstand the WDH forces?

No, they're not the same. When a rig without WD goes through a dip there is little change of the load on the trailer ball - and the more slowly you go, the less change (because there is minimal angular acceleration of the trailer around the lateral axis, if you're reviving old technical knowledge). With WD, the road bumps and dips change the pitch angle between the tug and trailer, changing the spring force applied by the WD bars and chains... even if stationary. Those forces are typically greater than the vertical load of the trailer weight on the ball (in a conventional WD transferring only 100 pounds to the front axle, each chain is under a few hundred pounds of tension, which can be doubled or reduced to zero by severe dips and humps).

Also, in normal use the vertical load on the ball (without WD) is always downward. That means that the ball mount is a lever which is always twisting the hitch receiver down at the rear (tending to lift the front of the tug); it varies with hitch load variation due to bumps, of course. A WD system applies a greater torque in the opposite direction - that's its primary purpose.

Originally Posted by thoer

While I can see that a poorly adjusted WDH that applies too much weight transfer to the front wheels could cause problems, it would seem the same would apply to all the weight applied only to the rear wheels.

There's a fundamental difference. A trailer without WD applies weight where the mass is. The rear of the vehicle needs more traction because it is controlling the horizontal movement of the trailer's tongue, and the hitch load provides vertical load which adds traction. With WD, the front tires get more traction but the mass isn't moved - it's still at the back. If WD systems just did the load redistribution (without sway-damping friction and return-to-centre spring action) they would substantially destabilize the rig, and that the reason that safety authorities and automotive manufacturers have been warning against over-application of WD systems for decades.

Yes, an overloaded rear axle is a problem, and it is the situation which WD systems exist to manage. Most people using or recommending WD systems seem to be using them for other reasons.

Originally Posted by thoer

In my mind, a vehicle/trailer combination with a properly adjusted WDH should more closely match an unloaded vehicle than that same combination with all the weight only on the rear hitch.

Why? The added load is at the rear, not distributed the way the load on the unloaded vehicle.

Try this - forget trailers for a moment...
A light-duty pickup truck weighs two or three tons when unloaded, and is front-heavy. Load it up with the maximum cargo in the box, and all of the cargo load goes on the rear axle so now it weighs three or four tons with more on the back than the front... which is why pickups normally have a greater Gross Axle Weight Rating for the rear axle than the front. Is that a problem? No, within reason - the traction is where the mass is, so handling is still suitable. No one suggests that cargo must be carried only in the middle of the wheelbase to avoid changing the distribution of load between axles.

Back to trailers...
Every responsible manufacturer of WD systems advises that the WD system be adjusted to at most return the front axle's load to the condition before the trailer is added. That means that the net change of load on the rear axle from no-trailer to hitched and WD applied is a substantial increase (typically perhaps 80% of the tongue weight), so the axle load distribution is substantially more rearward than without the trailer. The one set of instructions for WD adjustment that I've seen in a vehicle owner's manual agree that the front axle load should be restored to no more than the no-trailer condition.

[thoer]

Brian, my aging brain mostly understands what you are saying.

However, I can't visualize how your statement "When a rig without WD goes through a dip there is little change of the load on the trailer ball" can be correct in a sharp dip or hump like I described. A sharp hump would put upward pressure on the hitch, much the same as an WDH does - so wouldn't the vehicle have to be durable enough to take that stress?

Your example of a loaded pickup makes complete sense, but I don't see how it applies to the many of us who tow with large AWD or FWD SUV's? Are you advocating that the full tongue weight on the rear of an AWD or FWD vehicle would be better than using a WDH in all cases?

I was never proposing anything other than a correctly adjusted WDH with only some of the load being transferred back to the front.

[jennykatz]

I have decided before buying and installing a WDH on our 07 Sequoia towing a Escape 2017 model 3600lb plus what we put in and fluids over 4000lb for sure .We will install the Timbrin rubber stops or bellows that keep the rear end from sagging . We towed back from Chicago area to Naples, Fl. 1400miles with no problems but the rear end went down about 2 in . with the Timbrins that should be less then an inch . Hopefully get them on next week and get the welding shop to put in a shelf across the front end to put my big Cooler for storage ? Jim

[Brian B-P]

Originally Posted by thoer

...
However, I can't visualize how your statement "When a rig without WD goes through a dip there is little change of the load on the trailer ball" can be correct in a sharp dip or hump like I described. A sharp hump would put upward pressure on the hitch, much the same as an WDH does - so wouldn't the vehicle have to be durable enough to take that stress?

As the rear axle hits a speed bump (for instance), the tug bumps up, which lifts the trailer tongue, which means more load down on the ball to accelerate the trailer's nose up (that's the angular acceleration of the trailer); when the trailer axle hits the same bump, the trailer nose must come down so there is less downward force on the ball (or even force upward if the bump is severe enough). But unless you're racing over speed bumps, the force increase is minimal - a fraction of the tongue weight.

Yes, hitches are designed for variation in force due to bumps, but without WD they're not twisted the way they are with WD. The design forces are quite large: if I recall the values correctly, the SAE J684 standard requires the hitch to be able to withstand half the weight of the trailer (not just the tongue weight, the ton or two of an Escape) vertically or horizontally on the ball. The assumption is that the load without bumps is 10% or so of the trailer's weight, bumps could be a few times that vertically, and in extreme conditions it might take a similar amount of force to control the trailer's tongue side-to-side. That's 1750 pounds for a Class 2 hitch; 2500 pounds for a Class 3 hitch, all applied at the ball a few inches behind the opening of the receiver. A WD system routinely applies several hundred pounds over two feet from the receiver opening.

Originally Posted by thoer

Your example of a loaded pickup makes complete sense, but I don't see how it applies to the many of us who tow with large AWD or FWD SUV's? Are you advocating that the full tongue weight on the rear of an AWD or FWD vehicle would be better than using a WDH in all cases?

No, there's no right answer for all situations. On the other hand, there are many large SUVs and trucks which have a very reasonable load distribution with hundreds of pounds of hitch weight and no WD.

People can choose to use WD systems for pitch damping and other reasons (that's why Andersen developed a WD hitch which is primarily a sway-damping device and also a WD system) but load distribution is often not a good justification with high-capacity and long-wheelbase vehicles. Even my minivan has no need for WD with up to 350 pounds of hitch load, as far as axle load distribution is concerned.

Originally Posted by thoer

I was never proposing anything other than a correctly adjusted WDH with only some of the load being transferred back to the front.

Okay, but you were thinking of an axle load distribution closer to that of the unloaded vehicle than towing in weight-carrying mode, and anywhere near the same as without the trailer would require cranking the WD system up so that the front axle carries substantially more load than without the trailer... much more WD torque than appropriate.

[David H]

Originally Posted by thoer

Brian, my aging brain mostly understands what you are saying.

However, I can't visualize how your statement "When a rig without WD goes through a dip there is little change of the load on the trailer ball" can be correct in a sharp dip or hump like I described. A sharp hump would put upward pressure on the hitch, much the same as an WDH does - so wouldn't the vehicle have to be durable enough to take that stress?
.

Its a ball, and the coupling is allowed to move freely on it therefore a hump or dip in the road surface just changes the position and corresponding similar load to a slightly different location on the ball. The impact of the position change doesn't significantly change the tongue load. Unlike a WDH which in a dip amplifies the load substantially depending on how much tension had been applied to the chains when the tow and trailer were hitched up.

[thoer]

Ok, I give up - going back to my beers and I still plan on using a WDH with my Highlander.

The hitch ball still has to transmit forces somewhere - it can't just vanish because the load is on a different point on the ball.

To me it is still a matter of that there are significant forces applied to a tow vehicle regardless of whether is has a WDH or not. As far as I can tell a WDH cannot violate the first law of thermodynamics.

[Mike & Donna]

I don't really understand all this technical talk about the pros and cons of WD hitches. What I can take from the discussion is that with my 2016 F-150 with trailer package I will just take delivery of my 2nd gen Escape 19 and hook it up to the trailer hitch and pull it for a few hundred miles and see how it feels. If I have no issues and the trailer rides good then end of story. If I feel I don't have good control then I will see about getting some type of WD hitch. That's the best way I can explain it.

[Rossue]

Here's one camper's description in layman's terms: A WDH distributes the load, and on a tandem axle to the rear axle as well the front axle of the tow vehicle. Besides getting more even wear on all tires "You can feel it in the drive"- as Lee Trevino used to pitch.

[Steve Clark]

Originally Posted by Rossue

Here's one camper's description in layman's terms: A WDH distributes the load, and on a tandem axle to the rear axle as well the front axle of the tow vehicle. Besides getting more even wear on all tires "You can feel it in the drive"- as Lee Trevino used to pitch.

Due to his bad back, Lee Trevino used to wear a corset. During one press conference, a reporter noticed the corset and asked Lee how long he had been wearing it. He answered, "Since my wife found it in the glove box".

[Brian B-P]

Originally Posted by Rossue

Here's one camper's description in layman's terms: A WDH distributes the load, and on a tandem axle to the rear axle as well the front axle of the tow vehicle.

It doesn't matter whether the trailer is a single-axle or tandem-axle - the use of WD still transfers the same load back to the trailer axle(s), for the same transfer of load to the tug's front axle. I'm not suggesting that there is anything wrong with the transfer of load to the trailer - just a clarification of the single versus tandem situation.

With a tandem, the increase of load on the trailer's axles should be reasonably evenly split between the trailer axles, because the trailer is nearly level.

[Rossue]

So with a tandem axle without a WDH you may have more load on the front axle with more wear on those tires.

[Brian B-P]

Originally Posted by thoer

The hitch ball still has to transmit forces somewhere - it can't just vanish because the load is on a different point on the ball.

A conventional WD system prys on the tow vehicle though the spring bar sockets, with vertical reaction through the ball. If you're not accelerating, the coupler force is straight down on the ball, WD or not... and since the WD bars push up on the head, the force down on the ball is increased, rather than vanishing.

The Andersen Ultimate is differently configured, so while the chains pull back (with several hundred pounds of force each) on the hitch head, the reaction is to push the back of the coupler against the back of the ball with the same force... so the resulting net force on the ball is a different direction in this case. Net fore-aft force between the trailer and tug is unchanged.

Originally Posted by thoer

To me it is still a matter of that there are significant forces applied to a tow vehicle regardless of whether is has a WDH or not. As far as I can tell a WDH cannot violate the first law of thermodynamics.

Yes, but different forces.

The first law of thermodynamics is about conservation of energy - I don't get the relevance to trailer dynamics. Maybe Newton's Second Law (the famous F=m·a)? That makes sense to me - the net force on the whole tug and trailer rig or on each of the vehicles depends only on acceleration, so the WD system doesn't change that.

The tug doesn't pull on the trailer any more or less because of the WD system, but the way they are connected is certainly different. You can stand still with your hand on your friend's shoulder, or hold him still in a headlock, and the net force on the bodies is still zero (no acceleration, Newton's 1st law) and there no energy of motion either way (1st law of thermodynamics), but your friend will probably notice the difference.

[Brian B-P]

Originally Posted by Mike & Donna

... What I can take from the discussion is that with my 2016 F-150 with trailer package I will just take delivery of my 2nd gen Escape 19 and hook it up to the trailer hitch and pull it for a few hundred miles and see how it feels. If I have no issues and the trailer rides good then end of story. If I feel I don't have good control then I will see about getting some type of WD hitch. That's the best way I can explain it.

All I would add is to check the weight at each axle with the trailer hooked up (I use a roadside truck scale). If the rear axle load isn't excessive (compared to the GAWR shown on the placard at the driver's door), then there's no problem which needs to be fixed by a WD system - but you can still use one. If you don't have good control without WD, and the rear axle isn't too highly loaded, you have a problem that WD won't likely fix.

[Brian B-P]

Originally Posted by Rossue

So with a tandem axle without a WDH you may have more load on the front axle with more wear on those tires.

With or without a WD system, if the leading (or "front") trailer axle is carrying significantly more load than the trailing (or "rear") trailer axle, the trailer must be sitting nose-down... so raise the ball. You never need WD to get the trailer level, and adding a WD doesn't make the trailer level any more level than without it (you still need to configure the hitch correctly).

What am I missing, Ross?

[Rossue]

When tongue weight makes the tow vehicle sag you are thus loading more weight on the front axle of a tandem axle trailer.

[Brian B-P]

Originally Posted by Rossue

When tongue weight makes the tow vehicle sag you are thus loading more weight on the front axle of a tandem axle trailer.

... and then you raise the ball (less drop, more rise, next step up with an adjustable... whatever) and the trailer is level so there isn't more weight on the leading axle than the trailing any more. Makes no difference to the tug and still nothing to do with WD, right?

You can use WD to affect the pitch of the tug (including fixing excessive rear sag), but it's not the solution for trailer pitch errors - that's just a hitch adjustment.

[Rossue]

When calculating ball height you have both trailer and tow vehicle on level ground and match the top of the tongue to the bottom of the ball- right? Then if there is a sag- which results in other issues, not only uneven loading on the front axle of the tandem axle trailer. I suspect we could talk all night about this, however besides distributing the load across all axles of both trailer and tow vehicle, a WDH adds compliance which results in a less harsh ride by reducing porpoising for the tandem axle trailer. I can sure feel the difference with a Yukon, which doesn't need a WDH for a 4,000 lb. trailer for the reason most smaller SUV owners use one for- to be sure there is enough weight on the front axle for steering and braking control.

[Brian B-P]

Originally Posted by Rossue

When calculating ball height you have both trailer and tow vehicle on level ground and match the top of the tongue to the bottom of the ball- right?

No, I measure to the top of the coupler, and make the top of the ball match that height when loaded.

Ross, it sounds like you are expecting the ball to drop a specific amount under load (equal to the height of the ball), and that same value is not generally applicable to every tug and every trailer. More softly-sprung vehicles will drop more; vehicles with full air suspension will not drop at all. Heavier tongue weight (with the same tug) causes more drop; lighter tongues less. If your assumed amount of drop was wrong and the tug drops more or less than that, just set the ball higher or lower to start.

Originally Posted by Rossue

... besides distributing the load across all axles of both trailer and tow vehicle, a WDH adds compliance which results in a less harsh ride by reducing porpoising for the tandem axle trailer.

I'm not sure what term you're searching for - it wouldn't be added compliance, since the WD reduces compliance of the coupling - but perhaps 'adds damping' or 'reduces compliance'. Yes, taming down motion (especially in pitch) is a commonly-desired side effect of adding a bunch of linkage which doesn't move freely. One reason that Andersen chose polyurethane springs (rather than steel coil springs) for their "No-Sway WD" is that the material absorbs energy, damping pitch motions.

Europeans don't seem to have this same concern with pitching or porpoising - they generally don't use WD systems and don't have a direct alternative - despite towing trailers which typically weigh a greater percentage of the tug weight, with tugs having shorter wheelbase (because they don't use pickups and use few large SUVs). Perhaps this is due to the less common use of tandem-axle trailers, which respond to bumps by pitching as each axle goes over the bump separately. I've noticed that tandem cargo trailers buck much worse than the single-axle trailers that I have towed.