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 > Can I use a WDH on a DP

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ACDNate

Biloxi

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Posted: 01/11/19 04:39pm Link  |  Quote  |  Print  |  Notify Moderator

I'm not sure I understand why, those of you who said so, you don't think a wdh will help.

I've used a wdh with a trailer behind my truck and it significantly lightened the load on the rear end of the truck and put some weight back to the front axle.

Why would it not have the same affect here?

wildmanbaker

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Posted: 01/11/19 05:11pm Link  |  Quote  |  Print  |  Notify Moderator

Most DPs have auto leveling with the sir bag suspension. This raises the rear to its normal ride height.


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tropical36

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Posted: 01/12/19 07:18am Link  |  Quote  |  Print  |  Notify Moderator

ACDNate wrote:

I'm not sure I understand why, those of you who said so, you don't think a wdh will help.

I've used a wdh with a trailer behind my truck and it significantly lightened the load on the rear end of the truck and put some weight back to the front axle.

Why would it not have the same affect here?

I've often asked the same question, but have never heard from anybody using one and giving the actual weight transfer.
It's gotta do something towards that goal, so the question is how much.


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Posted: 01/12/19 12:52pm Link  |  Quote  |  Print  |  Notify Moderator

tropical36 wrote:

ACDNate wrote:

I'm not sure I understand why, those of you who said so, you don't think a wdh will help.

I've used a wdh with a trailer behind my truck and it significantly lightened the load on the rear end of the truck and put some weight back to the front axle.

Why would it not have the same affect here?

I've often asked the same question, but have never heard from anybody using one and giving the actual weight transfer.
It's gotta do something towards that goal, so the question is how much.


I asked that question at the beginning of my thread. From recall, a few people said that it probably wouldn't have much, if any effect on a DP, and possibly could make handling worse.

I have looked at lots of DP's pulling trailers and so far, have not seen any with a WDH. There must be a reason for that.


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Biloxi

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Posted: 01/12/19 01:28pm Link  |  Quote  |  Print  |  Notify Moderator

tropical36 wrote:


I've often asked the same question, but have never heard from anybody using one and giving the actual weight transfer.
It's gotta do something towards that goal, so the question is how much.


It would take a good bit of time at a scale to get weights. Only thing I know is w/o the WDH on my truck the rear was close to bottomed out and the front was rising 2.5". With the WDH setup the rear only dropped 1" and front only raised 0.75".

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Posted: 01/12/19 06:22pm Link  |  Quote  |  Print  |  Notify Moderator

I would think that if you are on here you would understand the leveling system that's on many DPs. This is not true with a class A on a Ford or GM chassis. Comparing a air bag suspended DP to a pickup truck is like comparing a banana to a orange. If your pickup dropped that much you didn't have enough truck for your trailer. My HEAVY trailer mentioned above don't drop my F-350 pick up hardly any....You never have to much truck, only not enough.

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Posted: 01/13/19 03:22am Link  |  Quote  |  Print  |  Notify Moderator

tropical36 wrote:

ACDNate wrote:

I'm not sure I understand why, those of you who said so, you don't think a wdh will help.

I've used a wdh with a trailer behind my truck and it significantly lightened the load on the rear end of the truck and put some weight back to the front axle.

Why would it not have the same affect here?

I've often asked the same question, but have never heard from anybody using one and giving the actual weight transfer.
It's gotta do something towards that goal, so the question is how much.


A WDH operates differently on a DP than on a spring supported chassis. The DP changes ride height of the DP automatically to compensate for additional weight on the coach. A WDH works to change the ride height of the vehicle it is attached to so that the rear doesn't drop with a heavy tongue weight.

In the end, all "parts" of the train weigh the same and the weight is distributed somewhere. If there is a weak link, and too much of the weight is distributed there, it will break. Since the DP automatically adjusts ride height, a WDH, which does the same thing, isn't necessary and may cause weight to be distributed to the wrong place when the DP readjusts what the WDH does when you crank on the spring bar.


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Posted: 01/13/19 08:19am Link  |  Quote  |  Print  |  Notify Moderator

Although it is true that a DP with air suspension will compensate for extra weight on the hitch receiver by raising the rear of the coach, that is not the primary purpose of a WDH.

The WDH does 2 important jobs.

1) Reduces the load on the rear axle and distributes it between the front axle and the trailer axle(s). Even though the coach may adjust the air to raise the rear of the coach, it does not affect the extra loading on the rear axle. If your tongue weight pushes you over the GAWR or your tire rating, then you should use the WDH.

2) The second purpose (and in my opinion the most important) of the WDH is to reduce the torque loading on the receiver. When the tongue weight exerts a downward force on the ball, it induces a twisting force (torque) on the receiver. The resulting torque has 2 components. First, is the static loading. This is the tongue weight when the rig is not moving. The second is the dynamic loading which is the resulting force on the ball when the vehicle is moving. For example, when the coach goes over a bump, the hitch moves down and then rebounds. This downward movement can exert many times the static loading as the suspension stops the downward movement and rebounds. The WDH counteracts this dynamic loading greatly reducing the torque on the receiver. This is especially critical if using a hitch extension of any kind.

So the bottom line, is that if your tongue weight exceeds the rating on your receiver OR you are using a hitch extension, you should use a WDH. This is true even if you coach air suspension can keep your coach level.

The following diagram illustrates the effect of a WDH.

[image]


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wildmanbaker

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Posted: 01/13/19 01:17pm Link  |  Quote  |  Print  |  Notify Moderator

bjbear wrote:

Although it is true that a DP with air suspension will compensate for extra weight on the hitch receiver by raising the rear of the coach, that is not the primary purpose of a WDH.

The WDH does 2 important jobs.

1) Reduces the load on the rear axle and distributes it between the front axle and the trailer axle(s). Even though the coach may adjust the air to raise the rear of the coach, it does not affect the extra loading on the rear axle. If your tongue weight pushes you over the GAWR or your tire rating, then you should use the WDH.

2) The second purpose (and in my opinion the most important) of the WDH is to reduce the torque loading on the receiver. When the tongue weight exerts a downward force on the ball, it induces a twisting force (torque) on the receiver. The resulting torque has 2 components. First, is the static loading. This is the tongue weight when the rig is not moving. The second is the dynamic loading which is the resulting force on the ball when the vehicle is moving. For example, when the coach goes over a bump, the hitch moves down and then rebounds. This downward movement can exert many times the static loading as the suspension stops the downward movement and rebounds. The WDH counteracts this dynamic loading greatly reducing the torque on the receiver. This is especially critical if using a hitch extension of any kind.

So the bottom line, is that if your tongue weight exceeds the rating on your receiver OR you are using a hitch extension, you should use a WDH. This is true even if you coach air suspension can keep your coach level.

The following diagram illustrates the effect of a WDH.

[image]

Tell you what, find a scale in your area and give this a try on you MH. Get back to us on what you find out, OK. There are a lot of things that work on paper, but not in real life.

bjbear

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Posted: 01/13/19 02:24pm Link  |  Quote  |  Print  |  Notify Moderator

wildmanbaker wrote:

bjbear wrote:

Although it is true that a DP with air suspension will compensate for extra weight on the hitch receiver by raising the rear of the coach, that is not the primary purpose of a WDH.

The WDH does 2 important jobs......................


Tell you what, find a scale in your area and give this a try on you MH. Get back to us on what you find out, OK. There are a lot of things that work on paper, but not in real life.


OK ... since you asked. Having spent my career in engineering, I am pretty careful to make sure I know what I am talking about before I put anything out......

Fortunately, I have a set of scales not far from where I live so when I wanted to engineer a hitch extension for my truck for use with a camper, I was able to verify my paper calculations with real life observations. Below is the results of some of work. Note that like my previous drawing, when you tighten the chains on the equalizer, more weight is added to the steer axle and the trailer axles and the equivalent weight is removed from the drive axle of the truck.

[image]

Although this data was from my truck, it applies equally to my MH.

Think of it this way.

On a motorhome, when you add weight to the hitch........
  • The rear of the MH goes down
  • The air level valves sense this change and add air to raise the rear of the MH back to the design level
  • The air bags are pushing upward on the frame. The increase in force is equal to the weight from the hitch
  • At the same time, the air bags are pushing down on the drive axle with an equivalent force, increasing the loading on your tires and axles.


The end result therefore is that when you measure the resultant weight on the drive axle, it will have increased. Doesn't matter if the rear of the MH is sagging or has been forced back to normal position. The weight has still been added to the axle and tires.

There is a reason that the manufacturers of hitches and receivers give different ratings for use with and without an WDH. If your actual tongue weight is over the rating for your receiver and hitch, then use a WDH.

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