Without the WDH, the Hitch Receiver carries all the load on the rearmost bolts. With the WDH, it pulls down on the front bolts and pushes up the same amount at the rear (this is how the WDH does it's thing with weight on the axles). The effect at the reciever attach points is to spread the weight so that the hitch receiver can handle a higher tongue weight with a WDH than it can without one.
This is universally true of all WDH applications, but not all manufacturers bother to provide separate ratings with and without a WDH. I can think of 2 reasons for a manufacturer to prohibit a WDH: 1) if the hitch receiver that comes with the vehicle is not rated for a WDH (WDH bars could be strong enough to bend a non-WDH rated receiver), and 2) there is no good way in the case of some particular vehicle to provide adequate front hitch receiver mounts which are able to support the WDH loads.
To the PopBeavers question
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Questions:
1 can a WD hitch fail in this manor?
2. How much safety margin would make one comfortable enough to tow without the spring bars.
The alternative is to leave the trailer where it is and drive to someplace that can fix the WD hitch.
Or am I just dreaming up a failure situation that can never occur?
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An overloaded hitch can bend metal, and everything has failure points. Though I've never heard of a WDH failure, you could visualize that concept pretty easily just thinking about missing/loose/damaged nuts/bolts/fasteners which forced you to temporarily put the WDH away and tow with a standard ball mount. A WDH still works with just one of the 2 bars, but a bit less than half as well. The question is, how far can you bend the envelope before it breaks?
The smarter the manufacturers get about cutting things closer with their computers, the less slop they will allow for in their designs. Given that tongue weights are static loads and dynamic loads are always the design limits because they have the potential to be substantially higher than static loads, you can probably get by with a whole lot just by taking things easy (staying on smooth roads and going real slow over bumps, for example). In some situations, that could be harder than others. I'd guess you might get by with a 50-100% overload, but I'm just guessing and of course this is highly application dependent.
To the Brulaz questions
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Why would the receiver be able to handle an extra 6500# of trailer with a WDH?
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Why can the receiver's bolts/welds support 2x the upward force as the downward?
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I'm also suspicious because all these non-wdh receiver ratings are max trailer weight divided by ten.
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Re extra 6500# of trailer: You're right, it's really about how the truck can handle more if the load is better distributed. But the WDH not only distributes axle loads, it also distributes loads at the hitch receiver attach points.
Re bolts/welds support 2x the upward force as the downward: Without a WDH, the forward bolts don't have to do much besides handling upward forces on bumps. The rear bolts have to carry the tongue besides the downward bump forces.
Re max trailer weight divided by 10: You're right, they are taking shortcuts by making the assumption that 10% of the trailer weight gets put on the hitch ball when reality should support something more like 7-14%.
Redhawk: Thanks for sharing the link. This is the first time I've read carefully about the new standards.
Some observations:
- Most of the methodology is about durability and power, not stopping and safety.
- Brake fade is specifically excluded!
- The bar is substantially lower for heavier vehicles (2x stopping distance is going to be allowed for trailer ratings over versus under 3000#)!