How does a weight distributing hitch help towing capacity?

[burningbaal]

Most hitches are rated as "X, 0.1*X weight-bearing, Y, 0.1*Y weight-distributing" where Y > X and the first number is trailer weight, second number is tongue weight. It has been described to me many times that the weight distributing hitch (with 'stabilizing/leveling' bars) "takes weight off the rear axle of the tow vehicle and puts it on the front axle". This sounds fantastic...in theory, but it seems like the fulcrum is the ball of the hitch, and I don't see a way to transfer weight from one end of the lever arm (the tow vehicle frame) to other with the fulcrum all the way at one end.

I DO understand that adding weight to the front axle is still worthwhile (greater steering ability/etc), but doesn't this just add even more weight to the rear axle?

 

[berkeman]

Most hitches are rated as "X, 0.1*X weight-bearing, Y, 0.1*Y weight-distributing" where Y > X and the first number is trailer weight, second number is tongue weight. It has been described to me many times that the weight distributing hitch (with 'stabilizing/leveling' bars) "takes weight off the rear axle of the tow vehicle and puts it on the front axle". This sounds fantastic...in theory, but it seems like the fulcrum is the ball of the hitch, and I don't see a way to transfer weight from one end of the lever arm (the tow vehicle frame) to other with the fulcrum all the way at one end.

I DO understand that adding weight to the front axle is still worthwhile (greater steering ability/etc), but doesn't this just add even more weight to the rear axle?

Welcome to the PF.

The key is to know that the "weight distribution" is the shifting of some of the trailer weight from the rear axle of the tow vehicle to the front axle. That helps to keep the front tires of the tow vehicle in good contact with the ground, especially under heavy braking. The article at HowStuffWorks is pretty good:

http://auto.howstuffworks.com/auto-...itches/towing-weight-distribution-systems.htm

 

[berkeman]

but it seems like the fulcrum is the ball of the hitch

The key is to gain leverage on the frame of the tow vehicle, to lift the rear axle a bit and transfer that weight to the front axle. Let us know if the article at the link helps.

 

[burningbaal]

Ya, I had seen the howstuffworks article before. I totally get that adding weight to the front axle helps handling/etc, but I just don't see how the laws of physics allow a fulcrum with that placement to relieve any weight from the rear.

Here's the drawing:
=F=====R==H======TT===
F = front tow axle, R = rear tow axle, H = hitch (fulcrum), TT is the tandem trailer axle.

the weight distrubuting bars torque the tow vehicle frame (F through R) counter-clockwise, which would push down on both the front and rear axles (though with larger displacement induced on the front, assuming a perfectly rigid frame).

So I totally understand that it increases the weight on the front axle, and I realize that's valuable, but I can't see any way this relieves weight from the rear axle.

 

[CWatters]

It torques the tow vehicle anticlockwise and the trailer clockwise. Thus "lifting" the tow hitch. I believe it must also "shift weight" rearwards in the towed vehicle if that had two axles.

 

[burningbaal]

@CWatters: I gather that the tow vehicle receives an anticlockwise torque, but wouldn't this push down harder on both the front and rear axles of the tow vehicle? how could this possibly remove force on the rear axle since the origin of the rotation is behind that axle?

 

[jack action]

You are effectively extending the tongue length, which reduces the tongue weight.

With a normal hitch, the tongue can pivot about the hitch. that makes the towing vehicle independent of the trailer and both entities' weight distribution can be analyzed separately.

With the weight distribution system, you basically solidly link the tongue and the hitch together as if they were one single piece and no rotation is possible (at least in one plane). This means that the car frame becomes a tongue extension which rest on both of its axles. You can then assume that the tongue weight is now resting midway between both axles of the towing vehicle. So if your tongue weight was 800 lb (which was probably split between the front & rear axles like -400 lb & +1200 lb) and you double the tongue length (from the trailer center of gravity), the tongue weight is now a lighter 400 lb, more or less split in half between both axles of the towing vehicle (both downward).

 

[CWatters]

@CWatters: I gather that the tow vehicle receives an anticlockwise torque, but wouldn't this push down harder on both the front and rear axles of the tow vehicle? how could this possibly remove force on the rear axle since the origin of the rotation is behind that axle?

Haven't you seen a JCB lift itself up? It presses down with the bucket/backhoe.
Watch this one lift it's wheels off the ground...

 

[burningbaal]

You are effectively extending the tongue length, which reduces the tongue weight.

With a normal hitch, the tongue can pivot about the hitch. that makes the towing vehicle independent of the trailer and both entities' weight distribution can be analyzed separately.

With the weight distribution system, you basically solidly link the tongue and the hitch together as if they were one single piece and no rotation is possible (at least in one plane). This means that the car frame becomes a tongue extension which rest on both of its axles. You can then assume that the tongue weight is now resting midway between both axles of the towing vehicle. So if your tongue weight was 800 lb (which was probably split between the front & rear axles like -400 lb & +1200 lb) and you double the tongue length (from the trailer center of gravity), the tongue weight is now a lighter 400 lb, more or less split in half between both axles of the towing vehicle (both downward).

that makes sense. it's by making the joint more rigid that there's less flex, thus less of the weight focused on the rear axle.
Thank you!

 

[burningbaal]

Haven't you seen a JCB lift itself up? It presses down with the bucket/backhoe.
Watch this one lift it's wheels off the ground...

I have done this on a mini-excavator (blade on front, bucket in back). However, this would be equivalent to putting the tongue jack on the trailer down, which would make driving...difficult.

 

[Ketch22]

One must remember that a weight distributing hitch is not connected all at the hitch. The end of the tongue is connected at the ball with the hitch end of the vehicle, The vehicle end of the transfer bars is connected at a different location that is separated by a small distance, The trailer end of the transfer bar is connected at a longer distance. It is the separation of these points that creates a couple.
In a standard hitch it is impossible to transfer force in fore and aft orientation as it is acting like a hinge. Only downward moments and tension/compression can be transferred. With the creation of a couple (which is controlled by the various offsets in the system) any other type of force can also be transferred but of specific concern is a torque applied to either vehicle. This can act on the balance of either affecting weight transfer.