Quote:
Originally Posted by Gilles
I understand that by high air pressure to help the back not too down because of the weight of the trailer, released the well leveled truck with trailer, except that in this way you are always with lack of weight on the front of your truck, so a lack of traction and control, right?
And in terms of flexibility between the trailer and the tow vehicle, use the appropriate model stabilizer bars for the weight of the trailer.
It is certain that if I use a template for a trailer of 10,000 pounds for a trailer 3,000 pounds, at times the trailer will suffer immense stress.
The reverse has the same effect, but on the towing vehicle this time, right?
It is my understanding on weight stabilization between trailer and tow vehicle.
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The air pressure lifts the rear of the vehicle by the original weight supporting point. The rear axle of the vehicle. It doesn't put any extra weight on the trailer axle, and merely shifts the weight BACK to the front
tires. It doesn't apply any extra. Depending on the distance between your rear axle and your hitch ball you can figure out how much weight is removed from the front wheels. The shorter the distance the better when it comes to sway and front wheel weight reduction. Yes, a WDH will push the front of the car down but it is almost NEVER needed. Just keeping the vehicle level while towing will keep it safely in control. I suggested softer spring bars when towing our lightweight trailers because they tend to have lighter gauge steel frames, and because quite honestly they are so
light that it really shouldn't be needed even for small vehicles. (Take mine for example.)
When you have a smaller vehicle and tongue weight is a concern, keep in mind that the weight limits are set mostly with dynamic loads in mind. That is to say, when you hit a bump that sudden upward movement tries to accelerate the mass of the trailer very quickly in a small time frame. This puts multiple thousands of pounds of force through the connection in rapid bursts. If you have a vehicle with a 150 lb tongue weight max, you probably wouldn't blink an eye if a couple of 220lb people got in the back of your car.
Unless it's a unibody where the connection points to the hitch themselves may be very weak, it's not the direct weight that causes the issue. It's the huge dynamic loads the develop over bumps and dips.
Here's where the WDH makes this so much worse. It's like it is ALWAYS under such stress. If you observe how they work you will find that they increase the effective tongue weight on the vehicle by multiples when in motion. Without such a system you can think of your trailer like a large teeter totter. It has a bias to weigh say 200 lbs more on one side (vehicle side) and thus puts a smooth and constant force of 200 lbs down. When you go over a speed bump for example, the front of the vehicle will raise, and the hitch point will simply swivel with no extra force being exerted on the trailer. The trailer tongue will drop down a little bit depending on the distance between the rear axle of the car and the ball. When the front of the car comes back down the same happens in reverse. No major shock load. When the REAR axle goes up over the bump the front of the trailer is lifted much more rapidly. this is where you get your first dynamic load. It is the weight of 200lbs being lifted, PLUS the mass of the entire trailer being shifted on its axle like a teeter totter. this however isn't all that hard on it since there is a lot of leverage to the wheels of the trailer from the hitch point. On the way back down you get another dynamic load as the vehicles suspension tries to decelerate the weight of the vehicle, 200lb tongue load, AND now downward traveling mass of the trailer. If your suspension is too soft and it bottoms out here, this becomes magnified by sometimes 10x or more. The shocks and springs will dissipate the energy over a longer period of time if they are well matched to the load. If they are not it will not slow down much until it suddenly bottoms out, causing it to have to dissipate ALL of that energy in a very short moment when you hit the jounce bumpers.
Air suspension fixes this by tuning the spring ratio to that of the increased weight. Once the trailer hits the bump it is lifted then dropped, all while exerting almost no more tongue weight or forces on the hitch. Some linear forces forward and backward are exerted when the trailer is pulled up the ramp of the speed bump and then rolls down it attempting to go faster than the car.
Now the same situation with weight distribution;
At rest you have the same hitch weight (plus the WDH which may be another 50 lb) Much of the constant force is converted into linear force in the form of compression. To the vehicle it thinks you are decelerating and that the trailer is pressing directly into the back of the draw bar, as well as down. To the trailer the force is as though it is being decelerated as pressure is traveling in line down the frame. This is okay so far if the system is balanced very well. Now lets drive over that speed bump.
When the front wheels of the tow vehicle rise and the rear end lowers. Instead of pivoting on the ball as normal, now look at the entire trailer frame as if it were a solid steel beam sticking out of your hitch. By the time you get to the position of the trailer axle it would likely already be only a few inches from the ground now that your vehicle is pointed upwards. This is where the spring bars flex to make it POSSIBLE for the vehicle and trailer to survive this. However in doing so, it's like you just threw 700 lbs of cargo (sometimes MUCH more) into the trailer. The trailers suspension compresses. It's actually worse yet than that though, because all of that force is being transferred through the frame rather than just sitting on it. Take a fork and hold it by the very end. Press it down against a scale while holding it nearly level like a trailer frame. you will find it bends quite easily. Now support that same end instead and then press down on the scale THROUGH the fork with your finger on top. It is now IMPOSSIBLE to bend as you are merely compressing the metal. That's the difference between the types of load. Oh, and we only got to the front wheels of the tow vehicle so far! Without the WDH at this point there is almost NO extra load from the front wheels going over a bump.
Now when the REAR hits the bump it is _EXACTLY_ the same as without the WDH. As in, it does no good at all. The chains only limit downward travel. Only this time, you have another 50+ lbs or steel and chain sitting on the hitch point to not only lift, but accelerate dynamically upward over the bump.
Here's where that goes from bad to worse. When the rear wheels roll down the bump the hit the flat ground suddenly and all of that weight is now traveling downward rapidly as last time. Only now the chains suddenly pull tight locking the trailer and vehicle together. This causes the trailer to squat under the load, as well as the front AND rear suspension of the vehicle. Remember the fork? This time it's worse. Much worse.
Most people will notice when driving almost any vehicle that you feel the downward impact after driving over a speed bump more than the initial upward movement. This is because on the way up the only thing that HAS to move is the wheels and axle of the vehicle. This is all unsuspended weight. This means it is not supported by the shocks and springs. It's also very little total weight, and so the dynamic load of accelerating that small mass quickly is low. For the shock this is called compression. Said shocks and springs absorb most of the impact with no issue, and the suspended weight, that is, the car and occupants are slowly lifted by the now smoothed force that comes through the shocks. Think of the force the shocks compress against as being the mass of the vehicle, unable to be suddenly moved. When the vehicle is coming back down the other side, this is BACKWARDS. Now you have the
light weight wheels and axles coming back down decelerated by the shocks (called rebound). When the weight and mass of the vehicle change direction and begin to
fall it is now MUCH harder for the springs and shocks to stop that force. On top of that, the new mass it is compressing against is the entire EARTH. It will NOT give at all, unlike the vehicles mass which moved away from the bump. This compresses the suspension way more than before. If the rear of the vehicle is compressed very far, that'a a lot like when the front is lifted. something the WDH physically resists. This means the entire weight of your car is now being applied to the TRAILER through the frame! At this point that's just unfair to the poor trailer. Without the WDH it would not have hardly even noticed three out of four of those impacts comparatively. The ones it would have felt have _ONLY_ been amplified by having a weight distribution system.
So far the WDH has caused massive stress on both tug and tow when the front wheels hit, and when the rear wheels drop back down. Now it's time for the trailer wheels.
When the trailer rolls up the hill, normally it would exert only a small amount of linear pulling force on the hitch as it is pulled UP the hill, then a small amount of linear pushing force as it rolled back down. And just like the second example with the fork, the frame will feel almost nothing as the trailers suspension does its job. As it rolls up the hill the trailer lifts, tilting the front end down. This is resisted by the WDH hitch which now attempts to LIFT the rear end of your car off the ground. All of that force going through the trailer tongue. The fork is now being flexed. The rear of the vehicle has been raised a bit, and the trailer is now higher. When it rolls down the other side its downward velocity and mass are accelerated and amplified once more by the VEHICLES rear end also dropping back down. Much like the prior example of why that is bad, it happens again. The car drops lower than perfectly level, with the trailer being crushed further down than it otherwise would have been. All through the frame again.
I could whip up a 3D model or animation as an example if you want.
Sorry for the long post, but it's filled with useful information to help understand what exactly a weight distribution hitch does.
Keep in mind it distributes the vehicles entire weight to the little trailer too.
This is why getting the softest spring bars possible is a critical thing, and why I will never use such a system with any
fiberglass trailer.
WDHs are stress CAUSERS. Using heavy spring bars causes stress for both vehicle and trailer. Using
light bars doesn't reverse this, it's just closer to not using WD at all. Meaning is causes less tress on both vehicle and trailer.
Lighter is better. They should just offer some support when set up correctly. they will ALWAYS exert more force across the hitch trailer and vehicle though. Particularly if you start to drive up a hill. Any time the vehicle points up, or the trailers rear end points up. They try not to allow that. A stiff enough one would literally lift the rear wheels off the ground on your vehicle if you tried to drive across a ditch for example. Problem is, the trailer or hitch mounting points WILL break before that. Using softer spring bars at least makes it possible to go up hills and driveways, but it will always stress the vehicle and trailer much more than without.
04-08-2016, 09:08 AM
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