This was second hand so no idea to the veracity.
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I​​​​t was stated that the 25% FALR on the 2015+ F150 is because in testing the truck entered an oversteer condition with heavier tongue weights and higher FALR.

If that was something that only occured in specific conditions or actually would have resulted in failure in J2807 at the current weights was not stated.

This sounds reasonable as the only other reason I could see is the unusually low reserve capacity of the FAWR on the typical F150.

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There’s not many that understand the concept and importance of the Understeer Gradient. It’s based on the principle that the addition of tongue weight to a tow vehicle shifts the steering response from understeer towards neutral. Once past neutral steering and into oversteer the combination is unstable. When you add a WDH with tension and remove weight from the rear axle, it shifts the steering response further towards neutral/oversteer. The vehicle manufacturers have to limit the amount of WDH/FALR used in order to meet SAE j2807 standards Understeer Gradient test.
The SAE Understeer Gradient test has very specific TV loading conditions. The TV must be loaded to the RAWR, GVWR, and GCWR simultaneously if possible. This loading requirement is different from other test requirements. There’s plenty more that can be discussed about the Understeer Gradient and why it’s important. I’m not in the mood to write up a full lesson right now.

 

This brings up a second question that puzzles me.

1320 lb tongue + 100 lb WDH = 1420 lb
1420 x .38 = 540 lb gross removed from front axle
25% FALR = 135 lb returned front axle
405 lb net removed from front axle.

1000 lb tongue without WDH.
1000 x .38 = 380 lb
380 lb net removed from front axle

If condition #1 is safe why wouldn't #2 be?

 

First off stop adding the weight of the WDH in the equation, take it out. The need for the WDH is probably more important in aiding the rear suspension of the truck then anything. It stiffens the suspension in the vertical plane to help react to road conditions and helps control body roll while cornering. That extra 125 lbs or so added back to the front axle does help as well.

 

Gene, here is some suggested areas to study on trying to understand what Static/Divergent instability is and why limiting FALR is important.

1) First understand that weight is a force which is completely different from mass, the amount of material contained in a object.
2) Understand the Kinetic energy of a mass at velocity.
3) Understand Tire cornering stiffness, a tires ability to grip/traction and how weight plays its roll.
4) Tongue weight % plays an important roll in Static instability. Lower is better for Static instability and worse for Dynamic(sway) instability.
5) Every towing combination is experiencing both dynamic and static instability simultaneously.
6) SAE j2807 sets limits to satisfy both Dynamic and Static modes of instability, most people completely ignore the static component.

 

The WDH Weight does result in front lift before the attachment of the bars/chains and you do usually take measurements before and after the bars/chains hooked up. I didn't see a way to estimate front lift without accounting for it's weight.

People seem to emphasize the effects of the WDH on the Steer Axle. My question as much as I had one was the implications of such a low FALR. Ford's recommendation doesn't really add a lot back (135 lb is certainly better than nothing but it still a lot lighter FAW when loaded). I do know it stiffens the rear suspension by acting as essentially a torsion bar. I had not considered the implications on rear roll control but it does make sense.

Sorry, I like to know how things work. Not just, "Because we said so".

 

You can't effectively predict the steering response change of the TV due to the addition of tongue weight by considering the front axle alone. Just because weight is removed from the front axle doesn't necessarily mean the TV has passed the neutral steer point and is now in a oversteer condition. Just because the steering feels lighter then before doesn't mean you are unsafe and reached a oversteer condition. When the TV reaches a oversteer condition it becomes safety issue, using high amounts of WDH tension causes the TV to reach oversteer sooner then one without tension. A towing combination with 0% FALR can withstand a higher amount of lateral acceleration and force then one with 50% FALR. Today modern ABS systems can effectively compensate braking bias so bias is now dynamic front to rear as needed.

 

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I'm trying to figure out how to ask the question I really want to know the answer to but find I don't have the engineering background or vocabulary to form an intelligent question. I will try anyway.

I'm puzzled by how when you apply throttle you lose front traction and gain rear traction. The opposite occurs during deceleration. This would make one believe more weight more traction. At the same time if you add static weight to one end of the vehicle you increase the tendency of that end to slide first. It seems counter intuitive.

 

Gene, if you add/increase the static load on the rear axle of a vehicle you will increase traction/grip at that axle. People add weight into the back of a pickup when driving in the snow to increase traction/grip and it’s not different on dry pavement. It’s not much different then the acceleration/deceleration scenario you described above.

The SAE will allow up to 100% FALR in j2807 testing but the vehicle manufacturers must limit FALR % to be able to pass the Understeer Gradient test. The fact that Ford has limited FALR on the F150 to 25% means it hits the instability point rather quickly, not a good thing. Here is quote from a engineer Richard Klein on the subject.

”The statement “too much tongue weight can force the truck down in the back, causing the front wheels to lift to the point where steering response and braking can be severely decreased” is not the real issue with heavy tongue weights. The real problem is that the tow vehicle's yaw stability, as measured by “understeer gradient”, is severely decreased. This increases the propensity of the tow vehicle to jackknife in turning maneuvers. Specifically, recent full scale testing conducted by the SAE Tow Vehicle Trailer Rating Committee (and now published in SAE J2807), determined that the use of weight distributing hitch torque should be minimized. In fact they recommend that the Front Axle Load Restoration (FALR) not exceed 100% (100% means that the front axle weight is brought back, via weight distribution, to a weight equal to its “no trailer” condition).

 

I think in the effects I'm struggling to articulate one may be an effect of mass and the other weight.

A vehicle with lots of front mass to rear mass like a pickup tend to have a natural affinity to understeer and a vehicle with lots of rear mass to front mass like a Porsche 911 will have a natural affinity to oversteer. Yet a Porsche will tend to get better traction than an unladen pickup.

My mind has trouble grasping increased traction yet an increased tendency to slide.


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