How important is tongue weight with lots of weight in bed

I tow with a genny, and and ATV in the box of my truck. To manage loading, I aim for 10%, because my trailer tows well at that number. Having the box loaded stabilizes the load slightly for sure. Tongue weight recommendations are what they are, partially due to the nature of vehicles to under steer when loaded correctly, and over steer, when not. A heavy load in the box changes that aspect. A balance of feel and proper numbers is the best way. IMO.

Note - This required me to recently add a WDH, (even after moving my 2 batteries to the back bumper) not because things were towing poorly, and not because the truck looked or felt overloaded. But simply because my axle weights were out of whack.

Or I could give you the standard RV.net advice. Go buy a 1 ton dually with snowplow package and diesel engine.

Ron Gratz wrote:
...If this procedure is used, the WDH does not have to redistribute the load which is removed from the TV's front axle due to adding weight behind the rear axle.

To avoid confusion to any who might be considering moment arms, weight added behind the rear axle does add to the moment arm a WD hitch works to oppose/reverse - that's just basic physics and geometry - it's just that there's no need to account for its minimal contribution as long as the procedure Ron laid out is followed.

In reality, it's very difficult to add cargo weight behind the rear axle of the TV without adding just as much or more weight in FRONT of the rear axle - negating any minimal moment arm entirely and actually HELPING load the front axle.

tluxon wrote:
---Whatever weight you have in the truck behind the rear axle IS going to add to the weight that a WD hitch has to redistribute,---

The normal procedure for adjusting a WDH is:
1) measure the load on the front axle (or height of the front end) with the TV loaded for camping and unhitched from the TT,
2) attach the TT to the TV, and
3) adjust the WDH to return some of all of the load removed from the front axle (or eliminate some or all of the front end rise) due to addition of tongue weight.

If this procedure is used, the WDH does not have to redistribute the load which is removed from the TV's front axle due to adding weight behind the rear axle.

Ron

Weight in the bed DOES make a small difference, but not enough to offset the importance of having weight properly distributed on the trailer. Whatever weight you have in the truck behind the rear axle IS going to add to the weight that a WD hitch has to redistribute, but it does next to nothing to resist side-to-side twisting (the "yaw" Ron mentioned).

Oh sure, it will add a little to the inertia of the tow vehicle and it may make it a little easier to maintain weight on the front axle, but inertia is of little help in resisting side-to-side twisting compared to having properly weighted tires on the ground and a trailer that isn't tail-heavy.

Taco wrote:
---I guess I should have said can anyone explain the physics involved in why tongue weight is important.---

I'll give it a try.

It's not tongue weight which is a critical factor -- it is the tongue weight percentage.
Tongue weight percentage is a measure of the ratio of distance from TT's axles to TT's center of gravity divided by the distance from TT's axles to the ball coupler.

The ratio is important because it is one of the factors which determine how much lateral force the TT might exert on the ball.
For given TT mass, polar moment of inertia, dimensions, and application of lateral force on the TT, there is an optimum value of tongue weight percentage which will minimize the lateral force which the TT imposes on the ball coupler.

When a right-directed lateral force is applied to a TT at a point behind the CG, two things happen:
1) the TT's CG begins to accelerate toward the right, and
2) the TT begins a counter-clockwise (when viewed from above) yaw acceleration about the CG.

The right-directed acceleration of the CG causes the ball coupler to tend to move to the right.
The CCW yaw acceleration of the TT about the CG causes the ball coupler to tend to move to the left.
With the optimum TW%, the right-directed acceleration of the coupler will be equal to the left-directed acceleration, and there will be little or no lateral force imposed on the ball.

So, the optimum tongue weight percentage is not controlled by TV rear axle loading considerations.
The optimum tongue weight percentage is controlled by TT yaw dynamics which determine how much lateral force is imposed on the ball.

Ron

So if the instability comes from the trailer bouncing being able to unload the rear tires then why should it matter if the weight loading the rear tires comes from tongue weight or from 2000 lbs of weight in the bed.

I guess I should have phrased the question differently. I guess I should have said can anyone explain the physics involved in why tongue weight is important. I know the conventional wisdom is that more tongue weight within reason makes it more stable. But there are other variables involved not just tongue weight. A short dump trailer will be stable with much less tongue weight percentage wise than a travel trailer. I would guess that length, how far away from the axles the weight is and even the frontal area of the trailer and how fast you are going all play into it. The wind pressure on the front of the trailer should actually have some ability to push back and rock the trailer back reducing tongue weight. The weight distributing hitch bars also transfer tongue weight back to the trailer. They can also be used to transfer weight to the front axle over and above what the tongue weight is. So even if the weight was in the bed the WD hitch can still transfer some of that bed weight to the front axles.

Taco wrote:
I tow a living quarters cargo trailer to haul atvs and dirtbikes. With it hauling a lot of heavy cargo i have a fair amount of leeway in how I load it so I can manipulate the tongue weight quite a bit.

If there is a 1000+ lbs in the bed of the truck how important is it to have the normal 13% or so tongue weight. Could I get by loading it to have less tongue weight because of the weight in the bed. Or is there some other reason that it is important to have the tongue weight?

NO, you can not "get by" just because you load up the back of the vehicle.

Trailering is all about proper "balance" of the load ON THE TRAILER.

Too much weight at or behind the axles of the trailer makes for an extremely unstable tow. This weight basically takes control of the trailer and makes the trailer act like a long, long lever.

This lever effect can remove enough weight from the rear of the vehicle and LIFT the tires right off the road (I personally have seen this happen once when I was loading a pickup truck onto my flatbed trailer, picked the tow vehicle rear tires up off the pavement when I forgot to put blocking under the ramps :S). Even if it doesn't lift the tires it will substantially remove enough weight that you will lose traction which can push you sideways .

It is for that reason you want to have a minimum of 10% and ideally 12%-15% of the trailer weight SITTING on your hitch at all times.

The trailer is a teeter totter with the axles almost in the middle. Unless it is heavier at one end - pin / tongue weight - you will have it rocking back and forth, swaying for a TT, porpoising for a 5er.

As stated you need balance, if you get the tongue weight to light it will sway. The lighter the tongue the lower the speed that it will start to sway. I've pulled a cargo trailer that started swaying at 30 mph and after moving weight forward 3 times, got it to where I could travel interstate speed with no problem.

The tongue weight and the load in the bed are effected by how much weight the rear tires can handle.

You need the correct weigh distribution for the trailer, or you are asking for a wild ride or worse.

I read about one guy who loaded the same ATVs in his trailer all the time, and had actually had the trailer weighed and shifted them a bit, to get the right distribution. Then he painted around the tires (or something like that) so that he'd know where to park them each time. Of course, if he added more stuff, he had to re-calculate, but he at least had a good starting point.