Towing a 7000# trailer requires a weight distribution hitch. I had used the famous "Equalizer" model when pulling my race car trailers behind a motor home and a Suburban. It worked well. But, it was rated at 6000# and their design requires a completely new "head" and "bars" to increase the capacity to 10000#. So, since I needed to buy a new hitch, I did some research and settled on the Blue Ox Sway Pro.
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| Equalizer |
I liked the way the Blue Ox achieved sway control without friction. It was quieter, cleaner and provided more flexibility in that capacity could be changed by simply changing bars.
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| Blue Ox Sway Pro |
There is an amazing lack of understanding of how these hitches actually work. The principles are easy to understand. You are not really reducing the tonque weight, but rather you are moving the weight from the rear axle of the tow vehicle to the front axle as well as to the rear axles of the trailer.
In addition to misunderstandings regarding how the hitch works, there is a bit of controversy as to how much weight should be transferred and how much "droop" or "sag" or "loss of rake" is desireable.
So, I spent some time analyzing my setup.
In the freshman year, engineering students take a basic "Statics and Dynamics" class. Perhaps the most useful of the entire 4 years curriculum. One of the basic concepts is called a "Free Body Diagram" that allows you to analyze forces on an object by complying with the law that for a body that is not in motion, all forces must cancel out for the total object and all torques or "force couples" must also canel out at any point of the object. This takes a bit of getting used to, but it is simple. For example, your standing body has two forces acting on it: 1) The force of gravity pulling you down; and 2) and equal and opposite force of the floor pushing you up.
The above figure is a "free body diagram" of the rig. The lighted loaded trailer is 6700# (6000 rear axles plus 700 tongue) and the truck is 5000# (2000 rear axle and 3000 front axle) before the trailer is connected. With no weight distribution, when connected, the rear axle of the trailer remains at 6000# but the 700# tongue weight is placed on the truck. Since the hitch ball is behind the rear axle, the tongue weight lifts the front proportional to the distance 12:5. 5 times 700 or 3500 ft pounds of torque is applied equal to 292 pounds applied to the 12 foot "lever" distance to the front. (All torques must be equal) So the front weight is REDUCED by 292# to 2708#. Since any weight reduced from the front must be borne by the rear, the rear weight is INCREASED by 292# in addition to the 700# for a total of 2992#.
The F150 has a built in "rake" of 2 in.--the rear is higher by 2 in. than the front. But once the weight is applied to the hitch, the front end will rise by around 1 in. and the rear will sag by around 3.5-4 inches. The rear will now be 2.5-3 inches BELOW the front.
Most old hands know that the trailer should be level or slightly lower in front when towing so to keep air from getting under and creating lift. This same concept applies to the truck. A nose high truck is unstable and the headlights are aimed improperly toward the sky. So the goal is level or a slight rake with front lower for the truck too.
The weight distribution hitch helps to accomplish this by pulling up on the hitch bars (and therefore down on the trailer frame) creating a TORQUE in ft-lbs. equal to WD x 2.5 feet to the trailer. (WD equal to the force applied by both hitch bars together.)
So for example, let's assume we apply 1000 pounds of WD. (500# per bar) The 1000# WD will produce a torque of WD x 2.5 feet or 2500 ft-lbs to the trailer. This in turn creates a force of 2500 divided by 17--adding 147 to the trailer axles. (17x147=2500=2.5x1000) It will also add 1000-147# of increased force, or 853# on the hitch ball.
Here is where the "free body diagram" concept is important. As a "coupled" unit, the 147# force on the trailer axles is "balanced" by a force on the front truck tires. Torque about any point must be equal (teeter-totter concept) so the torque around the rear truck tires is 147# x 22 ft. from the trailer or 3234 ft-lbs. countered by a force on the front tires of 269#. (269 x 12 = 3234)
It is very easy to get confused with a combination "body". The above analysis is the simplest, but another way is to look at a free body diagram of each unit separately. The 1000# WD creates 2500 ft-lbs torque on the trailer---pulling it down on the rear and the ball: 147# to the rear axles and the remaining 853# to the ball. (Teeter totter with long and short balanced.) Forces exactly opposite these are applied to the truck: 1000# up on the bars and 853# down on the ball. So this is a teeter totter about the rear axles--two forces on one side and one force on the other: 1000# x 7.5ft= 7500 ft-lbs less 853# x 5 ft= 4265 ft-lbs or 3235 ft-lbs. balanced by 269# x 12 ft. = 3235 ft-lbs with the 269# added to the front truck axle. Same result.
Since the weight of the total rig has not changed, the rear axle weight of the truck has been reduced by the 147# taken by the trailer axles, and 269# on the front axle of the truck. So the rear axles weight is reduced by 416# from 2992# to 2576#. The rear will now be about 2.0 inches lower than original and the truck will be about level This is acceptable and even recommended by Ford.
Some would be increase the WD by pulling up the bars more to 1400#. (205# to the trailer axles, 375# to the front (3083# total) reducing rear axle weight by 580# to 2412#. This essentially places the front about about 3/8 inch lower than orginal height and the rear about 2.0 inches below original height. * The truck will now have about 3/8-1/2 inch "rake".
(* This disproportionate drop or sag in the rear is why the top of the ball should always be 1 to 2 inches ABOVE the desired top of the trailer ball socket.)
In many people's opinion, this higher weight transfer would be "ideal". Some suggest more WD to put even more weight on the front--their argument would be that a top heavy trailer transfers weight to the rear truck axles under heavy braking, taking weight away from the front truck axles. My opinion is the trailer and the truck both being level or with 1/2 inch rake under normal operation is ideal depending on cargo loads. It is important to note that the weight distribution WD increases when the coupler is lowered, thus limiting much of this weight transfer under braking.
This variation in WD can be from changing the chain length or position of bar support brackets, or by changing bars. Using a hitch rated for 1000# tongue weight for a trailer having 700# tongue weight is probably close to the example above with 1400# WD. However, data as to actual force applied by the hitch bars is not provided by most manufacturers and often the WD produced by "factory" settings is less than what is required to bring the front weight and height back to "original". Just like there are some that suggest higher WD, others suggest only moving "half" of the weight. The WD can be reasonably estimated by the relative heights of the front and rear of the truck.
One must consider the GAWR figures. The GAWR for the front axle is 3525#. 4050# for the rear. With the "level" WD of 1000#, extra weight of 500# can be carried on the front and 1400# on the rear. But the GVWR is 7000#, so with 3500# on the front, only 3500# can be carried on the rear making the "capacity" for cargo of 500# front, 900# rear. Possibly you will reach the front max before the total of 7000# is reached if cargo is loaded to maintain a level attitude. Plus, adding more WD reduces the cargo capacity for the trailer. So, any WD above the 1000# level probably reduces cargo carrying capacity unless the cargo load is centered behind the truck rear axle.
If rake in the F150 towing vehicle is desired, probably the addition of an air ride "helper" would be a good mod. That feature is available on Ram pickups and the GM SUV's.
In addition to misunderstanding the principles of weight distribution on the truck, there may be even more misunderstanding regarding the function of the different WD hitch designs in regards to controlling sway. The "traditional" approach to sway control is to increase a frictional force to resist the rotation of the trailer about the hitch ball. Some actually use a friction bar--others use friction between the spring bars and the L brackets on the trailer. This is pretty intuitive, resist sway by applying a "braking" force to the rotation.The Blue Ox Sway Pro model uses a completely different principle--without friction. The Blue Ox uses the change in the three dimensional geometric position of the bar to introduce force in the spring bar itself to oppose the sway movement. In a sense--the Blue Ox produces forces to "center" the trailer and truck in the straight ahead position. This is accomplished by placing the trunnions for the bars at a significant incline---as the bars rotate, the "outside" bar moves down pulling on the chain, producing an increased force and increasing the bar's deflection. The "inside" bar moves up, decreasing the force on that bar. Also, the forces produced by the Blue Ox place a small "torque" on the truck and trailer since the "outside" bar is being pulled up more by the trailer than the "inside" bar. (The dynamics of this torque tends to offset the torque applied by a swaying trailer's hitch pushing the truck sideways toward the "outside".) The force bending the bar and the forces opposing the torque stabilize the combination. Unlike friction based anti-sway, the Blue Ox stabilizing force increases significantly with the angle of the "turn" so it more powerful in reducing sway.
The article below is excellent and may help in understanding hitch function. It leans toward the opinion that a large amount of weight should be moved.
The top of the 2" hitch receiver for the F150 is at 21 in. above ground without the trailer. The top of the trailer hitch ball socket with the Puma 25RBSC when level is 21.5 in. With the trailer connected, and the rear of the F150 dropped 2" (level), the top of the hitch receiver would be at 19 in. So, we would need the top of the ball to be at 3" above the hitch receiver.
In the lowest position on the 7 hole Blue Ox shank receiver, the ball would be 1" above top of the hitch receiver in the truck. Each hole moves the receiver up 1.25 in. so one hole showing below would place the ball 2.25 in. above the hitch receiver---close to perfect. (This would be 22 inches to top of the ball before the trailer is connected.)
The dealer set the hitch with two holes showing below the receiver, and the trailer was nose high. It also did not pull as well as I liked, so I moved it to the next lower position; with one hole showing---which would put the trailer with a slight 1/4 to 1/2" nose down attitude--most usually recommended as the "most stable".
With the hitch higher, I almost scraped the rear trailer stabilizer jacks entering a parking spot, so lowering the nose 1" will help with that too.
I tested the rig by hooking up without connecting the Blue Ox weight bars. Hitch ball was 20" so the F150 has dropped a bit more than 3" and the trailer showed a "nose down" attitude of about 1.5". A towing test at 50 mph indicated the rig was still very stable and towed quite well.
The Puma 25RBSC weighs 5700# empty, and 7300# maximum. The F150 weighs around 5000# empty and is rated at 7000# max. The 7000# max includes tongue weight from trailer.
I figure the maximum Gross Combination Weight is around 13,600# with 7000# on the truck tires and 6600# on the trailer axles (1650# per tire). Most likely, I will be operating at 13,000#.or less.
My first drive on "wavy" country roads, lightly loaded at 11,200#, the rig operated with excellent stability and control---great braking and stable pull. With one exception---the "wavy" road undulations translated into a "wavy" up/down oscillation almost like being on a boat. It appered to be more in the rear of the truck.
The trailer has no shock absorbers and the truck has "stock" OEM shocks. The truck also has "Standard Load" tires that tend to have more flexible sidewalls. I was running 40 psi in the tires.
First--I suspect that max inflation of the rear tires will be necessary when towing. And it is possible that LT 10 ply rated tires with stiffer side walls may reduce the oscillation. My first choice for tire change would be Bridgestone Dueller Revo that have the same diameter as the Goodyear Wrangler's on it now. (Nitto also offers a LT 275 55 20 but it is slightly larger (1.2%) larger.
Second, it seems clear that shocks with more damping will be a good mod. The F150 has a reputation of being "bouncy" in the rear. (Some call it "hopping") Stiffer shocks do not make the ride less "harsh but stiffer shocks do reduce "bouncy". Bilstein 5100 or Fox 2.0 are good candidates. I will make this mod first probably with the Bilsteins. The Fox 2.0 are more expensive and more for racing that street operation.
Third, oscillation of the trailer without shocks may simply be movement that the truck cannot completely absorb. The weight distribution hitch adds an additional "spring" that may affect oscillation as the tongue moves up and down increasing and reducing the weight bar tension. So, Roadmaster shocks for the trailer will be considered after evaluating the the improvement from the new truck shocks.
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