RV towing and weights

Fifth Wheel hitches for short bed trucks

Greetings fifth wheelers.  I’m going to tackle a difficult subject that is filled with a generous portion of emotion, not to mention the fervor that accompanies one’s choices and the marketing language one has been influenced by. I do realize that there is room for nuance and personal choice here — simply because every combination of truck, trailer, expected terrain, and tolerance for risk is different.  My goal here is to cut though the emotion and get to the science that drove my personal choice.

I’m talking about fifth wheel hitches for short bed trucks.

Make no mistake about it – all short bed hitch solutions make a compromise of some sort:  One only needs to figure out which ones are important, so for this post I’ll take an inventory of the important compromises and show how they laid out for me.  I will even give a spoiler alert up front:  My motivation in choosing a short bed hitch was to preserve the reason I bought a fifth wheel in the first place – the pin should be located over the tow vehicle’s rear axle (or even slightly forward). This heavily influenced my choice, as you will see.

Below is my 2018 Ram short bed truck, pulling the  35′ long Alliance Paradigm 310RL fifth wheel.  The combination weighs in at 24,000 lbs fully loaded.

The Compromises

Before I purchased my hitch, I prioritized  compromises in the order of their importance (to me) — at the top of the list are the things I just won’t accept:

(1) You can place the pin behind the axle.  By far, the majority of short bed solutions today do this,  whether there is a gooseneck type coupling, a traditional hitch head or a pyramid in the bed of the truck (even the venerable B&W Companions can go a small distance behind the axle, if you want).  The reason is obvious — the only way to avoid striking the cab is to move trailer away from it!  Whatever the configuration or design, however, there is just no free lunch with this solution:  If the king pin looks like it’s behind the axle, then the king pin is behind the axle — and  no amount of marketing language will be able to change the laws of physics:    The result is that the front axle is lifted slightly, which transfers weight to the rear axle.  In addition, such a solution gives some non-zero sway leverage to the trailer as well.

These compromises are small, of course,  if  the pin is only a short distance behind the axle.   I acknowledge that there are many who successfully tow with such solutions, but I decided that locating the pin behind the axle, by any amount, was not for me:  Especially for my 101-inch “wide body” trailer,  in order to really eliminate (not just reduce) the risk of striking the cab,  the pin has to be several inches behind the axle — where these compromises aren’t so small anymore!

The Andersen hitch is a good example of doing this.   This a pyramid style hitch that mounts up directly to the gooseneck mount point in the truck bed —  which for me (and most short bed trucks), is directly over the rear axle.  It is a very light hitch, which is fantastic for short  bed trucks because these are the most likely to exceed their payload ratings due to weight in the bed.  However, the Andersen’s geometry does a very interesting thing:  it re-locates the pin over 9 inches behind the axle!   To see this, note that the Andersen’s own coupler mount point (the ball facing upward) is located  5.4375 inches behind the hitch mount point (the rear axle).  In addition to this,  the Andersen coupler itself (the part that attaches to the trailer’s pin box) introduces an additional 4-inch offset of its own,  so that the total distance behind the axle is 9.4375 inches!   For sake of completeness, I note that the Andersen’s coupler can be reversed (applying the 4″ offset in the other direction) which would locate the pin only 1.4375 inches behind the axle — essentially eliminating the “behind the axle” problem, but also eliminating the benefit for short beds!  So in order to work in a short bed truck, the Andersen coupler has to place the pin 4-inches behind the Andersen ball, which itself is 5.4375 inches behind the axle for a total of 9.4375 inches.  No wonder these hitches are popular with short bed trucks!  Unfortunately, there are consequences…

Lets consider a truck with a wheelbase of 160 inches and a trailer pin weight of 3,000 pounds, as shown in the diagram below:

 

 

 

 

 

 

The physics here is just like an ordinary child’s teeter-totter:   Suppose you put 3,000 lbs of weight located 9.4375 inches to one side of the teeter-totter fulcrum, which in our case is the rear axle.  How much weight is required at the other end of that 160-inch teeter-totter (over the front axle) to balance it?  The answer is 177 pounds.  Translation:  our front axle is now 177 pounds lighter!  Now then, how much total weight does the teeter-totter fulcrum (the rear axle) actually see?  It sees 3000 lbs plus 177 lbs!  In other words,  3000 pounds of weight located 9.4375 inches behind the rear axle will transfer 177 pounds of weight from the front axle to the rear axle, effectively eliminating the weight advantage of the hitch.

In addition to the weight distribution consequences, there is also a sway leverage consequence, which I will describe by making a “first order approximation” (which means its really more complex than this, but this is good enough to get the point across).  Lets consider the situation where a 60mph wind gust hits the trailer from the side (perpendicular to the road).

The “wind load” of a 60 mph gust is about 30 pounds per square foot (there are a number of sites with wind load calculators that corroborate this).  This means if you were to hold up a 12-inch by 12-inch piece of wood “perpendicular to the wind”, that 60 mph gust would push back on you with a force of approximately 30 pounds.  On my “full profile” trailer, there is approximately 190 square feet of surface area forward of the axles — so that 60 mph wind wind gust translates to about 30 x 190 = 5,700 pounds of sideways force applied to the front of my trailer.  When this occurs, the trailer tries to rotate about it’s axles, and all of this force is applied to kingpin.  Normally, the kingpin would transfer this force to the truck at a point directly over the axle, in an attempt to scoot the tires over.  But in the case of the Andersen hitch, this force is applied at a point approximately three-fourths of a foot behind the axle, as shown below.  So what does that do?

 

 

 

 

 

 

As shown in the diagram, the trailer now has a “lever arm” — a mechanical advantage — that actually tries to rotate the truck instead of just scooting it over.  This is accomplished with a large twisting force (torque) of about 4,500 foot pounds, which tries to steer the truck back into the wind (to illustrate the strength of 4,500 foot-pounds, imagine you were pushing with a force of 100 lbs at the end of a 45-foot horizontal rod welded to the axle center).

(2) You can move the hitch pivot point to the other end of the pin box.   I haven’t kept up with all the market movements, variants and names, but fundamentally this solution is the Reese Sidewinder (or “Revolution” when installed as original equipment).   In this fascinating solution, the pin box is rigidly attached to the hitch (so it cannot rotate in the hitch) and the towing pivot point is relocated to the trailer end of the pin box. A huge advantage of this solution is that it reclaims a lot of space in the bed!  The “dead” or static weight of the pin is located over the rear axle, but there is an important consequence of this solution:    There is now a rigid “lever” (the pin box) attached to the fifth wheel hitch, giving the trailer 22 inches of leverage over the truck at the pivot point. Wait — I thought all would be fine as long as the pin weight is applied directly over the axle?  Not so fast — the difference is that side-ways forces applied at the pivot point will tend to twist the truck into the wind just like the Andersen does (see above)– only with a bigger lever.

Using our example above — where a 60 mph gust of wind pushes on the front of the trailer with a force of 5,700 pounds, consider  that as the trailer attempts to rotate about it’s own axles, it will try to push the truck off to the side so that it can rotate about its own pivot point (which is the Sidewinder mount point).   Only in this case, the Sidewinder applies this force at a point 22″ behind the axle.  This works out to be 6,219 ft. pounds of twisting force applied at the truck’s rear axle, attempting to steer the truck into the wind.

In actuality the movements of the trailer are complex, and in my opinion there is insufficient real engineering data to characterize the benefits and drawbacks of the turret-type geometry, but this first-order approximation was enough to convince me not to use this solution.

There is one other factor that must be considered in the use of the Sidewinder:  It introduces new side-to-side stresses to the trailer’s own frame and supporting members around the pinbox.  During a 90 degree turn, for example, all of the forces exerted by the pinbox onto the trailer frame are different now — they are applied at 90 degrees compared to the original intentions of the frame!  What that means is this:  unless your frame is certified by the frame manufacturer to accommodate those new forces, the Sidewinder has the potential to cause your frame to fail. not good.

(3) You can forget to engage your manual slider:  Manual sliders are great solutions if you can anticipate the need for more room before you have to use it: You simply stop and release the slider mechanism so that the trailer can shift rearward—away from the cab.  For normal driving, the king pin is located over the rear axle.  Another bonus is that you can even engage the sider when traveling over slow, undulating terrain to keep from hitting your truck rails.  Another advantage here is weight — manual sliders are considerably lighter than their automatic-sliding cousins.

(4) You can add more weight with an automatic slider.  If want to avoid the risks of  (1)  (2) and  (3) above, then the only solution of choice is the automatic slider.  These solutions are great because the king pin (and pivot point) is located over the rear axle for normal driving, and you don’t have to think about or anticipate the need to activate the slider mechanism.  The hitch does the thinking for you — by detecting the truck-trailer angle and automatically sliding the pin rearward for those tight corners.  There is no free lunch, however, as these hitches are heavy!  The other compromise here is that the hitching angle is critical – you can’t back into your trailer at 30 degrees and hitch up:  This is because the hitch is designed to detect the truck-trailer angle, and expects the hitching angle to be no more than 10 degrees to one side (in the case of the PullRite).  This adds the additional inconvenience that you can’t unhitch at these angles either:  The truck and trailer have to be very nearly in line with each other, or the hitch will not disengage.  Some find this irritating – I find it to be a minor inconvenience because there is a way to mitigate this problem:  if you are backed into the campsite and you can’t unhitch, due to the truck/trailer angle, simply remove two pins holding the hitch head onto the hitch — and use the trailer to lift the head off of the hitch.  Then you are free to re-position the truck, and you can then remove the hitch head from the trailer’s kingpin easily enough.  This is two-person job of course.  I should acknowledge here that there may be some campsites that simply require an angled approach, making it impossible to unhitch or to hitch.  In these situations, you’ll have to leave the trailer hitched up:  If you were to use the “remove hitch head” trick, you could certainly unhitch, but you’d never be able to hitch back up!

My solution

In the end, the only solution that gave me the peace of mind I wanted while locating the pin (and the pivot point) directly over the rear axle, was an automatic slider.  I purchased the 24K “Superglide” flavor from Pullrite because it provided a full 18” of travel and dropped into the factory puck system already present on my Turbo Diesel, making for easy installation and removal.   Here is Ronnie Williams of Bish’s RV, installing my PullRite.  It takes up a lot of space in my bed, but it is what it is — I just can’t live without any of the other compromises!

Below is a photo of PullRite’s use of the RAM puck system.  A simple 90-degree twist for each of the four posts is all it takes,  and the whole thing comes out.  The hitch itself is a little heavy (250 pounds!) so a ceiling hoist is probably in my future.

Here is a photo of my rig utilizing the PullRite slider. Notice that I’m not at 90 degrees, but this is the angle most likely to strike the cab, and I have several inches of comfort factor.  The Comfort factor is often over-looked in “slightly behind the axle” hitch solutions — they might give you an inch of clearance on flat pavement, but  provide no margin of error — you would probably still strike the cab in a deep driveway, for example, in combination with a sharp turn.

Happy hitching

-doug

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