Greetings RV enthusiasts. Todays post concerns some mis-information that appears to be surrounding a popular fifth wheel hitch option for short-bed trucks.
If you are reading this post, you know what I’m talking about — those popular light-weight pyramid style hitches that promise certain utopia for short bed owners. This post mentions Andersen only becuase they were the first to market, making the concept popular. By now there are other options that have entered the market, each with their own twist on the same concept, capitalizing on the market rush for a lightweight hitch that guards against striking the cab during sharp turns.
I touched on this topic in this post, where I outlined a set of viable hitch options for short bed trucks. The purpose of this post is to emphasize the weight penalty of such solutions. Wait — did I say weight penalty? I thought these lightweight hitches were the bees knees for weight?
Not so fast. Lets look at how much weight is actually applied to the rear axle with such solutions. we’ll use Andersen as an example because their geometry is well understood and published: In order to use this solution effectively for a short bed, one must “turn the coupler around”, maximizing the distance from cab to trailer, locating the pin 9.4375 inches behind the rear axle (I’m assuming here that the hitch itself is mounted over the gooseneck ball). All is good so far — 9.4375 inches provides wonderful clearance, even for wide-body (101″) trailers.
Its just that the numbers don’t lie.
Lets suppose your fifth wheel has a pin weight of 3,000 pounds (approximately that of my Alliance 310RL) and your tow vehicle has a wheelbases of 160 inches (the wheelbase of my 2022 Ram Mega Cab short bed). What you are about to see is that the laws of physics are not exactly in your favor with the pyramid-style hitches. Lets review the diagram that I used in the previous post:
As you can see, simple teeter-totter analysis shows 3,000 pounds sitting 9.4375 inches from the fulcrum on one end of the teeter-totter. How much weight has to go on the other end of the teeter totter?
The answer is 177 pounds. Of course, there’s a lot more than 177 pounds holding our front axle down (the engine, among other things), so this isn’t an issue. The problem is that this 177 pounds is actually transfered from the front axle to the rear. What is the weight that the teeter-totter fulcrum (the rear axle) actually sees? The answer is 3,000 + 177 or 3,177 pounds! Refer to the post in the above link for more detail.
Morover, the lightweight hitch itself weighs something too: 40 lbs for the Ansersen, so lets use that. Add about 15 pounds for the gooseneck ball itself that is required for such a mounting solution, and you have a 55-pound hitch. Add 55 pounds to the 177 pounds you just transfered to the rear axle and guess what: That super-light 40-pound hitch has just added about 230 pounds to the rear axle. Now, that heavy auto-slider isn’t looking so bad after all.
This is why I did not heasitate to put a 250-pound auto-slider hitch in my bed. The punch line is very simple:
My rear axle sees only 20 pounds more than it would if I used one of these lightweight hitches.
20 pounds is well within the margin of error for this analysis, and less than 10% of the weight of my hitch as well, so for all practical purposes the auto-slider has contributed no more weight to the rear axle than the Andersen hitch! That may be hard to swallow, but the numbers don’t lie. To recap the disadvantages of such a hitch, when used as a short bed solution:
- There is no weight advantage, as seen by the rear axle, compared to a heavy auto slider.
- As I pointed out in the previous post, the leverage that your fifth wheel has against the truck is significant, when you apply the pin that far behind rear axle. A gust of wind will try to steer the truck into the wind, but put the pin over the axle where it belongs, and that same wind only pushes sidways on the axle without trying to steer the truck.
One more point that should be clarified: The 177 pounds that is transfered from the front axle to the rear axle does not contribute to the total GVWR of the truck, so if your short bed truck has a particularly beefy rear axle with lots of margin in the axle and little GVWR margin, then feel free to load it up, because the Andersen accomplishes a simple transfer of weight from the front to the rear. I would only point out if you’re arguing with yourself over 200-ish pounds, you need more truck :-).
-Doug
