The pin weight listed is going to be the absolute most weight that will be loaded on the pin, when the trailer is properly configured. Trust me, if there is one thing I know, its kingpin weights. Drove a rig for a few years before the service.
You can't factor the empty weight with that kingpin weight and get the correct #'s. If your trailer weight is 13k, I doubt you will have 2k on the pin. maybe 1300. Like tongue weights, the rule is 10% of the total trailer weight (loaded or not). So if your trailer weighs 20k, you should have 2k of pin weight.
The reason your single rear wheel is rated to tow more than a dually is simple really. It fly's in the face of logic until you really think about it.
Picture your SRW with 2k on it, the tire is bubbled a little right? What happens when you apply turning force (TQ) to the wheel? It creates friction. Bu adding a second rear wheel to each side, you INCREASE friction, and therefore resistance. That resistance is what makes a difference in your towing capacity. All other things being equal, it comes down to, your SRW isnt turning 4 tires with a crapload of weight on them, it only has to turn 2. The reason's why many people run dually's eludes me, in most cases it is not nessecary. The only time you REALLY need a dually, is if most/all of your weight is centered over the axle, for the stability. Anyone who has hauled around a bed camper in a SRW and then hauled a camper in a DRW will tell you its night and day. It might feel better towing with a DRW, but when towing, DRW can actually hurt, not help.
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