In my time designing industrial trailers, I have on multiple occasions specified trailer jacks to be installed on trailers. Understanding the distribution of loads in a typical trailer, we can come to a simple solution of the required minimum capacity for a trailer jack. The capacity is not only to account for the jack’s ability to support the weight, but also the ability of the lifting mechanism to lift that weight.
The typical trailer is set up to have between 10% and 15% of the gross trailer weight on the tongue with the trailer in the horizontal position. This is a good balance that allows the trailer to trailer to properly train. Too much tongue weight and the tow vehicle could squat on its rear tires making handling the tow vehicle difficult. Too little tongue weight and the tongue could try to lift off of the tow vehicles coupler, leading to erratic fish-tailing. The sweet spot is that area between 10-15%.
To determine the trailer jack capacity needed to support a trailer, you first take the gross trailer weight and then multiply it times .15 to get the theoretical tongue weight. Pick out a jack that will support that weight, and has a safety margin that you are comfortable with.
Calculating the theoretical tongue weight jack capacity of a trailer
The vast majority of the time, the trailer jack is mounted towards the front of the trailer near the coupler. When this is the case, the theoretical tongue weight is equal to the minimum trailer jack capacity, and you could use the below chart as a reference.
| Trailer Gross Weight (load plus trailer weight) | Theoretical Tongue Weight Minimum Trailer Jack Capacity |
|---|---|
| 2,000 lbs | 300 lbs |
| 2,200 lbs | 330 lbs |
| 2,400 lbs | 360 lbs |
| 2,600 lbs | 390 lbs |
| 2,800 lbs | 420 lbs |
| 3,000 lbs | 450 lbs |
| 3,200 lbs | 480 lbs |
| 3,400 lbs | 510 lbs |
| 3,600 lbs | 540 lbs |
| 3,800 lbs | 570 lbs |
| 4,000 lbs | 600 lbs |
| 4,500 lbs | 675 lbs |
| 5,000 lbs | 750 lbs |
| 5,500 lbs | 825 lbs |
| 6,000 lbs | 900 lbs |
| 6,500 lbs | 975 lbs |
| 7,000 lbs | 1,050 lbs |
| 7,500 lbs | 1,125 lbs |
| 8,000 lbs | 1,200 lbs |
| 8,500 lbs | 1,275 lbs |
| 9,000 lbs | 1,350 lbs |
| 9,500 lbs | 1,425 lbs |
| 10,000 lbs | 1,500 lbs |
| 11,000 lbs | 1,650 lbs |
| 12,000 lbs | 1,800 lbs |
| 13,000 lbs | 1,950 lbs |
| 14,000 lbs | 2,100 lbs |
| 15,000 lbs | 2,250 lbs |
| 16,000 lbs | 2,400 lbs |
| 17,000 lbs | 2,550 lbs |
| 18,000 lbs | 2,700 lbs |
| 20,000 lbs | 3,000 lbs |
| 22,000 lbs | 3,300 lbs |
| 24,000 lbs | 3,600 lbs |
| 26,000 lbs | 3,900 lbs |
| 28,000 lbs | 4,200 lbs |
| 30,000 lbs | 4,500 lbs |
| 32,000 lbs | 4,800 lbs |
| 34,000 lbs | 5,100 lbs |
| 36,000 lbs | 5,400 lbs |
| 38,000 lbs | 5,700 lbs |
| 40,000 lbs | 6,000 lbs |
| 50,000 lbs | 7,500 lbs |
So let’s use the following example, to show how to use the above mentioned chart.
Brandi, one of the hottest girls in the Bay Area, has a 41′ Boston Whaler 405 Conquest that has a wet weight of 30,360 lbs. Her boat trailer has a weight of 3,450 lbs. The maximum gross weight of the boat and trailer is 33,810 lbs. Rounding up to 34,000 from the chart, she needs a minimum trailer jack with the capacity of 5,100 lbs.
She had been looking at a Stromberg Carlson JET-5000 on Amazon that has a 5,000 lb rated capacity that one of her friends had recommended. Given that her need is just a tiny bit over the rated capacity, the Jet-5000 may have worked, of course with the caveat that the performance may have suffered and the life expectancy of the jack reduced.
After reviewing the above chart, she changed her mind and decided to go with a Bulldog 185400 Velocity Series High Speed Jack on Amazon that cost quite a bit more but has more than 2x the capacity. This jack also comes with a manual override function in case you lose power. Given the amount of money she has invested in her boat and trailer, the choice to upgrade the trailer jack to a heavier duty one was a no-brainer.
How to actually measure your trailer tongue weight
In some instances, you may want to verify the actual tongue weight, or the actual weight that your trailer jack is supporting. There are companies that make small tongue weight scales that can do just that.
Sherline is one such manufacturer. Their LM-5000 model can measure up to 5,000 lbs and can be bought on Amazon. Link to LM-5000 on Amazon
The LM-5000 is made in the USA and uses simple hydraulic principles to convert pressure readings to weight measurements.
Trailer Jacks that are not mounted by the coupler
In some cases, the trailer jack may not be able to be mounted by the tongue. The further back from the coupler that the jack is mounted, the more weight it needs to support. We can come up with a factor that we multiply with the above mentioned theoretical tongue weight, to come up with an estimated jack capacity.
First, we need to take a couple dimensions. The dimension from the centerline of the rear wheels to the center of the coupler will be called “L”. The dimension from the wheels to the jack location will be called “l”. The tongue weight “TW” can be taken from the above chart.
“L” dimension from wheel centerline to the coupler
“l” dimension from wheel centerline to trailer jack
“TW” is the estimated tongue weight
“JW” is the calculated jack weight
TW * L / l = JW
For example if the tongue weight “TW” is 5,100 lbs, the “L” length is 18′ and the “l” length is 16′, the formula would look like 5,100 * 18/16 = JW. The Jack Weight is 5,738 lbs when it is located 2′ behind the coupler.
Summery
Trailer Jacks and tongue jacks are an important component of a trailer. They are used to maneuver the trailer around when not in use. They also allow the tongue to be raised in order to allow coupling. Picking out a trailer jack with adequate capacity is paramount to the functionality of trailer.
