Trailer tongue weight question

[Sailor Sue]

I'm wondering if any mathematic/engineer types know a formula to figure out how much the tongue weight changes by moving the axle an inch, or how much I would need to move an axle to change the tongue weight 25 to 50 pounds? In very general terms, boat and trailer weigh 4000 pounds, tongue weight is 400 pounds, axle point to hitch is 16 feet.
One option is trial and error movement, but that seems like a lot of work (the trailer is jacked up with the wheels off for summer storage). I imagine/hope this is probably a pretty simple problem for someone with a math or engineering background. Thanks in advance.

 

[kloudie1]

An interesting problem.. first, one must assume that the boat does not shift in its position on the trailer.. then the math is pretty straightforward.. first ya can find the center of effort of the boat weight.. since the tongue weight is 1/10 of boat weight, the center of effort is 1.6 feet forward of the axle... if ya move the axle toward the hutch 2 inches, that is 0.167 ft and the tongue is now 16 feet less 0.167 .. 15.833. the axle moves to (1.6-0.167=1.433) 1.433 feet behind the weight point.. so the tongue weight goes to
(4000 X 1.433)/15.833= 362 pounds at 2" movement toward the hitch
(4000X 1.35)/15.75= 343 pounds at 3"
(4000X 1.267)/15.67 = 323 pounds at 4"

 

[Meriachee]

Having just added a third axle, (see the tag below for the location of the pictures) I'd add to the equation that something else may get in the way. We did a whole bunch of math over a whole bunch of coffee cups over a whole lot of time (I had the axle for almost a year) and then when the trailer got flipped over and the whole assembly set in place, instantly discovered that there was a cross member in the way. In the end it didn't matter, the third axle spread the weight beautifully.
I'd be really interested if somebody comes up with a formula, but I suspect there's too many variables. Moving the boat an inch forward or aft seems like it would have a detrimental effect on the calculations.

cheers
Gary

 

[Meriachee]

p.s.
If you are only planning on adding 25 lbs to the tongue, you are probably far better off to just add a spare tire to the front of the trailer. or maybe move the boat forward an inch. That little addition is probably not worth the calculations on the blackboard.

 

[walt]

A typical 4000 pound boat and trailer are likely 2 axle.. which complicates things. How.. Im not sure but things could vary depending on how far apart the axles are, are you moving only one of two axles, different types of suspension between the axles may affect this (equiliazation).

One axle is much simpler.. and you also know what will happen with the balance and tongue weight if there is a blowout (it wont change). With 2 axles where you blow one out of four tires and you moved one axle????

 

[Scott T-Bird]

It is a little bit of a moving target, but Kloudie has it right. It is based on uniform distribution of weight so your objective may be accomplished by shifting weight on the trailer. What is your objective? Are you intending to lighten the tongue weight or increase it? It can be accomplished by redistributing the load on the trailer, possibly by moving shiftable weight to the rear of the boat or to the front. More weight back can make the trailer less stable, while lightening the load on the rear axle of the tow vehicle. It seems that at 10% you are about right already.

I thought this was interesting ... http://www.etrailer.com/faq-weightdistribution.aspx

 

[dscribner]

I've heard a lot of talk about tongue weight over the years. My boat/trailer combo is around 5500 lbs. It tows like a dream. I hardly know it's there. It doesn't mater the speed, the hills, the wind, the braking. I've towed it around 4500 miles, mostly on the highway. I basically centered my keel on the wheels when I built it. It seemed the thing to do based on measuring the same boat model on a trailer over at the marine. So I really didn't know what I was doing. The thing is my tongue weight is maybe 150 lbs. That's about 4% rather than 10-15%. But, I can't argue with the performance. It tows like a dream. Stable at all conditions. Never a quiver.

I guess what I'm saying (before the melee of "Don, are you STUPID?" starts) is don't worry about 25 lbs either way.

 

[Rick Webb]

There Is a Formula

I used to do it all the time in the Air Force getting vehicles and equipment ready to load on aircraft. I can't remember the formula right now I'll look for it but you will need:

1. Front Axle Weight (coupler)
2. Rear Axle weight ( you can use the center of the two axles
if you have more than one)
3. Distance between axles
4. Total weight of trailer

Using the formula you would be able the calculate the tounge weight if you had
the total weight and the axle weight using the distance between the axles. It is a simple moment calculation.

It has been a long long while since I had to do it by hand we got a computer program to do it and that is even a long time ago. I remember it being a DOS program on a 286 computer made by Zenith with a 10" monochrome monitor.

I'll look for the formula

 

[Rick Webb]

Found It

It is not as complicated as it looks

www.transcom.mil/…r/part-iii/dtr_part_iii_app_p.pdf

 

[Rick Webb]

Trouble Posting the Link

APPENDIX P
CENTER OF BALANCE (CB) DETERMINATION – FINDING CB

2. The following terms are used to calculate CB of a vehicle, see Figure P-1:
a. Reference Datum Line (RDL) (also called reference line). Predetermined point from which all measurements are taken. Normally, the RDL is established at the forward front edge.
b. Front Overhang (FOH). Distance in inches from front bumper to center of front axle.
c. Wheel Base (WB). Distance in inches from center of front axle to center of rear axle or center of tandem axles.
d. Gross Weight (GWT) (pounds).
e. Rear Overhang (ROH). Distance from rear or center of tandem axles to rear bumper.
f. Front Axle Weight (FAW) (pounds).
g. Intermediate Axle Weight (IAW) (pounds).
h. Rear Axle Weight (RAW) (pounds).
i. Front Forward Edge (FFE).
j. Moment. The product obtained by multiplying the weight by the distance (in inches) from the RDL.

C. COMPUTATIONAL FORMULA
The computation formula illustrated on the following pages shows examples of different types of vehicles and possible loads. Prior to beginning the process, the unit movement officer must ensure the scales are calibrated.
1. Use the following formula to compute the CB location of vehicles. Multiply weight by distance of each axle from the reference line (in inches), and then divide the total results by the vehicle gross weight. The resulting figure is the number of inches to be measured aft from the reference line to the CB of the vehicle.
Center of Balance Formula Wl -- Front axle weight. W2 -- Rear axle weight. DI -- Distance from RDL to front axle or center of articulated tandem axle D2 -- Distance from RDL to rear axle or center of articulated tandem axle.
2. The vehicle CB is computed to the nearest whole inch. Any answer with a fraction of .5 or higher is increased to next higher number. If .4 or less, drop the number.

EXAMPLE 1 – Determine CB of Vehicles.
Step 1. Weigh all axles individually.
Step 2. Mark weight above each individual axle.
Step 3. Establish the RDL at the forward edge of the vehicle.
Step 4. Measure all distances from RDL to center of each individual axle.
Step 5. Distance multiplied by weight equals a moment.

Example of basic formula for determining the CB. (Dl x W1) + (D2 x W2) GROSS WEIGHT = CB from RDL
EXAMPLE ONE
D1
W1 W2
RDL D I from RDL (from forward edge) to center of front axle = 20" W1 front axle weight = 2,870 lbs D2 from RDL (from forward edge) to center of rear axle - 150" W2 rear axle weight = 2,550 lbs 20" x 2,870 = 57,400 moment 150" x 2.550 = 382,500 moment 439,900 total moment W1 (2,870 lbs) + W2 (2550 lbs) = Gross Weight (5,420 lbs) Total Moment (439,900) divided by Gross Weight = CB (81" from RDL) [(D1 (20") x W1 (2,870 lbs)] +[ (D2 (150" x W2 (2,550 lbs)] Gross Weight (5,420 lbs) = CB (81" from RDL)
EXAMPLE TWO
RDL
W1
D1
D2
W2
[D1 (15") x W1 (250 lbs)] + [D2 (102") x W2 (2,250 lbs)] W1 (250 lbs) + W2 (2,250 lbs) = Gross Weight (2,500 lbs) = CB (93" from RDL)

 

[Rick Webb]

Maybe This Will Work

http://www.transcom.mil/dtr/part-iii/dtr_part_iii_app_p.pdf

 

[shemandr]

How does one weigh each of the axles individually? Or am I missing something - again?

 

[Rick Webb]

Axle Weights

Axle weight is the weight on both wheels (left and right) on an axle

There are portable scales used to get axle weights. You'll see the Highway Patrol, DOT or whoever enforces weights on the highways around you using them. Maybe for a box of Krispy Kremes they would weigh your trailer. Truck stops have scales and if they are not busy you can get a total weight and then just the weight on the axles to figure out the tounge weight. Recycling Centers or Moving and Storage outfits often have scales as well. Ask around.

Honestly for what you are trying to do the center of keel is going to be pretty close to the center of balance and moving 15# an equal distance from the aft portion of the boat towards the bow will get you the difference in tounge weight your looking for. Knowing the actual numbers is not as important as getting to know the trailer and how it handles. There are many more variables involved than just the tounge weight, it is just one of the easiest to manipulate.

 

[Crazy Dave Condon]

Although formulas are good, Manufactures will recommend a 10% of the towing weight of trailer and boat on the tongue for safety. It is more important with trailers on one axle as it will sway badly particularly without brakes. The addition of additional axles of course depend on the load requirements but going to a second axle, yes the trailer is more stable and so on with additional axles. I have owned trailers up to three axles pulling all types of boats over the years to also include launching and retrieving and that included a hydraulic trailer too. I was taught by the designers that the first key is the tongue weight is the first consideration. I learned this very well when I was a new dealer having purchased a keel boat without brakes on one axle. In addition, no one has mentioned center of gravity either which should be taken into consideration as well.

 

[Winkfish]

One thing boaters do often around here is to take their boat to a highway weigh station. They're pretty good about this. Put your trailer and boat, loaded as normal, by itself on the scales. Then hook it to your truck and position yourself so that the trailer, not the truck, is on the scales. Then simple math to get the %.

As for adjustment, my trailer hitch is the removable kind. The ball goes down from the hitch. I can, if needed, turn the hitch over and move the ball, so that the ball goes up from the hitch. That would decrease the tongue weight. If I need to get somewhere in between, I'm sure there are other things one could do.

 

[Benny17441]

Axle position is used to balance the load on a trailer so that it sits horizontally and parallel to a level road surface. Adjustments to the tongue weight are made by raising or lowering of the ball hitch on the tow vehicle. Some have pre-measured ball drops and others use adjustable. Only if the trailer sat inclined too much backwards or forward at the prescribed tongue weight would I change the axle position.

 

[Crazy Dave Condon]

Benny,

As a former sailboat dealer having worked on too many trailers, if you have a trailer that is balanced on the axles without the 10% of the total weight leaning forward, that trailer will sway if not kept under control which would also be adversely affected without trailer brakes more notably on a single axle. the height of the hitch does not address that. It does address the forward or back portion of the trailer if too low or high.

 

[Meriachee]

I always thought that those"drop thingies" were to accommodate the trucks (like the one next to me in the lot here) where you need a step ladder to get into.
And, to further Dave's point, the height of the lift kit has nothing to do with the tongue weight either.

 

[Ross]

It must be presumed that some of the load weight over hangs the axel on both side of the axel. The axle serves as the fulcrum in a simple load/lever problem. The wheels and axle weight do not figure into the calculation of load distribution. How much of the trailer tongue extends beyond the load bearing points on the trailer? Since the boat represents a distributed load shifting movable load in the boat from forward to aft could accomplish all that is needed.

 

[Rick Webb]

Center of Balance and Center of Gravity

Not necessarily the same.

A trailer would follow the tow vehicle best when the trailer is sitting level this is usually done by using the draw bar with the appropriate lift or drop.

Raising the tounge will usually shift the center of gravity and also the center of balance aft. Conversely lowering will shift them forward. The higher the CG the more difference it can make. It would be unusual for the attitude of the trailer to make a difference though I have towed trailers where it did.