Bending Moments

That depends on how the weight is distributed on the trailer. Most loads on trailers are not so there is no set formula to calculate the distance you would have to move the axle. We would need a better idea of what the load is, its dimension and the location on the trailer.

Hi

Thanks but the trailer and the fixed load are one intragal unit. The load can not be moved. we can only move the axle further back. The total weight of the is 4000 lbs and it only tales 100 lb force to lift the tongue with the present axle configuration, (ie) the axle is very close to the center of gravity. By moving the axle back will transfer weight to the tongue. The question is how far to move it to transfer an additional 500 - 600 lbs.

moments must be equal at balance pt, so (assuming load is equally distributed):

112" x 100# =11200 in-lb

3900# / 11200 in-lb = 2.87" fwd of axle (cg of load)

You want 600# on the tongue, and need to know how far to move axle (d),

solve for d:

600# x (112" - d) = 3900# x (2.87" + d)

112 - d = 18.655 + 18.655d

112 = 18.655 + 19.655d

d = 4.75"

Let d = distance to move axle back.

Let b = distance from axle to c.g. of load now

Let R_h = Reaction at hitch

At present, R_h = 3900*b/112 = 100#, so b=100*112/3900 = 2.87".

After the move, R_h = 3900(b + d)/(112 + d) = 700#

Solving this equation, d = 21" so distance from hitch to axle = 112" + 21" = 133".

Check Rh = 3900(2.87 + 21)/133 = 699.95# (close enough to 700#).

Conclusion: move the axle 21" back (or 133" from hitch).

Thanks very much for your help.

Mike Sidon

So - I've gotta ask ...........

Which dimension are you going with? The 4.75" or 21"? Quite a discrepancy there.

There is another way to solve the problem without calculating b. Since R_h = 100#, for equilibrium, the other reaction, say R_axle = 3900 - 100 = 3800#. We want to move the axle so that R_h is increased by 600#, i.e. 3800d/(112 + d) = 600. Solving, we again find d = 21".

Hi Bruce

I think that the 4.75 result is flawed in the math and also possibly the formula. Both of you however came up with the same initial value for CG. In the formula he took the target weight x 112-d; I think you are correct in taking the weight desired + d as the distance to the axle needs to be increased to shift more weight to the tongue. Do you agree?

Unfortunately I may run out of frame and only be able to move the axle about 6" so using the formula you provided that would give me a total of 286 lbs of tongue weight, correct? This will help somewhat (I have been blowing tires and need to do something to reduce the load on them.)

Thanks again for your help.

Mike

Hi Mark,

I agree that a move of 6" will reduce the tire load by about 190#. It may not be enough, but I guess every little bit helps. If you could extend the frame another foot or so, it would help, but I gather that is not practical.

Best of luck.

I don't think that you can solve this analytically without knowing the weight distribution of the load behind the axle. I'd jack the trailer up and put in jack stands a few inches behind the axle centerpoint and see how much load is shifted, and make adjustments as needed. A couple of tries should get you where you need to be.

It may not be a very elegant mathmatical approach, but it will get you the answer.

Dave

If the reaction at the hitch is known to be 100# and the total weight of trailer is known to be 3900#, and the distance from hitch to axle is known, then the weight distribution doesn't matter because the location of the center of gravity of all of those weights is known precisely. Place your trust in engineering statics. They will not fail you.

Aside from the constant threat of tire blow-out, how does the trailer "pull" now?

With regards to your tires, are you using P(passenger) type tires? If so, you would be well advised to go to a LT(light truck) tire. The internal construction of the tires is different, and is designed for load rather than smoothness of ride. 1900 lb per tire is overloaded on a tire designed for at most 1500 lb in the case of P type tires. LT tires have different load ratings as in "load range C" or "load range D", each designed to carry a different range of load, depending on if they are "single" or "dual".

The point I am trying to make is that if the trailer pulls fine as is, you should look at the tires themselves as the culprit for the blowout problem. Think Ford Explorer tire blowout problems of a few years past. Those were equipped with P type tires that were overloaded/under inflated to improve the ride of the vehicle. Buyers that spec'd LT tires when they bought the truck didn't have the blowout problems.

Charlie

Thanks for the response, however, tires are 14 ply, load range G, rated for over 3000 lbs each.

Mike