Visit our Popular Forums

Starstream · 11-20-2014, 12:19 PM

As we are planning the remodel of our 31' center bath '79 Land Yacht, my mind wondered back to some of my military days of packing equipment for transport. And how each pallet or package weight had to be in the right spot for maximum weight distribution and control of what ever the transport vehicle was.

So with that in mind, how should things be arranged so proper weight is distributed forward-aft-starboard-port and center as well as center port and center starboard? Things like batteries, refrigerator, stove, the holding tanks, water heater just to name a few. Or can it be more generalized by saying X amount of weight at the axles and decrease the weight by X every foot/yard going forward and X every foot/yard going aft?

Protagonist · 11-20-2014, 01:00 PM

The most important thing is the in-motion load case. If weight distribution is fine for over-the-road movement, it should be fine for camping as well.

You have two absolute limitations. (1) Maximum gross weight; (2) Maximum tongue weight. There is also an implied limitation: (3) Maximum load rating per tire.

The goal is to get the load on each tire to be the same while the load on the tongue is about 90% of it's maximum load, for best tread wear and braking action.

You will have six significant "point" loads that can't be uniformly distributed. Fresh, black, and gray tanks, propane tanks, battery box, and spare tire. The first three should be as close to the axles as possible, and with the balance point forward of the axles if you can't get it directly over them, while the last three will all be on the tongue and account for a fair proportion of the tongue weight.

Water heater and major kitchen appliances (stove, fridge) may also be considered point loads, depending on how big and how heavy they are.

Other than these point loads, if you can uniformly distribute the other loads fore-and-aft and side-to-side, you should do okay.

Starstream · 11-20-2014, 05:25 PM

Thank you for the response.

These are the figures according the owners manual and the plate on the Airstream.

GVWR = 7100
Axle total = 4255
Factory Tongue = 545

According to the owners manual. The fresh water tank is 50gal and 25gal gray and 25gal black. One of the waste tanks is aft of the rear axle and the other waste tank and half of the fresh tank is between the two axles and finally the other half of the fresh is forward of the front axle.

Should the liquid full weight be subtracted from the axles weight before working up the tongue weight?

Is there a way to reinforce the A-frame to increase the trailers tongue weight to at least equal the tongue weight of the tow vehicle?

To create the optimum distribution (tire wear and braking of the trailer and the tow vehicle as well as fuel economy), should the weight from the front axle decrease proportionately to the maximum tongue weight? And can that same proportion be used from the rear axle to the furthest point aft? Or does the proportion from the front axle go to where the A-frame starts and a different proportion used from the A-frame starting point to the coupler?

Protagonist · 11-21-2014, 06:13 AM

Quote:

Originally Posted by Starstream

These are the figures according the owners manual and the plate on the Airstream.

GVWR = 7100
Axle total = 4255
Factory Tongue = 545

According to the owners manual. The fresh water tank is 50gal and 25gal gray and 25gal black. One of the waste tanks is aft of the rear axle and the other waste tank and half of the fresh tank is between the two axles and finally the other half of the fresh is forward of the front axle.

Should the liquid full weight be subtracted from the axles weight before working up the tongue weight?

You may never choose to tow with all tanks full at the same time, but it's best to assume that it will happen someday so that you don't end up overloading your trailer when it finally happens. A hundred gallons is about 835 pounds, which is over 10% of the gross trailer weight. I would leave the tanks exactly where they are, if you have a choice. Placement seems to be optimum for distributing the weight of loaded tanks.

Quote:

Is there a way to reinforce the A-frame to increase the trailers tongue weight to at least equal the tongue weight of the tow vehicle?

Yes. It's called a doubler plate. But you shouldn't weld on doubler plates if you can avoid it. If your frame is galvanized steel, you have to grind off the galvanizing first before welding on a doubler plate; burning zinc from the heat of the welding rods is nasty to breathe! And you have to know exactly how big of a doubler plate you need and exactly where to put it or all you'd be doing is moving the stress points away from the tongue to some less accessible part of the frame. The basic Airstream frame and shell were designed by people who know what they're doing, and while everyone makes mistakes, including people who know what they're doing, you can't go very far wrong in assuming that the trailer tongue weight limit is already just about optimum for your Airstream. You can't change one design feature (trailer tongue weight) without changing a whole lot of others.

Quote:

To create the optimum distribution (tire wear and braking of the trailer and the tow vehicle as well as fuel economy), should the weight from the front axle decrease proportionately to the maximum tongue weight?

Using your numbers, 7100 - 545 = 6555. Now, subtract out the weight of full tanks: 6555 - 835 = 5720. Now allow about 10% of the gross trailer weight to account for all of the things you carry that aren't part of the trailer: clothing, groceries, cookware, portable appliances, etc: 5720 - 710 = 5010. Round down to 5000 pounds. Divide that by 4: 5000 ÷4 = 1250 pounds. If your average weight per tire is 1250 pounds on the remodeled trailer with no "live" (moveable/portable) loads applied, that's a good target to shoot for. How you achieve that, I'll leave up to you.

Quote:

Or does the proportion from the front axle go to where the A-frame starts and a different proportion used from the A-frame starting point to the coupler?

Your questions are starting to go beyond the limits of my expertise. I haven't done an engineering analysis of the typical Airstream frame and shell, let alone yours, and I don't want to provide engineering advice based on speculation. I've tried to limit my advice to engineering common sense; I'm not willing to get more technical.