timelinex

Below is a rough outline of the axles and tanks as I believe they are configured in the '21 381M. The red tanks are fuel, blue tanks water, black and gray should be obvious, and the black with the gray lines is the combined waste tank.

You could be right about adding water if the liquid migrates back to the Aux tank (the rear blue tank). Not sure if that can be avoided...a Momentum owner will have to chime in on that one.

Easiest way to add weight is putting water in the forward gray and black tanks until you're satisfied with the pin weight when the toy is in the garage.


Hope that helps somewhat...

Howard

Wow,

Thank you! That does help immensely.

Is there a repository of things like this for my model number or did you just find this one?

I made it. No end to what you can do in Photoshop.

Well thank you for taking the time! How did you find the actual locations, as your sig doesn't show you owning a 381m.

Anyways, 2 questions about your gray/black tank idea:

1. I'm guessing I have to add weight to both in similar amounts since they run perpendicular to the axles and it's probably not a good idea to make one side much heavier?

2. I know that people don't recommend filling up the fresh water tank too much on long trips as it just sloshes around and can even break the braces. Same issues with the black/grey, or are they reinforced better?

You're most welcome. As noted in the Welcome Letter, GD has allowed the moderators access to a limited number of drawings. No, we are not permitted to share them on the forum or via a private means (sorry). Each owner, if they desire something from GD related to their trailer, must call Customer Service and provide their VIN.

You might be surprised that there's already a weight difference from side-to-side on your camper. At least that is true on my 315RLTS and is the case for many rigs due to their configuration. The "easy" way to determine this is to find an RV Weigh facility and have the trailer weighed, fully loaded, by tire. https://rvweigh.com/ Note that the reliability of the scales have been questioned by many due to witnessing less-than-careful handling of the scales by the employees.

Another way was shown by Cate&Rob . His method is outlined in this Reference Material thread: https://gdrvowners.com/towing-and-hi...railer-and-how (If you haven't taken the time to browse all the Reference Material channels--there's a lot of outstanding information in them.)

Good point.

From your earlier posts an additional 540 pounds is needed on the pin to get to 3.2K. For argument's sake, assume all the water weight added goes to the pin and it is split between the two tanks. (Most of the weight will go to the pin--but probably not all of it in reality.)

A gallon of water weighs approximately 8 pounds, so 540 / 8 = 67 gallons. (Yikes!) To get to your minimum (3K) the number is 42.5 gallons. Even if split between the two tanks, that's not an insignificant amount of unconstrained liquid sloshing around. I can't vouch for whether it will (or won't) cause an issue. I doubt it, but that's an opinion. Given the option between a scary-light pin weight and risk of carrying water in tanks while towing, I'd choose the latter.

The other issue, too, is GVWR. The trailer is at 18,940 with the light pin weight. The trailer's GVWR is 20K. There's room here, but not a lot.

There's a lot of smarter folks here than yours truly. Hopefully they review all of this data and chime in with an alternate solution.

Howard

This may be insignificant, due to balance of the SxS. Looking at the floor layout, the front of the garage is over the trailer axles. Which axle on the SxS carries the most weight? I assume because you are backing it in that it has a rear engine setup? I'm not overly familiar with different configurations, but a couple hundred pounds behind the axles of the trailer could make a big difference.

I really appreciate the well thought out answers!

The SxS is 2100#, and the rear of it is ~1,200 of it. So that's why I back it in.

I see the fuel tanks are all the way in the back. This pin weight is with only 1/2 the generator tank full and the other tank completely empty! So unfortunately I can't be filling those bad boys up.

So I think the changes that will be made first seems to be:

1. Air bags, which may improve rode confort but unfortunately do the opposite of help with pin weight because it raises the trailer nose.
2. Lower the pullrite hitch to its lowest level to lower the front of the trailer
3. Reverse the pullright kingpin plate so that the ball is behind instead of in front of the kingpin. This should put the fulcrum few inches rearward and in turn lower the front whatever small amount.
4. Add 300-400lb in the front. Either thru gray/black tank filling, dead weight in the caninets, or something else similar.

See where that gets me . I do have a sherline 5k scale so maybe I will try to set it up between changes to see what each step does for other people's references.

timelinex I figured that was the case with backing it in.

The only other idea I have is to get some ballast bags to store in the front compartments while traveling. They could be emptied once you get to camp and filled again before traveling if needed. I'm thinking of the type that guy's use in wakeboats to generate a larger wake. That way you aren't concerned about the black and grey tank support and you wouldn't have to empty those before setting up camp. The obvious downside is loss of storage space.

For sure. In fact I have a wakeboard boat with those exactly bags May be a good start. But on the other hand, not sure I'd be confident they don't open and spill haha. I will have to think about it.

I was going to suggest adding a generator to the front storage compartment, but it looks like timelinex already has that covered. Another thought would be to add solar. All those lithium batteries, invertors, and controls would have to add some weight to the front.

I wouldn't put too much hope on raising or lowering the nose with air bags on the TV or moving the hitch. Weight distribution is determined by the distance from the supports raising or lowering the nose an inch or two is not going to make a significant difference in how the weight is distributed.

Looking back through the thread, one factor i didn't see or missed is how much cargo weight is left on the truck. Can the TV handle an additional 1,100+ lbs to get close to the 20% of the trailer weight?

Have you seen anyone do the testing to see how much weight lowering and raising adds to the pin? Physics dictates that lowering shoudl change the pin weight... But I haven't done the math or the actual testing to see if thats 20lb or 200lb.


I had 1,120# left on my payload. All the measurements were done with the truck fully loaded with gas,DEF, and travel things. The only thing that it didn't include is people which is <400# for my family. So after we get people inside, I have 720 lb left. You are right that I won't be able to get 20%, but that goal is pretty unreachable. I haven't seen anyone report being able to get 20%+ on the 381m with something in the garage. If I am remembering correctly, the highest I have seen someone report is 3,120#. So that would be a good goal for me and that is within my payload.

The listed length for the 381m is just under 44'. For simplicity let us say that from the king pin to the mid-axle, or center between axels (not sure if there is two or three axels) is 20'. If the king pin is lowered by 2" then the horizontal distance between mid-axle and the king pin is reduced by 0.008". Move closer to the king pin from mid-axle and this reduction reduces even further. As far as shifting weight towards the king pin the reduction in distance is pretty insignificant. Likely there will be more weight shifted by traveling up and down hills.

It looks like 720 lbs will get about 17% of the load on the pin. I run at about 18% without any problems, so hope that does the trick for you, timelinex however you decide to add the weight.

Whats the math you used to get that .008" to the right? So are you saying lowering by 2 inches may literally only change the pin weight by like a pound? I've been thinking over all my highschool and university physics, but it's been so long that I've lost most of it.


If there is such a TINY shift in weight from a a few inches up or down, why is it so bad pulling nose up vs level or nose down? I thought it was a proxy for % weight on pin?

Gravity always pulls straight down. When determining the moment arm from a support for a gravity load, use the horizontal distance. When a level beam is lifted or lowered at one end, then the beam becomes the hypotenuse of a right-angle triangle and the distance raised or lowered becomes the vertical leg. The horizontal leg can be determined using Pythagorean theorem or other trig functions. For the 20' (240") I used above the reduction in horizontal length is.

240 - (240² - 2²) ^1/2 = 0.008, all units in inches

Since the overall horizontal length of the beam does not change significantly, neither will the moment arm for load distribution change significantly.

As for not towing nose high, I've always thought this was more of a clearance issue for hills (and maybe wind resistance) than for load distribution.