Fun at the Cat Scale

[garyk52]

Jack, just thought of something, when I measured using the bathroom scale, the tongue jack is 10" behind the coupler.

 

[KnottyRig]

Gary said "I don't know why my [tongue] weight is so low

I suspect this is part of it:
"(Water tank) is 9' from the center of the first axle"

Your pivot point would be center between both axles. Even then, I suspect weight doesn't shift the same with 2 axles (complex pivot) vs 1 axle, though using that center measure should get us closer.

Really interesting reading guys. Going to bookmark this one.

 

[JackandJanet]

Dang! Just lost all of what I was typing! Here goes again.

Yes Knotty, the effect of two axles is somewhat different than a single one. As more weight is placed forward, the pivot point would actually move closer to the front axle. So would weighing things with the front of the trailer lower than the rear. In both cases, the front axle would carry more of the load.

This is also why it's important that a trailer with tandem axles be towed LEVEL, so that the load is distributed evenly over both axles.

The effect on the weight measurement at the tongue should be fairly negligible though, since the "moment arms" (distance from the pivot to the center of the water tank and distance from pivot to coupler) both change by the same amount.

But, Gary, in your last post you said your jack is 10" behind the coupler and this makes me think you measured the tongue weight at the jack? If so, you are getting higher numbers than you are actually "seeing" on your TV's trailer hitch. This would also change the effect of adding water, but it would increase it, because your equation would become: 9 x 250 = W x 14.17. And now the force addition due to water is 158.8#.

This is still fairly negligible though. But to find what your TV is experiencing, you should measure the tongue weight at the coupler. It's not easy with a bathroom scale, but that's where I did it.

- Jack

 

[garyk52]

But, Gary, in your last post you said your jack is 10" behind the coupler and this makes me think you measured the tongue weight at the jack?

That's were I measured from. Got that from the photo on e-trailer.com

 

[JackandJanet]

OK. The weight you measured then is actually higher than what your TV sees. Here's the "new" equation, assuming 1 foot more from the center of your front axle to a point halfway between them and your original figure of 780# at the jack: 15.17 x 780 = W x 16. And now, "W" the tongue weight at the coupler is 740#, or just under 15% of your trailer's weight.

This analysis, by the way, is what I used to determine if moving my axle forward 3" was a good idea. My tongue weight was too high, and, my axle was much too close to the step housing of my entry door. I had only about 1/2" of space there from my tire.

Anyway, I don't think your tongue weight is too high, even though I can't resolve the change due to adding water.

- Jack

 

[garyk52]

We got 2 different things going on here.

780 tongue weight was from the cat scale subracting with wd unhooked 2680 steer axle plus 3620 drive axle minus truck weight steer axle 3060 minus drive axle 2460 gives me 780 at the cat scale
750 tongue weight was from the bathroom scale which was under the jack. I'm going to redo this to double check my numbers with it under the coupler.

I'd rather believe the Cat scale than the Bathroom scale.

 

[JackandJanet]

Gets confusing!

- Jack

 

[garyk52]

You think that's confusing look what I found on the Airstream site.

Just for giggles; using the 2k Sherline model, my 25' Safari SS sitting winterized, i.e. no water, no food, 2 full LPG tanks, no adult beverages...

Measured at the coupler:
@15.5" (lower than towing) = 680#
@19.25" (normal towing height) - 790#
@ 22" (higher than towing) - 845#

So this means when I went to the Cat scale and got a tongue weight of 780lbs with my bars unhooked and the tongue at least 3" lower that level. So now I don't have a clue what my tongue weight really is

 

[piperjim]

Take a trip to an Escapees SmartWeigh location (FL, AZ or TX) and they'll weigh each tire so you'll have weights for each individual contact point, including your tow vehicle. Scales are very accurate and calibrated annually.

You need an excuse for a trip anyway, right?

https://www.escapees.com/knowledge/smartweigh

 

[hilltool]

That makes NO sense to me. You would think the higher it got weight would decrease if it hanged at all.

 

[JackandJanet]

That makes NO sense to me. You would think the higher it got weight would decrease if it hanged at all.

I agree - this makes no sense at all. Unless:

If you raise the front end, you shift the fulcrum of the balance toward the rear axle (I'm assuming he has two axles on the Airstream.) Consider what would happen if you lifted the front enough to lift the front wheels off the ground as an extreme. Now you have a longer length of trailer supported by the Sherline, in addition to the front axle weight, so it "sees" more weight.

And, if you lowered the front enough to lift the rear wheels off the ground, you see less weight at the Sherline, because more trailer plus the weight of the rear axle is BEHIND the new fulcrum.

Now, I'm using extremes here because it sometimes helps an analysis to look at things that way. But, a twin axle fulcrum IS complex, so things are probably less pronounced. Still, raising the front means the Sherline is supporting some of the weight of the front axle and lowering it adds some of the rear axle weight to the trailer behind the front axle.

So, this is my explanation and I'm sticking with it!

And Gary, to get your tongue weight at a CAT scale, you put the WHOLE trailer on the rear scale to get its total weight. Then, hitch it without using WDH bars and weigh only the weight of the trailer axles on the rear scale. Now you can subtract the hitched weight from the total weight, OR subtract the unhitched TV weight from the hitched TV weight. Either one should give a close approximation to the tongue load.

Yes, your tongue weight might be lower than normal, but I don't think the error will be that bad.

- Jack