Tongue Weight - How To Measure

[dave macrae]

This does seem like a hornets nest of a topic , I have always just used the jack on a commercial truck scale . I suppose that due to the jack being slightly further back from the ball it is slightly inaccurate .

[Jim Bennett]

There is an acceptable range for hitch weight, commonly put between 10-15% of the total trailer weight, so if you are anywhere near the middle of this range, to be a bit off with your weighing accuracy would not likely jeopardize the safety.

I know having towed lots in the past, and with some pretty heavy loads, I would much rather err towards the heavy end than the light end. Too light of a tongue weight will quickly result in out of control fish tailing, whereas as too heavy of a tongue weight can cause steering and front end traction problems with the tow, not to mention be real hard on the rear suspension. Still, it is best to avoid either situation.

[dave macrae]

I must admit the only issues I have had with tounge weight is to not have enough . Dealt with this issue with our first stick trailer. I guess. My Personel rule of thumb is a minimum of 10% . I havnt ever towed with a tow vehicle that was anywhere near capacity so too much weight has never been a issue.

[JohnB]

Originally Posted by cpaharley2008

I'm sorry, when I read you place the scale on your jack, I assumed you were lowering the trailer jack tube down on the scale, some measure this short cut way but it is only 90% exact.
So you are raising the scale on your jack up to the coupler. Is it possible the jack may have some give? can you get a cinder block or something solid upon which to place the jack?

I did a quick calculation using basic statics for a 15’ and 17’ single axle trailer.

Assumption for 15’, a tongue wt of 300 lbs at the ball.
Distance from ball to trailer jack of 10”, from axle to ball of 120”. (actual measurements from our trailer)
The weight at the jack calculates to be 327 lbs. - a 9% increase in load as measured at the ball.

Assumption for 17’, a tongue wt of 360 lb at the ball.
Distance from ball to trailer jack of 10”, from axle to ball of 144”. (assume extra 2’ length is ahead of the axle as it appears to be in photos on escape web site)
The weight at the jack calculates to be 387 lbs. - a 7.5 % increase in load as measured at the ball.

CPAHARLEY estimate of a 10% difference in load measured at ball and at trailer jack, closely agrees with these calculations.

[cpaharley2008]

Thanks for that information, John. As we used to say in the service, "close enuf for gov't work"

[float5]

Originally Posted by Jim Bennett

There is an acceptable range for hitch weight, commonly put between 10-15% of the total trailer weight, so if you are anywhere near the middle of this range, to be a bit off with your weighing accuracy would not likely jeopardize the safety.

I know having towed lots in the past, and with some pretty heavy loads, I would much rather err towards the heavy end than the light end. Too light of a tongue weight will quickly result in out of control fish tailing, whereas as too heavy of a tongue weight can cause steering and front end traction problems with the tow, not to mention be real hard on the rear suspension. Still, it is best to avoid either situation.

Exactly. What FirstJim said. For newbies, you get your total loaded weight of the trailer at a truck scale and then your tongue weight needs to be 10-15% of that as Jim gives above. When you make any significant changes to the load in the Escape, you then need another total weight or else you don't know what the tongue weight should be.

[Brian B-P]

The Sherline scale is a good design for the purpose, and if you don't mind sending a hundred bucks or so to them, I suggest buying one - it seems like a fair price for an effective and specialized piece of equipment. I didn't want to order for shipment across the border, and didn't see a need to spend that much, so I did my own. Okay, I really just wanted to play with a science project...

I bought a hydraulic body kit (hand pump and hydraulic cylinders) and picked up an extra short, small bore ("2 ton" size) cylinder plus a cheap pressure gauge to attach to it. I only use the pump to set the cylinder length - you could use the cylinder as a scale without it. The Sherline scale is just a hydraulic cylinder with a bore cross-sectional are of one square inch, so a pressure gauge marked in PSI reads the force pushing the rod in pounds. My cylinder is smaller, so I just measured it, calculated the area (it's a circle: area = pi x D squared / 4) and I read the tongue weight as pressure times area.

[Brian B-P]

The ground where you are measuring the the tongue weight needs to be level. If the whole rig is tipped back by pointing uphill, the tongue weight will be lower (just as it really is when climbing a grade) and conversely if it is tipped forward the tongue weight will be higher (just as it really is when descending a grade).

With a single-axle trailer, there is less load when the tongue is higher (trailer tipped back) and more load when the tongue is lower (trailer tipped forward)... just like being on a hill, and for essentially the same reason. The centre of the trailer's mass is above and forward of the point it's pivoting around (the wheel centre), so tilting back moves that mass up and rearward, putting it closer to the axle and further from the tongue.

A tandem-axle trailer does the same thing as the single-axle, but that effect on tongue weight is overwhelmed by a bigger factor: the axles don't share the load equally. If you lift the tongue the leading axle suspension extends (so it is less compressed and thus applying less spring force and supporting less load) while the trailing axle suspension is pushed in (so it is more compressed and thus applying more spring force and supporting more load). The effective centre of the axles' supporting force is further rearward, so more load must be carried by the tongue.

Most leaf-spring tandem suspensions have a rocker link between the leading and trailing springs which is supposed to equalize the load between axles, but this works poorly, and the tongue weight increase with tongue height (or much worse, decrease with lower tongue height) is very noticeable. Rubber torsion tandem axles are entirely independent from each other, without even an attempt at load sharing so this effect is even stronger, and thus the recommendation from the axle manufacturer (Dexter) and from conscientious trailer makers such as Escape to be careful to keep the trailer level when hitched.

If you want to know how much load your hitch is carrying, weigh the tongue with the trailer on level ground and the coupler at the height it is carried when towing.

If you want to know how much load your hitch should be carrying, and would carry if set up properly, weigh the tongue with the trailer on level ground and the trailer frame level so both suspensions are equally compressed (i.e. the trailer is level).


I realize that's a pile of text, and mostly repeating information in earlier posts, but I hope it is useful as a clear summary of the effects of "level" on tongue weight. Fire away with questions and corrections...

[Brian B-P]

Truck scales are great for getting all of the axle loads and total trailer weight, as others have described. The even work to get tongue weight, but they are not very good for that. They are typically designed to handle at least one big rig axle - which can be about ten tons - and more tyically designed for a set of up to three of those axles. They are impressively accurate for their capacity, but that means a precision of 10 kg or 20 pounds. That's a tiny fraction of a truck axle load, but a significant fraction of our tongue load... and that's if there is zero accuracy error, just limited precision. To make it even worse, most practical tongue weight measuring techniques combine two or more readings, and individual reading errors can accumulate.

I get axle and total weights at highway truck scales, and calculate tongue weight from them, but only believe the tongue weight I measure at home with a bathroom scale (my tongue weight is within the scale range so I can do that without levers) or with my hydraulic cylinder.

[gbaglo]

If the trailer ( single axel ) is level, how does the scale know that the ground below the trailer is not level?
My trailer is leveled and chocked. The ammo box I use to raise the jack and scale is actually on the road, so it is also pretty much level.
As long as the trailer is not nose up or nose down, I should get the same reading regardless of ground slope.
You're gonna force me to do it again so I can shoot pix and post them.

[float5]

Brian, I figured that the truck scales could be really off for just the reason you are giving. Our difference, however, has only been 20-40 lbs. compared to the Sherline so that doesn't seem bad. But when it comes to the bathroom scale, well, I don't like it a bit! I think I'll just stick with the Sherline.

Glenn, how unlevel are you? Did you ever take the TW at home and then get it at a campground to see the difference? If you were to do that a couple of times, you might then have a good idea of what your TW actually is at home. At least I'm assuming you would. Does that make sense?

[gbaglo]

I have to repeat. The trailer is level. The driveway is not. The scale is also pretty much on level. So, I fail to see what effect the slope of the driveway could have on the scale. Even though it is pouring rain for the next five days, I'll take photos.
And, before I got the Sherline, I tried a bathroom scale, now an ex-bathroom scale...

[Dixhiccup]

Gbaglo et all.. I've never weighed a trailer or a hitch weight but I now have 53 variations to consider before i go anywhere. Can it really matter if things are on level ground so long as everything is "levelled" or do the forces at the wheel chocks come into play?. Just thought I'd ask

[float5]

Glenn, what Brian is saying is that the trailer has to be on level ground. I believe he is correct. I get what you are saying about the trailer being level but apparently the ground has to be level also. It doesn't have to be perfectly level but at least more or less. I am not a scientist so I can't give you a good explanation for it but it's something to do with the way the stars are aligned above Canada.

[gbaglo]

Originally Posted by Dixhiccup

. Can it really matter if things are on level ground so long as everything is "levelled" or do the forces at the wheel chocks come into play?. Just thought I'd ask

I don't know about the chocks, but, I'm now a dog with a bone, and, I'm retired, so what else do I have to do?

Think about it this way though. If you have a teeter totter and the centre is on level ground, is it going to behave any different than if the centre is on a slope? One end can go up further from the ground, but whatever is sitting on that end is going to weigh the same as if it was sitting on the other end.

[Dixhiccup]

Glen. You have no idea how my iPad and a pencil have been angled trying to figure it out. Now I'll go dream teeter totters. Thanks for all the advice everyone. I still think "level" is good. Cheers

[dbailey]

Just to be clear, we're talking single-axle here, right? With a dual axle like the 19, the slope of the ground would be important.

[cpaharley2008]

Single and dual axle set up.

[Jim Bennett]

Originally Posted by Brian B-P

The ground where you are measuring the the tongue weight needs to be level. If the whole rig is tipped back by pointing uphill, the tongue weight will be lower (just as it really is when climbing a grade) and conversely if it is tipped forward the tongue weight will be higher (just as it really is when descending a grade).

In baglo's case (or any single axle), this is not true. As long as the trailer is level, the tongue weight acting straight down (towards the center of the earth) will be the same as it would on level ground.

Originally Posted by gbaglo

If the trailer ( single axel ) is level, how does the scale know that the ground below the trailer is not level?
My trailer is leveled and chocked. The ammo box I use to raise the jack and scale is actually on the road, so it is also pretty much level.
As long as the trailer is not nose up or nose down, I should get the same reading regardless of ground slope.
You're gonna force me to do it again so I can shoot pix and post them.

You know it pains me to have to agree with you, right?

[Brian B-P]

I agree that truck scale error in tongue weight may not be a big deal, but in my case 40 pounds is 13% of my tongue weight... it's all relative.

Originally Posted by gbaglo

If the trailer ( single axel ) is level, how does the scale know that the ground below the trailer is not level?

Trick question: the answer is "it doesn't", if

• the wheels are chocked (it is as if they are sitting in divots on level ground),
• the scale's base is shimmed level so it is measuring vertically, and
• the trailer is at the same angle to the horizontal as it is when in the situation you are trying to check - the towing position - which is rarely exactly level

I simplified the situation to level ground because

• if you start with the whole rig (tug and trailer) you need level ground for a reference coupler height
• Doug's original question was for a tandem axle trailer - applying Glenn's (correct) logic to this case would also require shimming under the trailer tires to get the two suspensions equally compressed (or actually in the same proportion as at the desied towing attitude)
• most people seem to have enough difficulty with this that only Baglo would want to make it more complicated

Pictures are always good, but don't bother on my account...