Weight of a 21

[TAfraser]

Chicken!

[cpaharley2008]

I'm retired and stop thinking years ago.

[TAfraser]

Does that not-thinking feel pretty good? I'm on the verge of retirement and looking forward to that frame of mind. I can drive myself crazy virtually all the time!

[Brian B-P]

Originally Posted by TAfraser

The Anderson was installed, but the chains were not tightened for the measurements I gave. I did run it over the scale twice with the chains set two different ways, but the weight measurements went a different way then what I expected.

Tighter chains means less load on the tug's rear axle and more on both the tug's front axle and the trailer axles; that's the point of a WD system. Is that what you were expecting, and is that what you saw?

This, of course, has nothing specifically to do with the weight of an Escape 21'...

[Brian B-P]

Sorry, I didn't read all the way to this post before posting my response above:

Originally Posted by TAfraser

Using the WDH, adjusted wrong, with the trailer only on the scale, the trailer weight went down by as much as 200 lbs to around 3160#. I thought that transferring weight off the tongue would put weight on the truck's front axle and the trailer's axles. The trucks front axle weight did go up by 40 lbs.

Your understanding of WD action is correct, and the front axle change is as expected. I agree that the trailer axle weight reading change does not make sense

Originally Posted by TAfraser

While the trailer was on the scale, I loosened the chains (just checking) while watching the scale weight, and the weight of the trailer went up. The truck weight, including tongue weight, also went down. So what I saw was both the trailer and truck individual weights going down, while the front axle weight went up. The only thing that remained constant was the total weight of truck and trailer. So where did that weight go to?

As you know, it has to go somewhere. Also, the average position of all the loads must stay in the same place - the weight of the mass of the truck and trailer is in the same place. There is no physical way to transfer load from one end of the combined rig to the other, only to move it between the middle (the tug's rear axle) and the ends (the tug's front axle and the trailer axles).

I suspect that the problem arises from (necessarily) making separate measurements at each axle (instead of simultaneously) and changing the height of the coupler on a tandem-axle trailer with non-equalized suspensions. Even slight (an inch or two) changes in coupler height cause significant shifts in the sharing of load between the axles, which changes the effective axle position, which changes how much load is carried on the hitch. If you are seeing only the trailer axle load, you might not be aware of the tug rear axle load change. You don't have separate scales under each trailer axle, so you can't see if the distribution between them is changing.

Here's a possibility: you crank up the tension on the chains, the coupler/hitch rises, the tongue gets so much more load (instead of the trailer axles) that the net change on the trailer is a decrease when you expect an increase. The balance works out because the effective trailer axle load is located further back. It's just a theory, but when you try this again I suggest observing coupler/tongue height carefully.

See why I think WD systems are interesting, but I avoid them? Also, why I'm not a big fan of tandems?

[Brian B-P]

Originally Posted by azjack

The Jeep weighs 5300# with 2 pax, When it had the equipment inside the jeep it was about 5800#, we then transferred the traveling equipment (we are travelers, not campers) into the trailer, Jeep was then about 5300#.

Thanks Jack - I just wanted to make sure that I understood that correctly.

Originally Posted by azjack

The sherline reading seemed to vary with level and chocking.

It absolutely will change with level, and I suspect TAfraser (Tom?) is seeing the same effect.

With a single-axle, tongue weight is moderately reduced by an increase in tongue height (the centre of mass of the trailer pivots back closer to the axle). The same thing happens in a tandem, but is overwhelmed by the transfer of load from the leading (forward) trailer axle to the trailing (rearward) axle, because they don't share load equally. The net effect is that tandem trailer tongue weight increases noticeably with the tongue higher, and vice versa, as the effective distance from the rear axle set to the centre of mass (weight) changes. In the extreme you could lift the leading axle right off the ground, leaving you with a very rear-set axle and the tongue taking a substantially greater share of the weight of the trailer.

Look at the tandem cargo trailers on a U-Haul lot - they don't have tongue jacks and the skid on the tongue is barely in contact with the ground when lowered down. I dropped one off after a rental and after unhitching it rolled across the lot with the skid barely skimming the pavement! The effect is even more pronounced with two Torflex suspensions which are independent of each other (leaf spring axles are usually linked together with an equalizer which helps them share load).

Now chocking is different. I think if the presence of a chock changes the Sherline scale reading that the scale may be under side load, not just straight up and down. That would be bad for almost any scale, and I expect it would cause binding in a hydraulic cylinder like the Sherline scale.

[Brian B-P]

Originally Posted by TAfraser

The reason I question the Sherline's accuracy is the following: I put the trailer tongue weight on the scale and received an indication of 380#. I then placed about 200 more pounds on the tongue and it indicated 580#. I then removed the 200# and the scale only returned to about 500#. If I do it again, but only use 100#, the Sherline doesn't return at all.

This sounds like it is sticking. The hydraulic pressure in the scale must correspond exactly to the piston area (which will have very little variance) and the load on it plus or minus friction in the seal. That friction and pressure gauge error seem like the only two likely candidates for significant error, and friction seems a far more likely cause of repeatability errors.

Originally Posted by TAfraser

I also received two different measurements depending on weather or not I lowered the trailer tongue onto the scale (only by 2") or raised the scale to the tongue with my hydraulic floor jack. Both methods are recommended in the Sherline directions.

I can't see why the method would matter, but the height would - see my previous post. Does lowering the tongue onto the scale end up 2" or more lower than raising the scale to the tongue?

Two inches of tongue height doesn't sound like much, but can makes a significant difference. Try raising the scale up to the tongue, then without changing anything else, raise it a couple more times in 2" steps, and see the readings... then to be thorough (because I think you will be, Tom ) go through the same heights on the way down to average out offsets due to scale cylinder friction. You could even start with the tongue lower than normal (lower the tongue jack), to see the tongue weight on both sides of the normal towing height. I expect a significant effect of height on tongue weight.


I think the Sherline scale is a brilliantly simple and effective design, which is why I adopted the idea for my own home-built scale. It does have its limitations, and it difficult sometimes to distinguish problems with the measurement from actual changes in the situation being measured.


And we were all told towing a travel trailer was simple...

[cpaharley2008]

Originally Posted by TAfraser

Does that not-thinking feel pretty good? I'm on the verge of retirement and looking forward to that frame of mind. I can drive myself crazy virtually all the time!

Don't you mean "drive yourself virtually crazy" because the other way, "drive yourself crazy, virtually", is what happens here on the forums!!!

Retirement is good, you do one thing a day and it takes all day to do it!!

[cpaharley2008]

I knew as soon as Brian got on he could answer the question, my right side brain was not working this morning. Have to adjust for the time zones.

[Brian B-P]

Originally Posted by cpaharley2008

Don't you mean "drive yourself virtually crazy" because the other way, "drive yourself crazy, virtually", is what happens here on the forums!!!

Hmmm... but I think "virtually" was intended to apply to "all the time", not "crazy", or "drive". It's all about where the comma is located.

For those who find this amusing - it may take an especially warped mind but there are lots of us - I recommend the book Eats, Shoots & Leaves by Lynne Truss.

[cpaharley2008]

Brian, that takes all the fun out of my dangling participle!!

[TAfraser]

Brian, thank you for the well thought out explanation. I now can see that I didn’t pay close enough attention to my surroundings. I did try to keep the level of the tongue consistent, but I can’t give you a measurement as to how level it was. That, to me, says that I didn’t pay close enough attention. I also tried to make sure there were no side loads on the scale by watching its’ body very close while lowering the tongue, or raising the lift. Any kind of movement meant that the cylinder or shaft inserted into the tongue wasn’t placed quite right. I didn’t however think of the dual axle issue. Removing weight from one or the other tends to change the weight levied on the other axle and the tongue. On top of all this, I didn’t think about the chocks. Shifting weight from one axle to another brings the chocks into play which could affect side loads. Boy did I miss a lot, and boy do I have a lot more testing to do.

Can anyone answer the question about how much calculated weight I’ll be towing? Is it axle weight only since the truck is carrying the rest, or is it the trailer weight + the tongue, whereas the tongue weight figures in both carrying capacity and towing capacity?
Oh, and Brian, you got my meaning about “virtually crazy” --- I think
Thanks again, and happy testing!
Tom

[Brian B-P]

I meant to respond to this earlier, but then, how much of me can you stand at a time...

Originally Posted by TAfraser

I've got a very basic question to ask. I should already know this, but here goes. With my trailer weighing in at 3360# when connected to the truck, am I only pulling that weight, and carrying the tongue weight , for calculation purposes? I know with this setup, on this day, the axles were only carrying 3360# - without the WDH - and the truck was carrying the rest, but what was I towing, again, for calculation purposes?

That depends what you mean by "calculation purposes", but I assume that means if you are calculating the weight of the trailer to compare to the vehicle's limits.
The tug's rated trailer weight limit is for the whole trailer, including both the portion supported on the trailer's axles (3360 lb in this case), and the portion (tongue weight) supported by the truck.

A reminder... keeping in mind that the ratings (limits) for these are the same term with an R on the end:

• Gross Combined Weight {GCW} - includes the tug with everything in it and all of the trailer, including tongue weight, but don't count the tongue weight twice! This is what the tug's drivetrain is moving down the road.
• Gross Vehicle Weight (of the tug) {GVW} - includes everything supported on the tug's axles, so that includes the tongue weight (only) of the trailer. This is what the tug's structure and suspension have to handle.
• Gross Vehicle Weight (of the tug) {GVW or GTW} - all of the trailer, including tongue weight. This is what the trailer's structure has to handle. The Escape product page for the 21' says the rating for this (GVWR) is no more than 4500 pounds.

[Brian B-P]

So...

Originally Posted by TAfraser

... is it the trailer weight + the tongue, whereas the tongue weight figures in both carrying capacity and towing capacity?

Yes. Weight on the tongue hurts you in every possible way, perhaps other than stability.

[float5]

The Sherline needs to have all of the weight removed for re-weighing to expect it to work properly. It needs to reset but it may possibly work sometimes without doing that depending on what you are doing. Looks as if you did not get it reset in some instances.

Your previous weighings indicate that the Sherline is working very well. When you get a big discrepancy, it is no doubt you. Even long-time trailer owners may sometimes forget what not to do, but they know that it is them and not the scale when they have such a big discrepancy. Someone may even have stabilizers down, for instance, and all kinds of things can goof up the weighing. I am sure you know not to pick it up by the piston as that will get air into it and mess up the weights. When it seems not to work, you need to step back and look at your set-up and think about exactly what you did. We cannot tell because there are so many possibilities but that scale looks good to me. You might even have a long-time trailer owner use it to see if there is a problem with someone else.



The reason I question the Sherline's accuracy is the following: I put the trailer tongue weight on the scale and received an indication of 380#. I then placed about 200 more pounds on the tongue and it indicated 580#. I then removed the 200# and the scale only returned to about 500#. If I do it again, but only use 100#, the Sherline doesn't return at all. I also received two different measurements depending on weather or not I lowered the trailer tongue onto the scale (only by 2") or raised the scale to the tongue with my hydraulic floor jack. Both methods are recommended in the Sherline directions. The difference was over 100#. In my old line of work, (Metrology) this scale would have been either rejected, or qualified as only being accurate in one direction, using one method. This phenomenon happened using two different, brand new Sherline scales.[/QUOTE]

[Brian B-P]

Originally Posted by Floating Cloud

The Sherline needs to have all of the weight removed for re-weighing to expect it to work properly. It needs to reset but it may possibly work sometimes without doing that depending on what you are doing. Looks as if you did not get it reset in some instances.

I can see why you would always want the direction of movement of the piston approaching the measurement point to be the same (in the instructions, adding load t the scale and so compressing the piston into the bore) for repeatability despite piston seal friction, but beyond that the idea of a "reset" of this device makes no sense to me.

It's not electronic, it doesn't do anything at zero load... it's just a hydraulic cylinder with a pressure gauge. The cylinder is made with a one-square-inch cross section so a pounds per square inch gauge will display the same number as the applied force in pounds, and it has an aluminum body and and end designed for convenient application of a load, but as far as the force measurement function is concerned any hydraulic cylinder would work. When the force goes to zero the gauge reads zero, but the gauge doesn't need to go to zero to accurately show pressure.

Neither Sherline's product page for the scale, nor the Care and Tips sheet, say anything about resetting or going to zero between readings. The sheet refers to a booklet that comes with the scale, but it is not available at the promised link (http://www.sherline.com/lmbook2.pdf) so I don't know what it says.

This is my small hydraulic cylinder: 2 Ton Porta-Power Ram. I put this (I think) pressure gauge on it: 0 to 2,000 PSI Glycerine Filled Pressure Gauge. The cylinder's cross-sectional area is about 0.5 square inches so I divide the pressure reading by two, and I have a 0-1000 lb scale. It would be a 0-4000 lb scale if I used a 0-8000 psi gauge, but I'm interested in Escape-sized trailers. It reads the force on it (so the tongue weight when turned upright and stuck under the coupler), and the reading changes to match the force, with no resetting required.

What am I missing?

[Brian B-P]

If my earlier definitions list seemed nonsensical to you... it was. Darn cut-and-paste errors! Too late to fix the original, corrected version below (with the last item corrected from "tug" to "trailer" GVW:

Originally Posted by Brian B-P

• Gross Combined Weight {GCW} - includes the tug with everything in it and all of the trailer, including tongue weight, but don't count the tongue weight twice! This is what the tug's drivetrain is moving down the road.
• Gross Vehicle Weight (of the tug) {GVW} - includes everything supported on the tug's axles, so that includes the tongue weight (only) of the trailer. This is what the tug's structure and suspension have to handle.
• Gross Vehicle Weight (of the trailer) {GVW or GTW} - all of the trailer, including tongue weight. This is what the trailer's structure has to handle. The Escape product page for the 21' says the rating for this (GVWR) is no more than 4500 pounds.

[daveandsandyclink]

For the sake of information...
Who can sum up the average real world weight of the new 21? This might be beneficial to those lurking around with the thought of trading up or getting their first Escape.

[TAfraser]

Brian, I agree with you on not having to go back to zero in order to attain an accurate reading. Nowhere in the Sherline manual does it state that zeroing is a requirement. I also talked to their maintenance/testing department lead. I gave him a sequential list of my tests, their setups, etc. and they thought that the scale I had was sticking (seal resistance) because of the return inaccuracies. This new scale does basically the same thing though. I'll bet my setup, lack of control on tongue height along with my lack of understanding about dual axle and chocks, is part, or all of the problem.

Before I call them again, I plan on doing more testing. Maybe I'll take the scale into my old calibration lab and test it under a strictly controlled environment. We would know for sure how accurate the Sherline is along with any and all limitations. I'll let you know the outcome.

[TAfraser]

Dave, I think for round numbers, we can go with Reaces' figures. They were loaded up for a long trip and 4 people aboard. Reace has stated that they were around 3900 lbs I believe. My 21 weight, lightly loaded (kitchen equipment, bedding, some towels, compressor, tools, chocks, leveling blocks, 18V drill) came out to 3360 lbs, not including the tongue weight. I'm still working on the tongue weight using the Sherline scale, but haven't refined my process yet for an accurate reading that I can believe in. My most consistent indication was 380 lbs.