Measuring forces at the hitch

[boondockdad]

the 65# in A/S are minimum safe operating... r u thinking reducing TV PSI should be explored?

[Road Ruler]

Originally Posted by finalcutjoe

the 65# in A/S are minimum safe operating... r u thinking reducing TV PSI should be explored?

No, not at all. Your 30' is much larger than a vintage 23' and probably needs that type of high pressure tire.

[slowmover]

Originally Posted by finalcutjoe

the 65# in A/S are minimum safe operating... r u thinking reducing TV PSI should be explored?

Even my Dodge 3/4T (with a 2,500-lb max bed load) doesn't call for 80-psi at maximum, but for 70-psi according to Dodge.

For what it's worth, I found no difference -- I did experiment -- by switching to 80, but reduced after a few hundred miles of heavy Interstate traffic (a 7500-lb TT, non-independent suspension). I had the rig at rear GAWR often (many scale tickets).

What I have noticed is that tire pressures beyond load recommendations (such as driving around with 70-psi unloaded) induces faster loss of control in fast lane changes. In other words, better to jerk this stiff-sprung truck around with the recommended 50/50 pressures all around. (Also, front pressure doesn't change for any load with this IFS truck, the 4WD's might be different).

I appreciate the insight from Andy T about the H/A. I do like mine, but it will not transfer load worthy of the idea to the front end. Granted, the truck rear suspension is too stiff, but I'd have thought I'd get some movement. Now can scratch off lighter bars and modifying rear springs until another approach is made to the problem (Hitch receiver angle?).

FinalCutJoe, sure appreciate the thread. Keep Gratz on retainer and have at it!!

[boondockdad]

OK, couldn't find another hitch for our experiment, so I stole a truck. Nice, soft sprung Expedition EL...

2007 Expedition EL Limited 4x4. Four wheel independent suspension, rear coils and air shocks. Auto-leveling rear suspension. 8,800lb tow capacity. 131” wheelbase. 1/2T truck chassis.

2005 Suburban LT 2500 4x4. Solid axle front and rear. Rear leaf springs. 12,000lb tow capacity. 130” wheelbase. 3/4T truck chassis.

Here's the constant between the two trucks:
Hensley Arrow 1400/14000
30’ Airstream Classic Slide. Goodyear Marathon ST225/75R15 ‘D’ 65psi.
Wheels balanced on 9/23/2009

3-Axis accelerometer mounted to ‘A’ frame, 1” fore of the tongue jack.

Captured 4 minutes of data at 5 samples/second, over 3.3 mile distance.
Capture started on eastbound S.Huron Drive, hard acceleration up I-275 entrance ramp, maintained 65mph southbound, exiting on Carleton Rd hard decel at stop sign.

Captured 4 minutes of data of reverse lap, northbound I-275.

I-275, both directions, was relatively smooth concrete highway (not new asphalt smooth). Carleton road was pretty rough. 10mph winds.

Ran each lap twice. Each dataset was converted independently, outputs (min, max, quartile, etc..) from each lap were averaged together.

1200 samples for each lap.
Each x,y,z acceleration sample was then translated into a vectored sum.
The 1200 vectored sum units were used for analysis.
The pounds of force unit is based on Force=mass*acceleration, assuming 850lb tongue weight.

................................

2500 Suburban.
Michelin 245/75R16 ‘E’ 75psi
NO spring bar tension
3020 STEER
5240 DRIVE
7640 TRAILER
15900 GROSS
850 TONGUE

MEDIAN 9.00 m/sec^2 779 lbs
MIN 5.02 435
25% QUARTILE 8.49 735
50% QUARTILE 9.00 779
75% QUARTILE 9.55 827
MAX 14.26 1235

.................................

2500 Suburban.
MAX spring bar tension
3440 STEER
4680 DRIVE
7780 TRAILER
15900 GROSS
850 TONGUE

MEDIAN 9.15 m/sec^2 792 lbs
MIN 5.39 467
25% QUARTILE 8.62 746
50% QUARTILE 9.15 792
75% QUARTILE 9.64 835
MAX 13.99 1212

.................................

Expedition EL
Pirelli 275/55R20 40psi
MAX spring bar tension
3300 STEER
4580 DRIVE
7860 TRAILER
15740 GROSS
850 TONGUE

MEDIAN 8.89 m/sec^2 769 lbs
MIN 4.69 406
25% QUARTILE 8.36 724
50% QUARTILE 8.89 769
75% QUARTILE 9.44 818
MAX 14.00 1213

.......................

Conclusion:

Driving the two different tow vehicles back to back, the difference in ride was obvious. The Suburban was definitely a ‘stiffer’ ride than the Expedition. The Exp'y 'floated' over rough road at Carleton, whereas the Burb bounced harshly over.

Overall, the Expy’s ride was significantly softer than the Suburban (with either MAX or NO weight distribution on the 'Burb). The quartile analysis supports what I expected, but to a much smaller degree. Comparing the data- there appears to be little difference in accelerations/ forces at the tongue between the two trucks (i.e. 10-20 lbs difference). This would lead me to believe the ride quality of the tow vehicle has little impact on forces experienced on the trailer at the tongue.

At this point, I would assume the tow vehicles suspension and/or hitch rating has very little to do with translating more or less punishment to the trailer frame- it appears the trailers suspension is the principle factor here.

[2airishuman]

Originally Posted by finalcutjoe

...At this point, I would assume the tow vehicles suspension and/or hitch rating has very little to do with translating more or less punishment to the trailer frame-
it appears the trailers suspension is the principle factor here.

OUTSTANDING fc!

really good info and a compliment to hiho's gyrations on a similar theme...

but WHY only 1200 samples of data per?
_________________

no doubt u considered this but why no runs with the motion gizmo INSIDE the trailer...

like over the axles or in the REAR?

that data (especially with the 2 different mules) would also clearly support the notion that ...

"the trailer suspension is the principle factor here..."

again GREAT effort and thanks 4 risking life and limb to do this.

cheers
2air'

[BillTex]

Originally Posted by finalcutjoe

At this point, I would assume the tow vehicles suspension and/or hitch rating has very little to do with translating more or less punishment to the trailer frame- it appears the trailers suspension is the principle factor here.

Wow! Deja vu all over again!

I have been saying this for years;

http://www.airforums.com/forums/f464...tml#post532978

http://www.airforums.com/forums/f464...tml#post532716

And a few other old posts I can't find right now...

This theory of TV's beating up your AS assumes that you have no functioning suspension on your AS!
Now that may be true for some Folks...particularly Vintage.
But for most of us this is NOT the case.

Joe, thanks for finaly taking the time to disprove this old Wives tale.
Vernon did a similar experiment when he installed new axles a while back.
Your test was more comprehensive.

Thank you-a job well done!

Bill

[Inland RV Center, In]

Shop service records, differ with those test results.

Andy

[boondockdad]

2Air, Bill- it was my pleasure. We (my boys and I) had a good time with this little experiment.

I'm really surprised by the numbers. With the dramatic difference in ride, I thought for sure there would be a larger disparity. I don't pretend to know exactly what's going on here, but I think it's safe to say, what one might consider predictable or obvious, might really not be the case.

I would have liked to have made a direct comparison of the 'z-axis' acceleration data- but this would require precise speed and timing on a fixed run. If I could just get into FORD's test track over here off Oakwood Blvd....

Someone mentioned multiple sensor position/acquisition... that would be superior... mounted at tongue, over-axle, and rear for simultaneous capture- except I can only run one instance of the logging software and data acquisition peripheral on my macbook

[markdoane]

Joe,

Thanks for an outstanding analysis. I guess the laws of physics prevail.

[boondockdad]

Andy- I suppose one could argue that:

a)an Expedition EL really isn't that 'soft' of a suspension
or
b).2 meters/second^2 (17 lbs) is actually a very substantial force when discussing 5" metal frames.

[boondockdad]

this is the sensor/software/peripheral used
with my Macbook Pro:

Vernier LabPro
Accelerometers
Logger Pro 3

[Inland RV Center, In]

Originally Posted by finalcutjoe

Andy- I suppose one could argue that:

a)an Expedition EL really isn't that 'soft' of a suspension
or
b).2 meters/second^2 (17 lbs) is actually a very substantial force when discussing 5" metal frames.

Using accelerometers, I think, is a good idea. What they say then, must be converted into forces that momemtarially are applied to the A-frame.

Somewhere I think you said, something like 24000 ft/lbs of force, was applied to the A-frame.

I don't know about you, but I would not want to get hit with that kind of force.

If the forces are truly any where near that, and applied thousands of times to the A-frame, as when it's being towed, I then don't question why the fatigue failures are occuring.

Airstream says, a soft ride is necessary. I have said that for over 40 years. There are opinions that differ.

A rough ride and/or extensive impact forces, to me,will cause damages. Again, some blame the design.

I feel you are on to some answers, with more testing, and let the test results speak out.

Contrary to some, who challenge the currect designs, 30 and 40 year old Airstreams had the same fatigue problems, but were seldom towed with rugged tow vehicles, as opposed to today, where the vast majority of tow vehicles are heavy duty trucks. So the damages, obviously, are more pronounced, greater in numbers, today than in the past.

I am looking into our repair records regarding damages to the front ends, in the absence of the front hold down plate.

Perhaps, we could come up with an update kit, if indeed it would prove as an advantage.

Please keep up the excellent work, with your testing.

Andy

[2airishuman]

Originally Posted by Inland RV Center, In

...Airstream says, a soft ride is necessary...

what document?
where is this written or said ?
who says this?
and when?

got a reference 4 ALL 2 C ?
_____________

and the point here is IF softness even IS an issue,

it comes from the axles/wheels/tires and INHERENT trailer design and materials.

where >85% of the RIDE originates.

cheers
2air'

[rluhr]

finalcutjoe ... Thanks. This is very interesting info, and as far as I know, something that hasn't ever been done before (or at least, never published). I wish we could enable you to test additional vehicles, to broaden the data set. It might be possible to draw more conclusions in that case.

But even if you were willing to lend out your setup for testing on other vehicles, it seems we'd need to run the exact same road and have the same trailer to get meaningful comparative results. Darn.

[2airishuman]

not to HJ your most excellent thread fc'

and i realize the TOPIC is force at the HITCH...

but since so little of this info exists, and the efforts to acquire such data are significant...

i thought some here might find the OTHER thread...

on 'jimmy legs' n airsteams interesting...

hiho' started posting at #60 in this axle thread....

http://www.airforums.com/forums/f437...les-39688.html

good stuff for about 20 posts, then again at #134 and BIG SURPRISE at #167.

the rest is typical axle replacement beechin'

but the graphs and shock analysis and MARGINAL improvement with new axles is good info...

cheers
2air'

[boondockdad]

Again, thanks to all for continued feedback here.

A few things-

This hitch setup is about as rigid as you’re going to get. I’m a svelte 210lbs, and jumping up and down (with spring bars at MAX setting) on the orange head produces slight movement at best (but detectable on the accelerometer as slight aberrations in z-axis).

I waded into this experiment expecting to see numbers that would support the ‘overhitching’ and ‘too much vehicle’ theories. I’ve towed my 30’ Slide with a Dodge 3500 CTD, an experience that I would consider teeth rattling. My 2500 Suburban being only slightly less 'jarring', and the Expedition riding like a silver cloud (note: I intend to run our ‘track’ with the sensor in the different mules- I think quartile feedback with those numbers will give us a more understandable scale of the acceleration data)

At this point, the numbers don’t lie. The ‘harsh’ ride, that one feels inside a 'heavy-duty' tow vehicle, are NOT transferred to the trailer thru the ball and bars. Is it possible there are frequencies, oscillations, vibrations occurring that are harmful to the frame/shell of my ‘stream? Certainly. But I would wager, based on this feedback, that these forces are present whether driving a heavy-duty truck or a passenger vehicle- they’re inherent in over-the-road travel. There are those that would attempt to discredit my findings by citing my earlier misinterpretations of the data. Again, I’m not going to pretend to understand exactly ALL of what the data is trying to tell us-- It would be great if an engineer familiar with this could jump in here. (I’ve got all the data in excel, .csv, whatever is convenient for review and comment.)

But at this point- I feel very comfortable knowing that I’m towing my rig safely within manufacturer specifications- and at the same time not subjecting my 'stream to excessive abuse.

[Maryw164]

This is great info! We just bought a new(used) to us TV. 2003 Dodge 3500 CTD, 4x4, dually. We have a 31ft '72 AS. What load bars should we be using? We used 1000# when towing with our Excursion. Any help would be great.

Mary

[Inland RV Center, In]

Originally Posted by finalcutjoe

Again, thanks to all for continued feedback here.

A few things-

Your abbreviated tests, prove the point.

The tests show huge weight impacts on the A-frame.

Those impacts cause the A-frame to flex.

Flexing of any metal, leads to fatigue.

Fatigue leads to cracks and failures.

You also claim a rough ride.

Flexing, and to what degree of the A-frame and it's effect on the bottom part of the front shell, can very easily be demostrated to the naked eye.

Use a fork kift, as we do, to move trailers in and out of our building, and lift the front of the trailer about 18 inches or so, above level.

Then, without letting the jack post hit the floor, drop the front end quickly, and observe the flexing, in the lower part of the shell.

If that doesn't cause long term problems, then I have wasted 43 plus years of my life, working on Airstreams.

Use Duallies, use 4 X 4's, use 1000 to 1400 pound bars, and you will keep service departments busy repairing front ends, guaranteed.

How do we know? Because we do a lot of that work, and then spend the time teaching the owner, what was wrong and how to fix the problem.

A very strange and interesting observation. Many of those customers, in time, come back for other services, none of which experienced a repeat problem with the front end, in spite of the fact that the trailer and tow vehicle, are the same as before.

What changed?

The rigidity was softened. That was it.

If that's not convincing information, then so be it.

To this day, Airstream says "soft ride". Do they take out pages of ads to say so, of course not.

It's overwelming, to read the insistance owners have of damaging their Airstreams, by over rigging, yet ignoring those that made the changes, and so reported, on this Forums, the improvements.

Wow. Unbelievable.

Andy

[2airishuman]

Originally Posted by finalcutjoe

... (note: I intend to run our ‘track’ with the sensor in the different mules- I think quartile feedback with those numbers will give us a more understandable scale of the acceleration data)

At this point, the numbers don’t lie...[/I]

good idea fc' and u might try the track loop with the sensor INSIDE the stream too.

doesn't need to be done at the same time as the tow vehicle and if done on the SAME loop and conditions...

each tv/trailer combo would be interesting info...
_____________

i agree there are low amplitude high frequency vibes being transmitted into the unit...

from the road via the tires/axles primarily and perhaps SOME via the hitch...

your data suggests NOT VERY MUCH via the hitch/a frame, ON YOUR 30 SLIDE...

i think ONE of the issues is this...

? is it better for a stream to move UP/DOWN somewhat level on all axles while connected (adequate w/d is needed here)...

? or is better for a stream to ROCK/roll like a teeter on soft sloppy w/d with loads shifting on the front/rear axles ??

following behind i've seen firm/flat units move very little UP/down,

while soft/sloppy units have rears that go WAY UP then way down...

where's that picture of plumber's crack?
___________________

Originally Posted by Maryw164

...What load bars should we be using?...

no offense mary but THIS thread is really about measuring forces and trying to make sense of the data.

it's NOT about how to set up any ONE rig (except maybe fc's rig)

so u might wanna START a thread on your needs and questions, or READ some of the 100s of threads on towing combos...

there are several folks here towing with 1 ton drw trucks who can offer help on tires, w/d bars, hitches and so on...

check out some of those threads in the tow vehicle/hitches section, good stuff is OUT there.
___________________

cheers
2air'

[Jammer]

Originally Posted by finalcutjoe

...
Captured 4 minutes of data at 5 samples/second, over 3.3 mile distance.
Capture started on eastbound S.Huron Drive, hard acceleration up I-275 entrance ramp, maintained 65mph southbound, exiting on Carleton Rd hard decel at stop sign.

Captured 4 minutes of data of reverse lap, northbound I-275.

I-275, both directions, was relatively smooth concrete highway (not new asphalt smooth). Carleton road was pretty rough. 10mph winds.

Ran each lap twice. Each dataset was converted independently, outputs (min, max, quartile, etc..) from each lap were averaged together.

1200 samples for each lap.
Each x,y,z acceleration sample was then translated into a vectored sum.
The 1200 vectored sum units were used for analysis.
The pounds of force unit is based on Force=mass*acceleration, assuming 850lb tongue weight.

My formal education, such as it was, was in physics.

While I agree with your conclusions generally, the forces at the hitch are much more difficult to model and are greater than what you conclude. The simplest way to model the forces is to view the trailer as pivoting over the wheels. To calculate hitch forces you would then use the moment of inertia which would be the integral of the trailer mass over the relative distance from the pivot point of the axles, and divide by the distance from the measurement point to the pivot point. You end up with, approximately, half the mass of the trailer in pounds, times the acceleration in Gs (m/s^2 divided by 9.81). Beyond some transitional point dependent on the stiffness of the axle springs, this force will be reduced slightly as the trailer pivots around its center of mass (which is up and to the rear of the axle pivot point) during application of a higher load.

This is important because it means that the typical hitch forces are about five times higher than you calculate, making the 200-400 pound difference in what the bars themselves apply insignificant.

I suspect that there's much more to the story though because driving down the road shouldn't be what stresses the rivets. It's the railroad tracks and potholes. How do the forces differ then? There are competing mechanics at work. The whole point behind WD bars is to keep the rear axle suspension of the truck in an area where the spring rate is lower. In extreme cases - pothole, railroad track - the WD bars could prevent the suspension from bottoming out by providing more distance to work with.

I can understand the impact the truck suspension has. It's obvious. Any increase in axle spring stiffness or shock absorber stiffness on the downstroke is going to translate directly into more force on the towbar. The WD bars, I still can't see it, because most of the hitch motion is vertical rather than angular. Maybe with heavy WD bars the trailer gets a little more force from the bumps the front wheels of the truck goes over but it's not much.

Nonetheless I have a great deal of respect for field experience and am inclined to believe what Andy says.