Measuring forces at the hitch

[Inland RV Center, In]

Originally Posted by Jammer

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.

Jammer

We have at this time a 1972 31 foot Airstream in our shop, for holding tank repairs.

The front end has been repaired before, because of missing rivets.

About 15 rivets are missing from the front hold down plate. The rub rail below the front window is very loose. In chatting with the owner, about these issues, we talked about his hitch and it's rating. He said he had an Equalizer hitch with 600 torsion bars. We looked at the labels on the bars, that clearly said, 1000 pounds. I then asked him to jump up and down on the tow vehicle bumper, without the trailer being attached. The bumper moved maybe 1/2 of an inch, in spite of the fact that the he weighs at least 225 pounds.

As it proves over and over again, the front, if not more of the Airstream, "WILL" sustain damage, when the hitch torsion bars are excessively rated, and/or with a stiff tow vehicle suspension.

It's so sad, that so many Airstream owners waste money repairing the trailer, when the damage could have been totally avoided.

Andy

[Jammer]

Andy,

I'm listening.

I wonder, though, if perhaps the tow vehicle suspension doesn't have more to do with the problem than the spring ratings. Obviously, they both affect what happens when you jump up and down on the towbar or bumper, more or less equally. But, if the rear tires on the TV hit, say, a pothole, do the hitch springs make much of a difference? I'm trying to visualize it and it doesn't seem like the angle between the TV and the trailer would change much -- it should be just a vertical movement of the rear body of the truck and the trailer tongue moving.

Can you think back to any serious repairs, where the hitch springs were strong but the TV rear axle springs were soft?

[2airishuman]

Originally Posted by Jammer

... 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...

not so.

the primary reasons for using w/d bars are...

- to RESTORE steering axle loads to the UNhitched/normal load...

-and because the RECEIVERs are rated higher than 'on the ball' ONLY when w/d gear is used.
_____________

the stream is stressed by high amplitude/low frequency bumps/holes, and low amplitude/high frequency stuff (road surface)

but MOST of the load rides on the trailer axles and tires, not the tongue.

one would need to measure the htz of each type of vibe over a LONG period and assess the reaction of the metal to this...

this can't be simply computer modeled, because of the hi VARIABILITY in build characteristics for each stream...

in any one year or decade, or model change or FRAME change or SHELL change (skin/rivets/ribs/floors) that's part of the product history.

the axle manufacturer has changed, the specs have changed, the tv have changed, tires have changed and so on...

60s or 70s data (if it existed and it doesn't) would NOT apply to ANY current 'stream.

they are built VERY DIFFERENTLY now.

this has been done for example with HUMANS riding/sitting in vehicles,

such that we now know when htz is most damaging to the human spine with prolonged exposures...

the differences between exposure to HIGH amplitude and LOW amplitude stuff on the spine.
____________

and the trailer frame flexes/extends/rotates when placed on the ball or with w/d tension...

this happens when PARKED as well as with movement.

but a 38 year old trailer with missing rivets or shell/frame issues is just an old trailer that's been used.

tired axles, old frame, old skin, old rivets and road weary and connected to a variety of mules over those 4 decades.

the skin/rivets are also stressed by WIND and air pressure while towing.

ANY aluminum riveted thing that old with miles would have loose/missing rivets or other signs of fatigue.

streams don't get inspected and regularly maintained like flying machines or boats as examples.

using 500 lb w/d bars with a 1000 lb tongue is POINTLESS for w/d unless the tv is otherwise modified.
_____________

much more on this stuff is posted here.

find the "body separation" thread and see hiho's work linked in post 35 above.

cheers
2air'

[HiHoAgRV]

FCJ,
Really cool thread...
I just typed a long response...now gone into cyberspace...

I think real hitch/overhitch issues won't come from accel number but from stress gaging the A-frame.

It's easy to argue both sides of it. Big, sloppy vehicles allowing things to whip and flop things into pieces. Not much acceleration but a huge amount of energy in the low frequency, high ampliture sinusoids.

Tight suspensions that keep the ball from jumping up and down (Use a locomotive and the hitch will never bounce or move, thus never transfering any rivit popping energy into our egg like shells). But it does transfer small amplitude higher frequency energy that will cause cyclic failure.

Tight WD bars that lock stuff together and minimize flopping. Spingy Bars that don't pull on the A-frame...

As we know, the numbers tell the truth, not Cause-and-Effect speculating. I can say that new axles/Old axles, Heavy WD bars/No WD don't make much of a difference in the energy transfer from the road to the trailer.

Keep up the good work...ya got a few strain gauges and some paint stripper?

[slowmover]

Still a great thread. Joe, did you continue with this test series elsewhere?

[rmkrum]

Great stuff!


Sent from my pocket Internet using Airstream Forums

[Larry C]

Originally Posted by ROBERT CROSS

Joe,

Just ordered this two days ago, tired of guessing.

Seems like a VG product, I'll let you know.

Sherline Trailer Tongue Weight Scales

I bought a Sherline last summer.....interesting, when I weighed the tongue the first time. I found that the weight on my 2001 30' Excella was about 125 pounds LESS than what some of the "experts" on here say it should be.

That being said, the next trip, we loaded a bunch of heavy stuff behind our recliners, and got the tongue weight up to 900 pounds. The trailer towed just fine before the increase in tongue weight, and there was no difference after I increased the weight...

For whatever it's worth

Larry C

[boondockdad]

Originally Posted by slowmover

Still a great thread. Joe, did you continue with this test series elsewhere?

Thanks Slow'

Haven't touched my accelerometer gear since this experiment.
Maybe my son and I could finagle this into his science fair project...

[slowmover]

Inquiring minds want to know.

Sure beats papier- mâché volcanoes.

You know who to make it appear was doing the driving. And flying across the RR tracks with an external shot.

Boy, that would be fun.

[oledawg]

Struggling with stuff shaking to pieces inside my 31' Excella. Uses a Reese hitch and TV is a Chevy 3500 HD dually. AS tech says axles and shocks are good. 1000 # bars, 6 links. Other than trading my TV any suggestions?

[boondockdad]

Originally Posted by oledawg

Struggling with stuff shaking to pieces inside my 31' Excella. Uses a Reese hitch and TV is a Chevy 3500 HD dually. AS tech says axles and shocks are good. 1000 # bars, 6 links. Other than trading my TV any suggestions?

I guess the first question I'd ask is; what do you mean by 'stuff shaking to pieces'?

- Tires balanced? I recently discovered tire balancing is like just about every other service out there- sometimes you don't get what you pay for. Had to go back to the tire shop twice, before they got it right on my Suburban.

The other item are your torsion bars. A '94 Excella 30' is listed as 750# tongue weight. I'm surprised you need (or are getting) any weight transfer at all with a 3500 dually.

[AWCHIEF]

I actually had one name brand national tire shop tell me that trailer tires do not have to be balanced. I took my tires out of there as fast as I could and went to another shop that is used by most of the RV/trailer dealers in my area.

[HiHoAgRV]

Originally Posted by AWCHIEF

I actually had one name brand national tire shop tell me that trailer tires do not have to be balanced. I took my tires out of there as fast as I could and went to another shop that is used by most of the RV/trailer dealers in my area.

I'm known at the local tire store as 'that guy that balances his trailer tires'

[slowmover]

Leaf spring has such short range of motion in some common co figs that tire balance is something of a nonstarter

[xrvr]

I use 800 pound bars with my one ton. Very little weight transfer us needed, but a little is just enough. Sway control is more important. The ride inside is okay with these bars. I wouldn't use any heavier bars than that. Jim

[JimAspinwall]

Great test subject!!! This could be VERY VERY informative for users, hitch vendors, etc.

Thanks for thinking of this!