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Old 07-02-2009, 04:06 PM   #1
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Measuring forces at the hitch

Discussions around here on "over-hitching" and "heavy-duty" truck suspensions coupled with our baby-soft A/S suspensions, prompted me to do some experimentation with a 3-axis accelerometer.

Here is a screen-cap of the data. This was just a quick pass to verify equipment was functioning. I also wanted some real world data to work with on finding the best way to analyze.

The +/- 5g accelerometer was velcro'd to the 'A'-frame just rear of the ball coupler.

So, all I need now are some rigs and a test track...
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Old 07-02-2009, 06:51 PM   #2
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So, all I need now are some rigs and a test track...
maybe Can Am RV? they are the only ones I have seen that do actual rig testing outside of some grad student ME efforts.

You need 2 accelerometers, one on the trailer side and one on the TV side of the ball. That would help infer the net force at the ball.

Then you could run some experiments for both road shocks and for sway. I don't know whether the 1 second time resolution would be sufficient for some of the vibration that loosens rivets, though.

It would probably also be good to put accelerometers where the A frame meets the body and at the rear of the trailer so you could, with an appropriate analysis, determine how a shock propagates down the frame and determine resonance issues.
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Old 07-02-2009, 07:14 PM   #3
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Now THATS kool....

Keep us posted on the results....

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Old 07-02-2009, 07:51 PM   #4
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I could up the samples/min, but I'm still trying to get my head around the data... I synced up the video to help- that's what you see in the screencap..

OK.. messed with the data some more...

I offset the "Z-Axis" numbers to cancel gravity's 9.8m/s^2 baseline, then recalculated the net forces using a vectored sum of all 3 axis

here's the screen shot

if I assume a 1,200 lb tongue weight, a spike of 6.23 m/s^2 (converting to inch/sec^2) calculates to approximately 24,000lb force at the tongue... is that right?
Seems like some serious force....
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Old 07-02-2009, 08:21 PM   #5
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Thumbs up 1200lb ????

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
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Old 07-02-2009, 08:27 PM   #6
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Originally Posted by ROBERT CROSS View Post
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
Absolutely... I use mine before every trip
(be sure your trailers well chocked before using... )
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Old 07-03-2009, 06:12 AM   #7
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maybe Can Am RV? they are the only ones I have seen that do actual rig testing outside of some grad student ME efforts.
Can Am, eh?
Anybody who hooks an 8600lb dry weight trailer to a pass car rated for 3600lbs, I have serious reservations about.
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Old 07-03-2009, 06:34 AM   #8
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Originally Posted by finalcutjoe View Post
Can Am, eh?
Anybody who hooks an 8600lb dry weight trailer to a pass car rated for 3600lbs, I have serious reservations about.

Amen to that!

I am interested in this data and what it exactly means.
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Old 07-03-2009, 06:40 AM   #9
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Yeah.....but it'll do 0-60 in 21 seconds.

Who am I to say...if it's in your comfort zone so-be-it.

TEHO
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Old 07-03-2009, 06:56 AM   #10
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Measuring forces at the hitch

Quote:
Originally Posted by finalcutjoe View Post
Can Am, eh?
Anybody who hooks an 8600lb dry weight trailer to a pass car rated for 3600lbs, I have serious reservations about.
Hey Joe, you are learning something about your question about hitch forces already. By reviewing the success of Can Am's set ups you can quickly realize the forces to some degree.

If you study a Can Am connection design, and the physics involved with the weight/forces as they apply to the hardware and its relationship to the TV, you can understand better the principles involves.

Look at and study a factory GM receiver. That may take about 2 seconds of casual thinking...lol.
Then look at the engineering, and build quality of a custom built and designed Can Am receiver/platform and you will continue to get answers to your topic.
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Old 07-03-2009, 07:02 AM   #11
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Originally Posted by Silvertwinkie View Post
Amen to that!

I am interested in this data and what it exactly means.
correction to my math:

Force = mass * acceleration

6.23 m/s^2= 20.44 ft/s^2 * 1,200 lb (tongue weight) = 24,528 lb/ft

the question is: how are the forces at the hitch affected with various TV suspensions and/or hitch bar ratings.

This data is based on a 2005 Chevy Suburban 2500/Hensley 1400 bars. I'd like to swap out my 1400 bars for, say 600lb- drive the loop again, check the forces. If we're spiking 6.23 m/s^2 net forces, the "over-hitching" argument is discredited (then it's on to the TV suspension debate...)

one hitch, no pun intended, is the 1400 rated hensley bars have a different end diameter/bearing size than the other ratings-- so it's not as simple as just switching bars....

anybody near detroit have a 600 (800?) rated hensley they want to advance for an afternoon?
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Old 07-03-2009, 11:31 AM   #12
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OK, I never professed to be a physics major...

keeping the original 9.8 m/s^2 as 'z' axis baseline, the peak acceleration is 16.03 m/s^2. converts to 52.59 ft/s^2 * 1,200 lbs= 63,108.00 lb/ft/s^2. then divide by 32.15 ft/s^2= 1,962.92 lbs

thanks Ron
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Old 07-03-2009, 08:26 PM   #13
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Interesting experiment.
You really should give me a call and come up with that gizmo and we will try it on some different tow vehicles. I have a perfect sideroad with frost heaves potholes and the combination of both as well as section with pavement strips. Where else are you going to find a 6 completely different tow vehicles of different types with several different suspension designs.

With a Hensley you cannot really get too much torsion bar pressure due to the lack of ball mount angle so the 600 bars will not be much if any different. In fact if the reduced torsion bar pressure causes the leafs to sit on the helpers the ride could be much rougher.

Who knows you might be surprized by those cars with 1000 lb. ratings. I don't know where you got 3600 from. Give me a call or send me an email, I would love to try that unit.

Andy
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Old 07-04-2009, 07:15 AM   #14
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Question

I'm not clear on the Hensley torsion bar.
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