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Old 07-02-2009, 03:06 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, 05:51 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, 06:14 PM   #3
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Now THATS kool....

Keep us posted on the results....

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Old 07-02-2009, 06:51 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, 07:21 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, 07:27 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, 05:12 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, 05:34 AM   #8
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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, 05:40 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, 05:56 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, 06:02 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, 10:31 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, 07:26 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, 06:15 AM   #14
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Question

I'm not clear on the Hensley torsion bar.
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Old 07-04-2009, 08:52 AM   #15
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Hi Robert

With a conventional hitch the pivot axis for the torsion bars is tilted rearwards. This lowers the ends of the torsion bars so they have more travel before they approach the frame. In this picture the red line is the correct torsion bar axis of movement for a conventional hitch.

With a Hensley the axis for the bars is completely vertical so the bars do not have as much range of motion. On long tow vehicles such as suburbans or pickups you will not get as much weight transfer as with a conventional hitch because of this. This is not critical as a Hensley does not require weight transfer to be stable, the converging link geometry does that.
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Old 07-04-2009, 10:34 AM   #16
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Joe, call him! I think it might be enlightening....
I don't believe Andy has anything to hide, and it would be nice to have a 3rd party person do an experiment.
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Old 07-04-2009, 11:16 AM   #17
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Quote:
Originally Posted by Andrew T View Post
Hi Robert

With a conventional hitch the pivot axis for the torsion bars is tilted rearwards. This lowers the ends of the torsion bars so they have more travel before they approach the frame. In this picture the red line is the correct torsion bar axis of movement for a conventional hitch.

With a Hensley the axis for the bars is completely vertical so the bars do not have as much range of motion. On long tow vehicles such as suburbans or pickups you will not get as much weight transfer as with a conventional hitch because of this. This is not critical as a Hensley does not require weight transfer to be stable, the converging link geometry does that.
hi andrew, thanks for all the explanations.

if you don't get as much weight transfer, doesn't this change the steering angles? also, in the case of gas engines with rear wheel drive doesn't this leave a little less traction on the front wheel?

thanks, ricky
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Old 07-04-2009, 06:42 PM   #18
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Quote:
Originally Posted by Andrew T View Post
Hi Robert

With a conventional hitch the pivot axis for the torsion bars is tilted rearwards. This lowers the ends of the torsion bars so they have more travel before they approach the frame. In this picture the red line is the correct torsion bar axis of movement for a conventional hitch.

With a Hensley the axis for the bars is completely vertical so the bars do not have as much range of motion. On long tow vehicles such as suburbans or pickups you will not get as much weight transfer as with a conventional hitch because of this. This is not critical as a Hensley does not require weight transfer to be stable, the converging link geometry does that.
Thank's Andy...I was thinking torsion... as in twist.

Maybe we could call 'em bending bars. We a little thick down here.

I can see from your photo the benefit of the head angle.
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Old 07-05-2009, 08:19 AM   #19
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The question I want to answer is: do lighter spring bars (torsion bars) reduce the forces experienced at the tongue?

If 'no', then I'd like to get with Andy from CanAm (thanks for the offer) or others and start fielding some data on various rigs/setups... (the reason I'm posting here is to get some brain storming going here as data from an accelerometer is inherently difficult to analyze.) I'm sure there's more concepts/theories that can be explored- various extrapolations can be made... etc...

If 'yes', then I'd like to explore how I configure my hitch to affect proper weight distribution without punishing the trailer.

It's been my experience that I could not get adequate weight transfer to the steer axle with my 1200# equal-i-zer (based on CAT scale data).... this was part of the reason for going to 1400# hensley. With the equal-i-zer, I experienced very pronounced 'wander' in the steering, and a 'twitchy' trailer feel (at highway speed, the trailer seemed to want to 'wag' the suburban back end, very unsettling) I attributed this to inadequate weight on the steer axle. As stated by other members, there's a lot of variables here.

It seems to me, the logical approach here, would be to set a 'baseline', then make only minor/single changes to the configuration.

Here’s my gameplan:


Equipment:

a) 2005 Suburban 2500. Full gas tank. No cargo. 80# tire pressure.
b) 2008 Airstream 30’ S/O. Full fresh water holding tank. Full 30# LP tanks. Both batteries. Empty gray/black tanks. No cargo. 65# tire pressure.
c) Hensley 1,400/14,000 hitch
d) Hensley 1,000/10,000 hitch


Testing:

1. Set data collection to 180 points/minute.
2. Anchor accelerometer to tongue 1” to the rear of the coupler lock.
Capture video feed from TV license plate camera of hitch. I can use the audio capture for ‘marking’. I can also download the GPS route/time data from my GARMIN directly to spreadsheet, however, I’m not sure how the accuracy of the coordinate/time data will effect our data- at minimum, it provides a good map route of run.
3. Weigh tongue using Sherline scale.
4. Weigh Suburban. Steer/Drive axles.
5. Setup rig with 1400/14000 Hensley, maximum tension on spring bars.
6. Weigh rig. Steer/Drive/Trailer axles.
7. Run rig through predetermined course. I think a five minute run is adequate. Include a hard acceleration, slalom, smooth surface, rough surface (speed bumps?), hard brake. Mark speeds and time on audio.
8. Setup rig with 1400/14000 Hensley, completely relaxed spring bars.
9. Weigh rig. Steer/Drive/Trailer axles.
10. Run predetermined course. Match and mark speeds and times.
11. Setup rig with 1000/10000 Hensley, maximum tension on spring bars.
12. Weigh rig. Steer/Drive/Trailer axles.
13. Run course. Match and mark speeds and time.
14. Setup rig with 1000/10000 Hensley, relaxed spring bars.
15. Weigh rig. Steer/Drive/Trailer axles.
16. Run course. Match and mark speeds and time.

Analysis.
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Old 07-05-2009, 06:50 PM   #20
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Equipment:

a) 2005 Suburban 2500. Full gas tank. No cargo. 80# tire pressure.
b) 2008 Airstream 30’ 65# tire pressure.
c) Hensley 1,400/14,000 hitch
d) Hensley 1,000/10,000 hitch

Wow, those high tire pressure numbers are not what I am used to seeing.

For comparison, we have a great handling rig (no Hensley) and when towing are running 36lbs pressure in the car an 36 on the trailer (P/XL's). The ride is great too.
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