Measuring forces at the hitch

[Andrew T]

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.

[3Ms75Argosy]

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.
Marc

[richinny]

Originally Posted by Andrew T

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

[ROBERT CROSS]

Originally Posted by Andrew T

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.

[finalcutjoe]

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.

[Road Ruler]

Originally Posted by finalcutjoe

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.

[finalcutjoe]

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.

[REDNAX]

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!!

[finalcutjoe]

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;

Bucking Bronco

Bucking Bronco

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

[finalcutjoe]

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