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Old 08-22-2014, 03:49 PM   #1
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Hensley and ProGlide Questions

https://flic.kr/s/aHsk26K5nZ

I am considering one of these hitches for my next trailer. If one was to be hit, either being struck on the side of the trailer or TV, could the vehicles be flipped more easily since these hitches basically make the two vehicles one solid unit? Instead of just the trailer as in the attached pictures.
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Old 08-22-2014, 03:57 PM   #2
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The 2 hitches you are questioning a sufficiently alike that the hitch along would not make a difference.

Now will could the TV and trailer ever both be flipped in an accident? That question can not be answered with a simple yes or no. The dynamics of the accident would govern that and there is no way to predict those conditions.
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Old 08-22-2014, 04:10 PM   #3
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I probably didn't ask the question correctly. Certainly the dynamics of the accident will govern the end result. However, its apparent from my recent accident that even though my TV was broadsided the configuration of the Blue Ox hitch actually kept my Yukon from flipping. I wonder if the use of either of these hitches would have actually resulted in both vehicles being flipped.
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Old 08-22-2014, 04:17 PM   #4
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I do not know how you trust any answer you receive. But both of those hitches will not allow sway. As I recall you were hit in the trailer first so perhaps that hit or your quick correction would not have resulted in the trailer flaying out to the side. I doubt that you will ever know.
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Old 08-22-2014, 04:43 PM   #5
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Larry, you are correct. I am just trying to understand more of the physics involved with the Hemsley and the ProGlide. It would seem to me that since the if these hitches make the TV and TT perform as if they were one solid unit, an impact such as the one I received would be more likely to flip both vehicles. However, I would also think that there would be a tremendous peace of mid knowing that a sway situation was impossible.
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Old 08-22-2014, 04:51 PM   #6
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Pat..
Here is an offer...

If you want to tow with a ProPride, come on up to Round Rock and I will hook up our AS to my truck and let you try it..
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Old 08-22-2014, 05:08 PM   #7
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When I was towing the 2013 25FB International Serenity home from Los Angles to Phoenix with the 2007 Mercedes ML 320 CDI diesel with a Hensley hitch and car receiver modified by Can-Am in London, Ontario, I drove 55 mph. When I hit the Arizona border, the semi-trucks sped up from 65 to 80 mph. I could sense the bow wave as they came up, but the trailer did not move sideways. Note that the tires were the OEM GoodYear Marathon ST model inflated to 65 psi.

When we discovered the Mercedes numbers did not work out right when the trailer was loaded for camping, we acquired the 2012 Dodge 2500HD Cummins diesel. I never sensed any sway with the 25FB and the Hensley or the new 31' Classic with the ProPride when the big trucks blow by on the intrastate.

Immediately after both the 25FB and the Classic were in the storage unit from the selling dealership, the tires were converted to Michelin LTX (P) 235/75R15 XL tires on the 25FB using the factory aluminum wheels and Michelin LT225/75R16/E LTX M/S2 tires mounted on Aluminum SenDel T03-66655T wheels on the Classic. All the new tires were inflated to maximum side wall pressure.

Would the entire rig turn over in a similar incident to the OP, I am not sure. The 2" steel bar connecting the two vehicles will not function as a swivel hitch. However, both hitched do have some rotational movement as they each have a ball that goes into the trailer socket.
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Old 08-23-2014, 12:21 PM   #8
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Originally Posted by Pat Cassity View Post
[URL]--If one was to be hit, either being struck on the side of the trailer or TV, could the vehicles be flipped more easily since these hitches basically make the two vehicles one solid unit?--
Pat, I see no reason to think your TV could have been flipped more easily if you had been using a HA/PP hitch.
The HA/PP hitch does not make the two vehicles one solid unit -- basically or otherwise.
Just as a fifth-wheel trailer can pitch, roll, and yaw independently of its TV, a HA/PP-equipped trailer also can pitch, roll, and yaw independently of its TV.

The difference between a HA/PP and a conventional hitch is that, when the TV and TT are aligned straight ahead, the HA/PP's 4-bar linkage effectively "projects" the yaw-axis pivot point approximately 50" forward of the ball location.
The linkage only affects yaw-axis connectivity. The roll-axis and pitch-axis connectivity essentially remain the same as for a conventional hitch.

Furthermore, the forward projection of the yaw-axis pivot point changes rapidly as the yaw-axis articulation changes from zero (TV and TT aligned straight ahead).
At an articulation of about 5 degrees the forward projection of the "pivot point" is reduced to about 25", and as the articulation increases beyond 5 degrees, the "pivot point" quickly returns approximately to the location of the ball -- i.e. the HA/PP hitch then functions basically the same as a conventional hitch.

IMO, if you had been using a HA/PP hitch, the likelihood of your TV remaining upright would not have been decreased.

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Old 08-23-2014, 01:12 PM   #9
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Either way, I am of the opinion that your specific accident was a relatively rare type of accident among the relatively rare RV major accidents.

Can lightning strike twice? It can , but probably wont.

The odds say this will never happen to you ever again. Would your TV have rolled in the same accident with a PP/HA? Maybe, maybe not, but in all reality statistically it pretty much matters not for the future.

My take....
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Old 08-23-2014, 01:33 PM   #10
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Furthermore, the forward projection of the yaw-axis pivot point changes rapidly as the yaw-axis articulation changes from zero (TV and TT aligned straight ahead). At an articulation of about 5 degrees the forward projection of the "pivot point" is reduced to about 25", and as the articulation increases beyond 5 degrees, the "pivot point" quickly returns approximately to the location of the ball -- i.e. the HA/PP hitch then functions basically the same as a conventional hitch.
Ron...

I'm not trying to start anything - that statement made me curious. Is it possible/common to exceed that 5 degrees on a curve?

Thanks,
John
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Old 08-23-2014, 01:44 PM   #11
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I would say when driving at highway speed a curve that demanded more than a 5 degree differential between the truck and trailer would never happen in normal driving,,, no way.

I say not even close.
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Old 08-23-2014, 02:20 PM   #12
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One other quick question... Given that the main gain in preventing swaying with the HA/PP is by moving the pivot point forward, does it make sense to have the stinger as short as possible on a ball riding hitch? It has to be better, but is an inch or two enough to make a meaningful difference?
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Old 08-23-2014, 03:16 PM   #13
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I'm not trying to start anything - that statement made me curious. Is it possible/common to exceed that 5 degrees on a curve?
It depends on the speed. For a typical TV/TT combination, the circular path corresponding to a 5 degree articulation might have a radius of curvature on the order of 300'. That would tend to limit the speed to about 35 mph.

However, I was addressing the OP's question:
" If one was to be hit, either being struck on the side of the trailer or TV, could the vehicles be flipped more easily since these hitches basically make the two vehicles one solid unit? "

A side impact, either on the TV or TT, probably would result in a nearly instantaneous articulation of considerably more than 5 degrees.

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Old 08-23-2014, 03:32 PM   #14
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One other quick question... Given that the main gain in preventing swaying with the HA/PP is by moving the pivot point forward, does it make sense to have the stinger as short as possible on a ball riding hitch? It has to be better, but is an inch or two enough to make a meaningful difference?
On a typical TV with a HA/PP hitch, the ball might be about 75" behind the TV's rear axle.
With TV/TT aligned straight ahead, the virtual pivot point might be about 25" behind the rear axle. That means the lateral force transmitted to TV is converted to a "steering torque" (referenced to the rear axle) via a moment arm of about 25".

If the HA/PP's stinger is shortened by 1-2", the moment arm is reduced from about 25" to 23-24" -- giving about a 4-8% reduction in "steering torque".

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Old 08-23-2014, 04:11 PM   #15
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I don't own either one of these hitches, (though I think I'd like to), and so haven't inspected one of them that closely.

I take it the geometry of the 4-bar linkage is not adjustable? In other words, it's a fixed projection amount? You can't dial it in to put the pivot point where you want it?

A lot of Fifth Wheel setups I've looked at have had the hitch point actually about a foot ahead of the axle.

To me, having it behind the axle just isn't as good. Certainly, any amount you can move it forward of the ball is helpful. But I would want to have it at the axle "pumpkin" at a minimum, and preferably a foot ahead of that.

I've never tried to make a hitch, but I have read quite a few books on auto suspension design and using long arm/short arm "double wishbone" suspensions to move the roll center as low as possible. These hitches are the same science, just rotated 90 degrees.

They really are a good idea, at least to my "P.E. in Mechanical Engineering Geek Brain" way of thinking

Although, I'm still not convinced that a modern version of the "Slimp Dolly" wouldn't be a great and wonderful thing too. I know there are at least two of them being produced right now. If you had one of those with a little electric motor on it, so that when you went to park the rig, you uncouple the tow vehicle and just back it on its own with the dolly....that could be pretty handy for getting into tight spots Not to muddy the waters though...just got me thinking here.

See ya on the road,
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Old 08-23-2014, 04:19 PM   #16
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Hensley and ProGlide Questions

As far as fifth wheel installations go way back when I first started doing one ton conversions, the rule of thumb was to put the center of the king pin one inch forward of the center of the rear axle,

Over time and experience I stated inching forward.

Now I put the king pin about 3" in front of the axle center line.

This improves steering stability going down the road and it helps to minimize front end lift of the tow vehicle when starting a heavy load from a stop. ( the second factor is moot with tongue mount trailers)

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The other day I did a very quick and imprecise measurement of the Hensley on my short bed Chevy, on my setup 54" from the ball is very close to the centerline of my rear axle.
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Old 08-23-2014, 10:17 PM   #17
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I take it the geometry of the 4-bar linkage is not adjustable? In other words, it's a fixed projection amount? You can't dial it in to put the pivot point where you want it?
That's correct, the geometry of the 4-bar linkage is not adjustable, and the "projection" distance is fixed.

Based on the information I have, the centers of the linkage pivots are 7.125" apart at the front and 7.875" at the rear. The linkage bars are 5" between the pivot centers at their ends.

The virtual pivot point is at the point of convergence of lines projected through the left linkage pivot centers and the right linkage pivot centers.
The virtual pivot point lies about 52.3" forward of the linkage bars' rear pivot points when the TV and TT are aligned straight ahead.

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Old 08-24-2014, 03:18 AM   #18
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Reducing the length of the shank with a Hensley or PP does improve handling somewhat. Mainly due to increased weight transfer to the front wheels and a reduction in the effect of the movement due to shank/receiver play. The ride is definitely improved.

With a pp it is hard to move it closer unless you convert the shank to a welded one. On a Hensley the straight shank can be shortened considerably but of coarse only if the heights work out to use a straight shank.

In some cases we have added a second receiver tube below the factory one so that a straight shank could be used. Though this was more to reduce the weight of the shank for the customer the handling was a secondary benefit.

Here is a picture of a straight shank shortened to reduce overhang.
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Old 08-24-2014, 06:35 AM   #19
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Great information guys! Thanks!

J, that's very interesting what you did with the fifth wheels. I have seen some that were a foot ahead, but most were just a little ahead, just like you came up with.

Ron, thanks for the measurements.

And Andy, with the short overhang on that sedan, a 52" forward projection should put you up near the back seat somewhere. I would think that would really make a huge improvement.

Good stuff!
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Old 08-24-2014, 07:49 AM   #20
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Thanks all for you very informative comments. If my future includes another Airstream, there is no doubt I will purchase one of these two hitches. I do have one more question, does anyone know where I might be able to have one of these installed in Houston area? I prefer not to do it myself.


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