Claude B
Montreal, Canada
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Joined: 12/14/2002
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PSDE,
And i guess that with a 41' TT, you don't need a water pressure regulator ! The hose is soooooo long that the pressure drop just take care of it !!! ![[emoticon]](http://www.coastresorts.com/sharedcontent/cfb/images/wink.gif "wink") LOL
Claude
2013 Gulf Stream VISA 19ERD
VW Touareg TDI 2013
VW Passat 2012 TDI
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tluxon
Kirkland, WA
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Joined: 08/12/2001
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Ron Gratz wrote:Can you tell from the video if any or all of the wheels are caster wheels? Is the front wheel of the "TV" always at 90 degrees?
Throughout the duration of the video, I never saw any of the wheels swivel. The front wheel of the "TV" was always at right angles to the other wheels. Based on how well it resisted his substantial lateral push at the hitch, I'd have to assume that at least the wheels on the "TT" and the rear wheels of the "TV" were not caster wheels.
We're heading out for the weekend. I'll have to rejoin the discussion on Monday.
Tim
Tim -
wife Beverly & 2 boys who love camping
2002 K2500 Suburban 8.1L 4.10 Prodigy
2005 Sunnybrook 30FKS HP Dual Cam
Replaced 2000 Sunnybrook 26FK on 8/6/04
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PSDExcursion
Millstone NJ
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Joined: 08/17/2001
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Claude B wrote:PSDE,
And i guess that with a 41' TT, you don't need a water pressure regulator ! The hose is soooooo long that the pressure drop just take care of it !!! LOL
Yep, first time out I had to go to Walmart and get an extra water hose. I use the brass water pressure regulator that in much stronger than the plastic ones.
2002 Chevy Express 3500 8.1 155" WB passenger van
41 Ft 2003 Thor Citation 41-ZBSR TT w/ Hensley Arrow
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Ron Gratz
full time RVer
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Joined: 12/27/2003
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Virtual Pivot Point versus Apparent Pivot Point
It appears that the virtual pivot point and the apparent pivot point converge to the same location when the angle between TV and TT is zero.
Define:
Virtual Pivot Point (VPP) is the location at which straight lines projected through the centers of the side links will converge.
VPPmax is the location of the VPP when the angle between TV and TT is zero.
Apparent Pivot Point (APP) is the location at which the longitudinal centerline of the TT intersects the longitudinal centerline of the TV.
APPmax is the location of the APP when the angle between TV and TT is zero.
My AutoCAD models indicate that VPP and APP converge to the same point, VPPmax = APPmax, when the angle between TV and TT is zero. Also, for non-zero angles, APP never lies forward of VPPmax. Please disregard my earlier incorrect statement about the APP being at infinity in front of the TV when the angle is zero.
I hope that Tim can use his modeling software to confirm these findings when he returns. If they are correct, our task of answering how the HA really works should be simplified.
Ron
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Jack Marley
Ft Lauderdale, FL
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I thot maybe some of you younger RV's might like to hear about an early type of hitch which did the same thing as the Hensly(in a way) at less money but with some drawbacks. When we moved to FL from IA in 57(yes l957) I bought a 35' trailer and a used hitch. It had a ball and A frame which went on the trailer tongue. It had long tube which went under the car to a saddle which clamped on the differencial. Thus all the weight was on the rear car wheels but was lessened by the length of the tongue. The car thus rode normally as no extra weight was on the body. It did, of course, have some drawbacks such as extra weight and poor clearance. I don't remember any trouble with turning radius, but must have limited it. I pulled it with a '57 Chev station wagon with the l65 hp v8 and it did fine, straight xmission and didn't use over-drive.
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Ron Gratz
full time RVer
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Joined: 12/27/2003
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Modeling the Hensley's 4-Bar Linkage
Some of you have suggested that having a physical model of the HA's linkage would aid the understanding of how the hitch works. I agree. The following is a way to construct such a model.
Start with a 3' piece of lath or wood of similar dimension. Cut four pieces with lengths: 1 @ 9 1/8", 2 @ 7", and 1 @ 9 7/8". Drill a hole on the centerline of each piece 1" in from each end; i.e. holes spaced at 7 1/8, 5, 5, and 7 7/8" center-to-center.
Insert a flathead bolt, pointing down, through one hole of a 7" piece and also put the bolt through a hole of the 9 1/8" piece with a washer between the pieces. Do the same for the other 7" piece placing that bolt through the other hole in the 9 1/8" piece. Put a nut on each bolt.
Insert a flathead bolt, pointing up, through the second hole of one of the 7" pieces and place one hole of the 9 7/8" piece over the bolt with a washer between the pieces. Do the same for the other 7" piece. Put a nut on each bolt.
You now should have lath pieces in three levels with each end of a piece pinned to another piece at a level above or below. The lower level (9 1/8" piece) represents the front link of the HA which actually is the hitch head. The middle level (two 7" pieces) represent the side links. The upper level (9 7/8" piece) represents the rear link to which the TT tongue is connected and restrained from yawing relative to the link.
Glue a short piece of lath to the top of the 9 7/8" piece at its midpoint and perpendicular to it. This will represent the TT's tongue.
If you got the bolts inserted in the right directions. You should be able to hold the front link and cause the other links to rotate about their bolts.
If you project a straight line through the longitudinal center of each side link, the point of intersection of the lines defines the instantaneous center of rotation of the linkage (the Virtual Pivot Point). When the rear link is parallel to the front link, the projected lines should intersect about 52" ahead of the rear link. As you allow the linkage to turn, the VPP will move to the side and rearward.
Ron
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tluxon
Kirkland, WA
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I've updated my model to show the dimensions Ron pointed out from the patent. I've also added points that represent many of the instantaneous center projections of the two side links as they go through their hinge angle sweep.
![[image]](http://home.comcast.net/~tbluxon/HensleyLink_InstCtr.jpg)
There are many ways that engineers use 4-bar linkages (http://designcenter.ucsd.edu/resources/subject.asp?subid=13 - go about 1/2 way down the page), but most of them won't remind you a whole lot of a Hensley. I did find one use that was quite interesting to me - the mechanism used in prosthetic knees. In an abstract titled, "The Influence of Four-Bar Linkage Knees on Prosthetic Swing-Phase Floor Clearance" on the American Academy of Orthotists and Prosthetists website, there is a figure that shows a path of projected instantaneous centers much like we've discussed here.
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Ron Gratz
full time RVer
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Tim,
Thanks for the very informative plot and the excellent references.
What parameter did you vary, and in what increments, to get the different instantaneous centers?
Ron
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tluxon
Kirkland, WA
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Ron Gratz wrote:Virtual Pivot Point versus Apparent Pivot Point
It appears that the virtual pivot point and the apparent pivot point converge to the same location when the angle between TV and TT is zero.
Define:
Virtual Pivot Point (VPP) is the location at which straight lines projected through the centers of the side links will converge.
VPPmax is the location of the VPP when the angle between TV and TT is zero.
Apparent Pivot Point (APP) is the location at which the longitudinal centerline of the TT intersects the longitudinal centerline of the TV.
APPmax is the location of the APP when the angle between TV and TT is zero.
My AutoCAD models indicate that VPP and APP converge to the same point, VPPmax = APPmax, when the angle between TV and TT is zero. Also, for non-zero angles, APP never lies forward of VPPmax. Please disregard my earlier incorrect statement about the APP being at infinity in front of the TV when the angle is zero.
I hope that Tim can use his modeling software to confirm these findings when he returns. If they are correct, our task of answering how the HA really works should be simplified.
Ron
My SolidWorks model shows me that everything you say here is 100% correct except for the statement, "APP never lies forward of VPPmax". According to the model, APP and VPPmax do in fact converge to where they are equal when angle TV-TT is zero. However, at all other angles, APP is actually further forward than VPPmax. This surprised me, so as an example, I've attached an image depicting the relationship when angle TV-TT is 5 degrees.
![[image]](http://home.comcast.net/~tbluxon/HensleyLink_VPP-APP.jpg)
I guess it doesn't matter too much when it comes to sway since most dangerous sway events seem to be triggered when the angle between the TV and the TT is quite small. For me, using either the APP or the VPP for figuring the pivot point when it comes to sway prevention would both be practical.
Tim
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tluxon
Kirkland, WA
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Ron Gratz wrote:Tim,
Thanks for the very informative plot and the excellent references.
What parameter did you vary, and in what increments, to get the different instantaneous centers?
Ron
I varied the angle parameter that is "0°" in the figure. I don't remember the precise angles I used for the plotted points, but I selected ones that I thought would give a good depiction of the curve. At the top and bottom of the curve, I used almost every single degree step (i.e. 1,2,3,4,5,6,7,8...70,71,72,73).
Tim
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