Ron Gratz
full time RVer
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tluxon wrote:...My SolidWorks model shows me that everything you say here is 100% correct except for the statement, "APP never lies forward of VPPmax".
Tim,
I'm sorry if my terminology was confusing. By "VPPmax", I mean the maximum forward position of the Virtual Pivot Point. The VPP varies with TV/TT articulation angle and the VPPmax is the value of VPP when the angle is zero. In your revised example, VPPmax is at 47.1" forward of the center of the rear link.
APP can lie forward of VPP; but I think it is correct to say that APP never lies forward of VPPmax.
Ron
P.S. That's another very informative plot. Please keep them coming.
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tluxon
Kirkland, WA
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Joined: 08/12/2001
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Ron Gratz wrote:tluxon wrote:...My SolidWorks model shows me that everything you say here is 100% correct except for the statement, "APP never lies forward of VPPmax".
Tim,
I'm sorry if my terminology was confusing. By "VPPmax", I mean the maximum forward position of the Virtual Pivot Point. The VPP varies with TV/TT articulation angle and the VPPmax is the value of VPP when the angle is zero. In your revised example, VPPmax is at 47.1" forward of the center of the rear link.
APP can lie forward of VPP; but I think it is correct to say that APP never lies forward of VPPmax.
Ron
P.S. That's another very informative plot. Please keep them coming.
Oops, you're right. I was thinking of VPP for each angle instead of VPPmax which doesn't change. Absolutely, neither APP or VPP are ever greater than VPPmax.
Ron, if you're interested in the Hensley video (either the whole thing or only the test model portion), I can put it on a disc and mail it to you if you want to PM me with a mailing address.
Tim
Tim -
wife Beverly & 2 boys who love camping
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tluxon
Kirkland, WA
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Jack Marley wrote: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.
Are you familiar with the Pullrite hitch? The one you describe sounds a lot more like it than a Hensley.
Tim
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Claude B
Montreal, Canada
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I remember those hitch back in the 60's, early 70's.
No, they did not look like a Pullrite. In fact they look like a big turkey wishbone and they where fix to the TV rear axle with U bolts. I know some people experience problem with their differential. The weight of the tongue applied to the diff make them bow. No weight was supported by the frame of the tow vehicule.
The pivot point was exactley like a standard hitch with WD bars etc.
Claude
2013 Gulf Stream VISA 19ERD
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Ron Gratz
full time RVer
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How the HA Transfers Force and Moment Between TT and TV
Having made good progress on defining the kinematics of the HA's 4-bar linkage, a next step might be to attempt to define how forces and moments are transferred between the trailer and the tow vehicle. Unlike a conventional ball hitch, the HA is able to transmit moment as well as force. This is critical to understanding how the HA works.
All interaction between TT and TV must take place via the two side links of the 4-bar linkage. Since these links have pins and bearings at both ends, a link can only transfer either a tension force or a compression force and the force will be in line with the centers of the pins at each end of a link. An individual link is not able to transmit a moment.
The easiest case to evaluate is the linkage configuration which exists when the TT and TV are aligned straight ahead. In this case, the two side links are oriented at equal but opposite angles relative to the longitudinal centerlines of the TT and TV. It also is practical to consider only the transfer of lateral force between TT and TV (i.e. pulling or braking forces will not be considered).
When the side links are at equal but opposite angles and only a lateral force exists, the magnitude of the axial force will be the same in both links; but one link will be in tension and the other in compression. For example, if the trailer is pushing to the left, the right link will be in compression and the left link will be in tension. There will be a longitudinal component of force pushing forward on the right front pin and an equal longitudinal force pulling rearward on the left front pin. These equal and opposite forces acting with a separation of 7 1/8" produce a counterclockwise moment on the TV. Also, there will be a lateral component of force acting toward the left on each of the front pins. These lateral forces will produce a clockwise moment on the TV. The magnitude of this clockwise moment will increase with increasing distance forward of the pins.
At some distance forward of the pins, the magnitude of the clockwise moment will equal the magnitude of the counterclockwise moment. This distance, measured along the longitudinal centerline of the TV, will define a point at which the resulting moment is zero. However, the lateral force at this point still will be equal to the lateral force on the hitch.
The distance to the point of zero moment can be determined from a free body diagram analysis of the forces in the side links. The distance also can be determined graphically by treating the forces in the side links as force vectors. At the point where the force vectors converge, there is no separation between them and there can be no moment produced by them. Since the force vectors must be co-linear with the centers of the pins, the vectors will converge at the point which previously has been defined as the Virtual Pivot Point.
If Tim concurs with this analysis, I would hope that he could produce a diagram which would do a better job of explaining .
Ron
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apljb
Vista, CA
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Joined: 02/11/2005
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Wow, after reading that, I'm even more grateful that I bought a Hensley. ![[emoticon]](http://www.coastresorts.com/sharedcontent/cfb/images/biggrin.gif "biggrin")
Seriously, this is a great thread and I'm trying hard to follow along.
John.
I have just one request, please can we make sure it gets wrapped up before page 18 ? ![[emoticon]](http://www.coastresorts.com/sharedcontent/cfb/images/awink.gif "awink")
Me (I just smile and pay the bills ">)
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DD (L-J, 15, knows she knows everything)
DD (Sarah, 9, just thinks she knows everything)
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Ron Gratz
full time RVer
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apljb wrote:Wow, after reading that, I'm even more grateful that I bought a Hensley.
You should be. It's a great hitch, and please remember I said that when we get into some of the more contentious stuff.
As for the 18 pages, I don't know why you would think anyone would let a thread go that long. This is Will's topic. Maybe he'll have better control.
Ron
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willald
NC
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Ron Gratz wrote:apljb wrote:Wow, after reading that, I'm even more grateful that I bought a Hensley.
You should be. It's a great hitch, and please remember I said that when we get into some of the more contentious stuff.
As for the 18 pages, I don't know why you would think anyone would let a thread go that long. This is Will's topic. Maybe he'll have better control.
Ron
Hehe, I don't mind if it goes 18 pages long, as long as it stays on topic, intelligent, and informative. And, so far it has, thanks to the excellent, very informative posts of both Ron and Tim. I've really enjoyed reading this, and following along. ![[emoticon]](http://www.coastresorts.com/sharedcontent/cfb/images/smile.gif "smile")
Moderator (LAdams): You think once we have somewhat 'concluded' this discussion, that it might be a good candidate, to lock and 'stick' to the top of this forum?
Will and Cheryl
2021 Newmar Baystar 3014 on F53 (7.3 V8) Chassis ("Brook")
2018 Jeep Wrangler JK ("Wilbur")
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Jack Marley
Ft Lauderdale, FL
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Claude B: I didn't suffer any diff. Problems, altho I worried about and expected it. I don't understand what you mean that the pivot point was the same as a standard hitch with weight dist. bars. Jack
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BurbMan
Indianapolis, IN
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OK, now let's see if the marketing guy can understand the engineers....
If I understand Ron and Tim correctly, the 4-bar configuration of the HA doesn't negate the lateral forces applied to the TT, but it eliminates the moment arm magnification of those forces.
So, if a 20mph crosswind hits your TT from the left, some force will be applied to the trailer, pushing it to the right. With a conventional hitch, the force applied to the TT will be transferred to the TV at the ball, which is some distance behind the TV. This force at that point is then multiplied by the distance from the ball to the TV, creating a moment arm that has the ability to influence the stability of the TV.
With the Hensley, the same lateral force is still present, and with the same magnitude, but the VPP created by the 4-bar linkage effectively moves the force forward in the TV, eliminating the leverage of the moment, and greatly increasing the TV's ability to resist the force.
How'd I do guys?
Don
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