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Old 12-13-2013, 01:18 PM   #141
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There are many conditions where the TT can end up pushing the TV, other than braking. Certain dips, downhill and other conditions could cause a temporary condition of the "TT pushing the TV."

Is this "misalignment" temporary? Does one have to stop and fix it?

I don't mean to open a can of worms. Just wondering.
If the ProPride slips from a brake operator problem, yes you need to stop and readjust the centering of the yoke.

If you have a severe enough event with the Hensley, it just brakes a strut rod.
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Old 12-13-2013, 01:18 PM   #142
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If they are still around they don't make them to fit any modern vehicles...
I don't know if it's the spare tire location or what...
If I could I would...
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Old 12-13-2013, 01:23 PM   #143
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Living in an agricultural area I have noticed something about farm trucks and trailers-
Some farm trailers have wheels at the front and rear like a railroad car- no tongue weight whatsoever- but could be extremely heavy when loaded- they pull them with 1/2 tons pickups-
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Old 12-13-2013, 01:43 PM   #144
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Yes, those four wheeled farm trailers were some of the first that I pulled, and I remember a couple of things about them, first you didn't drive over about 40mph, and second you did not back up.
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Old 12-13-2013, 01:48 PM   #145
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There are many conditions where the TT can end up pushing the TV, other than braking. Certain dips, downhill and other conditions could cause a temporary condition of the "TT pushing the TV."

Is this "misalignment" temporary? Does one have to stop and fix it?
With a 4-bar linkage hitch, when the TV/TT are aligned straight ahead and the TT is pushing forward against the ball, the linkage is in a state of unstable equilibrium. A small lateral force against the ball will make the ball and the rear link tend to move laterally. If there is any yaw-axis slack in the receiver/drawbar/pinbox/linkage system, the linkage will swing until the slack is taken up. Then, the linkage rotation might stop abruptly enough to be felt and/or heard. The misalignment is temporary (goes away when the TT stops pushing) unless the "bump" is severe enough to cause the frame bracket or struts to shift.

When the linkage is aligned parallel to the TV's center line, the virtual pivot point is located on the TV's longitudinal center line. If the linkage has swung enough to cause the rear unit of the hitch to shift by four degrees relative to the TV, the VPP will move rearward about 22" and laterally about 20".

If the linkage is pushing forward against the receiver when the VPP is 20" away from the TV's center line, the forward thrust from the TT will induce a yaw torque on the TV. This can result in an undesirable handling situation.

With a 4-bar hitch it's always a good idea to have a brake controller which can cause the TT brakes to "lead" the TV brakes, and always make sure the controller and brakes are working properly.

Ron
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Old 12-13-2013, 01:58 PM   #146
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Yes, those four wheeled farm trailers were some of the first that I pulled, and I remember a couple of things about them, first you didn't drive over about 40mph, and second you did not back up.
that is true- they are always driving slow and they always pull through the barn...
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Old 12-13-2013, 03:18 PM   #147
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With a 4-bar linkage hitch, when the TV/TT are aligned straight ahead and the TT is pushing forward against the ball, the linkage is in a state of unstable equilibrium. A small lateral force against the ball will make the ball and the rear link tend to move laterally. If there is any yaw-axis slack in the receiver/drawbar/pinbox/linkage system, the linkage will swing until the slack is taken up. Then, the linkage rotation might stop abruptly enough to be felt and/or heard. The misalignment is temporary (goes away when the TT stops pushing) unless the "bump" is severe enough to cause the frame bracket or struts to shift.

When the linkage is aligned parallel to the TV's center line, the virtual pivot point is located on the TV's longitudinal center line. If the linkage has swung enough to cause the rear unit of the hitch to shift by four degrees relative to the TV, the VPP will move rearward about 22" and laterally about 20".

If the linkage is pushing forward against the receiver when the VPP is 20" away from the TV's center line, the forward thrust from the TT will induce a yaw torque on the TV. This can result in an undesirable handling situation.

With a 4-bar hitch it's always a good idea to have a brake controller which can cause the TT brakes to "lead" the TV brakes, and always make sure the controller and brakes are working properly.

Ron
Thanks for this explanation. Unless I read this incorrectly, it means the operation of such a hitch is less predictable than that of a fixed pivot point hitch. Hence the term "virtual pivot point." It is only real under certain dynamics. I am not casting any aspersions, just trying to be clear on how things work.
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Old 12-13-2013, 04:35 PM   #148
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I was understanding that PullRite was out of business?
Not out of business, Steve- they're big in 5th wheel setups now and not making the under-the-bed hitch for modern trucks. I never bothered to think about a car version but odds probably aren't good there either...

I would have loved to have tried it! I even offered to pay a premium if they'd make one for my 2013 Silverado that they could use as a template for this model....I must be terribly uncompelling :-)
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Old 12-13-2013, 04:42 PM   #149
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...snip...
When the linkage is aligned parallel to the TV's center line, the virtual pivot point is located on the TV's longitudinal center line. If the linkage has swung enough to cause the rear unit of the hitch to shift by four degrees relative to the TV, the VPP will move rearward about 22" and laterally about 20".

...snip...
Ron - could this condition explain porpoising? You were SO helpful to me in evaluating scale data and hitch adjustments. I kept struggling with porpoising and we felt it was due to 1000# bars and 1100+# of tongue weight meaning not enough FALR. However, I later noticed that the hitch head was uneven (as installed by the dealer) and some kind folks here helped me figure out how to straighten it side to side (it was off by 1.5" to one side). Once I adjusted that, the porpoising went away - and that was on my way to the tire shop for the Michelins - not after those were installed. What do you think?
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Old 12-13-2013, 06:49 PM   #150
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Thanks for this explanation. Unless I read this incorrectly, it means the operation of such a hitch is less predictable than that of a fixed pivot point hitch. Hence the term "virtual pivot point." It is only real under certain dynamics. I am not casting any aspersions, just trying to be clear on how things work.
Certainly, as far as the 4-bar bump is concerned -- a pivot point projection hitch can provide some surprises of a type not encountered with a conventional hitch. However, the likelihood of a bump can be reduced if the user is forewarned and takes preventive measures.

Perhaps it would be better to refer to a "variable pivot point" (for which the acronym "VPP" also can be used) rather than a "virtual pivot point". The 4-bar linkage causes the trailer to swing about a point different from the ball coupler. The location of the VPP changes as the yaw angle between TV and TT changes.

If the linkage is centered with respect to the TV's centerline and the yoke is centered with respect to the TT's center line the VPP will be about 52" forward of the ball and should be on the TV's center line. But, it is important to recognize that the VPP does not stay at that location.

If the TV swings just 4 degrees, the VPP moves about 20" laterally and about 22" rearward. A significant amount of the "pivot point projection" distance is lost and towing forces are applied some distance from the TV's center.
If the swing angle is 9 degrees, the VPP's forward projection distance is only about 10".

However, at highway speeds, the TV/TT angle is necessarily small and the VPP remains sufficiently far forward of the ball to significantly reduce the steering torque imposed on the TV by lateral forces from the TT.

Ron
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Old 12-13-2013, 07:04 PM   #151
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Ron - could this condition explain porpoising? You were SO helpful to me in evaluating scale data and hitch adjustments. I kept struggling with porpoising and we felt it was due to 1000# bars and 1100+# of tongue weight meaning not enough FALR. However, I later noticed that the hitch head was uneven (as installed by the dealer) and some kind folks here helped me figure out how to straighten it side to side (it was off by 1.5" to one side). Once I adjusted that, the porpoising went away - and that was on my way to the tire shop for the Michelins - not after those were installed. What do you think?
A 1.5" lateral shift would cause the VPP to be about 15" off the TV's center line. I can see how that might affect steering and tire wear, but I cannot see how it might affect porpoising. Did you notice any fore-aft tugging/surging or were the porpoising motions in the pitch and heave modes?

The location of the VPP controls how yaw-axis torques and forces in a horizontal plane are applied to the TV, but moving the VPP does not change how the vertical forces and pitch-axis torques are applied.

A good experiment would be to return the hitch to the 1.5" mis-adjustment and see if the porpoising returns.

Ron
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Old 12-13-2013, 07:54 PM   #152
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...snip...
Did you notice any fore-aft tugging/surging or were the porpoising motions in the pitch and heave modes?
As I recall, it was more up/down than front/back. There were specific points on a local highway that magnified the issue so I thought it was always going to happen like that. After fixing the misalignment, I traveled on that highway to get to the tire dealer and there was almost zero porpoising! Has been that way since! So I have to assume this ha something to do with it.


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...snip...
A good experiment would be to return the hitch to the 1.5" mis-adjustment and see if the porpoising returns.
Well, you are 100% right and there's no way I'm going to do that :-)
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Old 12-13-2013, 07:56 PM   #153
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Is this a correct geometry for the type hitch in question?

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Old 12-13-2013, 09:41 PM   #154
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Is this a correct geometry for the type hitch in question?
I'm afraid you've found some incorrect information which has been confusing people for nearly ten years. The diagrams posted by Bill Basham are WRONG. I explained the error to Mr. Basham 2004 but he chose to leave the incorrect information on the internet.

The Virtual (Apparent) Pivot Point is located at the intersection of lines projected from the two side links as shown in blue below. The Virtual Pivot point is on the TV's centerline only when the TV and TT are aligned. If the TV and TT are not aligned, the VPP moves rearward and laterally as the relative yaw angle increases.



The next image shows the path of the VPP as the relative yaw angle varies from zero to the maximum of about 83 degrees. Each "x" on the path (except for the first and fifth on each side) represents an angle change of one degree.


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