Ron Gratz
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Stressor wrote:...Only the lower linkage moves, all moment is transferred through the linkage, and if the linkage greatly resists yaw, it effectively prevents the trailer from yawing.
Milt, at the risk of appearing obtuse, I don't understand this statement at all.
What do you mean by the "lower linkage"? I do understand how moment is transferred through the linkage; but, how does the linkage prevent the trailer from yawing?
Ron
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willald
NC
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tluxon wrote:Will, do you still have your conventional hitch drawbar and ball? That measurement from the rear axle would also be useful, but if you'd like to just estimate the distance from the rear axle to the receiver's drawbar pin hole, that'd be good to. I could just add the distance of my conventional drawbar to that for a good approximation.
Sorry, I dont have my conventional hitch drawbar anymore. It got sold, after the Hensley came along. But, I did get the measurements you wanted. I went out this morning, and did some measuring on the Excursion, Hensley drawbar, and the Citation (trailer). Here are some real-world measurements we can use:
Excursion measurements:
Excursion Rear axle (center) to receiver's drawbar pinhole: 48"
Drawbar measurements:
drawbar pinhole to drawbar plate that goes against hensley head: 11.5"
Trailer/Hensley measurements:
Hensley head opening, to trailer ball hitch: 12"
Center of trailer axles, to trailer hitch ball: aprx. 22'
Sooooo, for the purposes of our discussion, the distance from the rear axle to the trailer ball (with the Hensley & Excursion) is 48 + 11.5 + 12 = 71.5"
Note also, that the trailer hitch ball is at about the same place, as the rear linkage bar of the Hensley. That being the case, I think we can conclude that with the Excursion, the Hensley projects the pivot point to 71.5 - 47 = 24.5" behind the rear axle. That is, if the 47" we came up with earlier was measured from the rear linkage bar of the Hensley. If it was measured from the front linkage, then my original estimation of 20" was very accurate (right?)
Anyway, Tim: See what you come up with using these numbers, I know they are pretty accurate. I'll be curious to see the difference in 'moment' calculations we come up with from Tim's model, using these numbers. ![[emoticon]](http://www.coastresorts.com/sharedcontent/cfb/images/smile.gif "smile")
Will and Cheryl
2021 Newmar Baystar 3014 on F53 (7.3 V8) Chassis ("Brook")
2018 Jeep Wrangler JK ("Wilbur")
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Ron Gratz
full time RVer
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willald wrote:...Anyway, Tim: See what you come up with using these numbers, I know they are pretty accurate. I'll be curious to see the difference in 'moment' calculations we come up with from Tim's model, using these numbers.
Will,
Following are the results of my calculations using your numbers. Perhaps Tim can verify the values after the weekend.
RESULTS FOR WILL'S RIG WITH "CONVENTIONAL" HITCH
(assumes ball would be 12" forward of HA ball location)
Assumed Moment About TT Axles Center... 1000 ft-lbs
Distance TT Axles to Ball Coupler.................. 22 feet
Distance TV Rear Axle to Ball Coupler......... 59.5 inches
Distance Ball Coupler to Pivot Point................. 0 inches
Distance Pivot Point to TV Rear Axle........... 59.5 inches
Lateral Force Acting at Ball Coupler................ 45 lbs
Lateral Force Acting at Pivot Point.................. 45 lbs
"Steering Moment" about TV Rear Axle.......... 225 ft-lbs
RESULTS FOR WILL'S RIG WITH HENSLEY ARROW
Assumed Moment About TT Axles Center... 1000 ft-lbs
Distance TT Axles to Ball Coupler.................. 22 feet
Distance TV Rear Axle to Ball Coupler......... 71.5 inches
Distance Ball Coupler to Pivot Point............. 47.1 inches
Distance Pivot Point to TV Rear Axle........... 24.4 inches
Lateral Force Acting at Ball Coupler................ 39 lbs
Lateral Force Acting at Pivot Point.................. 39 lbs
"Steering Moment" about TV Rear Axle.......... 78 ft-lbs
HA Force / Conventional Force.......... 85%
HA Lever Arm / Conv. Lever Arm........ 41%
HA Moment / Conventional Moment... 35%
The calculations show that the HA would reduce the "steering moment" to 35% of the value for a "conventional" hitch.
Ron
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willald
NC
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Thanks, Ron. Wow - 225 ft lbs of 'moment' on the truck, down to 78 ft-lbs with the Hensley. Thats pretty amazing.
You know, this kind of has me thinking: Suppose I took the truck to a welder/'chop shop', and had them work it over. Specifically, suppose I had them move the rear axle backward, such that the distance from the rear axle to the ball hitch (with a conventional hitch) was about the same as the Hensley projects it to now - about 20". This would require the rear axle to be practically right up against the rear bumper and may well be impossible to do, but still, lets assume it IS done.
Would that mean, with the axles moved back that far, that the rig would handle essentially the SAME with a conventional hitch, as we do now with the Hensley? I mean - the 'moment' on the TV would be the same, right?
If I'm understanding this right, I think the answer is yes, it would handle the same. That being the case, it brings up a point that is really kinda upsetting - the ONLY reason we have to have a $3000 Hensley to make our rig stable, is because of the rear overhang the Excursion (and nearly every other tow vehicle) has.
Sheez, why can't they make tow vehicles with little or no rear overhang, and the back axle right up against the back end? Would sure make for a much better tow vehicle! Only vehicle I can think of built that way, would be the Hummer. Too bad the rest of that Hummer isn't very well designed for towing.
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BurbMan
Indianapolis, IN
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Sorry I've been away from the thread for a while, darn work always gets in the way....
Ron, I like your example (back on page 8) about suspending a semi tractor to illustrate that the trailer will swing like a pendulum around its hitch point. I also understand that if you hung my Suburban vertical, my trailer would swing around it's VPP. [[i]Interesting factoid about clocks being able to use shorter pendulua with 4-bar linkages.[/i]]
NOW.....if we turn these examples upside down (literally), and hold the semi trailer in the air, the tractor would still swing like a pendulum around the hitch point. BUT, if we hold my trailer in the air, would the HA let my Suburban swing like a pendulum?
Logic tells me NO, since the VPP is within the mass of the truck (not above it) it can't swing like a pendulum, it would have to rotate around the VPP, which it can't do because gravity is pulling the truck down, and the connection to the trailer is stopping the rotation.
I think this illustrates further how the HA contributes to stability by enabling the TV to resist forces imposed by the TT.
Note: Due to invalid formatting, all formatting has been ignored.
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2lMan
Brewer, ME
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Wow, this has been pretty technical reading in many instances, but a great discussion on how this hitch really works.
I will try to add a couple of points as well. Keep in mind, that I am a new Hensley owner, but have pulled with it about 2000 miles.
If everyone's discussion about the VPP is correct, and that the Hensley projects the VPP to an area about 20 inches behind the rear axle, wouldn't a Pull-Rite, or a fiver, allow less reaction from the TT to the TV, due to the fact that both a Pull-Rite, and a fiver, place their pivot point at the rear axle. That 20 inches has to still allow some lever action for movement, compared to the Pull-Rite or fiver. Still, no tests that I have ever seen will say that a TT equipped with a Pull-Rite or a fiver exibit less sway or reaction than a Hensley. In most instances, they are equal, or perhaps I should say, "Perceptibly Equal." Maybe that 20 inches of difference is reduced enough from the normal hitch distance to pivot point to make all of the difference?
Also, I don't have all of the technical terms for what I observe, but I will try to share my observations. With the linkages in the Hensley, The hitch almost has to pivot slightly backwards to make its pivot sideways, to allow turns. When the TV is pulling the trailer, the trailer, due to inertia, will try to resist forward movement, making the tendency of the TT to stay in place (or essentially backwards from the moving TV). This allows a natural backwards movement of the pivot in the black and orange parts of the Hensley.
Now, if the TT is to infulence movement on the TV, It must actually move forward relative to the TV. Now, since TTs are not equipped with engines, the only way that it can move forward is due to the TV stopping quickly, or when going downhill, if the TT tends to gain speed quicker than the TV.
Anyone with a Hensley Hitch knows, and is told in the manual, to set the boost higher on your brake control to prevent this forward (and sideways) movement of the TT on the hitch. I suppose the same thing could happen going down a steep grade if you were to decelerate, and engine braking was holding the TV back without the trailer brakes, although I have never heard of this being an issue. Perhaps this is because the differences in inertia between TV and TT are not great enough going downhill, and these differences only come in to play when the brakes on a TV are applied without the TT brakes coming on strong enough.
Perhaps an illustration could be used similar to a motorcross bike swingarm linkage type rear suspension. When the rear tire hits a bump, the arc of the swingarm is the same as the arc of the bump, which allows the swing arm to rise in an arc over the bump which has an arc going in the same direction.
Now, think of that motorcycle going very fast in reverse, hitting the same bump. The arc of the bump is going to be exactly the opposite of the arc of the swingarm, causing that swingarm to resist its movement. On a small bump (small force), if the motorcycle is going fast enough, the swing arm will still move, because the forces will overcome. If you hit a larger bump (more arc), the swingarm will tend to resist movement, due to the fact that the arcs are going in opposite direction. Oh, the swingarm will still move, but it would require much more force to do so (read greater speed, which will result in more force).
OK, now picture hitting a bump which has a 90 degree angle (straight up and down). If your motorcycle is going forward, the rear axle would still tend to go up and over it (it would be a rough ride, but it would tend to go over it.) The front might have an issue, but that is a different story. Now, if you hit the same bump in reverse, the wheel would tend to not go over it, due to the arcs being in opposite directions. Again, if going fast enough, the wheel might rise and go over this "bump", but after transmitting significant forces to the machine (probably would break something) and the rider (again, probably would break something).
I once had an ATV with a front swingarm suspension, and I noticed this tendency first hand.
So, perhaps the TT will resist the movement against the arc of the links in the same manner? The TT could force the links to turn, but it would require much more force than is routinely put out by a TT acting on a TV.
Maybe this is all manifested by the discussion on VPP and the like? Maybe this is the actual engineering explanation of what I have just attempted to explain? Could be. I am not an engineering student, nor do I even claim to be close to one. But, I do try to visualize things.
I hope I have been able to add to this enjoyable discussion, and hope that my response was not totally stupid. Carry on.
* This post was
edited 05/29/05 04:44am by an administrator/moderator *
2003 GMC Yukon XL 2500 8.1L 4.10 Axle - 2002 Sprinter Quad Bunkhouse
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2lMan
Brewer, ME
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I just had another thought...
With all of this talk about virtual pivot points, and hanging trucks up to illustrate this, I can see why the fiver would rotate at its pivot point, as there is a mechanical link which will allow it to pivot.
Try hanging my Yukon up like this, with the Hensley, and it would be impossible for it to swing on anything other than at the hitch. The virtual pivot point would not matter, as there is no place for it to pivot physically there.
Now, the question is, WOULD it pivot at the Hensley Hitch point?
My guess would be yes, because of what I said about linkages above. It obviously is not going to pivot at a point about 20 inches behind the rear axle, because, as mentioned, there is no physical place for it to pivot there, VPP or not.
This example assumes hanging my Yukon up by the front bumper, and applying swinging forces there.
A TT with a conventional hitch would swing easilly at the ball, Hung either way.
A TT with a friction control would swing at the ball, but tend to resist the movement, hung up either way.
The same is true with a dual cam.
The Hensley would swing freely, without noise or resistance from the hitch. And, it would swing at the location of the linkage, or where the black part meets the orange part.
If you hung it upside down, as mentioned by Burbman, it would tend to not pivot where the hitch is, but to swing as one unit, quite possibly due to the linkage discussion that I had above.
Therefore, I would maintain that perhaps it IS more about the linkages actually prohibiting (or preventing due to the force that would be required) the TT from initiating a pivot at the hitch point.
Really think about it... If the TT can not move the TV as a lever, it has to move the entire TV, across all four wheels. This requires a huge amount of force to do.
A fiver would actually have a better chance of swaying, because the fiver CAN act on the TV (nothing prevents this). Yes, it does have to move all four wheels, and it would take an incredible amount of force to do this. But, (I do not have calculations for this), I think it would take even more force to have the TT react on the TV, with a Hensley, due to the fact that the TT would have to exert even more force to overcome the linkages in the Hensley, and make a way for it to pivot.
Since the connection is "virtually" solid, couldn't it be considered that the "virtual pivot point" is non-existant, due to the excessive force that would be required to cause the Hensley to pivot at its hitch point when being reacted upon by the TT?
I hope I haven't totally butchered this discussion, as I am not an engineer, but simply trying to visualize in my mind how this concept works.
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Ron Gratz
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BurbMan wrote:... NOW.....if we turn these examples upside down (literally), and hold the semi trailer in the air, the tractor would still swing like a pendulum around the hitch point. BUT, if we hold my trailer in the air, would the HA let my Suburban swing like a pendulum?
Logic tells me NO, since the VPP is within the mass of the truck (not above it) it can't swing like a pendulum, it would have to rotate around the VPP, which it can't do because gravity is pulling the truck down, and the connection to the trailer is stopping the rotation. ...
Don, the instantaneous center of rotation (virtual pivot point) for each bar of a 4-bar linkage lies at the point of convergence of the two bars which are attached to it. This means that for the HA, the VPP for the rear bar (the rear, upper portion) and the VPP for the front bar (the front, lower portion) are at the same point.
If you hold your TT in the air, the HA will let the truck swing about the VPP. In this case, the HA's rear unit which is fixed to the TT does not move. However, the HA's front unit which is fixed to the TV is free to simultaneously rotate and move side to side as the TV swings about the VPP. The difference now is that, since since the VPP now is between the HA's front unit and the front of the TV, the front of the TV will move left when the HA's front unit moves right.
If any one bar of a 4-bar linkage is fixed in space, all of the other three bars still are able to simultaneously translate and rotate. Therefore, with the HA, it makes no difference whether you restrain the front unit or the rear unit. The other three bars are free to move. These kinematics are much easier to understand if one constructs a model of the HA's 4-bar linkage and observes how the bars move relative to one another.
Ron
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Ron Gratz
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MacMan wrote:... ...That 20 inches has to still allow some lever action for movement, compared to the Pull-Rite or fiver. Still, no tests that I have ever seen will say that a TT equipped with a Pull-Rite or a fiver exibit less sway or reaction than a Hensley. In most instances, they are equal, or perhaps I should say, "Perceptibly Equal." Maybe that 20 inches of difference is reduced enough from the normal hitch distance to pivot point to make all of the difference?
This is an excellent point. It is quite possible that, once the "steering moment" induced by the TT on the TV falls below a certain threshold, sway will appear to be eliminated whether the lever arm in 24" or 12" or zero. However, if unexpectedly large forces on the TT cause the moment to be above the threshold, then sway might occur.
...With the linkages in the Hensley, The hitch almost has to pivot slightly backwards to make its pivot sideways, to allow turns.
The ball is at is most rearward position when the TT and TV are aligned straight ahead. Any turning action will cause the ball to move to the side (up to 5") and slightly forward as the TT swings.
...When the TV is pulling the trailer, the trailer, due to inertia, will try to resist forward movement, making the tendency of the TT to stay in place (or essentially backwards from the moving TV). This allows a natural backwards movement of the pivot in the black and orange parts of the Hensley.
If lateral forces on the TT cause it to swing to the side, the pulling force (tension) in the linkage will tend to move the ball back to center.
...Now, if the TT is to infulence movement on the TV, It must actually move forward relative to the TV. Now, since TTs are not equipped with engines, the only way that it can move forward is due to the TV stopping quickly, or when going downhill, if the TT tends to gain speed quicker than the TV.
The TT can exert lateral force on the TV via the linkage even in the absence of any longitudinal force between TT and TV. There does not have to be relative movement.
...Anyone with a Hensley Hitch knows, and is told in the manual, to set the boost higher on your brake control to prevent this forward (and sideways) movement of the TT on the hitch. I suppose the same thing could happen going down a steep grade if you were to decelerate, and engine braking was holding the TV back without the trailer brakes, although I have never heard of this being an issue. Perhaps this is because the differences in inertia between TV and TT are not great enough going downhill, and these differences only come in to play when the brakes on a TV are applied without the TT brakes coming on strong enough.
It is important that the TT should not push on the TV. With the HA, if there is an angle of just a few degrees between TT and TV, the VPP can move as much as 19" to the side of the TV's centerline. This means that the forward thrust from the TT is acting on the TV with a moment arm of about 19". This can induce a large "steering moment" on the TV just as a lateral force can.
...Perhaps an illustration could be used similar to a motorcross bike swingarm linkage type rear suspension.
Off hand, I don't see how the HA linkage would be similar to a motorcycle swingarm; but, I'll have to give it more thought
...I hope I have been able to add to this enjoyable discussion, and hope that my response was not totally stupid.
Your comments were certainly of value -- nothing the least bit stupid.
Ron
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2lMan
Brewer, ME
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Good point Ron, about the hitch moving forward. I made a typo there, or I was just looking at it backwards. "
The other discussions about the movements of the vehicle, inertia, and the motorcycle swing arm apply to the hitch moving forward.
My mistake.
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