Claude B
Montreal, Canada
Senior Member
Joined: 12/14/2002
View Profile
 Offline
|
Ok, i am not a H-A owner but i surely agree on it's great technical quality. IMHO, it's a perfect hitch by desing. Now i see 2 discussions here. First, how doe's the H-A eliminate sway and second, the "apparent" or "virtual" point of pivot between the TV and TT. I made 2 hand drawing to explain my comprehension of the system.
I think we all agree that sway is the result of the TT pivoting aroud a point locate in the center of the axles forcing the tongue to the left and right (laterally) pushing the back of the TV sideways up to a point where control of the rig can be lost. Lots of factors are involved here like wheelbase, overhang of the hitch ball from the rear axle, tongue weight etc.
We also agree that sway only occur at speed over 45 to 50 mph (+/-)
So, at this speed, TT and TV are in straight line 99% of the time. This is where the HA desing is unique, it locks completely (from the TT point of vue) the TT and TV just like riding on a railroad. It has the same effect as welding a 6" I beam from the back of the TT to the front of the TV. Rotation is impossible if induce by the TT (sway). The HA trapezoïd 4 bars linkage combined with the 2 struts bars are locking the TV and TT. On highways or country roads, the TV and TT are always almost straight so with the HA, there is no apparent pivot point of rotation close, after or forward the TV axles. Lateral forces will be on the entire TV frame (from the HA to the receiver to the TV frame).
The apparent pivot point between the TT and TV appeers when and only when the TV induce a rotation and will occur only at low or very low speed in close manoevring (city driving, parking, backing-up) except on a panic and rapid lane change on highways.
Hand drawing of the HA without the strut bars : HA drawing 1
Hand drawing with the strut bars and what they do : HA drawing 2
This is also why with the HA, the TV has always the control of the TT. With this hitch, a jacknife situation is totally impossible in panic stop whatever the speed.
Now, here is my comment of a non HA owners 
Technically, it is a perfect hitch but it's to bad that in their advertising video, the let the impression that it's also a perfect "idiot proof" hitch. Don't take me wrong, HA owners are NOT idiots ! But when they claimed that you can load your TT with heavy stuff in the back not having any taughts about the tongue weight etc. After all, the HA will take care of any unstable rig due to an uninformed driver who know's nothing of the physics of towing (wide majority on the road).
P.S. The cherry over the cake was the ... lawyer (in the video)!
Best regards to all.
* This post was
edited 05/12/05 05:40pm by Claude B *
Claude & Lise
Nissan Pathfinder 4WD 2003, V-6 3.5L, auto tran, 4.36 diff., Palomino 2003 ultra lite TT (T-25FB) 25' (loaded: TT 4,200#, all together: 9300#) Reese WD hitch 750# bars, Dual-Cam, Prodigy
|
willald
NC
Senior Member
Joined: 07/15/2002
View Profile
|
Great points, Claude, and thanks for including the hand-drawn illustrations. 
Hmmm....Is it possible/fair to say, that the 'virtual pivot point' from the trailer's perspective might actually be an infinite distance forward from the ball, since the trailer cannot induce any swaying/lateral movement on the tow vehicle? Then, the 'apparent pivot point' would only be from the tow vehicle's perspective, and would be about 47" forward of the trailer ball (when in a straight line)? This would seem very consistent with the reality of how this hitch works.
On another note, I wanted to clarify one thing, about what Claude discussed:
Claude B wrote:This is also why with the HA, the TV has always the control of the TT. With this hitch, a jacknife situation is totally impossible in panic stop whatever the speed.
Thats true - with a Hensley, the TV always has control of the TT, as long as the tow vehicle is 'pulling' the TT, and the opposite does not occur (TT 'pushing' the tow vehicle real hard).
However, if during a panic stop, the trailer 'pushes' the tow vehicle too hard, it actually IS possible, for a jackknife to occur with a Hensley. During a panic stop, if the brake controller is not set aggressive enough, and the trailer 'pushes' the tow vehicle hard enough, the Hensley will let the trailer shift off to one side, resulting in a push slightly off to one side. This 'push' affect of the Hensley has been discussed, and is well known among Hensley owners. It just means that with a Hensley (like any other combination/hitch), you must have your brake controller adjusted right, to prevent too much trailer 'push' from occuring during a panic stop.
I will tell you, I once did not have our brake controller set right, and felt this 'push'. I was braking REALLY hard when it happened, and the push was not anything to worry about. Only way I can see it would cause a problem, is if it was a case of a VERY light/small tow vehicle towing a huge trailer, that it should not be towing in the first place, Hensley or not (like some of the combinations CanAm RV puts together..LOL).
Kind of goes back to what you said, Claude - even though the Hensley is such a perfect design, it should not be used as a 'cure-all' for a bad setup. You still need to set everything else up correctly, especially your brake controller. And, you still need to match the tow vehicle to the trailer appropriately.
Will
|
bryanl
Reno, NV
Senior Member
Joined: 12/25/2003
View Profile
|
Perhaps the best way to understand what is going on would be to put together a scale model and do a little bit of 'experimenting' with it. The model only needs to be 2 dimensional. You could do this easily enough with a few small sticks and some glue (and a hitch and rig to measure).
Ron has the best analysis of this I have seen so far. But the other talk about 'eliminating sway' and other absolutes should raise the skeptic meter. The hyperbole should always be a warning that something is missing.
Bryan
|
tluxon
Kirkland, WA
Senior Member
Joined: 08/12/2001
View Profile
Offline
|
willald wrote:...For starters: HERE is a basic illustration, showing the trapezoid/4 bar linkage, that the Hensley uses to eliminate sway. Some of the details of this description have been accused of being inaccurate, but this still provides a good starting point. I have actually measured our Hensley head, and found that the two side bars are about 4.5" long, the rear (longer) bar on the trailer side is about 8", and the front bar is about 7.25" long...
I sketched the geometry up in SolidWorks and plugged in the numbers you provided for the link lengths. The furthest forward the "virtual" pivot point (if we're using VPP to describe the projected intersection of the two side links) is of course when there is no angle between the TV and the TT. Using your numbers, that point is 43.35 inches in front of the "front bar" (the bar that makes a T with the TV).
Of course, this is using Will's numbers which I believe are only guestimates. We'd have to know the actuals to determine how far up the VPP really is.
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
|
willald
NC
Senior Member
Joined: 07/15/2002
View Profile
|
bryanl wrote:Perhaps the best way to understand what is going on would be to put together a scale model and do a little bit of 'experimenting' with it. The model only needs to be 2 dimensional. You could do this easily enough with a few small sticks and some glue (and a hitch and rig to measure).
I was actually thinking the same thing, except going to a bit more detail. Perhaps if you could find/build a scale model of a good size TT, and a truck/tow vehicle, then hitch them up with something similar to the 4 bar linkage. You could make the linkage with just 4 sticks like you mentioned here. Then, you could do things like 'push' on the side of the trailer to simulate crosswinds, etc, and see how any pivoting it does, affects the truck. My bet is you'd find that the whole combination would be pushed to the side as one unit, and that the trailer could NOT induce any pivoting. That would answer the question of where the pivot point is, from the trailer's perspective.
Of course, who has the time to do all that? lol! 
Quote:Ron has the best analysis of this I have seen so far. But the other talk about 'eliminating sway' and other absolutes should raise the skeptic meter. The hyperbole should always be a warning that something is missing.
I agree, Ron has a great analysis. And, I also agree that something is missing - from Ron's analysis. His analysis does a great job of showing where the pivot point is from the tow vehicle's perspective, but I can't help but wonder if from the trailer's perspective, that 'virtual pivot point' may be at a different place.
Will
|
|
|
LAdams
Northern Illinois
Moderator
Joined: 10/06/2000
View Profile
Offline
|
BarneyS,
BarneyS wrote:The rest have been "off topic" and if I had moderating privileges on this forum, they would be deleted!
DONE!!!
Moderator
|
tluxon
Kirkland, WA
Senior Member
Joined: 08/12/2001
View Profile
Offline
|
willald wrote:...Hmmm....Is it possible/fair to say, that the 'virtual pivot point' from the trailer's perspective might actually be an infinite distance forward from the ball, since the trailer cannot induce any swaying/lateral movement on the tow vehicle?...
No, the VPP could not be infinitely forward from the ball or you'd run into a situation where lateral forces could get leverage on the front of the TV. The only place where you have NO twisting leverage on the TV due to lateral forces is exactly halfway between the rear axle and the front axle. However, anywhere between or close to the axles (probably within 12" on a typical wheelbase TV) will probably make leverage due to lateral forces almost nil.
Tim
* This post was
edited 05/12/05 02:39pm by tluxon *
|
tluxon
Kirkland, WA
Senior Member
Joined: 08/12/2001
View Profile
Offline
|
willald wrote:bryanl wrote:Perhaps the best way to understand what is going on would be to put together a scale model and do a little bit of 'experimenting' with it. The model only needs to be 2 dimensional. You could do this easily enough with a few small sticks and some glue (and a hitch and rig to measure).
I was actually thinking the same thing, except going to a bit more detail. Perhaps if you could find/build a scale model of a good size TT, and a truck/tow vehicle, then hitch them up with something similar to the 4 bar linkage. You could make the linkage with just 4 sticks like you mentioned here. Then, you could do things like 'push' on the side of the trailer to simulate crosswinds, etc, and see how any pivoting it does, affects the truck. My bet is you'd find that the whole combination would be pushed to the side as one unit, and that the trailer could NOT induce any pivoting. That would answer the question of where the pivot point is, from the trailer's perspective.
I have the Hensley video where they use an actual hitch for demonstration purposes. They show that when the TV tries to create an angle with the TT there is no resistance and the angle is easily created. Then they demonstrate that applying strong lateral forces to the TT side of the hitch results in no angle being created - Yes, the combo moves as a single unit. I don't know that that answers where the pivot point is so well, but it sure addresses the concern with TT induced sway.
By the way, I'm not a Hensley owner - just a mechanical engineer who is impressed with how cleverly the Hensley addresses trailer-swinging induced sway. For the price, you'd think they'd be able to do it with a little less added weight by using more titanium parts.
Tim
|
Dogarms
California
Senior Member
Joined: 11/10/2001
View Profile
Offline
|
This is a very interesting discussion and I am beginning to understand more cleraly why the Hensley is often referred to as a "sway prevention" hitch instead of a "sway control" hitch. If I understand correctly, when driving in a straight line, the hitch essentially locks the trailer in column behind the TV as if they were a single, non-articulated unit. (I think the Dual Cam sway control cams provide some of this locking action, though from an entirely different approach.) When the TV and TT move out of column, for whatever reason, the hitch geometry projects the point of articulation forward toward the rear axle, in the manner of a fifth wheel, reducing or eliminating the tendancy of the two units to get out of phase with each other. Very clever.
My hitch, which is a straight forward "sway control" hitch simply dampens, but does not prevent, oscillations before they can become a problem. I works very well, but admire the genius of the Hensley hitch. I wish they were cheaper.
TC
'03 GMC ExCab, SB, Duramax, Allison, 4x4, Husky mudflaps, Procaps, Bilsteins, Leer shell, BedRug, Oilguard Bypass,
Caterpillar fuel filter, ISSPRO EV Pyro and Boost guages, Titan V, Equal-i-zer EQ1400, Arctic Fox 30U
|
tluxon
Kirkland, WA
Senior Member
Joined: 08/12/2001
View Profile
Offline
|
willald wrote:...His analysis does a great job of showing where the pivot point is from the tow vehicle's perspective, but I can't help but wonder if from the trailer's perspective, that 'virtual pivot point' may be at a different place.
If we really went crazy, we could break it down to free body diagrams and increase the scope from simple theoretical points to analyzing specific forces, points of application, centers of mass, moments of inertia, etc. Personally, I'd rather just look at the 4-link hitch as a sophisticated hinge that allows twisting forces to create an angle and prevents lateral forces at the "hinge" from doing the same.
|
|
|
|
