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All of you guys are correct (IMO).
I might have some answers.
In keeping with past admonishments to clearly state that I am expressing an opinion rather than "proven" fact, the following is only my reasoned opinion.
I think I can tell you more about why the "Hensley bump" could occur in sometimes in seeming correctly adjusted HA (Hensley Arrow) hitches. Part of my reasoning is due to experiences I had as a railroader. With hundreds of car couplers, each with a little slack, anyone can easily imagine how incredibly important it was for us to control the accumulated slack momentum in our trains. Other than that, and successfully planned critical stops, I guarantee trains are a lot easier to drive than
any truck/trailer combo.
Jack is correct that some folks have gotten a nasty bump surprise with their Hensley. I believe it
happened to Les Adams one time, and he was NOT careless with his set-ups. Les said he nearly lost it. I think there may be a Hensley set-up step that is not properly resolved for this and so the caveat is not fully emphasized. That the TT brakes lead is a good fix for it, but there must be a subtle underlying reason that makes it necessary, and I believe that reason can be partially avoided.
Keith made a point mentioning that big trucks have a bump, but I am thinking it is more like the railcar bumps than the Hensley bump (But they are related to Hensley because of the strut design - later). My thought is that there absolutely must be some play in the pin engagement on a large truck/trailer to avoid any bind when the 5th wheel jaws close, especially if the truck were off canter during coupling. Any bind could cause the latching mechanism to fail to latch. This "engineered-in" safety slack than manifests itself as an
end-to-end bump when the pin goes from towing tension to braking compression during a stop. It could occur smoothly, or occur suddenly if any plate stiction suddenly lets go. As little as 1/8 inch play would allow a lot of momentum for a heavy trailer, and the loaded rigs weigh up to 80,000 pounds including the tractor. (As a matter of trivia, many of the trains I had weighed in over 32 million pounds and just one car was over 200,000 loaded. Slack per
freight coupler is close to 3/8 inch each car end at best.)
When a tractor/trailer stops, the forward momentum is not in a straight line even when the truck and trailer are steering parallel. The 5th wheel hitch is located above the rear drive axles. So the forces created by the truck brakes are at the bottom of the rig (ground level) and the pin "thrust" is well above this plane. The result is that the trailer tends to lift the rear truck axle as weight transfers to the front axle. This is mitigated by trailer brakes and having a lengthy wheel-base on the tractor for "leveling leverage". Still, a side view of an emergency stop should reveal a slight lifting of the pin/5vr joint since the trailer axles heavily depend on truck brakes to assist stopping. I believe a similar thing occurs with our RV rigs.
Surprisingly enough, the same thing sometimes occurs in railcars. We have all seen the aftermath of train wrecks. Sometimes one sees some of the cars still jack-knifed with the yet-coupled ends of two cars up in the air as they came to rest. I have actually seen photos of such a thing occurring during movement. I live in Bismarck, but across the Missouri river is a town called Mandan that I grew up in. Mandan is a major railcar repair terminal. Because of this, there is a detector just east of Bismarck that "listens" for flat wheels. (Flat wheels occur if we skid the wheels by overbraking). If the flatspots are too long, they pound the rail too much creating broken rail, or even break the wheel. There is a camera set up to automatically take a picture of noisy cars to ID them.
Some cowboy came in hot one day, and as he slowed down for the municipality speed change, he "slammed" the brakes on the train. The proper way to apply brakes is a little at a time to even out buff forces since the air signal starts at the front and works back. He hard-braked by nearly locking the front cars while the back still rolled freely. The terrific noise from the slack run-in triggered the camera. The camera took a picture of two cars jack-knifed up in the air, wheels about two feet off the rail. Then apparently the cars set back down on the rail (non too gently) and he continued to Mandan. Sobering lesson for most of us operators.
The point is, though, that the cars lifted as they did. And why? Because there is more weight above the couplers than below the couplers. The thrust line is skewed. Connect that with the probability that the lifted cars were empty, and some cars behind them were heavily laden. It makes sense.
So here is what I think occasionally happens to the Hensley. When the TT brakes do not lead the stop, the tow vehicle tips forward and the hitch-point lifts. This "lift" axis runs right through the ball. The trailer does not steer on this axis, but it is the axis that gives when the rig goes through a dip. Then we come to the strut design.
Ideally, for this purpose, the struts would be exactly on this same ball axis. But they are not. The axis for the pair of struts falls slightly behind the ball axis. Those of you with HA's can go look. Lay a straight-edge across the two forward strut pins and note the ball-center is forward of this point. So when the HA goes through a dip, the struts go into a
very slight slack condition as their pin connections are stretched apart. Now stretch is a deceiving term here because these struts do their work by compression, not tension.
Stretch loosens them. The adjustment nuts are designed to expand the strut length until they drive the hitch-head welded ball tight against the front pocket of the TT coupler. Since the tow vehicle exerts tension in this same direction, the ball can be in near-zero clearance at all times. If it were not for dips, the ball could be permanently so tight as to not allow any movement.
But remember, I said that slack occurs in dips, maybe 1/16 inch or so. Minute slack can also occur when brakes are applied. This is because the rear of the truck can lift during braking, particularily if the TT brakes aren't applied. The opposite of a dip is when the ruck "tops" a sharp hill and again
some slack must occur from pivoting the other direction. (The off-axis arrangement of the ball and forward strut pins acts more like a 3-point farm hitch for those of you who are familar with that concept.) At any rate, a slight slack can occur during braking and I believe this hitch-point rise may be accentuated by downhill travel. So it is possible for a braking, downhill curve to cause some slack and this in turn, allows the hitch-head to swing a bit from side-to-side. The proposed definition of a "Hensley bump" then is: When an S turn is attempted, the slackened hitch-head suddenly over-centers to the opposite side and a bump is felt when it hits the stop (the slack limit). For an extra good feeling, the rise also removes some rear tire traction just as the bump hits and when it is needed most for maintaining directional steering.
What can be done? First religiously check struts for tightness. I think making sure the struts are installed as level as possible,
as per directions, is paramount,
possibly slightly down to the rear. This may be the Hensley set-up step I spoke of earlier, that is not properly resolved, and so the caveat is not emphasized enough in it's importance. Setting TT braking to lead can definately make up for other shortcomings. Carefully
not adjusting the WD bars too tight is another, as WD bars encourage lift during braking. I think this last part may now be mitigated by the newer WD method of keeping the front truck axle at the same stock height, rather than tightened to equal load compression to the rear, as I believe was commonly recommended when the HA was introduced.
There is always some non-solid strut clearance (the ball must pivot in a dip), but any increase, however small, is greatly multiplied by the Hensley leverage design. We may use reductionism logic to imagine this. Imagine that the more complicated leverage advantage of the HA is the same as a simple lever with the fulcrum nearer one end. On the HA, the truck steers the TT from the long end of the lever and pries the TT tongue sideways with the short end. Now imagine this fulcrum to become loose. The truck, on the long end of the lever, will flop the TT from side-to-side much more than it would if the fulcrum were centered... as it is with normal non-link hitches.
Wes
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