bettered
UpCountry SC
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Joined: 07/26/2004
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Ron Gratz wrote:bettered wrote:tluxon wrote:Somebody correct me if I'm wrong, but isn't the primary function of the strut to keep the TT square to the back "bar" of the HA?
I don't think so Tim. The rear link wants to be square to the TT centerline, but it's so the TT will track straight. The primary function of the strut bar(s) is to prevent all motion of the rear hitch bar (the orange part) relative to the hitch ball in the yaw plane; that is, to keep the orange part from turning on the ball thus forcing all turning through the linkage itself.
Ed
The function of the struts is to ensure that the TT cannot yaw relative to the rear (upper) unit of the HA -- i.e., so the ball coupler cannot pivot (yaw) relative to the ball.
However, it is of interest to consider what happens if the struts are adjusted so that the longitudinal axis of the TT is not perpendicular to the HA's rear link. Let's say the TT is 5 degrees out of alignment. If the HA locks up when the TV is moving straight ahead, then the misadjusted struts would cause the "locked" linkage to tow the TT at a 5 degree yaw angle. In reality, the linkage is not locked and the rear (upper) unit will rotate until the TT tracks at an angle between 0 and 5 (probably closer to 0) degrees.
Wow! An excellent observation Sir Ron. You are exactly correct. But I'm wondering, since you don't own a HA, how could you possibly know that this is how it works? I mean, that defies my opinion!
Regards
Ed
BetterEd
DW + 2 grandkids + Mini Schnauzer
2005 Chev 3500 Crew D/A 6.6L LLY, 6 x 6 DRW, 3.73
Tru-Flow + Banks, 2005 Flagstaff 831FKSS
Hensley + Prodigy
"Genius may have its limitations...." E. Hubbard 1856 - 1915
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Ron Gratz
full time RVer
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bettered wrote:The front link translates when the TV is turned. As long as the TT/TV combination is generally straight AND THE TT WHEELS ARE ON THE GROUND, no realistic side force imparted to the TT can be great enough to translate the rear link of the HA.
Ed, if the TT tires are rolling, then the necessary side force on the TT might be much less than you think. Following is a response I posted earlier:
"There is a difference between tire static friction (when the tire is not rolling) and the lateral force generated by a tire when it is rolling. A rolling tire does not generate lateral force unless is has some degree of "slip angle" relative to the direction of travel. This is why a moving TT swings when subjected to a lateral force. The TT must develop enough yaw angle to generate sufficient side force to counteract the imposed lateral load. The side force is proportional to the tire's cornering stiffness, the load on the tire, and the amount of yaw."
Ron
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Ron Gratz
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bettered wrote:Wow! An excellent observation Sir Ron. You are exactly correct. But I'm wondering, since you don't own a HA, how could you possibly know that this is how it works? I mean, that defies my opinion!
Regards
Ed
Ed, now I'm wondering if I really passed my college physics and statics courses since I didn't own a HA. ![[emoticon]](http://www.coastresorts.com/sharedcontent/cfb/images/awink.gif "awink")
Ron
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TnAnFLA
Nokomis FL
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Thanks for the kind words Ed B. And Tim, I think we mostly agree on the function of the strut bars, but I think the overlooked aspect of the assembly is that the struts keep everything aligned by placing all the pieces under compression. When you snug up the struts, keeping the hitch head, stinger, etc. all aligned you do that by "expanding" the strut. That means all the potential moving parts in the four bar linkage are being held in place by forces coming from the outside of the assembly, all pushing against each other in (approximate) balance. That, in conjuction with the fact that the front and rear linkage bars are different lengths, is what makes the hitch head to appear to be virtually "locked"
Now comes the hard part. How do you explain the "bump" that can happen if the TT brakes don't engage soon enough , or at all, when slowing down. It would seem that scenario would add even more compression to the hitch assembly. My only guess is that it's velocity dependant (i.e., it only happens when the TT slows down really quickly relative to the TT) and that the mass of the TT at those velocities is enough to cause the rear link to translate to one side or the other. And the beauty of it all is that since the head translates as opposed to the TT swinging, the net force vector on the TT/TV assembly is still pretty much straight ahead so even in a panic stop you don't end up swerving or swaying into your buddy in the adjacent lane.
Andy
31' Itasca Impulse
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Ron Gratz
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TnAnFLA wrote:--- One thing I don't recall seeing in this Odessey of a thread is that the strut bars are actually adjusted by placing them under a compressive stress. And when the TT trys to yaw, that places yet more compressive stress on that side of the hitch assembly.
Andy,
I think (but, cannot prove) that the strut bars are designed to be under compression. If the rear of the TT swings to the right, then the compression in the right strut decreases. However, a decrease of compression has an effect similar to an increase of tension.
Quote: All that prattle aside, I do think that Milt's obsequious (sp?) photo of the hitch with the come-a;ong has it backwards. The TT doesn't "pull" on the linkage when it trys to sway, it actually "pushes " on it (I think).
When the TT tries to yaw, it exerts a moment on the HA's rear (upper) unit. Assuming that both of the HA's side links are in tension due to "drag" from the TT, the imposed moment will reduce the tension in one link and increase the tension in the other. Due to the converging angles of the links, the increased tension on one side and decreased tension on the other will result in a net lateral force on the hitch.
Ron
On edit: Made changes in red to correct confusion between tension and compression
* This post was
edited 09/01/05 09:11pm by Ron Gratz *
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TnAnFLA
Nokomis FL
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Ron, it seems to me that if the struts were missaligned by 5 degrees or so then the TT would track straight, but off center along the arc defined by the 5 degree misalignment. In other words, the head would translate enough right or left to account for the misalighnment in order to balance out the forces on the four bar linkage.
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Ron Gratz
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TnAnFLA wrote:--- but I think the overlooked aspect of the assembly is that the struts keep everything aligned by placing all the pieces under compression. When you snug up the struts, keeping the hitch head, stinger, etc. all aligned you do that by "expanding" the strut.
Andy,
When you snug up the struts, I believe you are placing the struts in tension. The tension at the rear of a strut pulls forward on the TT's A-frame and exerts a compression load in the A-frame channel. The compression load in the A-frame channel causes a forward force on the ball coupler and ball.
The ball is rigidly connected to the HA's rear (upper) unit. The forward ends of the struts are connected to the rear unit via the strut pins. Snugging up the struts simply exerts a rearward force on the rear unit which is exactly balanced by the forward force from the A-frame. The net result is zero. There is no effect on the HA's side links and front link.
Quote:Now comes the hard part. How do you explain the "bump" that can happen if the TT brakes don't engage soon enough , or at all, when slowing down.
The "bump" probably results from yaw-related "slack" in the receiver/hitch/TT connections. Potential sources of "slack" are the receiver and stinger connection, the stinger and pin box connection, and the strut connections. A relatively small amount of yaw slack can translate into a relatively large translation/rotation of the HA's rear unit relative to the front unit.
Ron
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bettered
UpCountry SC
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TnAnFLA wrote:Now comes the hard part. How do you explain the "bump" that can happen if the TT brakes don't engage soon enough , or at all, when slowing down. It would seem that scenario would add even more compression to the hitch assembly. My only guess is that it's velocity dependant (i.e., it only happens when the TT slows down really quickly relative to the TT) and that the mass of the TT at those velocities is enough to cause the rear link to translate to one side or the other. And the beauty of it all is that since the head translates as opposed to the TT swinging, the net force vector on the TT/TV assembly is still pretty much straight ahead so even in a panic stop you don't end up swerving or swaying into your buddy in the adjacent lane.
Andy
I've experienced the bump only once, and I don't care to experience it again. If you're braking, AND in a turn beyond the "collapse point" of the side link on the inside of the turn, AND the trailer brakes aren't restraining the trailer (it would be difficult in a deep turn due to the statics of the forces) the trailer will actually move forward until the side links lock. The forces on the hitch (the part supplied by GM in my case) are not trivial. Other posts have commented on the problems with the GM hitch in this regard.
When it happened to me it was a slight bump, but enough for me to know what was going on. I immediately realized I had the boost turned off on my controller, so I quickly rectified that situation. With my 1/2 T p/u I used to run B3, but now I'm using B1 and that's fine. No boost at all is a problem...
The amount of kinetic energy difference between the TT and the TV in this situation cannot be great - as the relative speeds are similar. The additional travel might be 6" or so, but the decel forces involved in quickly stopping a moving 8,000 TT cannot be trivial. Hence the bump.
Ed
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bettered
UpCountry SC
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Initially when I set up my HA, I got a little too enthusiastic with the tightness of the struts. As a result, I was forcing some things to go where they really didn't want to go as the rig went across neutral from left to right (or vice versa). After some discussions with Hensley, I got it loosened up to just "snug" and it's been fine ever since.
This winter I'm going to do a dissassembly and clean up. I have an older hitch ($1500 used) so I suspect the grease is caked and the seals are probably shot. I'm going to clean and repack, then paint. Give me some nerdy thing to do while keeping out of the DW's hair..
Ed
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bettered
UpCountry SC
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Ron Gratz wrote:bettered wrote:The front link translates when the TV is turned. As long as the TT/TV combination is generally straight AND THE TT WHEELS ARE ON THE GROUND, no realistic side force imparted to the TT can be great enough to translate the rear link of the HA.
Ed, if the TT tires are rolling, then the necessary side force on the TT might be much less than you think. Following is a response I posted earlier:
"There is a difference between tire static friction (when the tire is not rolling) and the lateral force generated by a tire when it is rolling. A rolling tire does not generate lateral force unless is has some degree of "slip angle" relative to the direction of travel. This is why a moving TT swings when subjected to a lateral force. The TT must develop enough yaw angle to generate sufficient side force to counteract the imposed lateral load. The side force is proportional to the tire's cornering stiffness, the load on the tire, and the amount of yaw."
Ron
I remembered that Ron, and I am mindful of it. As you pointed out, the forces on both the TT tires and the TV are greatly reduced by the static forces acting through the HA, and I fully appreciated what you were saying at that time. (It ain't to late for this particular old dog to learn a trick or two.) But it's my opinion that there's a significant difference between some lateral force resisting sideways movement of the TT through the tires, and no resistance at all.
Because the sine and tangent for small angles are nearly identical, I'm of the opinion that a sideways movement at the tires (on dollies - i.e. no resisting force at all) would initially translate the rear link of the HA and thus get our rotational / translational sequence started. Now I am being very careful here to say that this is my opinion, it is not a fact - yet. But if we were to do it, a lot of the conjecture could be laid to rest.
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