BurbMan
Indianapolis, IN
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*YAWN*
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jdwhittaker
York, SC
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Joined: 06/01/2005
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Tim,
Excellent drawings(Figs. 1-3), they depict the only movement that the HA is capable of, well except for the opposite motion of course. It should also be noted that this type of motion can only be created by the TV, as in the case of a TV initiated turn...unless something is very wrong. Big hockey puck, or unsafe travel on very slick roadways or the like. It seems that alot of people posting in this discussion are interpeting other peoples posts as absolute statements...ie. can not happen, or can only happen this way, when in fact I think most people are trying to say something can't happen, or can only happen a certain way under normal circumstances. My interpetation to the forces and motion associated with sway are different to the forces and motion in the figures above. I see sway as a rotational moment with it's center at approximately the center of all of the axles on the TT, and it's affect on the TT and TV. The HA all but eliminates that type of motion except in the case of extreme conditions. I'll attempt to make and post a little sketch of the forces that I'm describing, as well as a short video of the HA moving while mounted only on the stinger(as Milt and I have done). I think the video will really clear up alot of misconceptions about the motion of the HA.
I am still looking for an explanation of the Virtual Pivot Point, and it's role in all of this, that I can follow. Anybody?
Tim, you must have 40 or more hours invested in this discussion, I don't know where you find the time. Thanks
Dave
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tluxon
Kirkland, WA
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bettered wrote:I agree with you Ron. But before we can get into sliding friction, we have to get past static friction. Now I'm not claiming to be an expert on tire dynamics, but even though the contact patch is moving (forward), I'm thinking it has a static friction coefficient in the lateral direction (sideways). As long as the tire is rolling, it is not considered static in any direction, because the contacted surface area is constantly changing.
bettered wrote:It is only after the TT has broken loose that it can apply any translation or turning movement to the rear bar of our 4-bar system. Forces yes, but movement of the rear link without tires going sideways - can't happen.
For the most part you're right, Ed. However, since in the real world there are always tolerance gaps and wear on interfacing materials, I'm certain that there is a slight amount of play in the linkage, some units more than others. This would allow some translation to occur without any rotation of the rear bar, resulting in no moment to be applied to the tires from the hitch. Besides, the rotational angle changes very little in the first inch or two of translation on each side of center anyway.
In addition, I'm having a hard time imagining what kind of forces outside of a broadside hit at the axles that could result in anything other than a moment on the CG of the TT. I've already shown above how a lateral force resulting from the moment of a TT results in a reactive COUNTER moment generated by the trans-rotation of the hitch linkage. We just don't know how much of a reactive moment the hitch CAN generate given its relatively minimal leverage and unknown compressive strength.
I think it might be more useful to identify specific hazardous conditions and analyze the various reactions using a HA. For example, if lateral slippage of the tires is a concern, pick a possible cause so we can start with those inputs.
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
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tluxon
Kirkland, WA
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jdwhittaker wrote:Tim,
Excellent drawings(Figs. 1-3), they depict the only movement that the HA is capable of, well except for the opposite motion of course. It should also be noted that this type of motion can only be created by the TV, as in the case of a TV initiated turn...unless something is very wrong. Big hockey puck, or unsafe travel on very slick roadways or the like. It seems that alot of people posting in this discussion are interpeting other peoples posts as absolute statements...ie. can not happen, or can only happen this way, when in fact I think most people are trying to say something can't happen, or can only happen a certain way under normal circumstances. I agree, but you've got to admit that Milt has not been so open to acknowledging those exceptional circumstances that result in a behavior that he hasn't himself seen or experienced.
jdwhittaker wrote:My interpetation to the forces and motion associated with sway are different to the forces and motion in the figures above. I see sway as a rotational moment with it's center at approximately the center of all of the axles on the TT, and it's affect on the TT and TV. How is that different than the moment diagram I posted a few days ago? The simple result of a 1000 foot-pound counterclockwise moment about the center of the TT's axles on Will's trailer is the 45.5 pound force I showed in Fig 1.
jdwhittaker wrote:The HA all but eliminates that type of motion except in the case of extreme conditions. I completely agree.
jdwhittaker wrote:I'll attempt to make and post a little sketch of the forces that I'm describing, as well as a short video of the HA moving while mounted only on the stinger(as Milt and I have done). I think the video will really clear up alot of misconceptions about the motion of the HA. I'm looking forward to seeing that.
jdwhittaker wrote:I am still looking for an explanation of the Virtual Pivot Point, and it's role in all of this, that I can follow. Anybody? Here's an earlier post where I showed the path of instantaneous centers inherent in a HA and linked to an interesting page of links discussing some of the many uses of 4-bar linkages. We're using the term Virtual Pivot Point to describe these Instantaneous Centers. Here's an interesting example of instantaneous centers in a prosthetic knee.
![[image]](http://www.oandp.org/jpo/library/images/1996_02_034/8234f1.gif)
jdwhittaker wrote:Tim, you must have 40 or more hours invested in this discussion, I don't know where you find the time. Thanks Well, I haven't been counting, but I'd guess its probably closer to 10 hours reading and 5 hours generating posts. Thankfully, it's been spread out a little over the course of nearly a month. I feel that being a non-Hensley owner and an engineer gives me a unique and objective perspective, but I'd be delighted to step back and let an engineer who's taken a Hensley apart take over ![[emoticon]](http://www.coastresorts.com/sharedcontent/cfb/images/grin.gif "grin") .
Tim
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tluxon
Kirkland, WA
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More on VPP and Instantaneous Centers:
Here's a very interesting piece on MIT's site on linkages in general.
Tim
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jdwhittaker
York, SC
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Joined: 06/01/2005
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Tim/Anybody,
I have a 3.48MB video file(zipped)that shows the motion of the HA lock to lock. However, I have no way readily available to post such a file for the group to see. Can any body help me out? Either by sending me a PM describing how to post the file myself, or telling me how to get the file to one of you who can post it for me.
Thanks,
Dave
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bettered
UpCountry SC
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Joined: 07/26/2004
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tluxon wrote:For the most part you're right, Ed. However, since in the real world there are always tolerance gaps and wear on interfacing materials, I'm certain that there is a slight amount of play in the linkage, some units more than others.
The links pivot on double tapered roller bearings - two sets under the domed caps on the orange part - like the wheel bearings on a car or truck. As a result, the operative word in your statement above is "slight".
tluxon wrote:This would allow some translation to occur without any rotation of the rear bar, resulting in no moment to be applied to the tires from the hitch.
Theoretically yes, but practically, it's nil.
tluxon wrote:Besides, the rotational angle changes very little in the first inch or two of translation on each side of center anyway.
This is absolutely true.
tluxon wrote:In addition, I'm having a hard time imagining what kind of forces outside of a broadside hit at the axles that could result in anything other than a moment on the CG of the TT. I've already shown above how a lateral force resulting from the moment of a TT results in a reactive COUNTER moment generated by the trans-rotation of the hitch linkage. We just don't know how much of a reactive moment the hitch CAN generate given its relatively minimal leverage and unknown compressive strength.
One of the more astounding testimonials on the Hensley website is from an owner who was involved in an accident where another vehicle lost control and rammed the TT at just about the axles. The TV and control of the TT were not affected in terms of directional stability. I find this hard to swallow, but that's what the fellow said.
tluxon wrote:I think it might be more useful to identify specific hazardous conditions and analyze the various reactions using a HA. For example, if lateral slippage of the tires is a concern, pick a possible cause so we can start with those inputs.
Tim
Obviously icy conditions is one such hazard. The TT must have some traction in order for the hitch forces to be reacted. On the other hand, the problem would be worse (I presume from the calcs) with just about any other hitch.
The other hazard is what Hensley refers to as "over-driving" the TV. I take that to mean improper brake control on the TT, combined with carrying too much speed into a downhill corner. A sharp corner where one of the links has moved 'overcenter' would be much worse than a gradual corner. In the former case, "too much speed" wouldn't necessarily have to be a huge amount. It's the momentum thing...
A comment on the MIT reference. BINGO! Thanks for that. I note that some are looking for the short courses in Kinematics to be provided here. I know it was a long time ago (for me), but I can remember the name of the course..
Regards
Ed B
BetterEd
DW + 2 grandkids + Mini Schnauzer
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"Genius may have its limitations...." E. Hubbard 1856 - 1915
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tluxon
Kirkland, WA
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jdwhittaker wrote:Tim/Anybody,
I have a 3.48MB video file(zipped)that shows the motion of the HA lock to lock. However, I have no way readily available to post such a file for the group to see. Can any body help me out? Either by sending me a PM describing how to post the file myself, or telling me how to get the file to one of you who can post it for me.
Thanks,
Dave
Here's a link to a compressed version of the video file Dave made.
http://home.comcast.net/~tbluxon/Hensley_Motion_sm.WMV
Tim
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bettered
UpCountry SC
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That video clearly shows the action of the 4 bar linkage with the back bar (orange part) free to move in any direction it can. Of course we know this is not possible when attached to the TT. But you can infer from the video what would happen if the rig were being driven forward slowly (can't happen at any appreciable speed) while the TV steering wheel is turned from lock to lock and back again, allowing the TT time to completely react to the directional changes. You can also envision the sideways translation of the orange part we've been talking about. Note that for small directional changes, it's much more translation than rotation, as Tim said in his previous post.
For normal highway towing (any roadway), the amount of movement is obviouisly much less that what we see here.
Ed
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Ron Gratz
full time RVer
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Joined: 12/27/2003
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bettered wrote:I agree with you Ron. But before we can get into sliding friction, we have to get past static friction. Now I'm not claiming to be an expert on tire dynamics, but even though the contact patch is moving (forward), I'm thinking it has a static friction coefficient in the lateral direction (sideways).
...
I'm very interested in your take on this postulation.
Ed B
Ed, here are my thoughts on lateral force generated by a rolling tire from a previous thread.
Lateral force generated by a rolling tire is different from either static or sliding friction.
Ron
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