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Old 11-06-2007, 03:03 PM   #71
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Unfortunately, I was a driver and not an engineer in my racing days. I can tell you that the rig feels a lot different. The most disconcerting part if the WD apparatus is how it behaves when the TV pitches before the camper... feels like the camper steers.
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Old 11-06-2007, 09:15 PM   #72
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Quote:
Originally Posted by markdoane
I've used the Crowhurst calculator for a couple of years and find it very useful. I'll add a little enhancement you might enjoy, which lets you calculate the vehicle balance front and rear, both with and without the spring bars hooked up.

If you are willing to spend some time and take a few measurements, it will even estimate the change in loads for driver and passenger, cargo, and fuel.

The first page is the Crowhurst calculations (in rough formatting), and the second page is the axle load calculator. . . .
Here is markdoane's application of Nick's spreadsheet. I'll let him take any questions or provide explanation:
Attached Files
File Type: xls markdoane's figures.xls (24.0 KB, 165 views)
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Old 11-06-2007, 09:28 PM   #73
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Old 11-07-2007, 11:24 AM   #74
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Originally Posted by nickcrowhurst
I find that these equations can be of great help in answering many further questions that arise about the use of Load Distribution hitches. One example is:
“Why is my hitch rated for 5000 pounds, but 10000 pounds with a Load Distributing hitch?”
We have seen, from equation 6, that the TV rear axle is lifted by a force U when the chains are tightened. This uplift is caused by the chain tension lifting the hitch upwards with a force which we will label Z. To calculate Z, take moments about the TV front axle:
U*W=Z(W+H), and hence
Z=(U*W)/(W+H) , equation 11, giving the load taken off the hitch when the chains are tightened.
If I input the figures for my rig into this equation, the uplift at the hitch, Z, is calculated at 370 pounds. The original hitch load (the tongue weight) was 800 pounds, so when the chains are tightened, the hitch load is reduced by 370 pounds from 800 pounds to 430 pounds. The load on the welds and bolts of the hitch are almost halved. Now I understand why my hitch capacity doubles when I tension the chains.
Nick.
Nick:

Lets assume a simple connection where the receiver is bolted to the TV with two horizontal bolts spaced say 2'-0" apart fore and aft with the center of the hitch ball being an additional 2'-0" behind the rear most bolt. Further assume, for simplicity, that the entire load will be transmitted to the bolts in shear. In a pure dead load configuration, with the vehicle not moving, it's obvious that the rear most bolt experiences a downward load which is twice that of the tongue weight while the forward most bolt experiences an upward load equal to the tongue weight. By tensioning the equalizer bars we can make the weight on both bolts equal and then theoretically continue until the load on the rear most bolt is zero and, ultimately, to the point where the loads are entirely reversed from their initial dead load scenario.

In the simplest scenario possible, if we were to substitute two bolts in place of the single rear most bolt, the capacity of the pair would determine the ultimate dead load capacity of the hitch, subject to the capacity of the attachment point on the TV - without reliance on the forward most bolt. The only effect the equalizer bars would then have is to transfer loads, as intended, to the front wheels of the TV. If the drawbar capacity of the hitch is ultimately rated at 12,000 lbs I fail to see where the use of the equalizer bars should have anything to do with it - unless, and just "unless" some damn cost-sensitive engineer designed the hitch so close to the wire that it had to have some value from equalizer bars in order to carry a given load - distributed over all of the attachment points! If so, I wish he would tell me exactly what value I have to provide with my equalizer bars to make the difference between a 7,500 lb capacity and a 12,000 lb capacity??? I cannot see where the equalizer bars have anything whatsoever to do with pure drawbar capacity (i.e. - pulling capacity.) If this is the way our OEM GM hitches were designed, I can certainly understand the failure rate! Imagine loading and unloading the hitch through a 4,500 lb range due to bumps in the road, railroad crossings, service station ramps, panic stops, etc. etc. etc. - while the dead load on the hitch oftentimes doubles while the equalizer bars dynamically unload!
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Old 11-07-2007, 12:15 PM   #75
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not shear alone...or at all!

The bolts holding the hitch to the TV frame are not acting solely in shear (or only minimally if bolted through the side of the frame) but more accurately (and most likely) acting in tension/compression because most aftermarket hitches are mounted to the bottom of the frame rails.

If the bolts are mounted horizontally, as in my '07 Dodge 3/4T OEM, they are benefiting from gobs (OK, not engineering term, but you get it, right?) of friction between the bolt's clamping force, the frame and the hitch...all forming a solid "extension" of the hitch mounting point to the TV frame.

A Grade 5 bolt properly torqued will provide more than enough clamping force to create a rigid assembly. Use a Grade 8 if you're an over-achiever like me.

Marc
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Old 11-07-2007, 02:15 PM   #76
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Cracker, my initial thoughts on this are as Marc states, above, but then I also started considering the forces involved. To have a full analysis we would need the original engineering drawings and a discussion with the design engineer. As that is not practical, what follows are my non-rigorous ramblings to establish my "best guess" at the reasons why the designers feel able to double the pulling capacity when WD is employed.

Let us examine the horizontal and vertical forces on the hitch ball. First, the horizontal loading. The horizontal force applied to accellerating the trailer along a level road is equal to the mass of the trailer multiplied by its accelleration. The accelleration will be high from rest to 20 mph, and the air resistance will be low, so let's ignore air resistance. Let's assume it takes about 5 seconds to get to 20 mph (approx. 30 feet per second). I suspect my truck would fail to pull this hard, even at wide open throttle. A trailer of mass 7500 pounds accellerating from rest at a rate of 6 feet per second per second must be acted on by a force of F pounds weight, where 32 feet per second per second is the accelleration due to gravity, hence :

F= (7500*6)/32 = 1400 pounds weight

We could look at other examples, for example climbing a 10% grade, but my conclusion is that the horizontal forces are nothing like the 7500 pound weight of the trailer. Also, intuitively, I feel that these forces will not be subject to rapid variations through shock-loading. (I said this was non-rigorous!) When we hit a bump in the road, it is the vertical forces on the hitch ball that will vary greatly and suddenly as the trailer and truck pitch independently.

Examining the vertical force on the hitch ball, we can expect the static hitch weight to be 10% to 15% of the trailer weight or 750 to 1125 pounds, before the WD chains are tightened.
If the trailer pitches rapidly over a hump in the road, this force will vary greatly. A wild guess would be that the force could vary from zero to double throughout the pitching. Meanwhile, I suspect that the horizontal loading from pulling the trailer along the road will stay relatively constant.
These steady horizontal forces are pulling on the hitch in a direction that the hitch is well constructed to resist. The vertical forces, on the other hand, have a bending moment on the welds and bolts holding the hitch to the frame. That is why, in my best guess, the vertical loading on the hitch ball is critical. From post 29, I show that the vertical forces are almost halved for my truck when WD is applied. That, I feel, is why the engineers permit a doubling of trailer weight (and hence hitch weight), when WD is applied.

I don't claim the above description to be strictly proven, but it's my best intuitive feel for the subject. You raise a good discussion point.
Nick
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Old 11-07-2007, 04:09 PM   #77
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B25 Guy:

You're absolutely right about how the forces are transferred to the TV chassis - but I purposely suggested a pin-type connection to remove those factors from consideration.

Nick:

I understand how the equalizer bars serve to distribute the loading among the various connections to the chassis - but the point I'm trying to make is that, if the TV/hitch can handle the horizontal forces created by a 12,000 lb load, and it can handle a 1,000 lb dead load, why not beef the hitch up enough to handle the 12,000 lb load without relying on the effect of the equalizer bars??? This should be a simple enough matter when dealing with heavy duty tow vehicles - however, I'll be the first to admit that lighter duty tow vehicles - and especially those without a chassis - may need all the help they can get to properly distribute the load to the connecting bolts. Again, looking at a more specific scenario regarding load equalizer bars, it's not inconceivable that the bars could completely unload at the point where the dead load is the heaviest - i.e. - a panic stop or a severe bump in the road with upward acceleration of the rear axle of the TV. I believe that there have been several cases of similar hitch failures, reported within the forums, where this very scenario may have been to blame. The 1,000 lb to 1,500 lb dead load range - where equalizers are required for the OEM GM hitch - seems somewhat reasonable, however the 7,500 lb to 12,000 lb range for trailer weights - where the mysterious, unknown value of equalizer bars comes into play - is, IMHO ridiculous.
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Old 11-07-2007, 04:23 PM   #78
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cracker...

it's a receiver not a hitch that you suggests is attached to the tv frame, right?

most (not all) receivers are rated with w/d equipment for loads over 5000lbs or 500 on the ball.

a 12k dead load (1200-1500 lbs on the ball) would still bob up and down...

perhaps more so and over a greater distance than without a w/d hitch.

so the frame/bolts/receiver would need even greater strength to handle a 12k load and proportional ball weight without w/d hardware...

regardless of side bolts or bottom bolts...

side bolts (pin type connectors) attaching receiver to frame, really just weaken the link regardless of hitch type

the equations and spreadsheets are very useful, i think, good job nick.

cheers
2air'
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Old 11-07-2007, 07:12 PM   #79
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Quote:
Originally Posted by 2airishuman
cracker...

it's a receiver not a hitch that you suggests is attached to the tv frame, right?

most (not all) receivers are rated with w/d equipment for loads over 5000lbs or 500 on the ball.

a 12k dead load (1200-1500 lbs on the ball) would still bob up and down...

perhaps more so and over a greater distance than without a w/d hitch.

so the frame/bolts/receiver would need even greater strength to handle a 12k load and proportional ball weight without w/d hardware...

regardless of side bolts or bottom bolts...

side bolts (pin type connectors) attaching receiver to frame, really just weaken the link regardless of hitch type

the equations and spreadsheets are very useful, i think, good job nick.

cheers
2air'
I apologize for the misnomer - yes, I'm talking about the receiver. I'm only using the descriptive "pin" connections or "shear" loads as a simple convention to address the fundamental loads involved. Note that I said to double up on the rear most bolts attaching the receiver to the TV as you suggested might be needed in bold face. It makes no difference whatsoever how the receiver is attached to the TV for purposes of this discussion. Perhaps I haven't been clear that the specifications I've been referencing are for the OEM hitch as installed on most late model 2500 to 3500 series GM vehicles. To wit, the hitch is dead load rated to carry 1,000 lbs without load equalizers, and up to 1,500 lbs with equalizers. It is further rated to tow up to 7,500 lbs without equalizers and 12,000 lbs with equalizers. Again, my point is that the equalizers should not be the element that permits you to go (---or that are required to go-)from a 7,500 lb tow to a 12,000 lb tow without specific reference to the load the equalizers must carry. On the other hand, why do equalizers have any effect on tow capacity at all when all they really do is serve to adjust the effective fore and aft position of the applied dead load? I, for one, very lightly load my equalizer bars as the truck does not need, nor can it effectively use the leverage offered by heavily loaded equalizer bars. The truck's wheelbase is too long, the overhang is too short, and I would be seriously overloading the trailer axle before I effectively transferred any significant weight to the front axle of the TV. That said, I would prefer that the receiver had been designed to simply handle a 12,000 lb tow, which the truck is perfectly comfortable with, without the caveat of needing equalizer bars to somehow make up the difference between towing 7,500 lbs and 12,000 lbs. I would still use the equalizer bars to cushion the hitch set up.

Nick has done a great job with his explanation of the static loads involved in the equalizer setup. Nothing I've said has questioned his presentation. I'm simply trying to resolve what I see as a flaw in the design of the GM receiver on the heavy duty trucks. A secondary question, for other TVs, is - "When do you know that you have loaded the equalizer bars sufficiently to evenly distribute the load to the receiver attachment points on the TV?"
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Old 11-07-2007, 08:10 PM   #80
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Quote:
Originally Posted by Cracker
my point is that the equalizers should not be the element that permits you to go (---or that are required to go-)from a 7,500 lb tow to a 12,000 lb tow without specific reference to the load the equalizers must carry. On the other hand, why do equalizers have any effect on tow capacity at all when all they really do is serve to adjust the effective fore and aft position of the applied dead load? I, for one, very lightly load my equalizer bars as the truck does not need, nor can it effectively use the leverage offered by heavily loaded equalizer bars. The truck's wheelbase is too long, the overhang is too short, and I would be seriously overloading the trailer axle before I effectively transferred any significant weight to the front axle of the TV. That said, I would prefer that the receiver had been designed to simply handle a 12,000 lb tow, which the truck is perfectly comfortable with, without the caveat of needing equalizer bars to somehow make up the difference between towing 7,500 lbs and 12,000 lbs. I would still use the equalizer bars to cushion the hitch set up.
well i'm no hitch expert or psychologist...

so from the bottom of this quote first, and working my way up...

-the 'equalizer bars' don't cushion the hitch set up, they generate a torsional force (greater than the tongue mass) that pushes the steering axle down, right?

so that suggests the hitch is 'stiffened' not cushioned, i think, and allows the drive axle load to lessen some, so is that the cushion part?

-the receiver capacity relates to OTHER capacities on the truck, as in payload, axle rating, wheel tire rating and so on...

-1800lb tongue (12k trailer) load is increased proportionally, by the ball to axle distance...

so the effective tongue load would be 2400lbs+? at the rear axle? and would really eat up the truck payload capacity, rear axle rating and so on...

so it seems, that IF the receiver was could handle a 12000 lb trailer the truck would need beefing up too...as in dually or 4500 series rear ends...

-i completely don't understand your statement...

"nor can it effectively use the leverage offered by heavily loaded equalizer bars" because of wheel base, overhand and transfer issues...

on the scales i've used the w/d bars to move SIGNIFICANT weight forward and very LITTLE back to the trailer.

this is with the longest tv wheelbase available in a light duty truck and the heaviest spring rating offered.

-it looks like you are suggesting that IF a vendor rating indicates 'w/d required for xxx load' they should also include the w/d parameters?

i don't see how that is possible without knowing what's IN the truck...

it's an interesting idea, however.

4 the most part we are using w/d bars to reload the front/steering axle right?

so 'set up' isn't about "evenly distribute the load to the receiver attachment points"...

it's about restoring the tv stance, and we accomplish that my measuring ground heights or axle loads.

the w/d bars by design don't stress the receiver attachment points evenly as they are tensioned, at least that isn't the goal of redistribution, is it?

no doubt i still don't get the question you have...

2air'
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Old 11-08-2007, 07:23 AM   #81
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2air':

Maybe it would help if I once again provided my loaded and unloaded scale weights - noting first that the truck's rear axle rating is 8,550 lbs and the front axle rating is 4,800 lbs. Light, including permanently mounted built-ins under the tonneau cover, the front axle weighs 4,400 lbs and the rear axle weighs 3,960 lbs. Loaded for the road, the front axle weighs 4,200 lbs, the rear axle weighs 4,900 lbs, the trailer axles weigh 7,400 lbs, and the tongue weight is 900 lbs. These weights are with the equalizer bars tensioned and my wife and I in the truck cab. Note that the front axle has unloaded 200 lbs and the rear axle has taken on 940 lbs. The truck rear has dropped about 1.25" - which makes it almost dead level. I'll repeat that I've made modifications to the rear springs which you (2air') commented on some time back herein - but that has nothing to do with this discussion. The unloading of the front axle has little or no effect on the truck handling and it would quickly return to it's unloaded weight of 4,400 lbs with the addition of a couple of passengers. As for the 940 lbs on the rear axle, that does nothing more than make the truck ride smooth right out. With 7,400 lbs on the trailer axles, cranking the equalizer bars up would simply bring their load closer to the trailer 8,000 lb axle rating along with adding to the TV's front axle load. Note that the front axle rating for these dualies, with the Duramax, A/C, 4wd, and crew cab doesn't have much of a reserve - only 400 lbs when light. That said, for me personally, and for many others towing with a heavy duty truck, the equalizer bars do, IMHO, serve more as a cushion and as a device for taking up mechanical slack. Since the chassis extensions on these vehicles should be more than capable of handling a 1,000 lb dead load (---and probably 1,500 lbs with the right receiver---) I cannot understand the reduction in trailer weight stated on the receiver placard (7,500 lbs without equalizers and 12,000 lbs with) when the only critical factor addressed by the inclusion of equalizers would appear to be tongue load.

This is bordering on a hijack of Nick's original topic and excellent presentation - which I never intended it to be, so, 'nuff said!
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Old 11-08-2007, 08:02 PM   #82
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Quote:
Originally Posted by 2airishuman
to restore the pre existing weight to the STEERING wheels and axle.
most of these tow vehicles understeer in normal driving...

reducing the weight on the front axle makes under-steering worse....

along with loss of grip and changes in alignment.

this is the primary reason to use w/d bars, even with a rear end that can 'handle it'...

i think.

cheers
2air'
Well, yes, of course (as I understand it also.) I meant for it to read rhetorically, "why bother with W/D?"

If the distribution is 2/3 forward and 1/3 backward to all axles then the TW just sort of disappears as I see it. The hitch and the receiver now have a spring to counteract the nastier vertical movements.

It'de be nice to mine Rogozinski's brain to see how those early 70's accident predictive factors were arrived at in Caravanner's offices.
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Old 11-08-2007, 10:50 PM   #83
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Well, yes, of course (as I understand it also.) I meant for it to read rhetorically, "why bother with W/D?"

If the distribution is 2/3 forward and 1/3 backward to all axles then the TW just sort of disappears as I see it. The hitch and the receiver now have a spring to counteract the nastier vertical movements.

It'de be nice to mine Rogozinski's brain to see how those early 70's accident predictive factors were arrived at in Caravanner's offices.
The predictive factors were arrived at by very closely examining over 1000 loss of control accidents while towing an Airstream trailer.

Each loss was examined as to how the tow vehicle was rigged, what kind and brand of hitch was used, what rating or ruggedness the suspension system had, what was added to the suspension system of the tow vehicle, type of sway control, if any, etc.

Gathering that information, very quickly raised "red" flags as to what setup was an accident looking for a place to happen, and what on a small scale was still a mystery.

We documented, "and proved," the cause of 90 percent of those accidents.

I will look in my very old records to see if I can find the original 12 questions that were used in each and every one of those losses.

However, posting them today, in this forums, would probably cause a rash of arguments, that I do not wish to start.

Perhaps I could post them as an article in our web site, for everyones perusal.

Andy
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Old 11-09-2007, 03:49 AM   #84
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Cracker, I believe your posts fit in well with this thread as they give a practical application of the hitch calculator. For example, can you put your TV figures into the calculator, but with a 12000 pound trailer weight and a hitch weight of 1460 pounds ( (900/7400)*12000), and then repeat the calculation with a hitch weight of the normally recommended maximum of 15%, namely 1800 pounds? I would roughly guess that the front axle of your TV would be lifted by about 325 pounds in the first case, and 400 pounds in the second, without WD.
It seems to me that the GMC recommendation may coincide with your view. That is, your dually can safely tow up to 7500 pounds without WD. Your trailer weighs 7400, so no problem. If, however, you wish to tow up to 12000 pounds, your front steering axle could be lifted up by about 325 to 400 pounds . The engineers stipulate that this is not safe, (I assume because it is outside the safe design parameters of the steering and suspension) and that you should use WD to reduce or eliminate the front axle uplift. Perhaps it's as simple as that, and perhaps the strength of the hitch is not an issue in the GMC 3500 towing weight limits?
Nick.
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