Load distribution hitches-an analysis

[nickcrowhurst]

I've been asking myself a few more questions about load distribution. I regularly read that the ideal distribution of "tongue weight" is an equal one third to each of the TV (tow vehicle) axles and to the trailer axle system. I have never seen any justification for this, either by practical experiment or by engineering calculation. Why should it be ideal to increase the "weight" of my trailer by the same amount as each axle of the TV? Why should I attempt to load the front axle of my truck by the same amount as the rear axle, when the truck is designed to take heavy and varying loads on the rear axle, and a moderate but fairly constant load on the front axle? If I go to a quarry for a load of stone, I center the weight over the rear axle, and just ensure that the front axle is not lifted. I decided to examine this issue with the aid of the equations listed at post #1 of this thread.

From equation 1, C, the corrective force required to load down the front axle of the TV is given by C= (T*H)/W , where T is the tongue weight, H is the rear overhang of the TV, and W is the wheelbase of the TV.
From equation 10, E, the final extra load on the TV rear axle, with chains tightened, is given by E=(T*L)/(H+L) , where L is the distance from the hitch to the center of the trailer axle system.

For these two loads to be equal, C = E, hence:
(T*H)/W = (T*L)/(H+L) , and this equation transforms to the quadratic:

(H^2) + (H*L) - (L*W) = 0 Equation 11, the condition for equal loading of the TV axles.

Given that W , the truck wheelbase, and L, the axle distance of the trailer, are fixed, what rear overhang will produce the desired equal loading? Putting my values of L = 170, and W = 160 into equation 11 and solving for H indicates that a rear overhang of 100 inches would be required, rather than the current 60. This huge overhang would be ridiculous and would dangerously increase trailer sway. I therefore conclude that, for my truck and trailer, equal TV axle loading is neither feasible nor desireable.
Those interested in playing with numbers may like to discover what TV wheelbase would be required for equal loading with my present rear overhang and current trailer. The answer is a wheelbase of 81 inches, instead of the current 160. Again, quite absurd. For more amusement, what length of trailer could I tow which would give equal loading, given my current truck? The answer is 36 inches from hitch to axle. Even more absurd.
To calculate the load on the trailer axle when the overhang is increased to 100 inches to equalize the TV axle loadings:

A= (T*H)/(H+L) , equation 4, giving the load transferred to the trailer axles

For my theoretical truck with the huge rear overhang of 100 inches, the TV axles are loaded with 500 pounds each, and the trailer axles by 300 pounds.

I conclude from all this that equal loading of the axles is neither possible nor desireable with my truck, and this is confirmed in practice. Others may wish to try out the numbers for their own rigs.

An Excel spreadsheet that performs these calculations has been posted at post 69 in this thread, http://www.airforums.com/forums/f464...tml#post475983

Nick.

[2airishuman]

Originally Posted by nickcrowhurst

I've been asking myself a few more questions about load distribution. I regularly read that the ideal distribution of "tongue weight" is an equal one third to each of the TV (tow vehicle) axles and to the trailer axle system. I have never seen any justification for this, either by practical experiment or by engineering calculation...

hi nick i agree.

no doubt this is another dictum without complete proof...

perhaps it comes from the days of towing with an impala or country squire...

but most of the towing guides provided by the auto companies and hitch makers STILL include this info...

my goal was to get the tv axles close to 50/50 when hitched and distributed,

and restore steer axle mass to at LEAST the original unhitched number.

with the diesel up front, the super duty was nose heavy un hitched.

so for me load redistribution isn't the same as simply transferring equal amounts to the 3 axle regions...

i've since tried allowing the drive axle to carry a greater load than the steer axle (44f/56r) while towing.

the ONE negative with this arrangement was 4/32nds GREATER tire wear on the drive axle between tire rotations (5kmiles)

cheers
2air'

[fastrob]

Impressive analysis folks.
My father was a truck driver. I spent a lot of time loading and unloading trucks. The way a person can load a trailer is amazing, distributing weight front to back and side to side. Just shifting weight can change everything to do with handling.
Also, when you consider what tire pressure can do to handling it is almost beyond words.
I like to look at a tow vehicle/trailer loaded for travel on a level surface. Level is best. Keep it simple.

R

[slowmover]

A good FF/RR weight bias would seem to be ideal for the TV.

Solo, mine is at 58%FF, not good. Bringing that down to 52-53% would seem to be acceptable no matter the actual rear weight (respectful of GAWR). Bringing it past 50% starts to raise questions in my mind.

Let's remember that the old cars were also nose heavy with a medium or big block engine. My recently departed 4,780# 1971 Chrysler was 56%/44% FF/RR after pains to get the suspension to stock or better on a mildly used family car; a number that was typical. I've seen this approximate bias also on a 5,650# 130" wheelbase Imperial.

In sense, why bother with W/D if the rear "can handle that and more"?

[2airishuman]

Originally Posted by REDNAX

...In sense, why bother with W/D if the rear "can handle that and more"?

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'

[HiHoAgRV]

Originally Posted by 2airishuman

...most of these tow vehicles understeer in normal driving...
2air'

Whoo hoo! the thought of oversteer while towing gives me the willies
and I'm the guy that made it hard to pass on the race track by spending more time sideways, thus using more track width

I use WD when-
1) the back squats
2) the f/r exceeds 50% rear
or 3) the rear approaches GVWR

Things have changed since the Estate Wagon TV

Top 10 thread Nick!

[Cracker]

Good discussion - but Nick has forced unpleasant memories of all-night sessions doing my Statics and Strength of Materials homework at the Univ. of Florida. In my own case I use 1,000 lb bars - no sway control - with 900 lbs on the hitch - measured with a Sherline scale. Unloaded my truck's front axle weighs 4,400 lbs and the rear axle weighs 3,960 lbs (---notwithstanding a fixed load in the bed.) Loaded, the front axle unloads 200 lbs to a net 4,200 lbs, the rear axle goes to 4,900 lbs, and the trailer axle reads 7,400 lbs. See the rig configuration below. I'm real happy with this setup but recently I was made aware of a factor that I had previously overlooked on my OEM GM hitch. i.e. - the hitch rating is reduced to 7,500 lbs capacity in a dead-load configuration, vs 12,000 lbs with equalizers. You have to look at the placard on the hitch to find this figure since the GM manual only addresses sway control with reference to the 7,500 lb figure. My primary use of the equalizers is to provide a cushion between the TV and the trailer - which they do very nicely. The 900 lb hitch load is within the dead load capacity of the hitch (---which, of course, goes to 1,500 lbs with an equalizer hitch - a change that would appear to be reasonable ---) and well within the rear axle capacity of the truck. What bothers me is that, by the simple incorporation of the very lightly loaded equalizer bars, the hitch towing capacity goes from 7,500 lbs to 12,000 lbs! With full understanding of the basic moment and torsion factors involved, what gives lightly loaded equalizer bars the ability to increase my towing capacity by 4,500 lbs??? I know that others have experienced failures with the OEM GM hitch and now, I'm personally beginning to question its design! My thinking is that the hitch should be capable of handling 12,000 lbs - with or without load equalization - subject only to the dead load restrictions.

[nickcrowhurst]

Originally Posted by Cracker

What bothers me is that, by the simple incorporation of the very lightly loaded equalizer bars, the hitch towing capacity goes from 7,500 lbs to 12,000 lbs! With full understanding of the basic moment and torsion factors involved, what gives lightly loaded equalizer bars the ability to increase my towing capacity by 4,500 lbs???

Cracker, I agree that this seems strange, so I examined this point at post #29 on page 3 of this thread. I investigated the issue with the equations, and proved to myself that the capacity of the hitch can indeed double when WD bars are used.
Nick.

[nickcrowhurst]

The forum administrator has kindly enabled .xls files, so the calculator should now be freely available to all who wish to download it. Just change the input values in the top cells to your own, and the outputs will be re-calculated. Have fun!
Nick.

[markdoane]

Originally Posted by nickcrowhurst

Have fun!
Nick.

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. . . .

. . . . sorry, I am over my download limit. If you want to see it, send me an PM with your email address and I will send it to you.

[flitzwhopper]

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.

[CanoeStream]

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:

[buzzkat]

....gee...we're outta beer again!.....

[Cracker]

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!

[B25guy]

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

[nickcrowhurst]

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

[Cracker]

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.

[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'

[Cracker]

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?"

[2airishuman]

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'