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Old 11-25-2008, 03:18 PM   #1
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Weight distribution - I still don't get it

I apologise for raising a subject that has been discussed thousands of times
before. I can understand the need for light w/d bars on a heavy duty truck
but I don't understand how I can transfer in excess of 1700 lbs on back axle
with light, say 700 lb, bars.

Airstream tongue weight 870 lbs, motorcycle 694 lbs plus spare lpg bottle,
generator, etc. in all over 1700 lbs.

Weight distribution hitch (Robin Industries - now deceased) with 1400 bars,
1.1/2" dia., recommended. Truck has heavy duty spring package, 11,400 lbs, which are longer than our last truck at 5' 8" long and initially loaded seem quite soft. Hitched up and loaded, rear of truck sinks 3", helper leaf just touching stops. Tensioning the bars two links raises rear of truck by only 1/2" clear of helper leaf stops. Truck and trailer are both level.

Ride, while not 70's Lincoln Town Car, is very smooth. Back roads can be harsh but we slow down.

Can anyone see anything wrong with the above set up?

Tow vehicle is 2008 F350 super duty, extended cab, long bed 6.4 diesel.

Thanks, John
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Old 11-25-2008, 03:46 PM   #2
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You have got 2 different issues here.

I assume the generator and bike are in the back of the truck. If so you are only transferring the the trailer tongue weight.

What I do to set up a trailer. Load the truck as if you were going out.

Tow to a level site and come to a stop over at least a 75 ft distance using the trailer brakes. This insures things are straight.

In place jack up the trailer off the ball. Measure and record the fender heights of truck down through the center of the wheel. I find it easier to put a piece of masking tape on the fender and marking a line on the tape and measuring to that line.

Now drop the trailer on and hitch up using the bars. If you are lucky the front fender will drop about 40% of the drop of the rear fender. If so you are good to go.

If not you need to adjust the hitch head. Tilting it back will increase the load transferred to the front axle and tilting it forward will decease the load. Make the adjustment in the head angle and not by increasing or decreasing the number of chain links. Keep in mind that as you change the angle of the hitch head you also change the way the bars sit on the cams so you may have to loosen the U bolts that hold the arms to the trailer.

Make the necessary adjustment till you have the weight distributed and the cams sitting in the saddles of the bars. Now tighten things to manufactures specs.

This description is for a Reese hitch but the principle is the same. You want the hitch adjusted in the straight ahead towing position and some weight added to the front axle to reduce sway.

Your bars will not deflect much because of their weight. The important thing is to place some weight on the front axle

This is not often a 5 min. job.
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Old 11-25-2008, 07:42 PM   #3
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With the light bars you may still want to consider th air safe hitch that has been discussed on past threads. I tow with a 3500 and use one. It gives the truck and trailer a soft ride with weight distribution. I use 600 lb bars with my Reese dual-cam. I like pulling a nat with an elephant without punishing the nat. Duallys are made to tow and do it well wIth maximum control and traction. It may be overkill but with the air safe hitch you eliminate the harsh ride.
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Old 11-25-2008, 08:29 PM   #4
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John, to help understand what occurs when the bars are loaded, you might like to read this thread:
http://www.airforums.com/forums/f464...sis-19236.html
Nick.
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Old 11-26-2008, 03:03 AM   #5
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You should only be trying to transfer half of the 870 pounds of trailer tongue weight to the front axles. This means only 435 lbs. which is only a fraction of your 1.400 pound rated bars. The stiff bars will add signifigant forces to your unit when the truck back wheels go through a dip or depression in the road. You need to buy lighter bars for your rig or buy a new hitch with lighter bars. If you do not, you can expect signifigant damage to the A frame and attachment of the frame to the front shell of your unit. The first signs are usually wear or loosening of the rivets holding the shell to the vertical plate (welded to the frame) in the front of the unit.
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Old 11-26-2008, 06:35 AM   #6
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Weight Distribution Hitch

John
I really didn't understand how the thing worked until I went to a truck scale and played with it. Here in the US there are CAT scales (Certified Automated Tare Weight Scales). They charge 10.00 for the first pass over the scales and 1.00 for each additional pass for the next 24 hours.
Anyway,
Here is a post I did about setting up my dual cam Reese.

Setup
The best way to set up the Airstream is with scales.
You need to go to a CAT Scale and expect to spend some time.
First you must adjust the ball height to make the weight on both axles the same (yes weigh each axle seperately). Make a reference mark on the tongue. This is what the tongue height must always be.
Next adjust the spring bars/trunion bars until the weight on the front axle and rear axle of the tow vehicle is the same with the tongue height noted before.
Then tweak the number of links under tension until the weight on all four axles is within 100 pounds of each other.
The change in overall handeling is dramatic.
I see no sway control on your unit.
This is a must.
Friction is a whole lot better than nothing.
Dual cam/dual cam high performance/equalizer is better.
Hensley Arrow/Pro Pride is better yet.
Pull Rite is best (according to simple physics) but it requires alteration of the tow vehicle.
I have the Reese Dual Cam (the old one) and have learned the hard way that that in conjunction with proper electric brake adjustment it works just fine.
When you finish the rig may not be perfectly level but what matters in the weight on the axles, not how it looks.
Beginner

Remember:
1. Equal weight on the two trailer axles is the first and most important consideration reguardless of appearance. There is no equalizer link between the front and rear axle so the ball height is what determines the load on each axle. It must be the reference height determined above when finished no matter how many times you have readjust the angle of the head on the shank.
2. Equal weight on all four axles is the best setup for braking and stability. This may not be attainable so get as close as possible.
Put your heavist toys in front of the rear axle of the pickup truck (between the front and rear axle).
3. The truck and trailer might be a little off level when the weights are right. This is caused by the different ACTUAL load capacity of the old axles on your trailer (after several years they get tired, mine are).
4. Use the lightest spring bars you can get away with. There should be 1 to 2 inches deflection from rest (on the spring bars) to be best to attain the reference mark height and equal weight on all axles. My tongue weight is 850 lbs and I use 750 lb bars. These spring bars provide a flexable link between the truck and the trailer. I learned years ago that unless you provide a flex point between the trailer and the truck the trailer or the truck will find one on its own and I guarantee you will not like it.
Beginner
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Old 11-26-2008, 06:43 AM   #7
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HowieE - many thanks for response. Our hitch is not the dual cam sway control type. I've set the ball mount tipping back slightly so that when hitched up the bars are parallel to trailer chassis (as per manufacturer's instructions). Will check all heights when somewhere level. I still feel that lighter bars will just flex and transfer no weight to steering axle. These 1400 bars flex just over 1/2" - measured by string line. I haven't strength to get more than 2 links under tension without using power jack to raise, which I am reluctant to do - too much strain.

PhilGobie - yes looked at that hitch but we don't have a harsh ride.

nickcrowhorst - thanks. Read your analysis some time ago but need to read it over and over for findings to sink in. The soft rear suspension of 08 trucks must change the picture.

dwightdi - thanks for input, but I can only see light bars flexing and not transferring weight. Our bed mass is over 1700 lbs which cannot be redistributed with bars rated @ say 600 lbs.

I am concerned about trailer damage but I don't want the truck sitting down 3" at the rear, that's why I'm using heavy bars. 90% of the trailer weight is on the axles not the hitch and we have a smooth ride. I am still not convinced that I need to change these bars. I think the Ford video on 2009 F150 truck suspension changes the w/d bar rules.

Not a good attached photo but hitched up everythings looks ok to me.

John
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Old 11-26-2008, 08:53 AM   #8
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Quote:
Originally Posted by stephenson View Post
I still feel that lighter bars will just flex and transfer no weight to steering axle.
The fact that they flex has absolutely no bearing on how much weight they're transferring; it just means that they have some give to them, which is what a "suspension" is supposed to do. Are your truck's rear springs not holding up the back of your truck, because they flex? Should I take them out, and weld a solid steel i-beam between the axle and the frame, to make sure that they are in fact holding the truck up?
I've used a 6' long steel pry bar to move boulders. Some were large enough to make the steel bar bend a bit, but the rock flipped over...it transferred the weight.
your WD bars are both a lever, and a spring. Certainly, if we tried to use a rubber hose, that would be "too light", and not transfer any weight. But on the other end of the spectrum would be like welding a steel beam from your truck frame to your trailer's a-frame...What would that do to your trailer? same thing that "too stiff" WD bars would do.

Quote:
Originally Posted by stephenson View Post
. I haven't strength to get more than 2 links under tension without using power jack to raise, which I am reluctant to do - too much strain.
...if you've got bars that are so stiff.

Quote:
Originally Posted by stephenson View Post
Our bed mass is over 1700 lbs which cannot be redistributed with bars rated @ say 600 lbs.
again: you're not re-distributing the entire bed mass; you're only re-distributing the trailer's tongue weight, and ONLY the trailer's tongue weight. And some of that is going back to the trailer's axles, too. What needs to go to the front axle of your truck is less than half of the TW, which can easily be done w/ 600lb bars.

Quote:
Originally Posted by stephenson View Post
I think the Ford video on 2009 F150 truck suspension changes the w/d bar rules.
has nothing to do with anything.
Quote:
Originally Posted by stephenson View Post
Not a good attached photo but hitched up everythings looks ok to me.

John
it looks close to level, which is all you can tell from a picture.
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Old 11-26-2008, 08:54 AM   #9
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You still do not get it. You are not trying to remove the load from the equipment in the box of the truck to the front wheels. The truck was engineered to handle properly with a load in the box. I am a mechanical engineer but this thread is not sufficient to train you on how to do a free body diagram and sum of moments equations to show you that just the absolute amount of the defection has nothing to do with the amount of weight transferred. If you are having a difficult time pulling up the bars, believe me, you are trying to transfer too much weight. If you do not mind destroying your trailer continue as you are. Otherwise look at the other threads suggested or take your unit to a high quality trailer hitch shop and have them help you modify your setup or sell you a new properly engineered one.
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Old 11-26-2008, 09:19 AM   #10
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OK Your picture looks pretty good.

Make sure to check the front axle drop with the bike in the bed before and after you hitch the trailer. This insures that the trailer will not have an adverse sway effect on the truck.

As for having to lift the tongue to pull up the bars that is normal. Those that claim they do not have to do this are just testifying that they have not set up the hitch correctly and not getting any effect out of the system.

As for your heavy bars. I have used 1,000 lbs bars on my trailer for over 120,000 miles and now using with newer 1,400 lbs bars. Yes the initial deflection in the bars will be less as the bar weight increases. I prefer the heavier bars and thus a reduced porpoising ride. If you lived in New Jersey you would understand
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Old 11-26-2008, 01:37 PM   #11
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Beginner, Chuck, dwightdi & HowieE, thanks you guys for coming back. Yes, I still don't get it and yes I'm trying to transfer tongue weight + weight of equipment carried in truck bed. I've made notes of all the things to do from posts and will get down to some measuring and weighing when at a level campground as suggested. As a relative newcomer to US travel trailer towing, I didn't have much information available as regards hitches. I knew the specs for truck and the weights and dimensions for trailer and I selected the hitch from the Reese formula. Hitch weight (spring bars) = tongue weight of trailer + cargo load (everything carried behind rear axle). Following the manufacturer's hitch selector seemed like a sensible idea.

John
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Old 11-26-2008, 01:43 PM   #12
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John,

I think you're going about this correctly. Despite what you may have read above, the weight in the back of the truck does play into the load equation, and you will find that heavier bars are needed to redistribute part of the weight to the other axles.
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Old 11-26-2008, 02:37 PM   #13
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John,

I think you're going about this correctly. Despite what you may have read above, the weight in the back of the truck does play into the load equation, and you will find that heavier bars are needed to redistribute part of the weight to the other axles.
Unless the truck bed load center of gravity is behind the rear axle and causing the front axle to rise before the trailer in hitched I would not consider that as part of the equation. The purpose of a WD Hitch is to insure that the trailer tongue weight attached well behind the rear axle does not cause the front axle weight to be reduced and thus reduce steering control imparted by the trailer. As I mentioned I like to see about a 40/60 ratio of reduction in fender height measurement when the trailer is hitched. Now this is clearly subject to the suspension of the TV. The Lincoln Continental with it's marshmallow soft springs would require a more equal distribution and conversly a Ford 350 truck with rock hard rear springs would want less depression forward.

Keep in mind the WD hitch was introduced when we were towing with Caddies and Lincolns and the front ends would have the headlights in the trees.

I did not have a WD hitch on my original trailer as noted in the attachment
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Old 11-26-2008, 06:42 PM   #14
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Quote:
Originally Posted by stephenson View Post
I apologise for raising a subject that has been discussed thousands of times
before. I can understand the need for light w/d bars on a heavy duty truck
but I don't understand how I can transfer in excess of 1700 lbs on back axle
with light, say 700 lb, bars.
John, you cannot remove 1700# from the rear axle using 700# bars and should not attempt to do it using 1400# bars. Furthermore, the amount of load added to your rear axle, based on the numbers you have given, will be closer to 2150#.

Let's assume your TT's tongue weight is 900# and you have a load of 800# in the bed of the TV with the CG of that load located exactly over the center of the rear axle.

Without any weight distribution being applied, the TW will cause a load of approximately 1350# to be added to the TV's rear axle and will cause a load of approximately 450# to be removed from the front axle. Exact load values will depend on TV wheelbase and ball overhang distance.

The cargo in the bed of the TV would cause an additional load of 800# to be added to the rear axle and zero load to be added to the front axle. The axle load changes, without WD being applied, would be 2150# added to the rear axle and 450# removed from the front axle.

Let's assume you apply just enough load to the WD bars to cause the front of the TV to return to its unloaded height. This would imply that the WD system has caused a load of 450# to be added to the front axle. This would further imply that a load of approximately 225# was "transferred to the TT's axles. Exact load transfer values will depend on TV wheelbase, ball overhang, and TT ball-to-axles distance.

For the above assumptions, if your WD system caused 450# to be added to the front axle and 225# to be added to the TT's axles, then 675# would have been removed from the TV's rear axle.

The net load change effect would be 0# (-450+450) added to front axle, 1475# (+1350+800-675) added to rear axle, and 225# added to TT's axles. For these assumptions, instead of removing all of the added load from the TV's rear axle, you have only removed less than 1/3 of it. So, the rear of the TV is still considerably lower than its unloaded height.

Also, for the above assumptions, the load on each WD bar would be approximately 700#. Exact values depend on WD bar length and TT ball-to-axles distance. To remove all of the added load from the TV's rear axle, the load on each WD bar would have to be approximately 2100#.

For many TV/TT combinations, about the best that one can hope to achieve from a WD system is to return the front axle to the "unhitched" load (i.e. the front wheel well height is the same before and after the TT is hitched and WD is applied. In fact, some TV manufacturers recommend that the front of the TV, after WD is applied, should not be lower than its "unhitched" height.

Ron
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