The unofficial pp users guide...

[Ron Gratz]

Originally Posted by Ag&Au

I don't agree with whatever manual you are quoting. The load placed on the tow vehicle ideally should be proportionally distributed between the front and rear axles.

For instance, lets say the difference between the rear axle's empty weight and max rated weight is 1000# and the difference on the front axle's is 500#, the load should be distributed in such a way so as to put 2/3 of the load on the rear axles and 1/3 on the front. If you unload the front axle at all, you will negatively affect the steering characteristics of the TV. If you unload it enough, it will be unstable and difficult to control.

A different opinion on the question of restoring load to the front axle is offered here for consideration:

Using a WDH to transfer too much load to the front axle can lead to undesirable TV oversteer. Some TV and WDH manufacturers, in the past few years, have changed their WDH adjustment recommendations.

I think the reason Ford, Chevrolet, Toyota, Equal-i-zer, and others have changed their weight distribution specifications is pretty well summed up in this Letter to Editor by Richard H Klein, P E printed in TRAILER BODY BUILDERS Magazine. The comment which specifically addresses front axle load is:

QUOTE
2. The statement “too much tongue weight can force the truck down in the back, causing the front wheels to lift to the point where steering response and braking can be severely decreased” is not the real issue with heavy tongue weights. The real problem is that the tow vehicle's yaw stability, as measured by “understeer gradient”, is severely decreased. This increases the propensity of the tow vehicle to jackknife in turning maneuvers. Specifically, recent full scale testing conducted by the SAE Tow Vehicle Trailer Rating Committee (and now published in SAE J2807), determined that the use of weight distributing hitch torque should be minimized. In fact they recommend that the Front Axle Load Restoration (FALR) not exceed 100% (100% means that the front axle weight is brought back, via weight distribution, to a weight equal to its “no trailer” condition).
UNQUOTE

A related explanation from a representative of the company which manufactures the Equal-i-zer hitch says:

QUOTE
In the past we had suggested that you should see a small drop on the front suspension. We are always trying to improve things here at Progress – our motto is “Safe and Happy Customers,” and so we are always reviewing our instructions and installation process. Recently, as part of this constant effort our engineers looked more deeply into this aspect of installation. We had always felt that a small drop was a sign that the trailer’s weight was being transferred to the front axle, and that this was essentially a good thing.

As our engineers reviewed the instructions for the last round of renewal of our instructions, they found research results that contradicted our prior thinking. There has been a substantial amount of testing conducted by experts from SAE and the RV Industry Association to find out what will produce the best stability when towing. This towing suggests that you want your front axle’s compression to be close to, but not lower than your free-standing height.
UNQUOTE (Underline added for emphasis.)

Ford now says the Front Axle Load Restoration should be approximately 50%.

Chevrolet/GMC now says the FALR should be 100%, 50% or 0% depending on TV model and TT weight.

Reese now says, "A new term in the industry is (“FALR” – Front Axle Load Return). 100% FALR Means the front fender is returned to the preload position. That is our recommendation for best performance."

Equal-i-zer says the Front Axle Load Restoration should be between 50% and 100%.

Equal-i-zer's revised instructions specifically state:
Good adjustment:
You have most likely achieved good weight distribution adjustment if your measurements show the following with the trailer coupled and the weight distribution engaged:
1. From the coupled without weight distribution measurement, the front wheel well measurement is at least halfway back to the original uncoupled measurement. See line C on Front Wheel Well Measure Chart.
2. The rear wheel well measurement is somewhere between the uncoupled height, and the coupled with no weight distribution height. It should NEVER be higher than the uncoupled height. See line C on Rear Wheel Well Measure Chart. See Figure 19.

Ron

[Ag&Au]

Wow,

I'm not going to argue with SAE or equalizer, but I am going to have to do a bit of thinking before that makes sense to me.

Do they carry the same logic over to loading the bed of a truck? In other words are they saying to load the truck so that the rear axle carries all or nearly all of the load or even unloads the front axle to some degree?

I would really like to read something that explains all of this in more detail.

Ken

[Ron Gratz]

Originally Posted by Ag&Au

Do they carry the same logic over to loading the bed of a truck? In other words are they saying to load the truck so that the rear axle carries all or nearly all of the load or even unloads the front axle to some degree?

Ken, the concern expressed by Richard Klein and the statement by the Equal-i-zer representative pertain to proper adjustment of a WDH when towing a trailer.
Carrying a load in a truck when not towing does not present the same problems.

The concern stated by Mr. Klein is that adding too much load to the TV's front axle via the WDH can severely decrease the "understeer gradient". In simpler words -- too much load transfer can increase the tendency toward "oversteer".

Oversteer can result if the TV's front tires are generating too much lateral force and/or the rear tires are generating too little lateral force. Lateral force depends, in part, on the load on a tire -- increasing load generates more lateral force and decreasing load generates less.

How the TV responds to lateral load depends on distance from a tire to the TV's center of gravity. Lateral load tends to make the TV rotate about its CG.

When a vehicle rounds a corner, lateral load on the front tires multiplied by the distance from front axle to CG tends to rotate the front toward the inside of the curve. Lateral load on the rear tires multiplied by the distance from rear axle to CG tends to rotate the rear toward the inside of the curve. The objective is to keep the front-induced torque equal to the rear-induced torque so there is no tendency for the TV to rotate about its CG.

For the case of a TV with no trailer attached -- if the CG of the TV plus its load is moved rearward (e.g. by adding load at the rear of the box), the load on the front axle is decreased and the load on the rear is increased. For a given tire slip angle, lateral force on the front is decreased and lateral force on the rear is increased. However, distance from front axle to CG is increased and distance from rear axle to CG is decreased. Therefore, the difference in product of lateral force times distance to CG for front and rear is relatively unchanged and there is relatively little tendency for oversteer or understeer -- as long as the load change on front and rear axles is not too great.

If we attach a trailer to the TV, we have, in effect, added a weight equal to the trailer's tongue weight onto the ball. Without WD applied, the effect is similar to adding load to the rear of the TV. The major difference is in the relative magnitude of load removed from the front and load added to the rear. For a 1000# TW, we might have 400# removed from the front and 1400# added to the rear.

Lateral force generated by a tire is not a linear function of load on the tire. If it were, the decreased lateral force at the front would be exactly compensated by the increased front-CG moment arm, and similarly for the rear.

In actuality, the rate of lateral force generation decreases as tire load increases. This means that when more load is added to the rear axle than is removed from the front axle, the lateral force generated by the rear axle is not sufficient to keep the front and rear torques in balance. The TV tends to oversteer (rear end wants to go the the outside of the curve), which might be hard to believe since load has been removed from the front axle.

When WD is applied, things get even worse. Application of WD causes load to be added to the TV's front axle without changing the location of the TV's CG. And, load is removed from the rear axle without changing the location of the CG. The lateral force generated by the front axle will increase and the lateral force generated by the rear will decrease.

Since there is no compensating effect due to changing location of the CG, the increased lateral force on the front will tend to make the front of the TV turn toward the inside of the curve.
The decreased lateral force on the rear will tend to make the rear of the TV swing toward the outside of the curve.
This can lead to a serious oversteer situation which could result in loss of directional control.
This all means that, from a yaw stability point of view, it is best to minimize the amount of weight distribution.
Of course this concern must be balanced by the concern for axle load distribution.
It appears that the RV trailer towing industry is moving toward a 50% FALR as being a good compromise.

I would really like to read something that explains all of this in more detail.

So would I. Then I wouldn't have to wonder if what I'm saying is really correct.

The Richard H. Klein Bibliography has some papers which might be of interest. Start with #5 under DOT Reports and B under SAE Papers

Ron

[Ag&Au]

Ron,

Thanks for taking the time to compose all of that. After reading through it, it is obvious that it will take some slow and cautious rereads along with drawing some pictures for myself as I go. Maybe this a good excuse to buy a toy truck and trailer. My brother inherited all my toys when we were growing up.

If I run into something I can't get my head around, I'll send you a PM in order to avoid having to do my thinking in this thread.

Ken

[SteveSueMac]

Ken - I love learning about stuff like this and Ron is a wealth of knowledge. Maybe we can start a new thread (if there isn't one already) so we can all learn together.

As for this thread, I'll bump my own question about instructions (hopefully video) of swapping out WD bars.

[Ag&Au]

Originally Posted by SteveSueMac

As for this thread, I'll bump my own question about instructions (hopefully video) of swapping out WD bars.

Check out these pictures of an installation someone posted on flickr.

http://www.flickr.com/photos/rests/s...7618684477177/

(section 6 shows the bars)

The WD bars are installed after the hitch head is mounted. I believe all you have to do is detach the bars from the jacks and then remove the retaining bolt, keeper, bushing ,and bar from the head. then reassemble with the new bushings and bars.

If the pictures don't help, let me know and I will dig my manual out and figure out the steps for sure.

I will be very surprised, but happy for you, if you find any specific bar swapping info, other than from Sean.

Ken

[SteveSueMac]

Originally Posted by Ag&Au

Check out these pictures of an installation someone posted on flickr.

http://www.flickr.com/photos/rests/s...7618684477177/

(section 6 shows the bars)

The WD bars are installed after the hitch head is mounted. I believe all you have to do is detach the bars from the jacks and then remove the retaining bolt, keeper, bushing ,and bar from the head. then reassemble with the new bushings and bars.

If the pictures don't help, let me know and I will dig my manual out and figure out the steps for sure.

I will be very surprised, but happy for you, if you find any specific bar swapping info, other than from Sean.

Ken

Thanks - those are good. I'll look along with my manual.

[chracatoa]

Sometimes when I'm turning after a full stop I hear something slipping in the hitch. I can't tell what it is - maybe it's the spring bar?

I was taking the trailer for service today and it was easier to reproduce it because it was raining which probably made the hitch parts more slippery.

As I said, this happens often but nothing seems wrong with the hitch afterwards. Did anybody notice something similar?

[Ag&Au]

Originally Posted by chracatoa

Sometimes when I'm turning after a full stop I hear something slipping in the hitch. I can't tell what it is - maybe it's the spring bar?

I was taking the trailer for service today and it was easier to reproduce it because it was raining which probably made the hitch parts more slippery.

As I said, this happens often but nothing seems wrong with the hitch afterwards. Did anybody notice something similar?

Check the nuts on the yoke frame brackets. If they are loose enough the yoke could be moving when you turn. As long as they are lubricated generously through the zerk fittings, the bars shouldn't be making any noise.

Ken

P.S, Do you have an explosive personality?
chracatoa sounds a lot like Krakatoa.

Krakatoa - Wikipedia, the free encyclopedia

[chracatoa]

Originally Posted by Ag&Au

Check the nuts on the yoke frame brackets. If they are loose enough the yoke could be moving when you turn. As long as they are lubricated generously through the zerk fittings, the bars shouldn't be making any noise.

Ken

P.S, Do you have an explosive personality?
chracatoa sounds a lot like Krakatoa.

Krakatoa - Wikipedia, the free encyclopedia

They are very tight - It actually moved at some point away from the center. But I fixed that.

I guess I'm going to draw some marks with chalk and check if anything moved.

Not explosive - it happens to be my alias for a computer game during the 80's

[ghaynes755]

So we picked up our new FC 25 Rear Bed this past week. Colonial did the PP install and after a bit of tuning we hit the road. I got to a CAT scale on our return to Colorado. Made some adjustments to the jack heights, settling on 6 inches. Towing was perfect, even when we hit a wind storm with 30-40mph cross winds and a dust storm brown out (dust in every upwind side of the openings).

While the data points on that initial weighing weren’t complete I was concerned about the totals.

Today I went to the CAT scales locally and ran the full list of data points. 5, 6 and 7 inch jack heights, WD loose and the truck.

My take away is that my 2014 GMC Sierra 1500 is maxed out, both with weight and axle limits. Weights were with full fuel, full fresh water, waste tanks empty, some but not all of our gear in the trailer and the truck bed empty. Wife and dog were not included.

So would appreciate a look at the data and your inputs and suggestions. Also wondering what the upper limit folks have seen on jack heights. That might transfer more to the front axle but won’t likely change the overall weights.

[SteveSueMac]

Ghaynes - can you post pics of the tickets?

[ghaynes755]

Steve here you go. The data was entered in Ron's format in the spreadsheet that is in the PDF file.

[SteveSueMac]

Sorry - I apparently don't see PDFs with the version I use. So I'm probably missing all the info about axle ratings and such.

As for weight distribution, looks similar to mine meaning, lots of jack lift without fully restoring the front axle load. What bars do you have and what does your truck manual say you need to restore (in my case it's 50%, not 100% ).

Since you're not restoring all the front, you are probably concerned about the rear and I don't know how far off it is. I'll see if I can see the rest of your info from a web version.

[Ridgerunner3]

I am not an expert on this but I do have a Propride hitch that I installed and setup myself and I have 25' Safari.

I looked through your data. My thoughts would be to increase the jacks to 8" and weigh again. You may also find that you need to change the tilt on the hitch head to give you even more range for WD. I think with your truck you need to restore more weight to the front axle to gain some capacity for the rear axle.

My setup is somewhat similar to SteveSueMac. I have a 2012 Chevy 2500HD with considerable payload capacity. I pull a 25' Safari with a calculated tongue weight very close to yours. I have 1000# bars on my Propride and restore about 60 - 70% of the weight to the front axle on purpose. I like the way the truck handles with the 60 - 70% restoration. But like I said this is a 2500HD with over 3000# payload capacity.

Looking at your data it appears you have a payload capacity of about 1300 - 1500 lbs. Is this correct? Your calculated I believe the payload capacity is on a sticker on the driver's door frame.

[ghaynes755]

Thanks for the replies. Steve, attaching a JPG of the PDF spreadsheet.

Bars are 1400#. Adjusting the hitch to put more to the front would be increasing the upward tilt, correct?

Max total weight for the truck is 7200#. Moving the weight from back to front wouldn't change the overall total weight would it? Total weight of truck by itself was 6120#. With the 1000# of tongue weight of the 2 truck axles is 7100# assuming I am following the math and logic of WD.

[ghaynes755]

Thanks for the replies. Steve, attaching a JPG of the PDF spreadsheet.

Bars are 1400#. Adjusting the hitch to put more to the front would be increasing the upward tilt, correct?

Max total weight for the truck is 7200#. Moving the weight from back to front wouldn't change the overall total weight would it? Total weight of truck by itself was 6120#. With the 1000# of tongue weight of the 2 truck axles is 7100# assuming I am following the math and logic of WD.

[m.hony]

Looks to me like the Cat scales increase/decrease in 40# increments...

[Ridgerunner3]

Moving more weight to the front axle of the truck through the WD system will also move weight to the TT axles. I am not sure how much weight will move. But it would worth a trip to the scales to find the best option to give you some payload capacity.

Your trailer axles are probably 3500# axles. I believe your trailer has a gross rating of 7300#.

Do you have any items packed in front of the trailer axles that could be moved back to reduce tongue weight?

[Alphonse]

Ghaynes,
Thanks for the spreadsheet. I printed it so I could mark on it and study the numbers.

A couple of questions:
1. when you weighed the truck alone, did you have the PP stinger in the receiver?
2. what is the front axle weight restoration specification on your truck. You are at ~50% as is.

Thanks,