Spring bar selection

Remember that a properly loaded TW will be around 12.5% of the trailers ready to travel weight. so lets say you loaded it up to it`s full GVWR. that would put the loaded, ready to travel TW at 1180lbs +/-. what you have will be a perfect match.

What is the trailers dry weight?

The published weight is for an empty trailer so that number is going to up as you load it. By how much? Depends on the trailer and how you load it. I'd suspect it will go up by more than 100lbs though, exceeding the 1200lb bars.

What's the empty weight of the trailer that they use to calculate that tongue weight? With a GVWR of 9500lbs I'm going to guess it's around 8000lbs empty. At 1100lbs you would be nearly 14% of the trailer weight on the tongue. If that stayed close to the same as you load it you would be over 1300lbs as you approach the GVWR.

You may also want to verify if published tongue weight includes full propane tanks and a battery or not. Most brochure weights are "dry" weights and do not include those or added options. I'd have the dealer weigh it for you before deciding.

MStat wrote:
Are the 1200 lb. bars stout enough for the "published" tongue weight of 1100lbs.

Spring bar capacity should be based on how much load transfer you want to achieve.

Since 2010, Ford has been specifying that the WD bars should be adjusted so you restore only about 50% of the load which was removed from the front axle.

If only 50% of load is restored, the WD bars only need to be rated for about 50% of the tongue weight.
If you plan to use the outdated approach of restoring 100% or more, the WD bars should be rated for 100% of the tongue weight.

In a different thread, you indicated the new trailer would have a loaded weight of 9000#.
If that's the case, a reasonable TW% of 13% would give a TW of just under 1200#.

If you want to go with 100% front axle load restoration, the existing 1200# bars should be okay.
If you want to go with 50% FALR, you could stay with the 1200# bars or switch to bars with a lower rating.

Ron

Ron Gratz wrote:

MStat wrote:
Are the 1200 lb. bars stout enough for the "published" tongue weight of 1100lbs.

Spring bar capacity should be based on how much load transfer you want to achieve.

Since 2010, Ford has been specifying that the WD bars should be adjusted so you restore only about 50% of the load which was removed from the front axle.

If only 50% of load is restored, the WD bars only need to be rated for about 50% of the tongue weight.
If you plan to use the outdated approach of restoring 100% or more, the WD bars should be rated for 100% of the tongue weight.

In a different thread, you indicated the new trailer would have a loaded weight of 9000#.
If that's the case, a reasonable TW% of 13% would give a TW of just under 1200#.

If you want to go with 100% front axle load restoration, the existing 1200# bars should be okay.
If you want to go with 50% FALR, you could stay with the 1200# bars or switch to bars with a lower rating.

Ron

I'm curious..
What is your source for using 600 lb WD bars for a 1200 lb tongue when doing restore of 50% of weight removed from front end of truck? My understanding is that you size the WD bars by tongue weight and then adjust properly to get front end of TV to the right spot.

Thank you for the reply's.

Unloaded trailer weight per sticker on door was 8054 lbs.

owenssailor wrote:
What is your source for using 600 lb WD bars for a 1200 lb tongue when doing restore of 50% of weight removed from front end of truck? My understanding is that you size the WD bars by tongue weight and then adjust properly to get front end of TV to the right spot.

First, we need to understand what is meant when a WD bar has a rating of, say, 1000#. Here, we need to make some assumptions because I've never seen a manufacturer's definition of bar rating.

We know that a WDH traditionally is called an "equalizer". And we know that a WDH traditionally was used to achieve equal load being added to the TV's front and rear axles.

Some WDH manufacturers still adhere to the "equal squat" approach when telling customers how to adjust the WDH.
Therefore, I believe it is safe to assume that you can use a WDH to achieve "equal squat" without exceeding a bar's working stress limit.

A WDH is a relatively simple mechanical device, and it can be shown using basic physics how much bar loading is required to achieve a certain amount of load transfer.
For typical TV/TT/bar dimensions (TV ball overhang = 60", TT coupler to axles' midpoint = 240", effective WD bar length =30"), it can be shown that each WD bar must be loaded to about 110% of tongue weight to achieve equal squat.
If the above assumptions are reasonable (including the "equal squat" design loading condition), it is safe to assume a bar rated for X# can handle an uplift force of 1.1X#.
Therefore, it is reasonable to assume that an uplift force equal to 110% of a bar's TW rating will not overload the bar.

If your objective is to achieve equal squat, you should choose a bar with a rating equal to or greater than the TW.
If your objective is to restore the front axle load to its unhitched value (100% Front Axle Load Restoration), the load on each bar only needs to be equal to about 80% of TW -- which we can assume would load the bar to about 80/110 = about 73% of its working stress limit.
For 50% FALR, each bar would need an uplift force equal to about 40% of TW.

For a rule of thumb with an added safety margin of about 25%, we can say that:
Minimum WD Bar Rating = FALR times Tongue Weight

Again, this all is based on the assumption that, with WD bars having a TW rating equal to the TT's TW, and with both bars subjected to an uplift force equal to their TW rating, the WDH would generate enough load transfer to achieve "equal squat" without overloading the bars.

Ron

Ron Gratz wrote:

MStat wrote:
Are the 1200 lb. bars stout enough for the "published" tongue weight of 1100lbs.

Spring bar capacity should be based on how much load transfer you want to achieve.

Since 2010, Ford has been specifying that the WD bars should be adjusted so you restore only about 50% of the load which was removed from the front axle.

If only 50% of load is restored, the WD bars only need to be rated for about 50% of the tongue weight.
If you plan to use the outdated approach of restoring 100% or more, the WD bars should be rated for 100% of the tongue weight.

In a different thread, you indicated the new trailer would have a loaded weight of 9000#.
If that's the case, a reasonable TW% of 13% would give a TW of just under 1200#.

If you want to go with 100% front axle load restoration, the existing 1200# bars should be okay.
If you want to go with 50% FALR, you could stay with the 1200# bars or switch to bars with a lower rating.

Ron

Ron,

Have you been able to find the recommendation you listed in blue above by an WD hitch manufacture? If so which manufacture?

To give an example, with a 1,200# loaded TW where the owner desires 50% FALR per TV mfg recommendations, which WD hitch manufactures states it is OK to use a WD bar with a sticker rating to be used on 600# TW or anything less than the loaded TW can be used at 50% FALR on a 1,200# Loaded TW?

Reason for my asking is what Reese told me a number of years ago. This was long before the new thinking of 50% FALR, where my dealer gave me 800# WD bars and my TT ended up have a 1,200# loaded TW. Reese declared they will not warrant this situation on any TW above the TW sticker rating on the WD bar and the 800# WD bar may break or permanently deform in certain situations.

Now thinking present day, 50% FALR is becoming a topic of discussion. If one does use 600# TW rated WD bars on a 1,200# TW, to achieve 50% FALR, what happens to the WD bar during dynamic situations where heavy back flex in the hitch under of some level of speed occurs? Like coming off a high up RR track or dropping out of a high up gas stations curb, pot hole etc.

The lighter bar will try and deliver all it can and the loaded TW has the ability to be 2 times the weight pushing down on the ball then the WD bar may have been tested for. Will the WD bar permanently deform? This goes directly against the Reese warranty.

If one uses a 1,200# WD bar on a 1,200# TW and adjusts the WD hitch to only deliver the force needed for 50% FALR, then the back flex situation should not affect the WD bar as it has been fully tested to comply with this situation. Or has at least been proven by consumers to not bend in most cases.

I have not yet found the WD hitch mfgs to change their method of how to size a WD bar per loaded TW. Yet some hitch mfg's now declare less than 100% FALR in their instructions. I may have missed a few which state it is OK to down size a WD bar to a lower FALR which is why I was asking.

Thanks

John

JBarca wrote:
Now thinking present day, 50% FALR is becoming a topic of discussion. If one does use 600# TW rated WD bars on a 1,200# TW, to achieve 50% FALR, what happens to the WD bar during dynamic situations where heavy back flex in the hitch under of some level of speed occurs? Like coming off a high up RR track or dropping out of a high up gas stations curb, pot hole etc.

John, the "back flex" angle will be determined almost entirely by the geometry of the "dip" which the TV/TT passes through. When the Front of TV and/or rear of TT are elevated relative to the ball coupler, the WD bars will experience additional deflection. The additional force exerted on a bar will be directly proportional to the stiffness of the bar. If a 1200# bar is twice as stiff as a 600# bar, the additional force on the 1200# bar will be twice as great as the force on the 600# bar.
If the "back flex" added loading on a bar were the same for all bar ratings (or greater for the bars with lesser ratings), I would expect that people with 400 and 600# bars would report much more bar damage when going through dips than people with stiffer bars would report.

The lighter bar will try and deliver all it can and the loaded TW has the ability to be 2 times the weight pushing down on the ball then the WD bar may have been tested for. Will the WD bar permanently deform? This goes directly against the Reese warranty.

Since the "back flex" induced added deflection will be essentially the same, the lighter and more flexible bar will experience less added load than will the heavier bar.

If one uses a 1,200# WD bar on a 1,200# TW and adjusts the WD hitch to only deliver the force needed for 50% FALR, then the back flex situation should not affect the WD bar as it has been fully tested to comply with this situation. Or has at least been proven by consumers to not bend in most cases.

I think it also is safe to say that a 1200# bar on a 1200# TW at 100% FALR has been proven by consumers to not bend in most cases.
A 1200# bar generating 100% FALR with 1200# TW will be loaded to about 960#.
A 600# bar generating 50% FALR with 1200# TW will be loaded to about 480#.
If "back flex" results in a doubling of the load on both bars, each bar will end up at 160% of its assumed static working load limit.
The load on a bar is not determined by TW -- it is determined by how much load it is asked to transfer.

I have not yet found the WD hitch mfgs to change their method of how to size a WD bar per loaded TW. Yet some hitch mfg's now declare less than 100% FALR in their instructions. I may have missed a few which state it is OK to down size a WD bar to a lower FALR which is why I was asking.

It was a huge step for Progress Mfg and Reese to move away from the "equal squat" approach and move toward a FALR-based approach. However, I think they don't have sufficient incentive to try to connect bar rating with FALR. The ones who do have the incentive are the customers who want to avoid the problems of excessive bar stiffness. There are many reports in the Airstream community about damage due to excessive stiffness. And, there are many Airstream owners who are using WD bars rated well below the trailer's tongue weight.
I've not seen any reports of bar damage from Airstream owners who are using bars rated for less than the tongue weight.

Ron

Ron Gratz wrote:

JBarca wrote:

The lighter bar will try and deliver all it can and the loaded TW has the ability to be 2 times the weight pushing down on the ball then the WD bar may have been tested for. Will the WD bar permanently deform? This goes directly against the Reese warranty.

Since the "back flex" induced added deflection will be essentially the same, the lighter and more flexible bar will experience less added load than will the heavier bar.

If one uses a 1,200# WD bar on a 1,200# TW and adjusts the WD hitch to only deliver the force needed for 50% FALR, then the back flex situation should not affect the WD bar as it has been fully tested to comply with this situation. Or has at least been proven by consumers to not bend in most cases.

I think it also is safe to say that a 1200# bar on a 1200# TW at 100% FALR has been proven by consumers to not bend in most cases.
A 1200# bar generating 100% FALR with 1200# TW will be loaded to about 960#.
A 600# bar generating 50% FALR with 1200# TW will be loaded to about 480#.
If "back flex" results in a doubling of the load on both bars, each bar will end up at 160% of its assumed static working load limit.
The load on a bar is not determined by TW -- it is determined by how much load it is asked to transfer.

Ron

Hi Ron,

Thanks for the response and detail. Sorry this took so long to respond back, life has been, busy.

I agree with your response for the most part however there is still in my mind dynamic situations that have me concerned using a heavy TW with a much lighter WD bar as a recommended setup.

When the road the TV is riding on is on an angle to the TT, the TV turns, while going up hill, that motion tilts the hitch head in 2 directions. The tilt and turn can unload a large percentage of one of the WD bar leaving the other WD bar heavily loaded. The DC users have this even worse then non DC users.

With the hitch head titled both back from the "back flex", one bar carrying the majority of the load as the head tilt created this, in this condition the TW provides the downward push on the ball to generate the tension that the WD bar is now positioned at. The bar is being asked to provide more force exertion. In this condition, the heavier TW will result in more bar flex then a lighter TW. Did I mix this up? The combination is asking the WD bar to deliver more force and thus more flex.

Granted these turns do not happen all the time, but they do happen. We never yet solved the heavy snap up failure of when the DC is added to a hitch so this force does exist on one WD bar as proven by several ORF members.

If we knew the working displacement limits of these spring bars before we approach the yield point, then I could agree using a 600# WD on a 1,200# TW and only shoot for 50% FALR providing the rest of the hitch can handle it. As long as the flexure of the WD bar does not exceed the working limit, it can flex many times to this limit and not deform. One too many times over that limit and it will stay bent.

The new guidelines for FALR have not yet caught up to how to rate the WD bar. 600# bar on a 800# TW, I'm not that concerned. 600# on a 1,200# or worse 800# bar on a 1,700# TW, I'm concerned. Maybe it is me and having seen too many fatigued out springs in machinery work that once the design allows the spring to go past the working limits, it breaks prematurely. Many spring failures come from exceeding the working limits with high duty cycles.

I agree we may not get the hitch mfg's to help sort this out for us for many years to come. And yes, I'm glad to see the shift from "equal squat" finally in print and especially from Progress Mfg. And in their case, would they warrant a 600# WD hitch system with 600# WD bars on a 1,200# TW? Good question. They make different heads per spring bar ratings. Until they test it and prove it, I would say no. Reese and others use the same higher rated hitch head and just change WD bars so the head and shank could be up to the task assuming the WD bar can take it.

Thanks

John

Hi John,

I'm not ignoring your post. A few days ago I began this thread on airforums.com.
So far, nobody has produced any reports of bar damage resulting from an "underhitched" setup.
That doesn't mean there has been no damage -- just means nobody has reported any.

In the meantime, I've reviewing our "back flex" discussions and "compound angle" discussions.

JBarca wrote:
With the hitch head titled both back from the "back flex", one bar carrying the majority of the load as the head tilt created this, in this condition the TW provides the downward push on the ball to generate the tension that the WD bar is now positioned at. The bar is being asked to provide more force exertion. In this condition, the heavier TW will result in more bar flex then a lighter TW. Did I mix this up? The combination is asking the WD bar to deliver more force and thus more flex.

First, let's go back to the condition where there is no "back flex" and no "compound angle".
Assume the TW is 1200# which causes 600# to be removed from the TV's front axle.
Assume the 1200# bar is adjusted to restore 600# (FALR = 100%) while the 600# bar is adjusted to restore 300# (FALR = 50%).
This might require an upward force of 1200# on each 1200# bar and 600# on each 600# bar.
Assume both the "1200# bar" and the 600# bar would have 3" of curvature. I.e., the 1200# bar has a "stiffness" of 400 lb/in and the 600# bar would be 200# lb/in.

The "compound angle" condition is a combination of a dip and a turn. The dip has the same effect as increasing the rearward tilt of the hitch head -- it increases the bar curvature.
The turn can have two effects. The tilt of the trunnion axis can cause a increase or decrease in bar curvature, depending on turn angle.
The turn also can cause the outside bar to "rise up" over the cam, having an effect similar to increasing the rearward tilt -- curvature increases.
The net effect of dip plus turn most likely will be an increase in bar curvature.

Now, a fundamental question: Is the change in bar curvature dependent on WD bar stiffness?
My belief is that bar stiffness does not have a significant effect on either the dip contribution or the turn contribution.
The dip contribution is determined primarily by the TV and TT dimensions and the geometry of the dip.
The turn contribution is determined primarily by the tilt of the trunnion axis and the slope of the rear end of the WD bar.

If the change in curvature is not dependent on bar stiffness, then the amount of change should be essentially the same whether using the 600# bar or the 1200# bar.
And, if the 600# bar is half as "stiff" as the 1200# bar, the added force exerted on the 600# bar should be half as much as that added to the 1200# bar.
The percentage increase in load for both bars would be the same.

Granted these turns do not happen all the time, but they do happen. We never yet solved the heavy snap up failure of when the DC is added to a hitch so this force does exist on one WD bar as proven by several ORF members.

I agree. I've calculated that the turn effect by itself can increase the load on a bar by approximately 50%.
But that does not mean the percentage increase in load for a 600# bar would be greater than for a 1200# bar.

If we knew the working displacement limits of these spring bars before we approach the yield point, then I could agree using a 600# WD on a 1,200# TW and only shoot for 50% FALR providing the rest of the hitch can handle it. As long as the flexure of the WD bar does not exceed the working limit, it can flex many times to this limit and not deform. One too many times over that limit and it will stay bent.

As long as the compliance (inverse of stiffness) of a 600# bar is twice that of a 1200# bar, and they are required to flex the same amount, I see no reason to believe the 600# bar would be more susceptible to damage.

I agree we may not get the hitch mfg's to help sort this out for us for many years to come. And yes, I'm glad to see the shift from "equal squat" finally in print and especially from Progress Mfg. And in their case, would they warrant a 600# WD hitch system with 600# WD bars on a 1,200# TW? Good question.---

Let's go back to the 1200# bar generating 600# of load restoration and the 600# bar generating 300# of load restoration. Assume both bars have 4" of curvature.
Assume a "compound angle" condition causes both bars to have an increased curvature of 1.5".
The load on the 1200# bar would increase by 50% -- 400*1.5 = 600#.
The load on the 600# bar also would increase by 50% -- 200*1.5 = 300#.
I don't see any reason to believe the 600# bar would be any more likely to sustain damage due to the "compound angle" condition than would the 1200# bar.

I have developed numerical models which predict "back flex" and "compound angle" curvature and load effects on WD bars.
I'll let you know if I come up with any different conclusions.
Of course, the conclusions are only as good as the assumptions on which the models are based. I'll also be giving the assumptions more thought.

Ron