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Old 10-15-2014, 08:05 AM   #15
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The pre-supposition is a high COG pickup. We went through some of this a few years back on J2807.

Very nice summation. Thanks for the thread and all of the time spent over the years in analysis.


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Old 10-15-2014, 08:23 AM   #16
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Although I don't use a WD hitch now, I totally agree with your findings. Whenever I've used WD I have found your conclusions the best for my situations. Many dif tv and trailers. Jim
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Old 10-15-2014, 10:52 AM   #17
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Very helpful info; thank you!
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Old 10-15-2014, 11:45 AM   #18
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I found that the front axle was 340 pounds lighter (4,640 vs 4,980 pounds) with the trailer attached via the ProPride hitch at the 6" mark and the rear axle was up 1500 pounds with a calculated tongue weight around 1,160 pounds. The trailer weight (total weight less pickup weight) was 9,120 pounds.

The next day I set up my portable individual wheel scales and my total trailer weight was 9,085 pounds. It showed a tongue weight of 1,346 pounds.

There is no degradation in steering input and the ride is smooth.

So this tends to support the more recent thoughts on the subject.
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Old 10-15-2014, 04:29 PM   #19
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Most vehicles I've seen have a higher rating for their rear axle, which I think implies most of the load should be carried by the rear axle. If this is correct,it would go against the 1/3 on each axle rule (which seems to be practically difficult to achieve also).


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Old 10-15-2014, 07:49 PM   #20
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Quote:
Originally Posted by Chuck View Post
Nice!
couple of questions:
What is "trailer damping?"

What does "conventionally sprung" mean?

What was the "compact" car?
I'll take the easy question first -- What does "conventionally sprung" mean?
In the context of the final report:
QUOTE
In this regard it was found that hookups resulting in a small hitch-low attitude (corresponding to 0 percent hitch load transfer to the front axle in conventionally sprung tow cars) appears more desirable than a level attitude which provides approximately 25 percent hitch load transfer.
UNQUOTE
"conventionally sprung" refers to NOT using suspension accessories (such as air shocks) to level the tow vehicle. The "small hitch-low attitude" refers to allowing some amount of rear-end drop when the WDH is adjusted to return the front axle to its unhitched load.

What was the "compact" car?
QUOTE
In order to cover a wide range of realistic vehicle/trailer combinations, it was desirable for the two vehicles to have diverse dimensions and characteristics, at the same time being representative of current and future vehicles.
Also, the tow vehicle needed to be of sufficient size to carry the experimental equipment and tow the range of trailers specified.
To accommodate the high weight end of this, range, one tow vehicle was specified to be a large station wagon.
The second vehicle was to be as small as feasible in recognition of the growing trend toward increased production of small model American cars.
A compact model, weight of about 4000 lb, appeared to provide the best compromise. This size could accommodate the onboard equipment space needs and the load of a utility trailer.

Accordingly, a 1974 Chevrolet Nova Hatchback and a 1973 Chevrolet Caprice station wagon were selected.
These specific makes and models were chosen because of availability and because considerable data were already on hand from prior handling tests. Also, both vehicles were equipped with air shocks and a Class III (frame-mounted) hitch receptacle.
UNQUOTE
The Nova hatchback was rated to tow 4000#, and the chosen test trailer was an enclosed 18' U-haul utility trailer with a test weight of 3000# and "hitch loads of 150# and 300#.

What is "trailer damping?"

If a trailer is displaced laterally by a wind gust, bow wave, pavement ruts, or some other source of lateral force, once the force subsides, the trailer will try to move back to a centered position in line with the TV.

If the trailer had no "damping", it would move back toward centered but would continue to move past center would continue to move in that direction until reaching an equal displacement to the other side.
Then it would continue to swing back and forth at constant amplitude.
Fortunately, the trailer tires provide an inherent "damping" which causes the amplitudes of the oscillations to decrease to zero if there is no further lateral forcing.

The pavement generates a lateral "damping" force against the tires as they move laterally. The magnitude of the force is directly proportional to the tire's lateral speed and inversely proportional to the forward speed of the trailer.
This means that, as the forward speed increases, the lateral damping force decreases.

If the forward speed is increased from 30 mph to 60 mph, the damping from the TT's tires is cut in half.
As the TT's forward speed increases, yaw oscillations will take longer to die out.
If the speed gets high enough, the TT will tend to oscillate like an undamped pendulum.

For a typical TV/TT combination, the loads imposed by the TT and the motions of the TT will affect the yaw dynamics of the TV.
The yaw stability of both the TV and TT will be affected by forward speed.

For the TV, there is a critical speed at which the handling changes from "understeering" to "oversteering".
For a coupled TV and TT, oversteer can produce negative damping in which energy is being added to the oscillating system causing the oscillations to rapidly increase in magnitude.
Adding load to the TV's front tires via a WDH also tends to move the vehicle from understeer to oversteer.

A plot of damping ratio as a function of speed is given as Figure 2 in Stability and Control Considerations of Vehicle-Trailer Combination.
The example TV/TT has negative damping above 70 mph indicating the combination would become unstable above that speed.

For most combinations, the wheelbase, tongue weight, load distribution, tire characteristics, etc cause the critical speed to be well above typical towing speeds.
For some combinations, this is not the case.

Speed does not have any significant effect on the natural frequency of the truck, trailer, or truck/trailer combination.
However, forward speed does have a significant effect on the damping generated by the trailer tires and on the yaw stability of the tow vehicle.

Quote:
I also wonder if the results would be different if done with a modern vehicle with independent suspension, fwd, lower-profile tires, etc.
Those are good considerations, and I don't have a good response.
I'll spend some more time reviewing other sources to see if I can find some answers.

Ron
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Old 10-15-2014, 09:01 PM   #21
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Great read. Obviously you care enough to find out the details. I've read quite a bit and experimented with my own setup. What really helped me was The Curt hitch setup guide and the Ford advice on front suspension. As others stated, each setup gives different results. The eyeball method that Curt uses is pretty close. They suggest setting up a WD hitch with these simple steps:

On flat ground measure the height of each wheel well center from ground to top.

Hitch up the trailer without using WD and measure the wheel wells again. (this allows for a corrective reference)

Ford F150 advice on my year state that the front end must not ever be lower than without a load or, the first measure of the front wells.

Then, the adjustment begins. At 5.5 inches on my PP, I have all but 1/8" from original front wheel well position. So, the front really determines the amount of overall distribution capability. My first attempt at this method indicated at the CAT Scale that I was within 100 lbs of original front axle weight. I have since shaved that down to less loss than that. It does not gain weight on the front end which blows the distribution formula of thirds but I regain my original front-end weight by WD and distribute the bulk of the weight across trailer and rear of truck. This is the result, more or less, that I have gotten from two different hitch systems. The point is that even the simple wheel well measure, checked by the CAT Scale, provided me a fairly accurate method for setting the weight distribution. When I first started doing this I contacted a Curt representative concerned that I could not get the weight evenly distributed. He told me that no one does across all three axle points.
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Old 10-15-2014, 09:45 PM   #22
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Exclamation

Quote:
Originally Posted by SteveH View Post
It's great to see some work done on the matter by the US DOT-NHTSA. However, I still believe not all setups are the same. A 1/2 ton pickup is different than a 3/4 ton, and they are both obviously different than a front wheel drive minivan. Each user must actually experiment with their own setup trying different adjustments to end up with the best driving, riding, and handling possible for their own rig.
Steve, I agree 1000% on this.

Many of the "rules" posted on this forum ignore too many important vehicle-specific factors.

Ron
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Old 09-12-2015, 06:54 PM   #23
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To avoid further hijacking, the following quoted text has been imported from Post #814 and Post #819 of the Equal-i-zer Hitch Thread.

Quote:
Originally Posted by Ron Gratz in another thread:
Quote:
Originally Posted by Inland RV Center, In
To remove weight from the front axle, will deem the steering wheel to become progressively useless, which in turn simply means your asking for a loss of control.
As I attempted to explain in response to Al's question about the weight of the WDH and bars:

Any mass added to a TV behind the rear axle will cause load to be removed from the front axle.
The added mass also will cause the TV's center of mass to move rearward.

The load on the front tires is reduced, but the distance from the front tires to the CM is increased.
Since the length of the moment arm (distance from axle to CM) is increased, the front tirescan cause the same amount of steering torque.

The steering ability of the front tires does not become progressively worthless, because the tires are exerting their force over a longer moment arm.

Now, if you're talking about load removed from the TV's front axle due to the downward force on the ball resulting from "tongue weight" -- you're talking about a completely different situation.
The reason it's different is because the "tongue weight" removes load from the TV's front axle, but doesn't cause a rearward movement of the TV's center of mass.

Ron
Quote:
Originally Posted by Al & Missy in another thread:
Quote:
Originally Posted by Ron Gratz

Now, if you're talking about load removed from the TV's front axle due to the downward force on the ball resulting from "tongue weight" -- you're talking about a completely different situation.
The reason it's different is because the "tongue weight" removes load from the TV's front axle, but doesn't cause a rearward movement of the TV's center of mass.
Ron
And this is the part i dont understand. Why is 900# of tongue weight any different from a 900# weight sitting on the hitch?

Taken to the limit, no matter how far aft the weight is, if it lifts the front tires off the ground, they won't provide any steering force. Not trying to be argumentative, just want to understand.

Al
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Old 09-12-2015, 07:05 PM   #24
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Al,

Yes, theoretically you can use tongue weight to lift the front tires off the ground.
If a TV has a 160” wheelbase and 4000# on the front tires, a 1000# tongue weight placed 640” (53.3’) behind the rear axle could completely unload the front tires.
Of course, you’d have to allow the tongue to drop a distance of 4 times the amount of front lift.

In reality, 1000# TW probably would remove only about 400# from the front axle. A 10% decrease in front axle load could be compensated by increasing the steering angle by about 10%.

But, the question is:
Why shouldn’t we just go ahead and use the WDH to replace all of the lost front-end load (or even replace more than 100% just for good measure)?
Doesn’t more load on the front tires make the front of the TV more responsive to steering input?
The answer to this second question is, “Yes”. However, more responsive steering is not necessarily a desirable objective.
Overly responsive steering (a.k.a. “oversteer”) easily can lead to loss of control.

When a TV and TT are rounding a curve, the pavement is exerting a force against the TT’s tires. That force is directed toward the inside of the curve.
Since the pavement force is acting rearward of the TT’s CG, the ball must exert a force (also directed toward the inside of the curve) to keep the TT in equilibrium.
According to Newton’s third law, since the ball is pushing the coupler inward, the coupler must be pushing the ball (and the rear of the TV) outward.

The outward force acting on the rear of the TV, tends to cause the TV to turn toward the inside of the curve. This tends to make the TV oversteer.
Simply placing a mass on the ball does not cause a lateral force on the ball, and doesn’t make the TV tend toward oversteer.

So, given that lateral force from the TT, when rounding a curve, will cause the TV to tend toward oversteer, why is it undesirable to use a WDH to add too much load back onto the TV’s front tires?
It is undesirable because adding load to the front tires (without moving the TV’s CG forward), also makes the TV tend toward oversteer.

Back in the 1970s, a scientific study conducted for the DOT-NTHSA (discussed in Post #3) demonstrated that increasing the load distribution degraded the tow vehicle understeer (made the TV tend toward oversteer) and reduced the speed for incipient jackknifing.
The study also concluded:
“---In this respect it was found that hook-ups resulting in a small hitch-low attitude (corresponding to 0 percent hitch load transfer to the front axle in conventionally sprung tow cars) is more desirable than a level attitude which provides approximately 25 percent hitch load transfer.---“

Circa 2010, a Letter to Editor contained the following:
---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).---"

Shortly thereafter, Ford indicated a desirable FALR for their trucks should be 50%. Other TV and hitch manufacturers then came forth with similar recommendations

Short story – TV and WDH manufacturers have concluded that allowing some load to remain removed from the front axle is preferable to restoring all of the load.

Ron
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Old 09-13-2015, 09:36 AM   #25
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I got it. From my towing experience I agree totally as to not adding all the weight back to the front end.
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Old 10-06-2016, 11:16 PM   #26
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FALR etc

I have not posted on this forum for a fair time as have been busy working on a (now 25 year!) project re a reasonably plain English guide to caravan & tow vehicle stability. It keeps getting too long.

Am currently attempting to get my head around the J2807 FALR performance metrics - as very applicable also to Australia (but where the WDH makers still push for 100% restoration - despite Cequant's recomendations).

There seems to be some confusion in that a few reports refer to 'reducing front tire weight' as increasing understeer' - that it cannot do. It can only decrease understeer.

That being so the suggested decrease appears to be such that a closer to neutral steer balance is achieved. What puzzles me though is that the then possible oversteer introduces a positive feedback loop - the last thing one needs in a close to infinite oversteer leading to jackknifing.

Or is it possibly that the full FALR introduces so much understeer that the desirable 0.4 g lateral cannot be achieved - and it is felt better that it should be (possibly enabling Dexter or AL-KO ESC to catch the rig if necessary).

I've read almost everything published re this since 1970 (particularly from Richard Klein) etc - and cannot see any other explanation.

(This is an increasing issue to us in Australia - where the caravan industry appears to have gone ape - with four tonne end-heavy megavans - with 5% tow ball mass - and towed by 2.5 tonne vehicles like a Toyota Hilux).

Would truy appreciate comment - particularly from Ron Gratz - whose comments I have been following for a long time.

My background is originally GM Applied Research (in the UK) mainly on suspension. I now write books and articles re all sorts of RV stuff (plus solar) but this area has always been my favourite.

I tend to speak metric and use esses where you use zeds (sorry zees?) but do understand pounds and mph! - no matter how archaic!

Collyn Rivers
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Old 10-07-2016, 03:16 PM   #27
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Weight Distributing Hitch Adjustment

The only expert around here (or on other RV forums) is Andrew Thomson of Can Am RV Centre, London, ON. Is a consultant to both Airstream and to the SAE committee on the above.

I'd recommend a thorough reading of his commentary at his website, on the Canadian magazine, RVLife, and his many posts at this site.

His refutation of "other than FALR" is solid. 31/1/2016 date at his website; Setting Torsion Bars.

Specifically, (and IMO) J2807 purposefully ignores whole classes of vehicles, both as potential TVs and wind loads on TTs versus the unrepresentative trailer employed.

Now, the differences between travel trailers here, in England or in Australia make for other problems in analysis.

I've enjoyed reading your contributions elsewhere and wish you well in your endeavor.
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Old 10-07-2016, 08:54 PM   #28
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The only expert around here (or on other RV forums) is Andrew Thomson of Can Am RV Centre, London, ON. Is a consultant to both Airstream and to the SAE committee on the above.

I'd recommend a thorough reading of his commentary at his website, on the Canadian magazine, RVLife, and his many posts at this site.

His refutation of "other than FALR" is solid. 31/1/2016 date at his website; Setting Torsion Bars.

Specifically, (and IMO) J2807 purposefully ignores whole classes of vehicles, both as potential TVs and wind loads on TTs versus the unrepresentative trailer employed.

Now, the differences between travel trailers here, in England or in Australia make for other problems in analysis.

I've enjoyed reading your contributions elsewhere and wish you well in your endeavor.
Thank you for that - will follow it up.

I agree there is a major difference between UK and US practice, but to many Australian companies build product that is much closer to US than UK.

The theoretical basis however is the same.
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