Ford - Unhitched vs Hitched Fender Height

Ron Gratz wrote:

ScottG wrote:
Don't be surprised if you can't get enough weight transferred forward to lower the front of the truck very much - let alone dropping it as much as the back. And if you manage to do so, it may handle strangely - feeling like it's pivoting in the middle. If the front is stiff enough you may be transferring weight but not pushing the springs down much.
The engineers are often wrong on this one.

I think Ford and others are using front-end height only as an indicator of axle load. They probably assume many more people will attempt to measure height changes than will go to a scales and measure load changes.
Unfortunately, it's the load changes which are the important parameter.

And, nobody should attempt to drop the front as much as the rear -- inspite of what some WDH sales people might tell you.

I'm not sure what you mean by the engineers often being wrong on this one -- but I'm going to disagree with your statement anyway.

Ron

And I agree with Ron. I think many people go wrong trying to either get the same amount of drop front to rear, or to just use the WD to make the truck ride level, and neither is the purpose of WD. I think the general opinion now is the get the front right and let the rear take care of itself.

ScottG wrote:
Don't be surprised if you can't get enough weight transferred forward to lower the front of the truck very much - let alone dropping it as much as the back. And if you manage to do so, it may handle strangely - feeling like it's pivoting in the middle. If the front is stiff enough you may be transferring weight but not pushing the springs down much.
The engineers are often wrong on this one.

I think Ford and others are using front-end height only as an indicator of axle load. They probably assume many more people will attempt to measure height changes than will go to a scales and measure load changes.
Unfortunately, it's the load changes which are the important parameter.

And, nobody should attempt to drop the front as much as the rear -- inspite of what some WDH sales people might tell you.

I'm not sure what you mean by the engineers often being wrong on this one -- but I'm going to disagree with your statement anyway.

Ron

Bob Landry wrote:
---I'm just curious why you wouldn't want all of the original weight transfered back to the front end for stability. Some manufacturers, like Toyota, tell you to do that..

Bob, I think the rationale for returning the front axle to its unhitched load (or even more than the unhitched) was more about removing load from the rear axle and less about adding load to the front axle.

Now it seems the concern for good yaw stability (positive understeer gradient) is receiving attention equal to the concern for rear axle load.

Ron

I agree with John.

I believe Ford's change of philosophy regarding how much weight distribution to apply stems from a desire to have the truck maintain a positive understeer gradient when towing.

Some of the rationale for changing the WDH adjustment specifications was cited in this post and others.

Most passenger cars and probably all trucks are designed to have a positive understeer gradient when a trailer is not attached.

When a ball-pull trailer is attached, two things happen:
1) The vertical force on the ball increases the load on the rear tires and decreases the load on the front tires. Both of these load changes cause an increase in the understeer gradient.
2) When negotiating a curve, the trailer induces a lateral force on the ball, and this force makes the TV want to turn toward the inside of the curve. This lateral force reduces the understeer gradient causing the TV to move more toward an oversteer condition.
The net effect of the (beneficial) change in axle load distribution plus the (detrimental) lateral force on the ball is a decrease in the understeer gradient.

As John has explained, using a WDH to remove load from the rear axle and add load to the front axle can make the TV transition from understeer (which is good) to oversteer (which is bad).
With a proper amount of load re-distribution, a positive understeer gradient can be maintained.

SAE J2807 includes specifications for the understeer gradient at Front Axle Load Restoration values of 0% (no WD), 50%, and 100%.
Perhaps Ford and others are responding to these new specs.

Ron

Bob Landry wrote:
Just got a new F250, so I have to redo my WD. I'm going to split the difference between the two heights because that's what Ford says to do, but I'm a little curious why you would not want to return the same amount of weight to the front axle.

From my research on this, part of the thought process has to deal with oversteer when towing.

http://en.wikipedia.org/wiki/Understeer

The concern is that with the mass of a trailer pushing the truck, if the front is heavy or extra heavy from more than unloaded weight, that the front wheels can bite in so to speak in a turn from a firm grip to the pavement and send the rig into jack knife from oversteer. This is a very negative thing.

SAE (Society of Automotive Engineers) have done tests on this in the last few years and are now recommending that when towing to try and avoid this oversteer condition. Equal-I-zer by Progress Mfg even changed their tune too. They use to recommend driving the front end down below unhitched. Now they too have changed theIr ways in light of the SAE findings.

This is not really a new thing, it was know long ago, just it is now coming to the forefront as a better method when towing. Back in the late 60's/early 70's an engineering firm was hired by the NHTSA to study the effect of towing trailers with certain vehicles. In this study this effect came up along with a lot of other things. If your really into this and want to read it, let me know I'll hunt it up. I have it printed out, about 1" thick and lots of detail. It is public information. Ron Gratz pointed me to this a long time ago.

On my 2005 F350 Ford or other SD's of the time, they allowed +/- 1/2" from unhitched height on the Front. In my case I found running the front about 75 to 100# lighter on the front created a more stable tow. This was not from the oversteer concern, but do to the F350 rear suspension. When I returned the front to unhitched height, the heavy TW camper rocked the back of the truck to a noticeable point.

When I backed off a little WD, the 1 ton helper springs (overloads) just kissed the frame brackets and created a global shift in left to right stability.

The F350 way up high in it's suspension when the truck is empty is fairly soft, when you load it, it then becomes more stiff.

The are objectives to meet when setting up WD, however there are also vehicle characteristics that one size fits all is not an absolute. My 2003 K2500 Suburban towed very well when just above front end unhitched.

Point in this, 1/2 ton, 3/4 ton and 1 ton PU suspensions are different. What you can do with a 3/4 or 1 ton is different than a 1/2 ton. Running the front end of a 1/2 ton real light and the rear close to max axle capacity is not good either. While oversteer will not be a concern, the trailer mass pushing the truck can affect front end steering. Not enough weight and you get understeer.

Good luck with your new truck.

Hope this helps

John

They may not know it all and they may occasionally be wrong, but as long as you follow their directions and something happens, the burden of blame is shifted to them if you followed instructions.

Bob Landry wrote:
I'm not arguing with them either.

They are the engineers.

I'm just curious why you wouldn't want all of the original weight transfered back to the front end for stability. Some manufacturers, like Toyota, tell you to do that..

Yup and think folks who do argue, are out to lunch in their Pollyanna land...

Nor or really wonder if they know that the OEM is now off the hook from
their contract (yes, that specification/glove box manual/labels/etc
are contractual) and have taken what would have been warranty but now
on their own pocket books...also know too many here and on there sites
that lie to claim warranty...even advise others to do the same

lbrjet wrote:
Curious as to why you got a new truck.

New truck smell.

Don't be surprised if you can't get enough weight transferred forward to lower the front of the truck very much - let alone dropping it as much as the back. And if you manage to do so, it may handle strangely - feeling like it's pivoting in the middle. If the front is stiff enough you may be transferring weight but not pushing the springs down much.
The engineers are often wrong on this one.

Curious as to why you got a new truck.