Ford - Unhitched vs Hitched Fender Height
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 t...
Forum Discussion
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
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
