Air Bags

[Campin_LI]

Hypothetical SUV towing travel trailer with 1200 lb tongue weight

Question / clarification:

Without weight distribution hitch - back of truck goes down and front of truck goes up.

With weight distribution hitch - back goes up a little, front goes down a little.

No weight distribution hitch - pump up air bags to bring back of truck up to unhitched height. Front of truck is not high. Why is this. The same weight is pushing down at the hitch. I would think the front of the truck would still rise because no weight is being transferred to the front of the truck.

Thanks for your ideas / opinions.

[babies_dadeo]

Long and short of it is that most front end rise due to hitch weight is related to the suspension, not the loss of weight from front axle due to the load behind the rear axle.

Makes me think that as far as a weight distribution hitch goes, it's really the act of raising the back of the truck that makes the difference in ride stability rather than the transfer of weight to the front.

Adding the airbags did not make or break my setup but I did notice that there is a lot less bounce in the back when I hit those type roads that have those wavy surfaces. More than likely, my WDH is doing less work now too.

Bingo!

Just my opinion, but with todays HD suspensions, the truck is built to handle so much more weight then they once were it makes judging vehicle angle and WD a much different process. I do know that with Heavy trucks, (Semis), weight movement is never an exact science, as you move a 5th wheel, sure you should move X kg from this axel to this axel, but it does not always work that way because of the dynamic nature of the units weight, length and suspension (hugely exaggerated due to air suspension). From what I am hearing you actually have a solid setup and the airbags are doing exactly what they are supposed to.
Happy Camping!

[Campin_LI]

Ron Gratz wrote:

Campin LI wrote:
So what causes the front to drop when air bags are pumped up?

The front should not drop.

If you can provide some actual before and after height measurements, I will try to interpret the numbers.

Axle load data would be even better.

Ron

Your right! I re-hooked up and it is more an illusion that the front went down. The truck bumper rolls slightly toward the road and the wheel well rotates slightly therefore giving an illusion that it goes down. After reading other posts on airbags, Although it rode real well last time I used it, I'm going to raise my hitch head back to where it was and only put enough air in the bags to see the rear of the truck start to rise. I will then decide which ride is better. Thanks for your help on this.

[Ron_Gratz]

Campin LI wrote:
So what causes the front to drop when air bags are pumped up?

The front should not drop.

If you can provide some actual before and after height measurements, I will try to interpret the numbers.

Axle load data would be even better.

Ron

[Campin_LI]

Ron Gratz wrote:

Campin LI wrote:
When WDH is engaged, front is higher than unhitched by about an inch and back is down by about 2 inches. When air bags are pumped up to get the rear back up to unhitched height, the front does not go down to unhitched height, it is still a little high.---

When you use the bags to lift the rear of the TV, you also are lifting the hitch and the trailer tongue. This changes the vertical angle between TV and TT which has the same effect as tilting the ball mount forward. The load on the WD bars will be reduced, and the amount of load transfer will be reduced.

Reducing the load transfer will cause load to be removed from the TV's front axle. The front of the TV will tend to rise.
Whether the front actually will rise depends on the amount of friction in the front suspension components. The front might not rise, but you should not expect it to drop.

On edit: To be more specific, No WDH - hook up trailer. Front is way up (not sure how high, maybe 3 inches?)and back is down. Pump up airbags and as back goes up, front goes down. This happening is what I don't get.

Does your TT have independent axles or does it have an "equalizer" link between the front and rear springs?

If you really want to understand what is happening to the front end you'll need to measure the axle loads under the various conditions. Height measurements often are not a reliable indicator of loading.

Ron

The trailer has an equalizer link between the 2 axles.

I realize scale numbers would explain a lot. When I posted the question, I kind of thought there would be 20 replies with the same answer. I'm actually surprised there is no overall consensus on this.

I have to believe the suspension has a lot to do with it. Or the WDH actually equally distributes the load across the entire TV chassis and the angle the truck makes with the road surface has something to do with it, ie by raising the back with airbags more load goes to the front axle because the truck is level. Not sure.

[Campin_LI]

Ron Gratz wrote:

transamz9 wrote:
---The natural arch in the leaf spring is flattened out when the load is applied so the front of the leaf spring is naturally going to lift the front of the SUV.---

Yes, if a downward force is applied to the hitch, there will be an increase in the upward force on the frame at the rear spring's front mounting point. And, if we assume the chassis is pivoting about the rear axle, the increase in upward force at the front mounting point will tend to cause the front of the TV to lift.

However, there also will be an increase in the upward force on the frame at the rear spring's rear mounting point. And, that upward force will tend to cause the front of the TV to drop. The net effect on the front end of the TV will be essentially zero.

The downward force on the ball will cause the load on the TV's front axle to be reduced, and that is what causes the front of the TV to rise.

Now when you add air to the bags, the weight on the truck is not changing but you are relieving pressure from the leaf spring allowwing the spring to relax also allowing the weight the spring was taking off of the front axle to return to the front axle.

Yes, you now are relieving load from the rear leaf spring. But, you must keep in mind the loads will be reduced on both the spring attachment point in front of the rear axle and the attachment point behind the rear axle. Again, the net effect on the front axle will be essentially zero.

Ron

So what causes the front to drop when air bags are pumped up?

[Ron_Gratz]

Campin LI wrote:
When WDH is engaged, front is higher than unhitched by about an inch and back is down by about 2 inches. When air bags are pumped up to get the rear back up to unhitched height, the front does not go down to unhitched height, it is still a little high.---

When you use the bags to lift the rear of the TV, you also are lifting the hitch and the trailer tongue. This changes the vertical angle between TV and TT which has the same effect as tilting the ball mount forward. The load on the WD bars will be reduced, and the amount of load transfer will be reduced.

Reducing the load transfer will cause load to be removed from the TV's front axle. The front of the TV will tend to rise.
Whether the front actually will rise depends on the amount of friction in the front suspension components. The front might not rise, but you should not expect it to drop.

On edit: To be more specific, No WDH - hook up trailer. Front is way up (not sure how high, maybe 3 inches?)and back is down. Pump up airbags and as back goes up, front goes down. This happening is what I don't get.

Does your TT have independent axles or does it have an "equalizer" link between the front and rear springs?

If you really want to understand what is happening to the front end you'll need to measure the axle loads under the various conditions. Height measurements often are not a reliable indicator of loading.

Ron

[Ron_Gratz]

transamz9 wrote:
---The natural arch in the leaf spring is flattened out when the load is applied so the front of the leaf spring is naturally going to lift the front of the SUV.---

Yes, if a downward force is applied to the hitch, there will be an increase in the upward force on the frame at the rear spring's front mounting point. And, if we assume the chassis is pivoting about the rear axle, the increase in upward force at the front mounting point will tend to cause the front of the TV to lift.

However, there also will be an increase in the upward force on the frame at the rear spring's rear mounting point. And, that upward force will tend to cause the front of the TV to drop. The net effect on the front end of the TV will be essentially zero.

The downward force on the ball will cause the load on the TV's front axle to be reduced, and that is what causes the front of the TV to rise.

Now when you add air to the bags, the weight on the truck is not changing but you are relieving pressure from the leaf spring allowwing the spring to relax also allowing the weight the spring was taking off of the front axle to return to the front axle.

Yes, you now are relieving load from the rear leaf spring. But, you must keep in mind the loads will be reduced on both the spring attachment point in front of the rear axle and the attachment point behind the rear axle. Again, the net effect on the front axle will be essentially zero.

Ron

[transamz9]

With the air bags or not the tt is still going to remove weight from the front. If you don't want to weigh the rig then I would get a measurement from the ground to the top center of the front wheel well when the SUV is unloaded and then return it back to that measurement once the tt is hooked and you get your bags at the pressure you want using the WD system.

[Campin_LI]

transamz9 wrote:
If your tongue weight is 1200# then that will be between 300 and 360# your tt is taking off of your front axle not counting for the rear suspension taking weight off also. If I knew your actual measurements I could get that number closer to exact.

I don't have the measurements of the truck off hand. I realize that weight was removed by the hitch which is why I use the WDH. It was the change in front end height that had me stumped.

With the airbags, when I tilted the hitch head back a washer, once engaged the front was lower than before the airbags were installed, so I knew that was too much tilt and went back to the original tilt. It's good to know that there is a good amount of leeway in the adjustment as our trailer is always loaded differently and so is the truck for each trip.

[transamz9]

If your tongue weight is 1200# then that will be between 300 and 360# your tt is taking off of your front axle not counting for the rear suspension taking weight off also. If I knew your actual measurements I could get that number closer to exact.

[Campin_LI]

transamz9 wrote:
If your SUV has leaf springs in the rear (don't know much about the Ex's) the front half of the leaf spring is attached to the frame with a solid mount. When you add the weight of the trailer on the SUV the truck squats down. The laws of physics states that when you have a reaction there is going to be an opposite reaction. The natural arch in the leaf spring is flattened out when the load is applied so the front of the leaf spring is naturally going to lift the front of the SUV. Now when you add air to the bags, the weight on the truck is not changing but you are relieving pressure from the leaf spring allowwing the spring to relax also allowing the weight the spring was taking off of the front axle to return to the front axle. Make sense?

Yes, perfect sense. Thank you for your explanation. I was thinking that it is vehicle dynamics (suspension). I started thinking about trucks that can handle 1200 lb tongue weight without weight distribution. The fronts of those trucks don't rise much. Your explanation clears that up. The Ex has leaf springs and solid rear axle, same as an F250 or 350 of that vintage. The Ex has "SUV" springs to give it a better ride, not that they are bad for towing - just a little squishy. The air bags stiffened them up.

Long and short of it is that most front end rise due to hitch weight is related to the suspension, not the loss of weight from front axle due to the load behind the rear axle.

Makes me think that as far as a weight distribution hitch goes, it's really the act of raising the back of the truck that makes the difference in ride stability rather than the transfer of weight to the front.

Adding the airbags did not make or break my setup but I did notice that there is a lot less bounce in the back when I hit those type roads that have those wavy surfaces. More than likely, my WDH is doing less work now too.

[transamz9]

If your SUV has leaf springs in the rear (don't know much about the Ex's) the front half of the leaf spring is attached to the frame with a solid mount. When you add the weight of the trailer on the SUV the truck squats down. The laws of physics states that when you have a reaction there is going to be an opposite reaction. The natural arch in the leaf spring is flattened out when the load is applied so the front of the leaf spring is naturally going to lift the front of the SUV. Now when you add air to the bags, the weight on the truck is not changing but you are relieving pressure from the leaf spring allowwing the spring to relax also allowing the weight the spring was taking off of the front axle to return to the front axle. Make sense?

[Campin_LI]

Ron Gratz wrote:

Campin LI wrote:
No weight distribution hitch - pump up air bags to bring back of truck up to unhitched height. Front of truck is not high. Why is this. The same weight is pushing down at the hitch. I would think the front of the truck would still rise because no weight is being transferred to the front of the truck.

In your hypothetical scenario, when you say, "front of truck is not high" -- to what are you comparing the front height? Unhitched height, hitched height prior to adding air to bags, or something else?

Ron

When WDH is engaged, front is higher than unhitched by about an inch and back is down by about 2 inches. When air bags are pumped up to get the rear back up to unhitched height, the front does not go down to unhitched height, it is still a little high. I can't say for sure that it is an inch high but it is around an inch higher. I would think that it should rise in the front as much as it rose in the back since weight distribution on the truck is the same with or without the bags pumped up and no WDH engaged.

On edit: To be more specific, No WDH - hook up trailer. Front is way up (not sure how high, maybe 3 inches?)and back is down. Pump up airbags and as back goes up, front goes down. This happening is what I don't get.

Please remember that this is all an after thought. I was on a mission to re adjust hitch due to addition of airbags. That night, I started thinking of how the front was reacting so there are no real measurements to that effect....But it did happen.

In the end, I dropped the hitch head 1 adjustment to keep the trailer level. Tilt remains the same and 10 lbs of air in the bags equates to 20 lbs when hitched. The back of the truck is up about an inch higher (not high enough to be equal to unhitched height). The front is about the same as pre-airbags (didn't measure). Ride is good and firm, no bounce and front is not near the jounce bumpers, so it is not overloaded at all.

[Ron_Gratz]

Campin LI wrote:
No weight distribution hitch - pump up air bags to bring back of truck up to unhitched height. Front of truck is not high. Why is this. The same weight is pushing down at the hitch. I would think the front of the truck would still rise because no weight is being transferred to the front of the truck.

In your hypothetical scenario, when you say, "front of truck is not high" -- to what are you comparing the front height? Unhitched height, hitched height prior to adding air to bags, or something else?

Ron