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Old 07-07-2015, 02:14 PM   #1
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Be very cautious adding rear airbags

This discussion was started on another thread and thought it was better to have its own thread. So here it is:

There seems to be a belief by some that one can add airbags on the rear suspension of their tow vehicle and get additional cargo capacity. This is not true and as a matter of fact they may well be creating a dangerous scenario from a vehicle dynamics scenario. Here is a quote from a tow vehicle dynamics expert:

"Stiffening the rear springs or adding rear airbags without doing so proportionally at the front, transfers more of the roll couple (roll resistance) to the rear suspension. This yet further loads up the outer tyre whilst cornering, thus increasing its slip angle. Here again if that footprint collapses or slides there is a very real risk of jack-knifing. This is not theoretical conjecture. It happens.

Because of the above, vehicle manufacturer's intended front/rear slip angle relationship should not be changed. If the rear suspension is stiffened (or air bags fitted), so should the front to the same degree (or a stiffer roll bar added) to maintain the intended roll resistance balance."
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Old 07-07-2015, 02:25 PM   #2
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Makes sense. Who is the "tow vehicle dynamics expert" ?
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Old 07-07-2015, 02:36 PM   #3
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The specific quote is from an author named Collyn Rivers.
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Old 07-08-2015, 09:07 PM   #4
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Al -

The 2011 to current F250 4x4 tend to sag in the rear under load. I've been thinking about adding the Firestone airbag system, not to add cargo capacity but to reduce the rear end sag and make the ride less stiff for the trailer. Our friends "Luckyducks" have done this and seem happy with the modification.

Others with the F250 4x4 have used mechanical devices to accomplish the same goal. One mechanical solution is to add an additional leaf to the rear springs, giving it the same spring setup as the F350. I haven't seen discussion of making adjustments to the front suspension on any of the rear sag solution forum threads I've read. I'm not saying it isn't needed, I'm just saying there are many people making changes to the rear suspension of their F250 trucks without doing anything to the front end.

There are many discussions about this F250 rear end sag issue, and solutions, on the Ford Truck, Powerstroke, and the Diesel Stop forums. I believe it has been covered here on AirForums as well. In the Ford truck world this topic generates as much passion, controversy, and emotions as the tire, hitch, tow vehicle, and WBCCI threads on AirForums.

Sadly, the one place I would not go for perspective on this topic is the service department of my local Ford dealership. If it isn't stock, they don't want to deal with it.
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Old 07-08-2015, 11:04 PM   #5
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Chuck,
The first suggestion I would give is to reduce some of the sag with restoration of load to the front of the truck with the WD hitch. I know you are familiar with hitch weight distribution so we won’t go into that. Of course the best is to also use truck scales to assure you are getting what you need.

The air bag vendors do market their systems to reduce sag. Unfortunately there is some anecdotal evidence here on the forum that they are also telling folks they can increase cargo capacity and that is simply not true. It also appears that some well-meaning folks here readily advise to add airbags to solve tow vehicle cargo capacity deficits. Both of these concern me because people come here to the forum to get information and they may rely on something that is promoted here that is simply not true.

In the pursuit of reducing sag with an airbag, you are modifying the rear suspension of the truck. The experts tell us that the addition of an airbag in the back suspension should also include corresponding changes to the front suspension. Without this you may well be creating the propensity for oversteer in your tow vehicle. This is dangerous territory from a vehicle dynamics perspective. This is why I plucked the quote from the author on the subject and posted it here to kick this thread off.

I am not surprised that Ford dealerships want no part of changing a suspension design because they would assume responsibility for the modification.

The static condition of raising the back of the truck to reduce sag with an airbag looks good and likely gives us a sense of comfort. But in fact you have changed the suspension system on the rear. The propensity for oversteer that may have been created will not necessarily show up until you get into an emergency condition such as a sway condition or the trailer beginning to push you going downhill causing a yaw in the tow vehicle. It is in these unstable conditions where this oversteer we may have inadvertently created bites.

I certainly do not claim to be a vehicles dynamics expert but I have had a deep enough interest in this topic to inform myself to some extent. My agenda with starting this thread here is to point out the danger of simply adding an airbag to a tow vehicle without an engineered change to the whole suspension system. We should be very cautious and challenge the competency of the salesman who is promoting a change to our suspension. I will again repeat a quote from a friend on the forum here, “Sometimes we must know more than the salesman.”

Sorry for the long winded answer. Hope it helps.
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Old 07-08-2015, 11:34 PM   #6
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Quote:
Originally Posted by Alphonse View Post
Chuck,
The first suggestion I would give is to reduce some of the sag with restoration of load to the front of the truck with the WD hitch. I know you are familiar with hitch weight distribution so we won’t go into that. Of course the best is to also use truck scales to assure you are getting what you need.

The air bag vendors do market their systems to reduce sag. Unfortunately there is some anecdotal evidence here on the forum that they are also telling folks they can increase cargo capacity and that is simply not true. It also appears that some well-meaning folks here readily advise to add airbags to solve tow vehicle cargo capacity deficits. Both of these concern me because people come here to the forum to get information and they may rely on something that is promoted here that is simply not true.

In the pursuit of reducing sag with an airbag, you are modifying the rear suspension of the truck. The experts tell us that the addition of an airbag in the back suspension should also include corresponding changes to the front suspension. Without this you may well be creating the propensity for oversteer in your tow vehicle. This is dangerous territory from a vehicle dynamics perspective. This is why I plucked the quote from the author on the subject and posted it here to kick this thread off.

I am not surprised that Ford dealerships want no part of changing a suspension design because they would assume responsibility for the modification.

The static condition of raising the back of the truck to reduce sag with an airbag looks good and likely gives us a sense of comfort. But in fact you have changed the suspension system on the rear. The propensity for oversteer that may have been created will not necessarily show up until you get into an emergency condition such as a sway condition or the trailer beginning to push you going downhill causing a yaw in the tow vehicle. It is in these unstable conditions where this oversteer we may have inadvertently created bites.

I certainly do not claim to be a vehicles dynamics expert but I have had a deep enough interest in this topic to inform myself to some extent. My agenda with starting this thread here is to point out the danger of simply adding an airbag to a tow vehicle without an engineered change to the whole suspension system. We should be very cautious and challenge the competency of the salesman who is promoting a change to our suspension. I will again repeat a quote from a friend on the forum here, “Sometimes we must know more than the salesman.”

Sorry for the long winded answer. Hope it helps.
The purpose of air bags is not to increase carrying capacity, but to level the truck when carrying varying amounts of payload in the bed in addition to the trailer tongue weight.

In the limiting case, oversteer is caused by rear axle weight share being in excess of 50%. Yes, roll angles, etc. can be used to adjust this initially, but in the end mass wins. If you're carrying this much weight, keep your speed down to avoid problems, esp. in slippery conditions.

My 1996 F250 has a 8800# gvw, and ~6000# capacity rear and ~4500# capacity in the front. Empty, the truck weighs 7200 lbs w/ 4000 or so on the front axle. When we're running at/over GVV, the rear has prob. something like 5000 lbs, and the front is 4200 or so, so if you took the truck
on a skid pad, you'd see oversteer - whether we used the airbags or not.

The airbags allow us to pull the trailer level, a good thing with Airstreams, and they cut down on porpoising significantly - important in CA, where the freeways are often heavily worn.

I do want to add air bags in front, but that's because the front suspension is heavily taxed w/ that 1000 lb diesel engine, and could use a little bracing after 200k miles.

As to leveling the truck w/ the WD hitch, the amount of force required to move a significant amount of weight onto the front axle of a 20' long crew cab pickup would not be good for vintage Airstreams.

- Bart
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Old 07-09-2015, 07:10 AM   #7
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Question Sag & wd

"As to leveling the truck w/ the WD hitch, the amount of force required to move a significant amount of weight onto the front axle of a 20' long crew cab pickup would not be good for vintage Airstreams."

Why?


I tend to agree with the WD option, the purpose of WD is to restore FA weight, weight equals sag, the restoration of which eliminates the 'sag'.

Bob
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Old 07-09-2015, 11:57 AM   #8
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I have a GMC 2500 HD with Firestone Air Bags. I installed them originally when I had a 2000 lbs slide in camper. I installed them for two reasons, #1 to bring the truck back to level (i.e. get rid of sag), #2 to control sway. My 2004 2500 HD would sway/rock with the camper before the air bags. With air bags at about 40 lbs pressure it took out all the sway. We have now sold the camper and upgraded to a 76' Overlander. When my fresh water tank is full I see some sag on the truck. I put about 25-30 lbs in the bags to bring the truck back to normal height, then do up the w/d portion of the hitch. Raising the back end up with the bags will transfer some weight forward simply by geometry.

Air bags are not for additional capacity, but they do allow the truck to handle the capacity better.

With respect to the oversteer, I drive pretty conservatively so I cannot comment. I have two young kids in the truck. My particular 2500 HD has the 6.0l gas engine which is about 800 lbs lighter than the Duramax so right there my front end has additional capacity.

Not an expert but it is what I have experienced.
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Old 07-09-2015, 12:12 PM   #9
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Wow

Many Airstreamer's want to push the envelope, tow with s jeep or a Honda van, just on the hitch, cheap hitch, build my own, modify the TV, buy a expensive hitch

hitching and towing is sort of like riding a motorcycle everyone has their own throttle and handle bars.. screw up and your skin is in the fire or follow good recommendations and quit pushing the envelope adding airbags, hitch modifications and on and on....Life is a highway ride at your own risk.

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Old 07-09-2015, 12:43 PM   #10
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Amen Earl! Everyone is over complicating this. This is a simple issue with a simple solution.... Don't try to tow with a vehicle that is underrated or near it's max. Bottom line. Airbags are not a legitimate solution. If it was tow vehicle manufacturers would put them on vehicles or offer them as an option. They don't for a reason. If you have excessive sag then you are either outside of the manufacturer's specifications or there is a problem with your suspension. Springs get old and weak with time and weight on them. Do yourself and everyone else on the same road as you a favor and address the issue directly.
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Old 07-09-2015, 12:49 PM   #11
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Aftermarket Air Bags are like a girdle...if you need one, somethings wrong.

IMHO AB's do nothing to improve towing, and yes I have tried them.

But TETO...if they make YOU feel better when towing, that's all that matters.

Bob
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Old 07-09-2015, 12:52 PM   #12
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I added rear bags to my 2000 Dodge Ram 2500 4x4 with a HD tow package. Not for added weight carrying capacity but to reduce the jarring of 1000# of tongue weight going across expansion joints on the super slab. Run about 50 #'s in them and it makes my stiff suspension truck ride like a Lincoln Town Car.
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Old 07-09-2015, 12:56 PM   #13
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I've had Firestone AB's on my 2008 F-250 for the past 7 years with the on-board compressor. For me it has definitely helped with leveling my TV and AS, and also as important, it has basically eliminated the dreaded "interstate hop" . The naysayers that have not used AB's just don't know all of the advantages and instant differences a set will make on handling, sway control and overall "feel" of installed AB's.
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Old 07-09-2015, 04:28 PM   #14
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I find it interesting too that almost everyone who has posted on this (me included) have 3/4 ton trucks. Not small TV's.

My truck is a 2012 2500 HD and I really like driving with the truck level. The 2014 and 15 GMC 2500's c/w with factory AB's to reduce sag now.

Should also note the sag on my 2012 is far less than my 2004 was. Weigh capacity on the rear axle is similar.

My father runs a 2008 Chev 2500 HD with a 29 ft Dutchmen Denali and he relies a lot on his W/D hitch to keep him level.
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