VNL Air Bag Replacement

bigredhdt · February 27, 2018

It is time to replace air bags on my non singled volvo vnl. Do i really need to replace all 4 bags since the trailer load is low or can I replace 2 and disconnect the supply to the other bags?

Quickthrotl · February 27, 2018

Im not positive but if you disconnect the bags on one axle it would not hold the axle on the road well and cause it to bounce for lack of better words

Imurphy907 · February 27, 2018

Don't unplug the other axle. As stated above the axle will bounce and ruin those tires in a hurry. Not to mention make a rougher ride. Fix all 4 or single the truck.

chooseopen · February 27, 2018

If it is any consolation the job is pretty simple. I recently replaced the bags on my singled truck in a couple hours. While I had the area torn apart I also cleaned up the frame and spray painted to tidy things up. The bags for my truck cost $115 each from class8truckparts.com

Air up your truck (assuming your bags still hold air)
Place jack stands under frame.
Wait for the old bags to deflate. This didnt take too long with my old leaky bags. Once deflated you will be able to collapse the bag and pull it out.
Use breaker bar to loosen the nut on the bottom of the bags.
Use breaker bar to loosen the nut on the top of the bags. My new bags came with new brackets too.
Unscrew the air supply fitting
Put it all back together in reverse.

NOTE: When I was all done, my bags would not inflate until I jacked the truck up and lowered it off the jack stands. There must be some sort of valve inside them that causes this to happen.

Pics from my project: https://photos.app.goo.gl/GyOFbMmSP3yC0ctG2

lockmup68 · February 27, 2018

good idea to replace shocks at same time.

chooseopen · February 27, 2018

5 minutes ago, lockmup68 said:

good idea to replace shocks at same time.

Yeah it is. For me though I recently had a nightmare of a time replacing a front shock. Since I was doing the bag replacement at an RV park, I didnt want to get into that situation again!

Chad Heiser · February 27, 2018

36 minutes ago, chooseopen said:

NOTE: When I was all done, my bags would not inflate until I jacked the truck up and lowered it off the jack stands. There must be some sort of valve inside them that causes this to happen.

Pics from my project: https://photos.app.goo.gl/GyOFbMmSP3yC0ctG2

There is no valve inside the bags, it was your ride height leveling valve doing it’s job. Since the frame was supported at ride height on jack stands, it would not add air to the air bags. You could also have manipulated the leveling valve to get it to pass air to the bags or what you did made it work as well.

Cotreker · February 28, 2018

At what point do we replace the air bags, years, miles or signs of cracking?

Recently camped and wanted to level the rear of the HDT so i drove up on ramps. Air leaked out of one bag most not have liked to be in that angle.

chooseopen · February 28, 2018

3 minutes ago, Cotreker said:

At what point do we replace the air bags, years, miles or signs of cracking?

I researched this a bit before I replaced mine. I found that cracking is normal. At some point I found a technical bulletin from Goodyear that showed this "normal" cracking. It basically looks like dry rot on tires. I decided to change mine at the point that the "dry rot" cracks started to peel open into what I would call "splits". Flaps of rubber were actually starting to pull away from the surrounding rubber.

rickeieio · February 28, 2018

2 hours ago, Cotreker said:

At what point do we replace the air bags, years, miles or signs of cracking?

Recently camped and wanted to level the rear of the HDT so i drove up on ramps. Air leaked out of one bag most not have liked to be in that angle.

Backing onto blocks may have caused the leveling valve to try to compensate.

beemergary · February 28, 2018

I replaced my last original @ 950,000mi. and 28 yrs. old on my Peterbilt. On mine there's an internal stop so when there's no air truck will come too far down. Bought the cheaper bag and Whoops. On older Freightliners there's an external stop so any generic bag will work-not on mine. It only cost a few dollars more to go 1st. class. Nice thing about old rubber lines with flare fittings just clamp off or put in dime.

beemergary · February 28, 2018

On ‎2‎/‎27‎/‎2018 at 12:56 PM, lockmup68 said:

good idea to replace shocks at same time.

On my dual axle Pete I swear the axles don't really move. Only a little over an inch to fenders and they haven't rubbed. I 'll save my money on the back ones.

bigredhdt · February 28, 2018

I'm having a hard time understanding why the axle without bags would bounce as there is a leaf spring and shock. This suspension setup is what you find on pickup trucks.

My understanding is that the sole purpose if the air bag is to maintain drive shaft alignment.

Thoughts?

DesertMiner · February 28, 2018

:wacko: It appears you are confusing the operation of an air bag in an “air ride” suspension and the need of said air bag being inflated correctly to maintain proper drive line angles. Apples and oranges!

Have you ever driven your truck with the air bags deflated?

noteven · February 28, 2018

If we were to call them air springs which is what they are maybe less confusions might occur?

The level valve maintains ride height as load changes on the chassis.

“Half steel spring” air suspensions still rely on the air spring to carry the load.

Level valve is usually on the rear drive axle and maintains the same psi in all springs to hold ride height when the truck is sitting on level surface.

Some trucks might have dual level valves but they will be side to side on the same axle. They help with lopsided loads. And they will be happily out of synch with each other most of the time ...

beemergary · March 1, 2018

4 hours ago, bigredhdt said:

I'm having a hard time understanding why the axle without bags would bounce as there is a leaf spring and shock. This suspension setup is what you find on pickup trucks.

My understanding is that the sole purpose if the air bag is to maintain drive shaft alignment.

Thoughts?

The air bag is the same as the shakles on the back of a leaf spring. The front of the spring pivots off the front spring hanger. More or less air changes the pinion angles. The secret is to have the pinions or yokes parallel to each other or the back a hair down so when torque is applied the back will go up and be parallel. There's a geometry therm or something that relates to this.

VegasFlyer · March 2, 2018

On 2/28/2018 at 3:16 PM, bigredhdt said:

I'm having a hard time understanding why the axle without bags would bounce as there is a leaf spring and shock. This suspension setup is what you find on pickup trucks.

My understanding is that the sole purpose if the air bag is to maintain drive shaft alignment.

Thoughts?

With no air bags, you do not have a leaf spring, the axle will simply move up and down, pivoting on the front of the leaf spring.

The air bag (Air Spring) does maintain frame height with varying weight loads, however, it also works, in conjuction, with the leaf spring and shock absorbers to absorb axle movement forces.