Rear Suspension Air Bags Experiment

The stock suspension on our 2015 Airstream Interstate was leaving much to be desired. Over bumps, we were experiencing plenty of dipping, bouncing and swaying. Too much of a spongy feel for my taste.

I then installed a Hellwig rear anti-sway bar, Koni shocks and Sumo Springs but did not like the result in the rear. Cornering was very nice, but we were not soaking up the bumps. Entering or leaving a parking lot became worse… the side to side rocking had less oscillation than before but it was more violent and harsh.

By late Spring 2018 we were living in this ClassB RV full time. Our home was rented out for a year and we were packing up to leave on a cross country trip to New England. I needed a better solution. A company in Tualatin Oregon that proclaims to be Sprinter experts convinced me to let them remove the Sumo Spring rubber towers and put in a pair of Boss airbags. Air bags are great, right? Maybe this low-cost ($1000 installed) system would be a good solution… after all, these guys really know Sprinters, right? Not so fast.

The Boss airbags went in and we were sent on our way with 35 lbs of pressure. The next morning we went on a short trip to test everything out and discovered we were banging hard on every little bump. Something was wrong. I checked the air pressure in the bags and the pressure was gone. I got out the air compressor and inflated them back up. It was Saturday and that shop was closed.

I crawled underneath to view the installation and learned that the air bags are not very tall and, like the Sumo Springs, they occupy the space between the axle tube and the chassis frame. The Boss air bag features thick metal mounting plates above and below a black rubber pillow. Due to the geometries of the Sprinter chassis, the top metal plate sits at about a 10 degree angle and this uses up some of the space. Imagine squeezing a marshmallow at just one edge to get a visual on that. Only two inches or so of travel can take place before the two plates strike each other. With OEM rubber bump stops removed, the airbag top and bottom metal plates now crash together if there is not enough air pressure. The picture attached shows the bags with 50lbs of pressure.

Then the battery fell out.

I stopped to fill the air bags several times. As we neared our destination, we crossed a couple of short bridges that had abrupt pavement seams. I didn’t know the airbags were low on air and then BAM!

I heard a terrible racket and pulled off the road immediately. That’s when one of the house batteries, mounted underneath the coach behind the rear wheels, had decided it was time to abandon its moorings and hurtle itself to the pavement. I crawled underneath to find one of the batteries laying on the ground, still in its metal cage and attached by one of its two massive battery cables. I disassembled the mess and noticed the rivets designed to hold the battery cage to the chassis had completely sheared off. If this had happened a few minutes earlier on this trip, the 55mph twisty mountain road we had been on had no place to pull off and it could have been disastrous to stop, even deadly. And what if the second battery cable had snapped? That’s a 65lb battery that would have caused major damage to another vehicle at the very least.

What happens next? Grab the popcorn.

There was only one RV repair place close by. Fortunately they were willing to squeeze us into their schedule and we decide the best short term fix was to put both batteries into the rear hidden storage compartment that exists under the floor of the Interstate Grand Tour. Originally, the battery mass was centered at 30 inches behind the rear axle, it is now about 55 inches. With 135 lbs of batteries sitting that far back it acts like a kind of lever on the rear suspension and I can definitely feel the effect. It’s like driving a small pickup truck with a few bags of cement sitting just inside the tailgate. This can’t be helping the airbag situation.

We went back to the company in Tualatin that installed the airbags... calm, but not too happy. They convinced me that once they repair or replace the air bags the chassis really needs to have a lift kit, consisting of two extra leaf springs plus a spacer that would lift the body to increase the ground clearance and allow those air bags more vertical room to travel.

Two extra leaf springs for more load support

Their theory being that, with the added stiffness of the leaf springs, the air bags could then serve more to dampen motion rather than acting as load support, and I could run a lower pressure setting.

Sounds great, right? On that day, they did the right thing, as far as customer service goes, and installed the lift kit at a price I couldn’t refuse since I had just spent $400 on emergency battery repairs thanks to a defective product and/or poor workmanship.

And then the cabinets started falling down.

Now the bags are losing 4-5lbs per day. The extra leaf springs produced an absolutely terrible ride but now we were on a schedule, heading for New England. We found ourselves cringing every time we saw a pothole looming into view. Now I know what it’s like to drive a dump truck with nothing loaded in the back. And yet we were traveling fully loaded, at the manufacturer’s rated weight capacity!

The big, gradual wallowing bumps were getting soaked up very nicely but the stiffness from the leaf springs could not cope with any abrupt elevation changes, like you find on many pavement seams, broken pavement and bridge overpasses. We’re feeling every ½ inch. The whole coach was being tortured toward an early demise. By the time we got to South Dakota, the upper cabinets over the galley started showing signs of separating from the ceiling at one end. I installed a heavy duty tension rod to arrest their descent, where it remains to this day.

My wife started talking about the thousands of jolts causing back pain and she was threatening to buy a plane ticket if I didn’t get this fixed... PRONTO! I diverted our route a little and we rolled into Kankakee Spring Co. in northern Illinois to get help. They had the same opinion… we had “too much spring” (leaf spring) and we collectively decided to remove just one of the two extra leaves but leave the spacer that gave some lift to the chassis. The end result was a reduction in road violence of about 30%, but would it be enough? Oh, and they found the air line had not been trimmed cleanly by the installer so they snipped the ends fresh… now I lose “only” about 2lbs of air per day.

On we trudged through Indiana, Ohio, Pennsylvania, New York, Vermont, New Hampshire and Maine. Central Iowa’s I-80 had been terribly rough… but southern New York’s I-88 was even worse. I concluded the ride is just not good enough; there’s still too much hammering on sharp bumps. It pains me to admit that we would have been better off leaving it all original, except for the upgrade to better shocks, that is.

Maybe if we take out the other extra leaf spring it will be all better. Keep the airbags and the extra height from the spacers that had been installed. Right now the airbags like to be at about 40 psi. Less pressure is just too bouncy-bouncy on medium bumps and a bad hit can still cause the airbag plates to collide.

Next stop: Palmer Spring in Portland, Maine

Over the next six weeks and 5000 miles I concluded the ride is just too rough. I’m ready to end this nightmare. The folks at Palmer Spring agree that taking out the remaining extra leaf spring is the best next choice. This returned the leaf springs to the original setup but again we kept the extra spacer to help those airbags have some room to compress before the mounting plates could meet and bang together.

I had originally been told back in Oregon the spacer was one inch… it’s actually ¾ of an inch or less. The height of the bag between the top and bottom plates is about 2-3/8” with 50lbs of pressure, and only 1-7/8” at 25lbs. I think I’d better get accustomed to keeping plenty of air in them, which of course affects ride quality. I wish I could run them with just 15-20 lbs for a gentle assist and let the Koni shocks do more of the work.

I know that Air-ride equipped Interstates use the VB air system for about a $7500+ upcharge. And someone I met in Arizona went to Advanced RV and got the same system installed on his RoadTrek for about $8000. It’s a completely different and well engineered design. Those bags are much larger and have lots of luscious cushioning air volume. Maybe someone reading this can comment whether that system dampens the rebound stroke as well as the compression stroke? These little Boss airbags are just dumb pillows that only assist with compression… and then propel the chassis upward, causing a secondary bounce when gravity takes hold again.

The Journey Back to Oregon

We traveled south through Kentucky, kissed the NW corner of Tennessee, then straight west to southern California and back to Oregon. I could finally tell what is happening with the airbags without the offensive harshness of the extra leaf springs. I’ve been running with 40 lbs of pressure, thinking this was the optimum level. They are losing about two lbs of air per day so I use a bicycle pump to quickly check and add air as needed. There is so little air volume in these bags due to their small size that it only takes one stroke of the tire pump to add 2-3 lbs.

So what do I think about the Boss airbags? They are not up to the task. Oh, they are a tough product for sure, but the top and bottom plates use up half the vertical space between the axle and the frame so the bags can only offer about 2 ½ inches of travel, making it necessary to run higher pressure in the bags. Have you ever sat and bounced on one of those really firm gym balls? Yeah, it’s like that. Really freaking annoying.

I am tempted to have these bags removed. I never did experience what it would be like to have just the upgraded Koni shocks with no Sumo springs and no airbags. That Sprinter shop in Tualatin Oregon wants $200 to remove them… but they haven’t returned three phone calls in two weeks. They also admitted they’d only installed this setup on one other vehicle after saying there are hundreds of these installations on other Sprinter based motorhomes (suggesting, what, I’m imagining a terrible ride?). No, they won’t be getting any more of MY money. Yes, they tried to make things right by installing the lift kit, but that is the worst Sprinter "upgrade” you can install in my opinion. The added leaf springs caused me great angst and $600 in expense to remove one pair, and then the other, while on our 12,000 mile journey away from Oregon.

As soon as we returned to Oregon, I had the Airstream dealer remount the batteries underneath the coach, closer again to the axle, but this time with a re-engineered sturdy system of holding them in place.

Since we run fully loaded, carrying 4000 lbs in the front and 7000 lbs in the rear with all tanks half full. I’ve been trying tire pressures of +4 to +7 above the 61lbs Mercedes recommends because I thought it was needed with our weight. I decided to reduce my tire pressures to +2 so they would help absorb more shock. It’s not ideal, but it is an improvement. Some of the bouncy bouncy action is gone. It’s like I’ve just got really, REALLY firm shocks back there. I may still have these airbags removed or I might crawl underneath and try to do it myself. I’ll let you know.

Conclusion? Upgrade your shocks, absolutely, big improvement. I like the Koni shocks but I would try Bilsteins next time or, better yet, go with the expensive Fox shocks that some have raved about. Tip? Just my opinion, but I say avoid Sumo Springs, avoid small airbags and never, ever let an “expert” Sprinter shop talk you into adding leaf springs.

UPDATE: JUNE 10, 2019

Well Hallelujah, and pass the biscuits !!

I got those damn Boss air bags excised this morning. Wow, what a positive difference. It's no longer a bar fight going on back there; in fact, the rear is a little more supple on bumps now than the front. To recap: now I have Konis all around and the Hellwig rear sway bar.

Note to self: when it comes to suspension modifications, make one change at a time and evaluate the ride before you take another $tep.

Oh, and it cost $95, not $200, at a different shop that specializes in suspension work. One more reason not to use the Sprinter Store ever again.

© 2019 by WinstonsAdventures

  • Instagram Black Round