Read the bag, not the trailer
Air springs are identified by a number moulded or stamped on the bead plate, and that number is the fact to order by. Trailer make and model narrow the field but never settle it, because the same trailer model runs different bags across production years and across suspension makes. The suspension manufacturer specifies the spring, not the trailer builder.
Four numbering systems cover almost everything you will meet. Firestone runs W01-358-xxxx and W01-681-xxxx. Goodyear runs 1R12-xxx, 1R13-xxx, 1R14-xxx for reversible sleeves and 1T17 or 1T19 for the taller ones, plus 2B12-300 style numbers for double convoluted bags. Hendrickson uses its own S- and C- numbers such as S-28929, S-21800 and S-21966. Meritor uses plain part numbers like 202796. Any one of those four identifies the bag, and a good listing carries all of them, which is why searching the number off your old bead plate beats searching your trailer model.
If the old bag is destroyed and the number is unreadable, the suspension itself carries an identification tag, and the mounting style tells you a lot: the stud pattern on top, the bottom mount type, and the collapsed and extended dimensions. Those together identify the bag when the number is gone.
Collapsed and extended height is the fit
Two numbers decide whether a bag physically fits your hanger: how short it goes and how tall it goes. The spread across common trailer bags is wide. A Meritor 202796 replacement, Firestone W01-358-8729 or Goodyear 1R13-130, works between 4.7 and 14.0 in. A Hendrickson HT-190T bag, W01-358-9640 or 1R12-367, runs 5.4 to 15.8 in. An INTRAAX AA230 or VanTraax HKA-180 bag, W01-358-8709 or 1R13-159, runs 6.0 to 19.4 in. A Hendrickson INTRAAX HT bag, S-28929, runs 7.9 to 17.5 in, and a VanTraax HKA-200 bag, W01-358-8708 or 1T17CA-6.5, runs 9.1 to 21.5 in.
Taller again are the reversible sleeves that serve Hendrickson, SAF-Holland and Ridewell suspensions: W01-358-9177 or 1R14-028 covers 7.2 to 21.7 in, and the Neway and Holland AR120 and AR125 bag, W01-358-9158 or 1R14-030, stretches from 8.7 all the way to 27.3 in. At the other end, a trailer lift axle usually takes a short double convoluted bag rather than a sleeve, and W01-358-7424 or 2B12-300 does that job in 3.4 to 11.0 in.
That range, 3.4 in at the low end to 27.3 in at the high end, is why a bag that looks similar is not similar. Check both dimensions against your hanger before you order, and check the bellows type while you are there. A single convolution reversible rolling lobe and a double convoluted bag behave differently even when their heights overlap.
Ride height is the setting that matters
The single most misunderstood thing about air suspension is that pressure is not the setting. Ride height is. The levelling valve adds and dumps air to hold your frame at a fixed height, and pressure simply lands wherever the load requires. A trailer that rides low is not short of pressure. It has a levelling valve out of adjustment, a linkage that has come loose, or a leak the valve is chasing.
That matters when you replace a bag, because a fresh bag on a trailer with a mis-set valve fails early. Set the ride height to the suspension maker's dimension after the change, and check that the bag is not contacting the frame or a crossmember anywhere through its travel. Almost every premature bag failure is contact wear rather than a manufacturing fault.
What kills bags, and replacing in pairs
Bags die from chafing against something they were never meant to touch, from running at the wrong ride height so the rubber works outside its designed travel, and from ozone and heat cracking over years. A bag that has been run flat for any distance is finished even if it still holds air, because the piston has been working against folded rubber.
Replace across an axle rather than one corner. Bags stiffen as they age, so a new one beside an old one changes how the load splits across that axle and shows up as uneven tyre wear before it shows up as anything else. While the corner is apart, look at the shock, because a bag that failed from bouncing usually has a dead shock behind it.
Frequently asked questions
How do I identify the right air spring for my trailer?
By the number on the bead plate, not by the trailer make and model. Look for a Firestone W01-358 number, a Goodyear 1R12, 1R13 or 1R14 number, or a Hendrickson S-number. The same trailer model runs different bags across production years and suspension makes, because the suspension manufacturer specifies the spring.
What do collapsed and extended height mean when ordering?
They are how short and how tall the bag works, and together they decide whether it fits your hanger. Common trailer bags run anywhere from 3.4 to 11.0 in for a short lift axle bag up to 8.7 to 27.3 in for a tall Neway AR120 sleeve, so two bags that look alike can be nowhere near interchangeable.
What is the difference between a reversible sleeve and a double convoluted bag?
A reversible sleeve, or rolling lobe, is a single convolution that rolls over a piston and gives a softer, longer stroke, which suits air-ride axles. A double convoluted bag is shorter and stiffer for its height and is what you normally find on a lift axle, such as the 2B12-300 pattern that works in 3.4 to 11.0 in.
My trailer rides low. Do I need more pressure?
No. Ride height is the setting and pressure lands wherever the load requires. Riding low points to a levelling valve out of adjustment, a loose linkage, or a leak the valve is chasing.
Should I replace air springs in pairs?
Across an axle, yes. Bags stiffen with age, so a new one beside an old one changes how load splits across that axle and usually shows up as uneven tyre wear first.
Why did my new air spring fail early?
Most often contact wear or ride height. A bag rubbing a frame rail or crossmember through its travel, or working outside its designed travel because the levelling valve is mis-set, will not last regardless of how good the bag is.
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