Every trailer suspension job starts with the same question at the counter, and it is almost never the one the caller expects. Not “what is it doing.” Just “air ride or spring ride?”
More often than not, that is where the call stalls. The person on the phone is rarely the person who spec’d the trailer. The unit may be on its second or third owner. And until something goes wrong, nobody has any reason to crawl under it.
At Ressorts Laval / C.T. CAM, we have been rebuilding truck and trailer suspensions for more than 50 years. Our other trailer articles all tell you to have the suspension type ready when you call. None of them tells you how to find it. That is the entire job of this one.
One part settles it, and it only exists on air ride
Start here, because it is the fastest answer and it leaves no room for interpretation.
Look at a crossmember above the axle for a small body with air lines running into it, connected to a metal arm that drops down and links to the axle or the trailing arm through a rod. That is the height control valve, also called the leveling valve, and the arm is what keeps it honest: it reads the distance between the frame and the axle, then adds or dumps air to hold the trailer at one ride height whether it is loaded or empty.
A spring suspension has nothing to level. There is no valve, no arm, no rod. Find that linkage on any axle and you have air ride. Walk the whole group without finding one and you have spring ride. That is the check, and it takes under 1 minute once you know what shape you are looking for.
What does the load-carrying part look like from the side?
If you want to confirm it with your own eyes, stand at the side of an axle and look at what sits between the axle tube and the frame above it. There are only two pictures, and they are nothing alike.
Spring ride. Steel. A pack of curved leaves stacked on each other, clamped to the axle at the center by U-bolts, with a hanger at each end of the pack and often a shackle swinging at one of them. On a tandem, a steel equalizer ties the two axles together. Everything is mechanical, everything is visible, nothing moves while the trailer sits.
Air ride. Rubber. A black cylinder about the size of a five-gallon pail, sitting between a steel trailing arm and the frame, one per side per axle. That is the air spring, commonly called the air bag. There is a piston under it, and the arm carrying the whole assembly pivots on a bushing at a front hanger.
Both types have steel and both types have rubber somewhere. What decides the answer is the part actually carrying the load: a leaf pack, or a bag with air in it.
A trailer carries no compressor of its own
Trailer air comes from the tractor. Drop a trailer in the yard and leave it, and an air ride unit will eventually bleed down and settle, bags wrinkled and squashed flat, looking exactly like something failed. Nothing failed. It has no supply. Any real look at an air suspension happens hooked up and charged, never on a unit parked alone at the back of the lot.
Why the shop asks before anything else
This is not paperwork. 3 practical things hang on the answer.
The parts have nothing in common. A leaf pack and an air spring share no supplier, no shelf and no lead time. Whatever is in stock for one is useless for the other.
The inspection is a different walk. Spring ride means leaves, shackles, spring pins, equalizer bushings and U-bolt clamping force. Air ride means bags, valves, lines and fittings. 2 lists, 2 amounts of bay time.
The trip is the expensive part. Getting a trailer to us costs a tractor moving in and a tractor moving back out. Prepping the wrong list and the wrong parts shelf does not cost an hour. It costs the round trip.
Three clues that fool people
These three signals look convincing, but none of them confirms the suspension type on its own. Only the parts under the axle settle it for certain.
| The clue | Why it proves nothing |
|---|---|
| “There are air lines under it” | There are air lines under every trailer, feeding systems that have nothing to do with the suspension and that fall outside our six specialties. The question is not whether lines exist. It is whether any of them end in a bag. |
| The AIR RIDE lettering on the side | That is a build record, not a current condition. It went on at the factory and nobody scrapes it off when something changes. On a second or third owner unit, treat it as a hint and nothing more. |
| “It rides rough, so it must be springs” | Behaviour does not identify the system. An air ride trailer that is no longer holding its height rides rough too, and that is precisely the kind of fault worth finding. Judging the design by the symptom gets the answer backwards. |
The case nobody plans for: a converted trailer
This is where the build sticker and the original paperwork stop being evidence, and we know the case well because we do the work.
Converting a spring suspension to air, and air back to spring, is something we perform. Which means there are trailers running today whose suspension is not the one they were built with. A unit that was sold, bought back, or moved to a different kind of freight may well have been converted without a single visible change to the outside of the box.
The rule that falls out of this covers every other clue on this page: only what is under the trailer counts. Anything printed on it is history.
The answer worth more than “air” or “spring”
Knowing the type gets the visit prepped. There is a better answer, and it is stamped on the trailer.
Trailer suspensions are built by specialty manufacturers such as Hendrickson, and the assembly almost always carries a plate or a decal with a model number. Look on a crossmember above the suspension, on the trailing arm itself, or near the trailer’s own identification plate. Most of them are filthy enough to look blank until somebody wipes them.
That number beats a category, because it points at one specific assembly with its own components, dimensions and part numbers. Photograph it with your phone instead of copying it out by hand, and file the photo against the unit number.
For a fleet this is a one-time job. One walk around the yard with a phone, one photo of the suspension plate per unit, the type written beside the unit number. The next time a trailer goes down on a Friday afternoon, that question costs nobody twenty minutes.
Still have a suspension question this page doesn’t answer?
Choosing between the two systems. This page identifies what you already own. Deciding what to spec on the next unit is a fleet decision with its own criteria, and we covered it separately.
Symptoms of a trailer suspension problem. Sitting low on one side, bottoming out over bumps, knocking when empty and uneven tire wear are handled in our article on trailer suspension maintenance.
Shock count. An air ride trailer necessarily has shocks; a spring trailer may have been built without any. That distinction carries consequences big enough to need its own page, and it has one.
What do drivers usually ask about identifying a trailer suspension type?
Do both axles on a trailer always run the same suspension type?
Within one axle group, yes, the system is engineered as an assembly. What does vary is something else entirely: two trailers bought the same year for the same fleet may not have been ordered with the same suspension. Never use one unit as the model for another.
My trailer is sitting very low in the yard. Is the suspension dead?
Probably not, and this is the most common false alarm on an air ride unit. With no tractor hooked to it, the system has no supply and bleeds down. Hook it, let it charge, then look again. If it does not come back up, or does not come up evenly side to side, now you have something worth checking.
Can I tell just by driving it?
No, for two reasons. Nobody sits on a trailer, so whatever you feel in the cab arrives through the tractor first. And an air system that is faulty rides hard, which would lead you to exactly the wrong conclusion. This one gets looked at, not guessed at.
The suspension manufacturer’s plate is unreadable. Now what?
Wipe it with a rag and some degreaser first; most of them come back. If it is truly gone, it is not a dead end. The type, the number of axles and a few side-on photos of the suspension are enough for us to prep the visit.
Can a trailer have been converted from one system to the other?
Yes, it happens, and we do that work. That is exactly why the lettering on the box and the original paperwork are not proof. On a unit that has changed hands or changed jobs, only what is underneath settles it.
Does the suspension type change anything about axle alignment?
The measurement targets the same thing either way, meaning where the axles sit relative to each other and to the trailer. What changes is what gets adjusted to get there, and what turns up when a reading is out of tolerance. So the suspension type is useful information before a trailer axle alignment.
Do you work on both types of trailer suspension in Laval?
Both, and we are a spring shop first. On a trailer, the specialties that genuinely apply are suspension and axle alignment, since there is no steering, no manual transmission, no clutch and no differential on that kind of unit. We serve Laval, the North Shore and Greater Montreal.
Under there and still not sure?
Take 2 side-on photos at axle height, one of the plate if you can find it, and call with the unit number. We will tell you what you have and what to plan before you tie up a tractor bringing it in. Ressorts Laval / C.T. CAM, at 450-661-5157.