Trailer tires: ST is not a truck tire, and pressure is the whole game

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Research. We have not hauled anything here yet — no trailer, no miles. What follows is how trailer tires are specified, which of those specs mislead a first-time buyer, and the maintenance that decides whether a set lasts. When we have run a season on our own tires, this page gets updated with the date.

A trailer tire almost never wears out. It ages out, or low pressure destroys it, and both of those are on you.

ST is a different tire, not a cheaper one

ST stands for Special Trailer. Stiff sidewall, shallow tread, built to be dragged in a straight line under a constant load. It never steers and never puts power down.

A light-truck tire is built for the opposite job. Its sidewall flexes, which is what you want under a steering axle and exactly what you do not want under a trailer — that flex aggravates sway — though sway is a tongue-weight and load-distribution problem first, and no tire choice fixes a badly loaded trailer. At the same nominal size it usually carries less load, too. The truck's takeoff set is a downgrade both ways.

Speed rating catches people. Unless the sidewall carries a speed symbol, treat an ST tire as rated to 65 mph. Faster ones exist and are marked: L is 75, M is 81. Above the rating heat builds faster than the casing sheds it, and the failure is a tread separation, not a slow leak.

Do not mix radial and bias tires on one trailer.

Load range is a pressure, not a promise

Load range C, D and E correspond to 6, 8 and 10 ply ratings — a construction equivalence, not a count of anything. Nobody is stacking ten plies in there.

What matters is the capacity stamped on the sidewall, and it carries a condition printed right beside it: at a stated cold inflation pressure. Run that tire 15 psi low and you no longer own the tire on the sticker. You own a smaller one.

Size against the axle rating and the trailer's GVWR from that certification label, not against what you plan to haul. Two 3,500 lb axles give a 7,000 lb rating, so four tires need 1,750 lb each — and that is the floor. Load never splits evenly: crowned roads load one side, hay stacks off center, animals stand where they want. Ten to twenty percent of margin costs little.

Two things this audience has to hold on to. A live load moves — cattle shift under braking and in corners, and the transient load on one tire goes well past its share of the static weight. And fitting a bigger tire does not raise the trailer's GVWR or its axle rating. The tire stops being the limit; the trailer still is. The only way to know what you are actually running is to weigh the loaded rig on a scale.

The date code, and why tread depth lies

Every tire carries a DOT serial whose last four digits are the week and year it was built: 2419 is the 24th week of 2019. Only one sidewall has to carry the complete serial, so a partial code on the outside means the full one is inside — under the trailer, with a light.

Trailer tires age out before they wear out. Three to five years of service is the working expectation, and most ST manufacturers cap service life around five years from the date code regardless of tread. Six is the outside, and for a trailer carrying animals it is worth erring short. Trailers sit for months while sun, ozone and a static load work on the casing, and the tread stays deep through all of it. A used trailer advertised with "good tires" usually means tread is present.

Check the code on tires you buy new, too. One that sat in a warehouse two years arrives two years old.

Cold pressure is the whole game

Cold means the trailer has not moved in three hours, or has gone under a mile.

There is no door placard, but there is a label. US trailers carry a federal certification tag, usually on the left front, listing GVWR, axle ratings, tire size and the cold inflation pressure the trailer was built around. Find it before you guess. ST tires generally run at the maximum cold pressure on the sidewall rather than a lower number picked for ride — but check the wheel too. Rims carry their own maximum pressure rating, and inflating an 80 psi tire on a rim rated 65 will burst the wheel, usually while you are standing over it.

Underinflation causes most blowouts. A soft sidewall flexes, flexing makes heat, heat separates the belts from the casing. None of it shows on the outside first.

Pressure falls with temperature, and the familiar "1 psi per 10 degrees" comes from passenger tyres at about 30 psi. It scales with absolute pressure, so at ST inflation the loss is closer to 2 psi per 10 degrees. A set aired to 65 psi in August is nearer 57 on a January morning — not 60 — and that is the trip where the trailer is loaded. Check them cold in winter, every time. Do not bleed pressure out of a hot tire. It is supposed to go up.

One failure gets blamed on the tire unfairly: a dragging brake or dying bearing cooks it from the hub outward. Check each hub at every fuel stop — but a failing hub reaches temperatures that will burn you on contact, so hold the back of your hand near it rather than pressing a palm to it. An infrared thermometer costs little and is the right tool; it also gives you a number to compare hub to hub, which is what actually tells you something. A hot hub is a warning that arrives before the blowout — the same arithmetic as a gate latch, with a worse ending.

What we are buying

Tires are not the first purchase on this page. The trailer is, and there is no trailer here, so the set that matters is still on somebody else's lot with date codes we have not read. When one is in the yard, this section records what came on it: size, load range, and the last four digits of every DOT serial, spare included. The cold pressure we settle on goes here too, against the sidewall figure and the certification label, with what the spare, jack and torque wrench cost and the date. Whether the trailer's lugs take the same socket as whatever tows it gets answered standing over a wheel, not in a catalog. Buying does not move the Research tag at the top. That waits on a season run on our own tires: whether the pressure held through a winter, and what the hubs ran at fuel stops in summer heat.

Before you buy

• Read the date code before you pay, new or used. It is the one spec on a tire that cannot be improved later.
• Buy the spare, a jack rated for the load, and a torque wrench in the same order. Re-torque new wheels to the trailer manufacturer's spec, in a star pattern, after the first 25 to 50 miles and again around 100. A wheel leaving a loaded stock trailer is somebody's fatality.
• Set every tire to sidewall cold pressure before the first trip, at home, not warm at a gas stop.

— Chris D., McEwen, Tennessee