R, D and B: the construction letter in a tire size

In 265/70R17 the R is easy to skim past. It is the construction code, and it identifies how the body plies inside the casing are laid — which determines almost everything about how the tire behaves.

R — radial

Body plies run radially, straight across the tire from bead to bead at approximately 90 degrees to the direction of travel, with a belt package of steel cords beneath the tread running at a shallow angle.

The two structures do different jobs, and that separation is the point. The sidewall flexes independently of the tread band, so the tread can be held flat against the road while the sidewall deflects over a bump. Radials dominate for that reason: longer tread life, lower rolling resistance, more consistent contact patch under cornering load.

Every passenger tire and effectively every light-truck tire sold new for road use is radial.

D — diagonal bias

Also written as a plain hyphen: 7.50-16 is a bias tire, and the hyphen is doing the same job the D does.

Body plies run diagonally, at roughly 30 to 40 degrees to the centreline, in alternating layers that cross each other. There is no separate belt package, so the sidewall and the tread are a single interlinked structure — flex one and the other moves with it.

The consequences are the mirror image of the radial's:

• The casing is stiffer as a whole and squirms more under lateral load.
• Rolling resistance is higher and tread life shorter.
• The sidewall is more resistant to cutting and puncture, being made of the same crossed plies.

Bias construction is still current in specific places: trailer tires, agricultural and industrial equipment, some competition applications, temporary spares, and reproduction tires for historic vehicles where the original appearance and behaviour matter.

B — bias-belted

A bias casing with a belt package added under the tread. It was the transitional technology of the 1970s, taking some of the radial's tread stability without redesigning the casing, and it has largely disappeared from new production.

ZR, and the letter that is not a construction code

245/40ZR18 looks like a fourth construction type and is not. The Z is a speed rating that historically was declared inside the size rather than in the service description. The construction is radial; the R is still doing its job with a Z in front of it.

Where a tire carries both a ZR in the size and a service description like 96Y, the service description is the specification. The speed rating table covers why Z ended up in two places at once.

The older European marking, where the letter had company

Sizes from before the current service-description convention frequently print two letters where you now expect one: 175 HR 14, 185 SR 14. That is not a construction code of its own. The first letter is the speed symbol and the R is the ordinary radial marking, run together because the speed rating had not yet moved out of the size and into a separate figure after it. Read them as 175R14 plus a speed symbol of H and the marking makes sense immediately.

The same logic explains ZR above. Both are the same historical habit — a speed letter parked against the construction letter — caught at different points in its retreat.

Reading construction off the sidewall when the code is unclear

The letter is not the only place the casing is declared. Regulations covering tires sold in the United States require a radial to carry the word RADIAL moulded into the sidewall, spelled out, separately from the size. On a worn or overpainted tire where the size is hard to read, that word is often the easier find, and its absence on a tire that otherwise looks modern is worth a second look rather than an assumption.

Alongside it, most sidewalls carry a plies-and-materials statement in plain words, listing the tread and sidewall composition separately. That statement is where ply counts and cord materials actually live; the construction letter never carries them.

Where the D still turns up for an ordinary driver

Two places, and neither is under the car. Trailer tires marked ST are frequently bias, which is why a utility or boat trailer is where most people meet a hyphenated size for the first time. The other is the temporary spare in the boot of an older vehicle. In both cases the tire is doing a job where casing stiffness and puncture resistance count for more than tread life, which is the argument for the construction rather than an accident of age.

Why mixing constructions matters

Radial and bias tires respond to load and steering input so differently that mixing them across an axle, or front to rear, produces genuinely unpredictable handling. This is not a subtle compatibility question — it is the reason the construction letter is part of the size code rather than a footnote in a catalogue.

The geometry is unaffected: a bias tire and a radial of the same nominal dimensions compute to the same diameter and the same section width. Everything this site calculates would report them as identical, and they are not.

Where the letter sits in each notation

• Metric: between the aspect ratio and the wheel diameter — LT285/70R17.
• Flotation: between the section width and the wheel diameter — 33x12.50R17.
• Old inch sizes: in place of the separator — 7.50-16 for bias, 7.50R16 for radial.

In all three the position is the same relative to the wheel diameter, which is the reliable way to find it on an unfamiliar marking. The size code page walks through the rest of the elements in order.

What the code does not tell you

Nothing about the number of plies, the cord material, the belt count or the load rating. Those come from the load range letter, the load index and the sidewall's own construction statement — which on most tires lists the ply composition of the sidewall and the tread separately, in words. Load range and ply rating covers what those markings actually encode.