Radial vs bias tractor tyres

One of the first choices when buying a tractor tyre is whether it is radial or bias (cross-ply). They look similar from outside, but their construction and field behaviour differ. The right choice depends on how much field versus road work the tractor does, and on budget.

How to tell from the sidewall

The size itself tells you. A size like 520/85R38 with an “R” is radial. A size like 18.4-38 with a dash, or no letter, is bias. Bias tyres usually show a ply rating (e.g. 12 PR); radial tyres show a load index and speed symbol (e.g. 146A8).

Construction

In a bias tyre the carcass plies cross each other diagonally, so sidewall and tread work as one stiff body. In a radial tyre the carcass cords run straight from bead to bead, with separate belts under the tread. The sidewall can flex while the tread stays flat, giving a larger footprint.

Radial advantages

• Larger footprint: more traction, less wheel slip.
• Works at lower pressure, so less soil compaction.
• Runs cooler on the road and usually lasts longer.
• Smoother ride and less operator fatigue.

Bias advantages

• Usually cheaper to buy.
• The stiff sidewall can resist cuts better in stony or stumpy ground.
• An economical choice for older or lightly used tractors.

Can they be mixed?

Never mix radial and bias tyres on the same axle; their rolling circumference and flex differ. On 4WD tractors the front-to-rear ratio also matters, and mixing constructions between axles can stress the driveline when front drive is engaged. Keeping all four tyres the same construction is the safest approach. See the 4WD front–rear tyre ratio guide.

Converting sizes

When moving from a bias size to a radial one (e.g. 18.4-38 to 520/85R38), manufacturers publish equivalence tables. The replacement must have a similar outside diameter and width and must clear fenders and frame. Tractor-Size shows your model’s factory size; always confirm the outside diameter with your dealer before switching.