Wheels and tyres: construction, designation and tyre wear

Why it matters

Every force that steers, drives, brakes or supports the vehicle passes through four small tyre contact patches. The tyre's construction decides its grip, rolling resistance, comfort and life; its designation tells you whether a replacement fits and can carry the load at the speed; and the wear pattern on an old tyre is one of the quickest diagnostic tools in the workshop.

Key ideas

Wheel. Pressed-steel disc wheels (cheap, tough — commercial vehicles and base cars), cast or forged aluminium-alloy wheels (lighter, better heat dissipation from brakes), and wire-spoke wheels (two-wheelers). The rim is drop-centre (well-base) so the bead can drop into the well when the tyre is fitted; tubeless rims have humps that keep the bead seated if pressure drops. A rim is designated, for example, 6.5J × 16 ET45: 6.5 in rim width, J flange profile, 16 in bead-seat diameter, offset (ET) 45 mm between the rim centre plane and the hub mounting face. Offset affects scrub radius and track.

Tyre construction.

• Bead — high-tensile steel wire hoops wrapped in rubber; anchor the tyre to the rim and seal a tubeless tyre.
• Carcass (body plies) — rubberised textile (polyester, nylon, rayon) or steel cords running from bead to bead; they contain the air pressure, which actually carries the load.
• Belts (breaker) — layers of steel cord under the tread in radial tyres; they stiffen the tread and keep it flat on the road. A nylon cap ply restrains growth at high speed.
• Tread — the wearing rubber with its pattern of grooves and sipes, which clears water from the contact patch and gives grip on wet roads.
• Sidewall — flexible rubber protecting the carcass; carries the markings.
• Inner liner — butyl rubber layer that keeps air in a tubeless tyre.

Bias-ply versus radial. In a bias (cross-ply) tyre the cords run diagonally from bead to bead at roughly 30–40° to the centre line, alternate plies crossing; the tread and sidewall flex together, so the tread squirms. In a radial tyre the carcass cords run at about 90° to the direction of travel (radially, bead to bead), and a stiff belt stabilises the tread. The sidewall flexes freely for comfort while the tread stays flat: lower rolling resistance (better fuel economy), less heat, longer tread life, better grip and cornering. Bias tyres survive mainly on some trucks, tractors and off-road vehicles because of their tough sidewalls.

Tubed versus tubeless. Tubeless tyres seal on the rim with an inner liner; a puncture usually leaks slowly instead of deflating at once, and they run cooler.

Designation — 205/55 R16 91V.

• 205 = section width (mm).
• 55 = aspect ratio: section height is 55 % of the width.
• R = radial construction (B for belted bias; a dash or D for bias).
• 16 = rim (bead-seat) diameter in inches.
• 91 = load index → 615 kg per tyre (from the load-index table; e.g. 95 → 690 kg).
• V = speed symbol → 240 km/h maximum (S 180, T 190, H 210, V 240, W 270, Y 300 km/h). Also on the sidewall: manufacturing date code (week and year), tread-wear and traction grades on some tyres, maximum load and pressure, and tread-wear indicators — small bars in the grooves that show when about 1.6 mm of tread remains.

Tyre wear patterns.

• Both shoulders worn → under-inflation (or overload).
• Centre worn → over-inflation.
• One shoulder worn → camber fault (outer edge: positive; inner edge: negative), or hard cornering.
• Feathered (saw-tooth) ribs → wrong toe.
• Cupping or scalloping → worn dampers, wheel imbalance or loose bearings.
• Flat spots → wheel lock-up under braking. Rotating tyres at the recommended interval evens out front/rear differences.

Rolling resistance comes mainly from hysteresis in the flexing rubber; it rises with under-inflation and load and accounts for a significant share of fuel used at urban speeds.

Formulas

H = AR × W / 100

• H = section (sidewall) height (mm), AR = aspect ratio (%), W = section width (mm).

D = d_r × 25.4 + 2·H

• D = nominal overall (unloaded) diameter (mm), d_r = rim diameter (inches).

n = 10⁶ / (π · D) (revolutions per km, D in mm)

• Nominal value; the loaded rolling radius is a few percent smaller than D/2, so real revolutions per km are slightly higher.

v_true / v_ind = D_new / D_old

• Speedometer reading after changing tyre size; the speedometer counts wheel revolutions.

F_rr = C_rr · W, P_rr = F_rr · v

• F_rr = rolling-resistance force (N), C_rr = rolling-resistance coefficient (≈ 0.008–0.015 for car tyres on asphalt; take from tyre data), W = vehicle weight (N), P_rr = power (W), v = speed (m/s).

Worked examples

Example 1 (standard) — dimensions from a designation. Find the sidewall height, overall diameter and nominal revolutions per km of a 205/55 R16 tyre.

1. H = AR × W / 100 = 55 × 205 / 100 = 112.75 mm.
2. D = d_r × 25.4 + 2·H = 16 × 25.4 + 2 × 112.75 = 406.4 + 225.5 = 631.9 mm.
3. n = 10⁶ / (π · D) = 10⁶ / (π × 631.9) = 503.7 rev/km. H = 112.75 mm, D ≈ 632 mm, n ≈ 504 rev/km.

Example 2 (GATE level) — upsizing and its effects. A car's 185/65 R15 tyres are replaced with 205/55 R16. (a) When the speedometer reads 100 km/h, what is the true speed? (b) For a 1200 kg car with C_rr = 0.012, what power is spent on rolling resistance at that true speed?

1. Old tyre: H = 0.65 × 185 = 120.25 mm; D_old = 15 × 25.4 + 2 × 120.25 = 381 + 240.5 = 621.5 mm.
2. New tyre: D_new = 631.9 mm (Example 1).
3. v_true = v_ind × D_new / D_old = 100 × 631.9 / 621.5 = 101.7 km/h.
4. F_rr = C_rr · W = 0.012 × 1200 × 9.81 = 141.3 N.
5. P_rr = F_rr · v = 141.3 × (101.7 / 3.6) = 141.3 × 28.25 = 3.99 kW. (a) ≈ 101.7 km/h (the speedometer under-reads by about 1.7 %); (b) ≈ 4.0 kW. The new size also changes overall gearing by the same 1.7 %.

Common mistakes

• Taking the aspect ratio as a fraction of the rim diameter; it is section height ÷ section width.
• Mixing inches (rim) and millimetres (width) — convert the rim with 25.4 mm/in.
• Using only one sidewall height in the overall diameter; there are two (above and below the rim).
• Reading the load index (91) as kilograms; it is a code to be looked up.
• Calling "R" the rim; it denotes radial construction.
• Blaming every edge wear on alignment — check inflation first.

For GATE ME

Tyre topics appear as designation and dimension questions (sidewall height, overall diameter, revolutions per km, speedometer error), as rolling-resistance and tractive-effort inputs to vehicle-performance problems, and as conceptual questions on radial versus bias construction and wear diagnosis. Practise converting a designation into rolling radius for gear-ratio and speed calculations.

Quick check

1. What is the sidewall height of a 195/60 R15 tyre?
2. What is its overall diameter?
3. Which wear pattern indicates over-inflation?
4. What does the speed symbol H stand for?
5. Why does a radial tyre have lower rolling resistance than a bias tyre?

Answers: 1. 117 mm. 2. 381 + 234 = 615 mm. 3. Centre wear. 4. 210 km/h. 5. Its stiff belted tread does not squirm, so less rubber is flexed and less energy is lost as heat.