Wheels and Tires

Construction of a tubeless radial tyre

 Tread ----------- (grooves)
 Belts (steel) -- under tread
 Carcass plies (radial cords, 90 deg to bead)
 Inner liner (air-tight, replaces tube)
 Sidewall      Bead (steel wire) seats on rim

• Tread: thick rubber with a pattern for grip and drainage; the part that wears.
• Belts: 2 steel (or fabric) layers under the tread, which stiffen it.
• Carcass (body plies): polyester or steel cords that give strength and hold the air pressure.
• Inner liner: butyl rubber, air-tight layer in a tubeless tyre.
• Sidewall: protects the carcass and displays markings.
• Bead: high-strength steel wire bundle that locks the tyre to the rim.

Bias-ply vs radial-ply

Factors affecting tyre life

• Inflation pressure, vehicle overloading and speed.
• Wheel alignment (toe, camber) and balance, suspension condition.
• Driving habits: sharp braking, cornering, fast starts.
• Road condition and temperature; tyre rotation and storage.

Effects of incorrect inflation

• Under-inflation: wear on both shoulders, higher rolling resistance, heat and possible failure, poor steering.
• Over-inflation: wear in the centre, harsh ride, reduced grip and more risk of impact damage.

(a) Wheel alignment angles

Wheel balancing (static and dynamic) corrects uneven mass of the wheel and tyre by adding weights, to prevent vibration.

(b) Tyre size

Marking 205/55 R16: width $w = 205$ mm, aspect ratio $55\,\%$, radial, rim diameter 16 in $= 406.4$ mm.

$$\begin{aligned} h &= 0.55 \times 205 = 112.75\ \text{mm} \\ D &= 406.4 + 2(112.75) = 631.9\ \text{mm} \\ C &= \pi D = 1.985\ \text{m} \\ \text{rev/km} &= \frac{1000}{1.985} = 504 \end{aligned}$$

New tyre 215/60 R16:

$$\begin{aligned} h' &= 0.60 \times 215 = 129\ \text{mm} \\ D' &= 406.4 + 2(129) = 664.4\ \text{mm} \quad (5.1\,\%\text{ larger}) \\ \text{rev/km} &= \frac{1000}{\pi \times 0.6644} = 479 \end{aligned}$$

The speedometer counts wheel revolutions, so it reads according to the old diameter:

$$v_{actual} = 100 \times \frac{664.4}{631.9} = 105.1\ \text{km/h}$$

Answer: sidewall $= 112.75$ mm, $D = 631.9$ mm, $504$ rev/km; new $D = 664.4$ mm; actual speed is $105.1$ km/h when the speedometer reads 100 km/h (it under-reads by about 5 %).