---
title: "Wheel alignment angles and tyre size numerical"
subject: "Automotive Technology"
chapter: 8
key: "semester_5/automotive_technology:alignment-tyre-size-numerical"
source: https://www.bctengineer.com/q/semester_5/automotive_technology/alignment-tyre-size-numerical
site: BCT Engineering
---

# Wheel alignment angles and tyre size numerical

Automotive Technology · Chapter 8: Wheels and Tires

- Asked in: year not given, 4+4 marks
- Video: [Wheel alignment explained & animation: camber, caster toe | toe in toe out explained](https://www.youtube.com/watch?v=OVJ39zW0kJg) — Animated Engineering | Explore the World
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/automotive_technology/alignment-tyre-size-numerical
- Whole chapter: https://www.bctengineer.com/md/semester_5/automotive_technology/chapter-8

## Question

(a) Define camber, caster, king-pin inclination and toe-in, and state the purpose of each. (b) A tyre is marked 205/55 R16. Find the sidewall height, overall diameter, and the number of revolutions per km. If the tyre is replaced by a 215/60 R16, find the new overall diameter and the actual speed when the speedometer shows 100 km/h.

## Short answer

Camber is wheel tilt from vertical (front view) for load distribution; caster is kingpin tilt (side view) for self-centring; KPI reduces scrub radius; toe-in compensates for camber thrust. For 205/55 R16: sidewall height 112.75 mm, diameter 631.9 mm, 504 rev/km. With 215/60 R16, diameter 664.4 mm, actual speed at indicated 100 km/h is 105.1 km/h.

## Answer

#### (a) Wheel alignment angles

| Angle | Definition | Purpose |
|---|---|---|
| **Camber** | Tilt of the wheel from vertical when viewed from the front; positive if the top leans outward | Reduces steering effort and keeps tyre flat on the road when loaded |
| **Caster** | Tilt of the king-pin axis forward or backward when viewed from the side; positive when the top leans back | Gives straight-ahead stability and self-centring |
| **King-pin inclination (KPI)** | Inward tilt of the king-pin axis from vertical, seen from the front | Reduces scrub radius, steering effort, and helps wheel return |
| **Toe-in** | Front of wheels closer than rear (a few mm) | Compensates for the outward push of camber, prevents tyre scrub and wear |

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 %).

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What does positive caster give?
   A. Lower rolling resistance
   B. Straight-ahead stability and self-centring
   C. Reduced tyre scrub
   D. Compensation for camber thrust
   Answer: B. Backward tilt of the king-pin axis at the top makes the wheel trail and self-centre. (Key point: Caster)

2. Why is toe-in provided?
   A. To increase steering effort
   B. To make tyres wear in the centre
   C. To compensate for the outward push of camber and prevent tyre scrub and wear
   D. To give self-centring
   Answer: C. A few mm of toe-in offset camber's outward push so the wheels roll parallel. (Key point: Toe-in purpose)

3. For 205/55 R16, what is the overall diameter?
   A. 631.9 mm
   B. 406.4 mm
   C. 112.75 mm
   D. 518.6 mm
   Answer: A. Sidewall = 0.55 x 205 = 112.75 mm; D = 406.4 + 2 x 112.75 = 631.9 mm. (Key point: Tyre diameter)

4. A 215/60 R16 tyre (D = 664.4 mm) replaces it. When the speedometer reads 100 km/h, the actual speed is about
   A. 110.0 km/h
   B. 100 km/h
   C. 95.1 km/h
   D. 105.1 km/h
   Answer: D. The speedometer counts revolutions calibrated for the old size; actual speed = 100 x 664.4/631.9 = 105.1 km/h (it under-reads). (Key point: Speedometer error)

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