Automotive Engineering · CHAPTER 1 / 5
Level: introductory undergraduate · Study session: 15–20 minutes including practice.
Learning outcome: Separate rolling resistance, drag and acceleration demand.
Before you begin: Basic algebra, SI units and a willingness to sketch the problem.
1. Understand the idea
A longitudinal vehicle balance adds forces opposing motion and the force needed for acceleration. On a level road, a simplified model includes rolling resistance and aerodynamic drag. Wind, gradient, tyres and drivetrain losses can matter. State whether speed is relative to the ground or air.
2. Write the model
F = ma + Crr mg + ½ρCdAv²
Define the symbols from the worked case before substituting values. Keep units consistent, record the assumptions and round only after the calculation.
3. Follow a worked example
At steady 20 m/s with m=1000 kg, Crr=0.01, ρ=1.225 kg/m³ and CdA=0.6 m², rolling resistance is 98.1 N and drag is 147 N. Total road load is 245.1 N.
4. Explore the model
| Speed (m/s) | Drag (N) | Total road load (N) |
|---|---|---|
| 10 | 36.75 | 134.85 |
| 20 | 147 | 245.1 |
| 30 | 330.75 | 428.85 |
Sweep speed with no wind and zero gradient. Compare the fixed rolling term with the rising drag term.
5. Reproduce it yourself
- Copy the input cases into Excel or a notebook and name each quantity and unit.
- Recalculate each result from the stated equation or rule. For qualitative cases, explain the state or partition decision instead.
- Change one input, predict the direction of change, then calculate again.
- Write one sentence explaining where the model stops being valid.
6. Check your understanding
How much extra force is needed for 1 m/s² acceleration at 1000 kg?
Reveal the answer and check your work
1000 N, ignoring rotating inertia.
Compare your result, units and reasoning. If they differ, return to the equation and test the worked case first.
7. Common mistake
The simple rolling coefficient is treated as constant only for this exercise.
Further study and academic note
Explore the discipline reference and further learning resources. This is an original IIMEEE introductory teaching note with synthetic examples; it is not a peer-reviewed research result or a professional design specification. Figures were constructed from the listed teaching cases. Resource links support further study and do not imply endorsement.
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