Torque, Gearing and Wheel Force

Automotive Engineering · CHAPTER 2 / 5

Level: introductory undergraduate · Study session: 15–20 minutes including practice.

Learning outcome: Translate motor torque through an idealised gear reduction.

Before you begin: Complete earlier chapters in this discipline; revise algebra and units as needed.

1. Understand the idea

A reduction gear increases wheel torque while lowering rotational speed. Efficiency reduces transferred power. Wheel force is wheel torque divided by effective rolling radius. Available acceleration may be limited by tyre grip rather than motor torque.

2. Write the model

Tw = Tm i η; F = Tw/r

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

For motor torque 100 N·m, ratio 4, efficiency 0.9 and wheel radius 0.3 m, wheel torque is 360 N·m and tractive force is 1200 N.

4. Explore the model

Figure: calculated teaching cases; straight segments connect sampled points, not experimental observations. Read exact values below.

Illustrative scenario sweep — synthetic educational data
Gear ratio	Wheel torque (N·m)	Wheel force (N)
2	180	600
4	360	1200
6	540	1800

Increase ratio with torque fixed. Compare higher force with lower wheel speed for the same motor speed.

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

What force follows if radius increases to 0.4 m at 360 N·m?

Reveal the answer and check your work

900 N.

Compare your result, units and reasoning. If they differ, return to the equation and test the worked case first.

7. Common mistake

Gearing trades torque and speed; it does not create energy.

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.