Which instrument is used for the measurement of the longitudinal coefficient of friction (of a road surface)?
Try it first: pick an option, then read the explanation.
Asked in SSC JE 24/01/2018 (Morning)
Detailed Explanation
Skid resistance of a road depends on the friction between tyres and the pavement surface. Highway engineers measure both the friction and the unevenness of pavement surfaces, and different instruments are used for each.
Why “Roughometer” Is Correct
The bump integrator is a towed wheel unit that adds up vertical movements to give an unevenness index (cm/km). It says nothing about friction. Of the options given, the instrument the exam expects for the longitudinal coefficient of friction is the roughometer, option (c). (In practice, skid resistance is often measured with skid-test trailers or the portable pendulum skid-resistance tester.)
| Instrument | What it measures |
|---|---|
| Bump integrator | Pavement unevenness (unevenness index) |
| Roughometer | Surface roughness / friction (exam answer for this question) |
| Pendulum skid tester | Skid resistance of the surface |
| Speedometer | Vehicle speed |
Why the Other Options Are Wrong
- A. Bump integrator: The bump integrator measures unevenness, not friction.
- B. Both bump integrator and roughometer: The bump integrator does not measure friction, so ‘both’ is wrong.
- D. Speedometer: A speedometer measures speed.
Key Point
Bump integrator → unevenness; roughometer → friction (as per this question's key).
Key Concepts for Students
- IRC gives a longitudinal friction coefficient of 0.35–0.40 for design.
- Lateral friction coefficient for curves is taken as 0.15.
- Wet and polished surfaces have lower friction.
Practice more: this is question 1 in Friction MCQs – Part 1. Try the whole set and check every answer instantly.