How is slip expressed as a percentage?

• A. [(Nr - Ns) / Nr] × 100
• B. [(Ns - Nr) / Nr] × 100
• C. [(Ns - Nr) / Ns] × 100
• D. [(Nr - Ns) / Ns] × 100

Answer: (C)

Slip is how far the wheel’s rotation lags behind the vehicle’s actual motion, expressed as a percentage of the vehicle’s speed. It compares the wheel’s peripheral speed Nr to the vehicle speed Ns and takes the difference (Ns minus Nr) divided by Ns, then times 100. This makes slip 0% when the wheel perfectly matches the vehicle speed (Nr equals Ns) and 100% when the wheel isn’t turning relative to the road (Nr is zero). If the wheel somehow spun faster than the vehicle (Nr greater than Ns), the result would be negative, which signals a different operating condition. The other forms are not correct because they either divide by the wheel speed Nr or reverse the subtraction, which misrepresents how much of the vehicle’s motion isn’t being captured by wheel rotation.