A cylinder rolling down an inclined board is a commonly seen and interesting object to study and it
is also easy to experiment with and model. Following what has become a popular practice, we use
smartphones to measure the angular acceleration of a cylinder rolling down a plane of different
inclining angles. The friction force deviates from the theoretical result near the critical angle,
showing an anomalous bump for the angular acceleration in the neighbourhood of the transition point
from pure rolling to rolling with slipping. To give a concise and plausible explanation, we assume
that the friction coefficient bears a random component along the rolling down process, which has
both an average value and a random component. Based on the formulation, the numerical simulation
matches the experimental data well, which confirms the validity of our assumption.